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Combi boiler - condensing - or not?
Hi - I'm new to this forum but would appreciate some helpful advice.
I live in a flat and am having the kitchen refitted, which means moving or replacing our existing boiler (Alpha Ocean Style FF - about 12 yrs old). I'm told that if we replace it we have to have a condensing boiler (new building regs), but I understand that there are problems with the plume from this type of boiler as we have a balcony roof above where the flue would exit, and it's close to a door and a window. The cheaper option is to just move the Alpha, but I'm worried this may affect it (cause leaks). Or can we get a new non-condensing boiler fitted? I've spoken to 4 different heating companies (incl BG) and have had different comments and advice from each. Any help much appreciated. Thanks. |
Combi boiler - condensing - or not?
"CharlieR" wrote in message oups.com... Hi - I'm new to this forum but would appreciate some helpful advice. I live in a flat and am having the kitchen refitted, which means moving or replacing our existing boiler (Alpha Ocean Style FF - about 12 yrs old). I'm told that if we replace it we have to have a condensing boiler (new building regs), but I understand that there are problems with the plume from this type of boiler as we have a balcony roof above where the flue would exit, and it's close to a door and a window. The cheaper option is to just move the Alpha, but I'm worried this may affect it (cause leaks). Or can we get a new non-condensing boiler fitted? I've spoken to 4 different heating companies (incl BG) and have had different comments and advice from each. Any help much appreciated. Thanks. Look on the odpm site. There is the check list. If the plume is a nuisance, then a non-condensing boiler can be fitted. |
Combi boiler - condensing - or not?
CharlieR wrote:
Hi - I'm new to this forum but would appreciate some helpful advice. I live in a flat and am having the kitchen refitted, which means moving or replacing our existing boiler (Alpha Ocean Style FF - about 12 yrs old). I'm told that if we replace it we have to have a condensing boiler (new building regs), but I understand that there are problems with the plume from this type of boiler as we have a balcony roof above where the flue would exit, and it's close to a door and a window. Limitations on proximity to doors and windows are not that different to existing boilers. There are some issues with distances to neighbouring properties though since some folks may find the plume objectionable. What you have been told is correct basically - in that in order to meet the energy efficiency requirements of the building regulations, you will need a condensing boiler. However there are exemptions allowed in certain circumstances. You can find out if these apply to you by reading this: http://www.odpm.gov.uk/pub/731/Guide..._id1130731.pdf http://tinyurl.com/p6n3v The cheaper option is to just move the Alpha, but I'm worried this may affect it (cause leaks). Moving it would count the same as installing - so unless you are prepared to do it yourself (or can find a CORGI who is prepared to ignore the regs) then you would be back to requiring the exemption as above. Or can we get a new non-condensing boiler fitted? See above. I've spoken to 4 different heating companies (incl BG) and have had different comments and advice from each. Any help much appreciated. Thanks. Hardly surprising - the regulations in this area are a bit of a pigs breakfast. -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
Combi boiler - condensing - or not?
"John Rumm" wrote in message ... CharlieR wrote: Hi - I'm new to this forum but would appreciate some helpful advice. I live in a flat and am having the kitchen refitted, which means moving or replacing our existing boiler (Alpha Ocean Style FF - about 12 yrs old). I'm told that if we replace it we have to have a condensing boiler (new building regs), but I understand that there are problems with the plume from this type of boiler as we have a balcony roof above where the flue would exit, and it's close to a door and a window. Limitations on proximity to doors and windows are not that different to existing boilers. There are some issues with distances to neighbouring properties though since some folks may find the plume objectionable. What you have been told is correct basically - in that in order to meet the energy efficiency requirements of the building regulations, you will need a condensing boiler. However there are exemptions allowed in certain circumstances. You can find out if these apply to you by reading this: http://www.odpm.gov.uk/pub/731/Guide..._id1130731.pdf http://tinyurl.com/p6n3v The cheaper option is to just move the Alpha, but I'm worried this may affect it (cause leaks). Moving it would count the same as installing It isn't. |
Combi boiler - condensing - or not?
Owain wrote:
CharlieR wrote: Hi - I'm new to this forum but would appreciate some helpful advice. I live in a flat and am having the kitchen refitted, which means moving or replacing our existing boiler (Alpha Ocean Style FF - about 12 yrs old). I'm told that if we replace it we have to have a condensing boiler (new building regs), but I understand that there are problems with the plume from this type of boiler as we have a balcony roof above where the flue would exit, and it's close to a door and a window. Provided the regulations and the manufacturer's instructions re distance from windows/walls are complied with there is no "problem" with the plume - it's only water vapour - apart from aesthetically. Personally, I And financially! Are there any add-on units that take the exhaust down from ~80C to ~20C, and use it to warm air to come into the house? |
Combi boiler - condensing - or not?
Ian Stirling wrote:
And financially! Are there any add-on units that take the exhaust down from ~80C to ~20C, and use it to warm air to come into the house? I've often wondered why manufacturers don't fit a much longer insulated co-axial flue. This would reclaim much more of the heat lost in the exhaust by using the counter-current priciple to warm the incoming air to the boiler. Warmer incoming air would surely reduce the work the boiler has to do to heat the water? Sounds too simple so there must be something wrong with the idea although I can't see it. Tim |
Combi boiler - condensing - or not?
On Fri, 17 Mar 2006 08:19:35 -0000, "Tim Downie"
wrote: Ian Stirling wrote: And financially! Are there any add-on units that take the exhaust down from ~80C to ~20C, and use it to warm air to come into the house? I've often wondered why manufacturers don't fit a much longer insulated co-axial flue. This would reclaim much more of the heat lost in the exhaust by using the counter-current priciple to warm the incoming air to the boiler. Warmer incoming air would surely reduce the work the boiler has to do to heat the water? Sounds too simple so there must be something wrong with the idea although I can't see it. Cars often have a temperature controlled flap that diverts the incoming air via the radiator in order to warm it up. sponix |
Combi boiler - condensing - or not?
On 17 Mar 2006 07:28:43 GMT, Ian Stirling
wrote: Owain wrote: CharlieR wrote: Hi - I'm new to this forum but would appreciate some helpful advice. I live in a flat and am having the kitchen refitted, which means moving or replacing our existing boiler (Alpha Ocean Style FF - about 12 yrs old). I'm told that if we replace it we have to have a condensing boiler (new building regs), but I understand that there are problems with the plume from this type of boiler as we have a balcony roof above where the flue would exit, and it's close to a door and a window. Provided the regulations and the manufacturer's instructions re distance from windows/walls are complied with there is no "problem" with the plume - it's only water vapour - apart from aesthetically. Personally, I And financially! Are there any add-on units that take the exhaust down from ~80C to ~20C, and use it to warm air to come into the house? In effect, this is what some early UK designs of condensing boiler did - manufacturers added a secondary heat exchanger. The trouble was that the whole set up was not well designed to cope with the acidic condensate and corrosion resulted; leading to a poor reputation in the trade for condensing technology in general. German and Dutch designs which were made from the ground up as condensing, did much better and have been in use for many years. The second aspect is that balanced flue boilers with no fan are not designed to have an extended flue because they use natural draught to bring in air and expel exhaust. Even those with a fan may not be designed to run with a long flue since the fan may have been selected to run with a designed and limited length flue. -- ..andy |
Combi boiler - condensing - or not?
On Fri, 17 Mar 2006 08:19:35 -0000, "Tim Downie"
wrote: Ian Stirling wrote: And financially! Are there any add-on units that take the exhaust down from ~80C to ~20C, and use it to warm air to come into the house? I've often wondered why manufacturers don't fit a much longer insulated co-axial flue. This would reclaim much more of the heat lost in the exhaust by using the counter-current priciple to warm the incoming air to the boiler. Warmer incoming air would surely reduce the work the boiler has to do to heat the water? Sounds too simple so there must be something wrong with the idea although I can't see it. There are several issues: - People may not *want* to have a long flue even though the boiler may be able to support it. - Condensing boilers have a low exhaust temperature. Since the rate of heat transfer would depend on the temperature difference between incoming and outgoing air, this would be a lot less than on a conventional boiler in the first place. - Plastic flues are not good conductors of heat Because of these first issues, to make this worthwhile whould probably need something similar in concept to a plate heat exchanger where there is a lot of contact area between the incoming and outgoing flows. There needs to be good conductivity as well. There would be additional condensation from outgoing flue gases and this condensate would be acidic. There would be an increased resistance to flow for gases in and out, and the fan would need to cope with that. The net of all of this is that I suspect a physically large heat exchanger with quite large stainless steel plates would be needed in order to make much differene as well as standing up to the conditions. I don't suppose that this would be inexpensive. -- ..andy |
Combi boiler - condensing - or not?
Sponix wrote: On Fri, 17 Mar 2006 08:19:35 -0000, "Tim Downie" wrote: Ian Stirling wrote: And financially! Are there any add-on units that take the exhaust down from ~80C to ~20C, and use it to warm air to come into the house? I've often wondered why manufacturers don't fit a much longer insulated co-axial flue. This would reclaim much more of the heat lost in the exhaust by using the counter-current priciple to warm the incoming air to the boiler. Warmer incoming air would surely reduce the work the boiler has to do to heat the water? Sounds too simple so there must be something wrong with the idea although I can't see it. Cars often have a temperature controlled flap that diverts the incoming air via the radiator in order to warm it up. Cold air is denser so you get more air per unit volume to support combustion so they also fit intercoolers to cool the air before it enters the engine. MBQ |
Combi boiler - condensing - or not?
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Combi boiler - condensing - or not?
wrote in message oups.com... Sponix wrote: On Fri, 17 Mar 2006 08:19:35 -0000, "Tim Downie" wrote: Ian Stirling wrote: And financially! Are there any add-on units that take the exhaust down from ~80C to ~20C, and use it to warm air to come into the house? I've often wondered why manufacturers don't fit a much longer insulated co-axial flue. This would reclaim much more of the heat lost in the exhaust by using the counter-current priciple to warm the incoming air to the boiler. Warmer incoming air would surely reduce the work the boiler has to do to heat the water? Sounds too simple so there must be something wrong with the idea although I can't see it. Cars often have a temperature controlled flap that diverts the incoming air via the radiator in order to warm it up. Cold air is denser so you get more air per unit volume to support combustion so they also fit intercoolers to cool the air before it enters the engine. On start up better warm, after the engine is warm the colder the better. |
Combi boiler - condensing - or not?
Andy Hall wrote:
On Fri, 17 Mar 2006 08:19:35 -0000, "Tim Downie" wrote: Sounds too simple so there must be something wrong with the idea although I can't see it. There are several issues: - People may not *want* to have a long flue even though the boiler may be able to support it. True but they might be convinced if the savings were worthwhile. - Condensing boilers have a low exhaust temperature. Since the rate of heat transfer would depend on the temperature difference between incoming and outgoing air, this would be a lot less than on a conventional boiler in the first place. Again true but there is still wasted heat going out that could be regained. I've no idea of the amount but there's no technical reason why the exhaust temperature couldn't be cooled right done the the incoming air temperature. Surely that's got to be worth something? - Plastic flues are not good conductors of heat Which would be good for the outer intake pipe. The inner exhaust pipe could be made of stainless steel. Because of these first issues, to make this worthwhile whould probably need something similar in concept to a plate heat exchanger where there is a lot of contact area between the incoming and outgoing flows. I think that without testing designs it would be premature to dismiss a simple coaxial counter-current heat exchanger. Keeping it straight & relatively long would keep it cheap making the payback period proportionally shorter. There needs to be good conductivity as well. Duh... There would be additional condensation from outgoing flue gases and this condensate would be acidic. So the stuff that condenses at the moment and trickles back through the boiler isn't acidic? I would presume that the boiler's materials already take that into account. There would be an increased resistance to flow for gases in and out, and the fan would need to cope with that. Well that's true and might limit what can be retro fitted to an existing boiler but many boilers can already accept quite long flue extensions. The net of all of this is that I suspect a physically large heat exchanger with quite large stainless steel plates would be needed in order to make much differene as well as standing up to the conditions. I don't suppose that this would be inexpensive. Maybe. I remain unconvinced. Tim |
Combi boiler - condensing - or not?
"Ian Stirling" wrote in message ... Are there any add-on units that take the exhaust down from ~80C to ~20C, and use it to warm air to come into the house? Yep. A company in Plymouth, Zenex, has come out with a "top box" that fits on a condensing boiler's flue - out of the boiler into the box and out again to outside. Not new as Johnnson & Starley has one for their forced air units converting them to condensers. This appears a leap though making condensers more efficient. They announced it pre-Xmas, but now it is on sale. It extracts even more latent heat from the flue of a condensing boiler. They claim, with some independent validation, that a "condensing" boiler can improve fuel consumption by 30%. Currently the best non-condensing boiler is 80% SEDBUK, and the best condensing 92% - only 12% leap. They claim it will go to the near 109% available (yes you can get 109% efficiencies from a condensing boiler). There are no moving parts. They claim that a combi can deliver 50% more water flow for the same input - so a 12 litres per min job will be 18 litres, a big hype that fills a bath pretty fast instead of a leisurely fill. They are developing a 15kW combi that delivers 12 litres/min - a little better than the average combi around. 12 litres is normally only achieved by a 28kW boiler. So, at 15Kw it is not oversized for the CH in flats, and the case size can be kept down. They plan one for a non-condensing boilers too, that will bring non- condenser into condesning territory. They claim the price will fall from £595 as production gears up. If it get to half, and it does what they say, then this can save a lot of fuel, the bigger the house the more fuel saved. They may have versions available for oil and LPG boilers. The version available may suitable for LPG boioers. This box may well make an LPG boiler equal, or better, than an oil boiler to run and with £600 on top probably still lower capital costs of installation. Worth looking at and assessing. http://www.zenexenergy.com/downloads/HVR10-REPRINT.pdf http://www.zenexenergy.com/ Buy here for alimted flue implementation. More flues to come.: http://www.discountedheating.co.uk/shop/acatalog/Zenex_Gas_Saver.html Another way of eliminating a plume and yet have higher efficiencies than non-condensing boiler is to intall a bledning valve on the boiers flow-return, set to 60C. The returning wtaer will always be about dew-point. |
Combi boiler - condensing - or not?
"Andy Hall" wrote in message ... On Fri, 17 Mar 2006 08:19:35 -0000, "Tim Downie" wrote: Ian Stirling wrote: And financially! Are there any add-on units that take the exhaust down from ~80C to ~20C, and use it to warm air to come into the house? I've often wondered why manufacturers don't fit a much longer insulated co-axial flue. This would reclaim much more of the heat lost in the exhaust by using the counter-current priciple to warm the incoming air to the boiler. Warmer incoming air would surely reduce the work the boiler has to do to heat the water? Sounds too simple so there must be something wrong with the idea although I can't see it. There are several issues: - People may not *want* to have a long flue even though the boiler may be able to support it. A long plastic mPVC flue can have a pipe around it. A pipe in pipe arrangement. This is legal as the inside of flue is not touched at all. It could run for 10 to 15 foot across a loft and have water in the gap between the pipes. Feed cold mains water through this and it pre-heats the water. Zenex, pre-heat water this way with their top boxes and claim 50% greater flow rate through a combi. So, a 12 litres/min combi can be zipped up to 18 litres/min. Big jump. Using a pipe in pipe means it is cheaper than their top box. But they are incorporating it all inside one combi box. They are talking to the big boiler manufacturers to licence their technology. If a 12 l/min combi gives 18 litre/min for the same kW input then this combi will sell like hot cakes. Also, the plume is eradicated. Councils are now having to deal with complaints of nuisance pluming. This appear to be the answer. |
Combi boiler - condensing - or not?
Doctor Drivel wrote:
"Andy Hall" wrote in message ... On Fri, 17 Mar 2006 08:19:35 -0000, "Tim Downie" wrote: Ian Stirling wrote: And financially! Are there any add-on units that take the exhaust down from ~80C to ~20C, and use it to warm air to come into the house? I've often wondered why manufacturers don't fit a much longer insulated co-axial flue. This would reclaim much more of the heat lost in the exhaust by using the counter-current priciple to warm the incoming air to the boiler. Warmer incoming air would surely reduce the work the boiler has to do to heat the water? Sounds too simple so there must be something wrong with the idea although I can't see it. There are several issues: - People may not *want* to have a long flue even though the boiler may be able to support it. A long plastic mPVC flue can have a pipe around it. A pipe in pipe arrangement. This is legal as the inside of flue is not touched at all. It could run for 10 to 15 foot across a loft and have water in the gap between the pipes. Feed cold mains water through this and it pre-heats the water. Zenex, pre-heat water this way with their top boxes and claim 50% greater flow rate through a combi. So, a 12 litres/min combi can be zipped up to 18 litres/min. Big jump. Using a pipe in pipe means it is cheaper than their top box. But they are incorporating it all inside one combi box. They are talking to the big boiler manufacturers to licence their technology. If a 12 l/min combi gives 18 litre/min for the same kW input then this combi will sell like hot cakes. Also, the plume is eradicated. Councils are now having to deal with complaints of nuisance pluming. This appear to be the answer. |
Combi boiler - condensing - or not?
On Fri, 17 Mar 2006 08:19:35 -0000 Tim Downie wrote :
I've often wondered why manufacturers don't fit a much longer insulated co-axial flue. This would reclaim much more of the heat lost in the exhaust by using the counter-current priciple to warm the incoming air to the boiler. Warmer incoming air would surely reduce the work the boiler has to do to heat the water? I'm not sure that warmer air would improve combustion efficiency - cars are reputed to run better in cool weather as the air is denser. Re the flue, ISTM that the problem is that if heat can easily migrate from the flue into the room then the reverse will be true too, and when the boiler isn't running the flue would act as a heat sink losing heat from the room. Are there any scientists here who can tell us how much heat could potentially be recovered if you could condense all the water vapour in the combustion gases. I remember enough school chemistry to know that it's CH4+ 4O = CO2+ 2H2O - so every 12g of gas burned makes 36g of water vapour. And the latent heat of water is easy to find. But I couldn't find what gas weighs given that it is under pressure when metered. -- Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk Free SEDBUK boiler database browser http://www.sda.co.uk/qsedbuk.htm [Latest version QSEDBUK 1.12 released 8 Dec 2005] |
Combi boiler - condensing - or not?
"Tony Bryer" wrote in message ... On Fri, 17 Mar 2006 08:19:35 -0000 Tim Downie wrote : I've often wondered why manufacturers don't fit a much longer insulated co-axial flue. This would reclaim much more of the heat lost in the exhaust by using the counter-current priciple to warm the incoming air to the boiler. Warmer incoming air would surely reduce the work the boiler has to do to heat the water? I'm not sure that warmer air would improve combustion efficiency - cars are reputed to run better in cool weather as the air is denser. The denser air means more oxygen. The same for any combustion process. |
Combi boiler - condensing - or not?
"Tony Bryer" wrote in message ... On Fri, 17 Mar 2006 08:19:35 -0000 Tim Downie wrote : I've often wondered why manufacturers don't fit a much longer insulated co-axial flue. This would reclaim much more of the heat lost in the exhaust by using the counter-current priciple to warm the incoming air to the boiler. Warmer incoming air would surely reduce the work the boiler has to do to heat the water? I'm not sure that warmer air would improve combustion efficiency - cars are reputed to run better in cool weather as the air is denser. In co-axial flues the incoming air is heated by the exhaust air. Two pipe flue systems are more efficient as the incoming cold stays cold until burnt. On large trucks there were experiments to us heat pumps to cool the incoming air. The larger the engine, and more fuel used, the more cost effective these things are. |
Combi boiler - condensing - or not?
The message ews.net
from "Doctor Drivel" contains these words: Councils are now having to deal with complaints of nuisance pluming. Hah, I read that as plumbing and had visions of gangs of unruly plumbers fitting a bathroom on the street corner and giving the wife a loutish plungering. -- Skipweasel Pay no attention to the man behind the curtain. |
Combi boiler - condensing - or not?
The message ews.net
from "Doctor Drivel" contains these words: In co-axial flues the incoming air is heated by the exhaust air. Two pipe flue systems are more efficient as the incoming cold stays cold until burnt. On large trucks there were experiments to us heat pumps to cool the incoming air. The larger the engine, and more fuel used, the more cost effective these things are. I'm fairly sure that the coolth of the incoming charge is only of consequence on an expansion engine, where the idea is to get the smallest most dense charge into the engine, then expand it to the maximum amount practicable to extract the most energy possible. Boilers are different - provided the fuel is fully burnt at the stochiometric point or better (leaner), then all the thermal energy will have been liberated and now the task it to extract as much as practicable. This means that the closer the exhaust temperature is to ambient (above, of course) the more energy has been retained for useful purposes. If you look at the design of boiler systems for power stations you'll see that they take great trouble to preheat the imcoming air for combustion to the maximum practicable. This is a much closer analogy to a domestic boiler than an internal combustion engine. -- Skipweasel Pay no attention to the man behind the curtain. |
Combi boiler - condensing - or not?
On Fri, 17 Mar 2006 11:00:19 -0000 Tim Downie wrote :
Yeah but cars are internal combustion engines, What with twiddling screws to get the gas-air mix right, poking things in the exhaust pipe and spark plugs that don't they are going that way! -- Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk Free SEDBUK boiler database browser http://www.sda.co.uk/qsedbuk.htm [Latest version QSEDBUK 1.12 released 8 Dec 2005] |
Combi boiler - condensing - or not?
"Guy King" wrote in message ... The message ews.net from "Doctor Drivel" contains these words: In co-axial flues the incoming air is heated by the exhaust air. Two pipe flue systems are more efficient as the incoming cold stays cold until burnt. On large trucks there were experiments to us heat pumps to cool the incoming air. The larger the engine, and more fuel used, the more cost effective these things are. I'm fairly sure that the coolth of the incoming charge is only of consequence on an expansion engine, where the idea is to get the smallest most dense charge into the engine, then expand it to the maximum amount practicable to extract the most energy possible. Yep. Boilers are different Not really, although external combustion is far more efficient than internal explosions. - provided the fuel is fully burnt at the stochiometric point or better (leaner), A pre-mix fan can only deliver a ratio of air/gas because of the fan design. Put more oxygen insthe air and more efficient combustion. Cold air is denser containing more oxygen. then all the thermal energy will have been liberated and now the task it to extract as much as practicable. All the sensible heat has been liberated, not to recover the latent heat. This means that the closer the exhaust temperature is to ambient (above, of course) the more energy has been retained for useful purposes. It means more latetent heat has been extracted If you look at the design of boiler systems for power stations you'll see that they take great trouble to preheat the imcoming air for combustion to the maximum practicable. This is a much closer analogy to a domestic boiler than an internal combustion engine. It depends on the engines being used. Turbines are different again. |
Combi boiler - condensing - or not?
Doctor Drivel wrote:
A long plastic mPVC flue can have a pipe around it. A pipe in pipe arrangement. This is legal as the inside of flue is not touched at all. It could run for 10 to 15 foot across a loft and have water in the gap between the pipes. Feed cold mains water through this and it pre-heats the water. Zenex, pre-heat water this way with their top boxes and claim 50% greater flow rate through a combi. So, a 12 litres/min combi can be zipped up to 18 litres/min. Big jump. Yes *but* the Zenex design looks to be very much optimised for gas to water heat exchange, not gas to gas. Now, what proportion of a typical domestic combi boilers gas consumtion goes on heating hot water for the taps and how much on heating for the radiators? 5%, 10%, 20%? The water to gas heat exchange is going to do absolutely diddly if you're not drawing hot water and would be useless on a non-combi boiler. (Okay, you could run the SH return through it but the gains are going to be small). A gas/gas heat exchanger would improve the efficiency *all* the time the boiler is running. Tim |
Combi boiler - condensing - or not?
"Tim Downie" wrote in message ... Doctor Drivel wrote: A long plastic mPVC flue can have a pipe around it. A pipe in pipe arrangement. This is legal as the inside of flue is not touched at all. It could run for 10 to 15 foot across a loft and have water in the gap between the pipes. Feed cold mains water through this and it pre-heats the water. Zenex, pre-heat water this way with their top boxes and claim 50% greater flow rate through a combi. So, a 12 litres/min combi can be zipped up to 18 litres/min. Big jump. Yes *but* the Zenex design looks to be very much optimised for gas to water heat exchange, not gas to gas. Now, what proportion of a typical domestic combi boilers gas consumtion goes on heating hot water for the taps and how much on heating for the radiators? 5%, 10%, 20%? The water to gas heat exchange is going to do absolutely diddly if you're not drawing hot water and would be useless on a non-combi boiler. Read their web site and HV article again. |
Combi boiler - condensing - or not?
Tony Bryer wrote:
Are there any scientists here who can tell us how much heat could potentially be recovered if you could condense all the water vapour in the combustion gases. I remember enough school chemistry to know that it's CH4+ 4O = CO2+ 2H2O - so every 12g of gas burned makes 36g of water If you take the density in its gaseous state at 1.82 kg/m^3 - your 3:1 water to gas weight ratio above would yield 5.46 L of water vapour. Which seems a bit much... Are you sure that 36g is the mass of just the water? If it were the total mass of the combustion product (i.e. inc CO2) that would make more sense with a water vapour yield of 3.64 L (if you take the latenet heat of vapourisation of water as 2.26 MJ/kg the 5.46L figure would give you a total of 12.3MJ as oposed to 8.23MJ for the lower figure) Since the figures are normally quoted at about 36 / 43MJ for the nett / gross energy yield of natural gas the latter one seems closer. vapour. And the latent heat of water is easy to find. But I couldn't find what gas weighs given that it is under pressure when metered. It is under very little pressure when metered (0.021 Bar) so you can probably discount that. -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
Combi boiler - condensing - or not?
On Fri, 17 Mar 2006 11:14:30 -0000, "Tim Downie"
wrote: Andy Hall wrote: On Fri, 17 Mar 2006 08:19:35 -0000, "Tim Downie" wrote: Sounds too simple so there must be something wrong with the idea although I can't see it. There are several issues: - People may not *want* to have a long flue even though the boiler may be able to support it. True but they might be convinced if the savings were worthwhile. and that the logistics of running the flue worked. - Condensing boilers have a low exhaust temperature. Since the rate of heat transfer would depend on the temperature difference between incoming and outgoing air, this would be a lot less than on a conventional boiler in the first place. Again true but there is still wasted heat going out that could be regained. I've no idea of the amount but there's no technical reason why the exhaust temperature couldn't be cooled right done the the incoming air temperature. Surely that's got to be worth something? It depends on how much and the context of the rest of the house. The amount of surface area required to achieve this amount of temperature drop could become quite substantial. - Plastic flues are not good conductors of heat Which would be good for the outer intake pipe. The inner exhaust pipe could be made of stainless steel. Although the contact area is then not that substantial in comparison to something with baffles etc. Because of these first issues, to make this worthwhile whould probably need something similar in concept to a plate heat exchanger where there is a lot of contact area between the incoming and outgoing flows. I think that without testing designs it would be premature to dismiss a simple coaxial counter-current heat exchanger. Keeping it straight & relatively long would keep it cheap making the payback period proportionally shorter. It comes back to whether people would want the extra cost and hassle of running a long flue for the small amount of energy involved. There needs to be good conductivity as well. Duh... There would be additional condensation from outgoing flue gases and this condensate would be acidic. So the stuff that condenses at the moment and trickles back through the boiler isn't acidic? I would presume that the boiler's materials already take that into account. We were looking originally at conventional boilers. There would be an increased resistance to flow for gases in and out, and the fan would need to cope with that. Well that's true and might limit what can be retro fitted to an existing boiler but many boilers can already accept quite long flue extensions. Generally those that are already condensing models. The net of all of this is that I suspect a physically large heat exchanger with quite large stainless steel plates would be needed in order to make much differene as well as standing up to the conditions. I don't suppose that this would be inexpensive. Maybe. I remain unconvinced. I would look elsewhere first Tim -- ..andy |
Combi boiler - condensing - or not?
On Fri, 17 Mar 2006 11:10:08 -0000, "Doctor Drivel"
wrote: wrote in message roups.com... Cars often have a temperature controlled flap that diverts the incoming air via the radiator in order to warm it up. Cold air is denser so you get more air per unit volume to support combustion so they also fit intercoolers to cool the air before it enters the engine. On start up better warm, after the engine is warm the colder the better. Wrong (partly) again. I guess you've never experienced "carb" icing then? -- |
Combi boiler - condensing - or not?
On Fri, 17 Mar 2006 11:15:58 -0000, "Doctor Drivel"
wrote: A company in Plymouth, Zenex, has come out with a "top box" that fits on a condensing boiler's flue. As you seem to have such a close relationship with this company, would you care to locate the relevant patents that they claim to have? The mumbo jumbo quoted on their website, together with the performance "test" that has more holes in it than a colander hardly inspire confidence in the claimed performance. -- |
Combi boiler - condensing - or not?
Doctor Drivel wrote:
A company in Plymouth, Zenex, has come out with a "top box" that fits on a condensing boiler's flue - out of the boiler into the box and out again to outside. Not new as Johnnson & Starley has one for their forced air units converting them to condensers. This appears a leap though making condensers more efficient. Sounds like you have been reading glossy brosures again, get ready for the incoming barrage of dodgy statistics.... It extracts even more latent heat from the flue of a condensing boiler. They claim, with some independent validation, that a "condensing" boiler can improve fuel consumption by 30%. Currently the best non-condensing What did I say! What does that mean exactly? 30% of what? From the description it sounds like the best that would be able to achieve would be recovery of 30% or the un-recovered heat remaining in the flue gasses - or put it another way, for a top efficiency boiler you are only talking about perhaps 3% over and above what the boiler can extract anyway. Note also the gain is only applied to HW use and not CH - so again there is a proportionate reduction in return there. boiler is 80% SEDBUK, and the best condensing 92% - only 12% leap. They 100% - 92% = 12% hmmm... The idea looks ok in priniple, but I can't see how you will ever achieve any payback given its £800 cost (probably more like £1100 once fitted for most buyers). Unless you have a gerriatric old boiler with very poor efficiency - in which case the cost of the top box would buy you a whole new boiler. claim it will go to the near 109% available (yes you can get 109% efficiencies from a condensing boiler). Only if you put the wrong calorific value (i.e. the nett value) for gas into the equations... do the sums correctly and you can't. They claim that a combi can deliver 50% more water flow for the same input - so a 12 litres per min job will be 18 litres, a big hype that fills a bath pretty fast instead of a leisurely fill. Hype is certainly the word.... They are developing a 15kW combi that delivers 12 litres/min - a little better than the average combi around. 12 litres is normally only (I suppose we are talking about 35 degree rise over incoming temp here) Are you sure that is not 12 lpm *peak* rate - i.e. by dumping its reserve of stored heat into the first few litres demanded? Otherwise where is the other 14.4kW coming from if you are talking about susstained performance? and the case size can be kept down. "Straws", clutching at perhaps ;-) Another way of eliminating a plume and yet have higher efficiencies than non-condensing boiler is to intall a bledning valve on the boiers flow-return, set to 60C. The returning wtaer will always be about dew-point. Why not leave it up to the control systems in the boiler? They seem to do a pretty good job of modulating the gas rate to achieve best efficiency anyway, and the efficiency ought to take precedence over the visibility of the plume. -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
Combi boiler - condensing - or not?
Andy Hall wrote:
On Fri, 17 Mar 2006 11:14:30 -0000, "Tim Downie" Maybe. I remain unconvinced. I would look elsewhere first Andy, I don't *know* if my idea would be viable but you seem to be taking the position "It hasn't been done so it can't work" which may be right most of the time but certainly not all of the time. There's nothing logically wrong with my idea that you've pointed out. I see no reason why a cheap co-axial flue extension shouldn't reclaim a significant amount of waste heat. It doesn't need to be complicated and if the bore diameters were reasonably dimensioned, it seems unlikely that it would greatly increase the amount of backpressure over what a modern fan flues boiler can cope with. Tim |
Combi boiler - condensing - or not?
"Matt" aka Lord Hall wrote in message ... On Fri, 17 Mar 2006 11:10:08 -0000, "Doctor Drivel" wrote: wrote in message groups.com... Cars often have a temperature controlled flap that diverts the incoming air via the radiator in order to warm it up. Cold air is denser so you get more air per unit volume to support combustion so they also fit intercoolers to cool the air before it enters the engine. On start up better warm, after the engine is warm the colder the better. Wrong (partly) again. I guess you've never experienced "carb" icing then? Lord Hall, the point is combustion. I doubt the penny will drop. |
Combi boiler - condensing - or not?
"Matt" aka Lord Hall wrote in message ... On Fri, 17 Mar 2006 11:15:58 -0000, "Doctor Drivel" wrote: A company in Plymouth, Zenex, has come out with a "top box" that fits on a condensing boiler's flue. As you seem to have such a close relationship with this company, would you care to locate the relevant patents that they claim to have? Lord Hall, I have no direct relationship with them. The mumbo jumbo quoted on their website, together with the performance "test" that has more holes in it than a colander hardly inspire confidence in the claimed performance. GasTec, a testing house in Holand, confirnen their claims. |
Combi boiler - condensing - or not?
"Tim Downie" wrote in message ... Andy Hall wrote: On Fri, 17 Mar 2006 11:14:30 -0000, "Tim Downie" Maybe. I remain unconvinced. I would look elsewhere first Andy, I don't *know* if my idea would be viable but you seem to be taking the position "It hasn't been done so it can't work" which may be right most of the time but certainly not all of the time. There's nothing logically wrong with my idea that you've pointed out. I see no reason why a cheap co-axial flue extension shouldn't reclaim a significant amount of waste heat. It doesn't need to be complicated and if the bore diameters were reasonably dimensioned, it seems unlikely that it would greatly increase the amount of backpressure over what a modern fan flues boiler can cope with. Johnson & Starley sell a unit that converts a non-condensing conventional flued forced air unit into a condenser doing what you proposed. It is a box in the flue pre-heating air. |
Combi boiler - condensing - or not?
The message
from Tony Bryer contains these words: Are there any scientists here who can tell us how much heat could potentially be recovered if you could condense all the water vapour in the combustion gases. I remember enough school chemistry to know that it's CH4+ 4O = CO2+ 2H2O - so every 12g of gas burned makes 36g of water vapour. And the latent heat of water is easy to find. But I couldn't find what gas weighs given that it is under pressure when metered. ISTM you don't need be a scientist to solve the problem. Being an engineer is quite sufficient. The key is the difference between the Higher and Lower Calorific Values of North Sea Gas. 38500 - 34500 = 4000 or about 10.4% of the higher value. I could be wrong but I thought the figures usually bandied about for boiler efficiency were based of the HCV. Dribble says different but he can't be trusted at that level of competence. As Tim pondered it should be worthwhile reducing exhaust gases to input air temperature and while that would appear impossible in an gas to air heat exchanger surely if achieved that would mean a fuel efficiency of 100%. But if that is the case then in the manner of heat engines if some way was used to reduce the temperature of the exhaust gasses to less than that of the incoming air then efficiency in excess of 100% does become a possibility. Not technically true of course but on a par with using the LVC to calculate boiler efficiencies. -- Roger Chapman |
Combi boiler - condensing - or not?
"John Rumm" wrote in message ... Doctor Drivel wrote: A company in Plymouth, Zenex, has come out with a "top box" that fits on a condensing boiler's flue - out of the boiler into the box and out again to outside. Not new as Johnnson & Starley has one for their forced air units converting them to condensers. This appears a leap though making condensers more efficient. Sounds like you have been reading glossy brosures again, get ready for the incoming barrage of dodgy statistics.... Chav, this is your way of envy of my knowledge, as you tend to know sweet FA about thsis game. It extracts even more latent heat from the flue of a condensing boiler. They claim, with some independent validation, that a "condensing" boiler can improve fuel consumption by 30%. Currently the best non-condensing What did I say! What does that mean exactly? 30% of what? From the description it sounds like the best that would be able to achieve would be recovery of 30% or the un-recovered heat remaining in the flue gasses - or put it another way, for a top efficiency boiler you are only talking about perhaps 3% over and above what the boiler can extract anyway. Note also the gain is only applied to HW use and not CH - so again there is a proportionate reduction in return there. boiler is 80% SEDBUK, and the best condensing 92% - only 12% leap. They 100% - 92% = 12% hmmm... The idea looks ok in priniple, but I can't see how you will ever achieve any payback given its £800 cost (probably more like £1100 once fitted for most buyers). £600 and they hope to half this price once production is up. Unless you have a gerriatric old boiler with very poor efficiency - in which case the cost of the top box would buy you a whole new boiler. claim it will go to the near 109% available (yes you can get 109% efficiencies from a condensing boiler). Only if you put the wrong calorific value (i.e. the nett value) for gas into the equations... do the sums correctly and you can't. They claim that a combi can deliver 50% more water flow for the same input - so a 12 litres per min job will be 18 litres, a big hype that fills a bath pretty fast instead of a leisurely fill. Hype is certainly the word.... Chav, tell that to GasTec in Holland who did the testing Why not leave it up to the control systems in the boiler? Chav, you clearly haven't got it. You could buy a two pipe plastic flue combi and make you own pipe in pipe flue, mains water pre-heater. Totally legal to do so. These plastic flue boilers are expensive. Or buy one of these top-boxes for an Ariston and the total cost is less than the Keston, Eco-Hometec, etc. And you claw back a hell of a lot of wasted energy, converting 12 l/min combi to an 18 litres/min combi and fill your bath pronto Then, add a GFX drain water heat reclaimer, solar panels too and DHW will not cost too much at all. Have all this in a highly insulated and pretty air-tight house, where DHW is the bulk of heating energy used, and your overall gas bills will be very, very, low. |
Combi boiler - condensing - or not?
Doctor Drivel wrote:
Chav, this is your way of envy of my knowledge, as you tend to know sweet FA about thsis game. You probably are self deluded enough to believe that... The idea looks ok in priniple, but I can't see how you will ever achieve any payback given its £800 cost (probably more like £1100 once fitted for most buyers). £600 and they hope to half this price once production is up. +VAT + the adaption kit for connecting to the boiler gets you to £788. Still 'kin expensive at half the price. If there are worthwhile gains to be had then you can expect a boiler manufacturer to include the same concept in the box without needing anything like the price jump proposed here. Chav, you clearly haven't got it. Chump, if it is what you have, then I clearly don't want it! -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
Combi boiler - condensing - or not?
"John Rumm" wrote in message ... "Straws", clutching at perhaps ;-) Chav, you really are a dumbo. Here is the GasTec report: http://www.zenexenergy.com/downloads/Final%20GasTec%20report%20for%20Zenex.pdf |
Combi boiler - condensing - or not?
On Fri, 17 Mar 2006 15:54:23 -0000 Doctor Drivel wrote :
Chav, you really are a dumbo. Here is the GasTec report: http://www.zenexenergy.com/downloads/Final%20GasTec%20report%20for%20Zenex.pdf Which says that during the summer season you can expect to cut your DHW energy usage by 12%. On a modern house DHW is responsible for about one third of energy use and most of this will be during the heating season (greater part of year and colder water coming in). "Added to this figure should be the instantaneous saving of ~2.47% that this achieved whilst the boiler is operational in DHW mode. The total saving is thus projected to be ~(38+2.47) % ie fractionally in excess of 40%. This figure will only be valid if the boiler operates for sufficient hours to release the required amount of flue gas energy. If this amounts to 2.47%, then energy that the boiler must consume is:- 2.24/0.0247 = 91kWh This is equivalent to about 3 hours operation per day for a ~30kW Combi, although this period should be reasonably uniformly distributed through the day. This operational period and such an operational profile is not unreasonable during the winter months." For most people this assumption is totally unreasonable. If you have a 30kW combi it's almost certain that this is determined by DHW needs. For the typical house we use as a model (108m2 built to 2002 standards) the annual heating energy is 7325kWh - averaging about 30kW/day across the heating season. You can't recover what's not going out the flue. -- Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk Free SEDBUK boiler database browser http://www.sda.co.uk/qsedbuk.htm [Latest version QSEDBUK 1.12 released 8 Dec 2005] |
Combi boiler - condensing - or not?
"John Rumm" wrote in message ... Doctor Drivel wrote: Chav, this is your way of envy of my knowledge, as you tend to know sweet FA about thsis game. You probably are self deluded enough to believe that... Chav, it is clear you are very lacking. £600 and they hope to half this price once production is up. +VAT + the adaption kit for connecting to the boiler gets you to £788. Still 'kin expensive at half the price. Payback could be 3 years. They aim to get it into mass produced combis as an integral part of the combi. Many makers are assessing it, and a housing associtions in Liverpool is testing out a number of them. Chav, tell them they are all wrong, as you know better. If there are worthwhile gains to be had then you can expect a boiler manufacturer to include the same concept in the box without needing anything like the price jump proposed here. See above. Chav, you clearly haven't got it. Chump, if it is what you have, then I clearly don't want it! Chav, you reall want it, but I never dance in Essex. |
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