It occurs to me that the current condensing boilers rely on the low
temperature of the returned water flow to get their efficiency. Those
of us with old fashioned systems designed to run non-condensing have a
high return temp., so the boilers won't condense. But there's a nice
supply of cold air available to cool the exhaust further - which is the
intake. An intake-exhaust heat exchanger would surely recover at least
some of the waste heat in the exhaust. Not all of it, firstly because a
heat exchanger, even counter current, is never completely effective, and
second because the heat capacity of warm steamy air that's condensing is
far higher than that of dry, cool air - but it's worth a go, isn't it?

so waffle mode off, does anyone make such a beast? And is it any good?

Andy

Andy Champ wrote:
It occurs to me that the current condensing boilers rely on the low
temperature of the returned water flow to get their efficiency. Those
of us with old fashioned systems designed to run non-condensing have a
high return temp., so the boilers won't condense. But there's a nice
supply of cold air available to cool the exhaust further - which is the
intake. An intake-exhaust heat exchanger would surely recover at least
some of the waste heat in the exhaust. Not all of it, firstly because a
heat exchanger, even counter current, is never completely effective, and
second because the heat capacity of warm steamy air that's condensing is
far higher than that of dry, cool air - but it's worth a go, isn't it?

so waffle mode off, does anyone make such a beast? And is it any good?


THhey are called balanced flue boilers. I am sure there are one or two
out there ;-)


Andy

On Wed, 11 Nov 2009 20:24:33 +0000 Andy Champ wrote :
An intake-exhaust heat exchanger would surely recover at least
some of the waste heat in the exhaust. Not all of it, firstly because a
heat exchanger, even counter current, is never completely effective, and
second because the heat capacity of warm steamy air that's condensing is
far higher than that of dry, cool air - but it's worth a go, isn't it?

so waffle mode off, does anyone make such a beast? And is it any good?

http://www.zenexenergy.co.uk/Zenex1/...w=article&id=9

--
Tony Bryer, Greentram: 'Software to build on' Melbourne, Australia
www.superbeam.co.uk www.superbeam.com www.greentram.com

On 11 Nov, 20:24, Andy Champ wrote:
It occurs to me that the current condensing boilers rely on the low
temperature of the returned water flow to get their efficiency. Those
of us with old fashioned systems designed to run non-condensing have a
high return temp., so the boilers won't condense. But there's a nice
supply of cold air available to cool the exhaust further - which is the
intake. An intake-exhaust heat exchanger would surely recover at least
some of the waste heat in the exhaust. Not all of it, firstly because a
heat exchanger, even counter current, is never completely effective, and
second because the heat capacity of warm steamy air that's condensing is
far higher than that of dry, cool air - but it's worth a go, isn't it?

so waffle mode off, does anyone make such a beast? And is it any good?

Andy

what's the advantage of preheating combustion air? I thought colder
combustion air = more dense= more O2= better combustion? or do my
elderly A levels let me down somewhere?

JimK

The Natural Philosopher wrote:

THhey are called balanced flue boilers. I am sure there are one or two
out there ;-)

I was not aware that a standard balanced flue boiler had a heat
exchanger in the inlet. I know mine doesn't.

Andy

Tony Bryer wrote:

http://www.zenexenergy.co.uk/Zenex1/...w=article&id=9


Vague memory of this being mentioned before. And the price-performance
was called into question. It's a preheater for the incoming cold water
supply though, rather than the incoming cold air.

Andy

JimK wrote:

what's the advantage of preheating combustion air? I thought colder
combustion air = more dense= more O2= better combustion? or do my
elderly A levels let me down somewhere?


You A levels don't let you down, but cold inlet air is a solution to a
different problem.

If you have an internal combustion engine, especially one with a turbo-
or supercharger, hot inlet air reduces the mass of air available for
combustion. So the bang is smaller. Cooling the air wastes energy -
but the extra power is generally desirable (if you are a car salesman!).

It won't help us with a boiler, where 10% less output is probably not an
issue, but 10% less efficiency matters.

Andy

Andy Champ wrote:
The Natural Philosopher wrote:

THhey are called balanced flue boilers. I am sure there are one or two
out there ;-)

I was not aware that a standard balanced flue boiler had a heat
exchanger in the inlet. I know mine doesn't.

Andy
A balanced flue IS a primitive heat exchanger..;-)

On Thu, 12 Nov 2009 21:27:15 +0000 Andy Champ wrote :
If you have an internal combustion engine, especially one with a turbo-
or supercharger, hot inlet air reduces the mass of air available for
combustion. So the bang is smaller. Cooling the air wastes energy -
but the extra power is generally desirable (if you are a car salesman!).

It won't help us with a boiler, where 10% less output is probably not an
issue, but 10% less efficiency matters.

Nothing like 10% I think - gas delivers 10.8kW/m3. Specific heat of air is
1.25kJ/m3/K = 0.004kW/m3/K, so raising the air temp by 40C saves
0.016kW/m3. This needs to be multiplied by the air/gas ratio.

40 years since I did O-level physics so these numbers probably wrong!

--
Tony Bryer, Greentram: 'Software to build on' Melbourne, Australia
www.superbeam.co.uk www.superbeam.com www.greentram.com

"The Natural Philosopher" wrote in message
...
Andy Champ wrote:
The Natural Philosopher wrote:

THhey are called balanced flue boilers. I am sure there are one or two
out there ;-)

I was not aware that a standard balanced flue boiler had a heat exchanger
in the inlet. I know mine doesn't.

Andy

A balanced flue IS a primitive heat exchanger..;-)

It's not its a flue.

"Andy Champ" wrote in message
...
Tony Bryer wrote:

http://www.zenexenergy.co.uk/Zenex1/...w=article&id=9


Vague memory of this being mentioned before. And the price-performance
was called into question. It's a preheater for the incoming cold water
supply though, rather than the incoming cold air.

It will heat incoming air. This box is used for combis, so not applicable
for CH. It can also be used to assist in heating a heat bank/thermal store
while the boiler operates, using a pump, and increasing efficiency.

They are one of the few eco devices that have a short payback time, along
with rainwater recovery.

To warm incoming air you need long flue lengths in concentric flue pipes.

"Andy Champ" wrote in message
. uk...

What you mean is there a way to use the waste heat from the exhaust of the
boiler.

Tony Bryer wrote:
On Thu, 12 Nov 2009 21:27:15 +0000 Andy Champ wrote :
If you have an internal combustion engine, especially one with a turbo-
or supercharger, hot inlet air reduces the mass of air available for
combustion. So the bang is smaller. Cooling the air wastes energy -
but the extra power is generally desirable (if you are a car salesman!).

It won't help us with a boiler, where 10% less output is probably not an
issue, but 10% less efficiency matters.

Nothing like 10% I think - gas delivers 10.8kW/m3. Specific heat of air is
1.25kJ/m3/K = 0.004kW/m3/K, so raising the air temp by 40C saves
0.016kW/m3. This needs to be multiplied by the air/gas ratio.

40 years since I did O-level physics so these numbers probably wrong!

efficiency of a heat engine is pretty much down to the ratio of
combustion temperature to exhaust temperature.

That's the rationale behind the condensing boiler.


It doesn't really matter what you cool the exhaust with, as long as its
heat that ends up inside the house rather than outside.


I.e. you could pass exhaust gases through a hot air heat exchanger, and
us it to preheat ventilation air.

As I said, a balanced flue does act as a rather small heat exchanger,
and could be improved with, say, fins..

But my guess is that the gains are very small, or people would have gone
that route instead of condensing.

On Fri, 13 Nov 2009 10:27:24 +0000 The Natural Philosopher wrote :
efficiency of a heat engine is pretty much down to the ratio of
combustion temperature to exhaust temperature.

That's the rationale behind the condensing boiler.

Surely the rationale of a condensing boiler is that the amount of
heat released when the water in flue gases condenses is vastly
greater than any amount of heat gained by just cooling them a bit.

--
Tony Bryer, Greentram: 'Software to build on' Melbourne, Australia
www.superbeam.co.uk www.superbeam.com www.greentram.com

Tony Bryer wrote:
On Fri, 13 Nov 2009 10:27:24 +0000 The Natural Philosopher wrote :
efficiency of a heat engine is pretty much down to the ratio of
combustion temperature to exhaust temperature.

That's the rationale behind the condensing boiler.

Surely the rationale of a condensing boiler is that the amount of
heat released when the water in flue gases condenses is vastly
greater than any amount of heat gained by just cooling them a bit.


I am not sure there is enough water IN the flue gases to make that a
dominant factor.

Its more an issue with steam turbines were the steamm is ALL water.

If you think about a typical gas like methane, CH4, then two oxygen
atoms make the CO2 molecule, and two make the two H2O molecules. The
longer chain hydrocarbons (butane, propane, pentane, heptane etc) make
even less water per unit CO2.

AND remember, we are not feeding them a pure oxygen stream anyway, its
mainly nitrogen.

I would say, without actually researching it, the bulk of the heat is in
the hot CO2 and nitrogen.

Not the water vapour.

In article ,
Doctor Drivel wrote:

Vague memory of this being mentioned before. And the price-performance
was called into question. It's a preheater for the incoming cold water
supply though, rather than the incoming cold air.

It will heat incoming air.

It will?.. Can't really see how it's much better than a normal flue.

This box is used for combis, so not applicable
for CH. It can also be used to assist in heating a heat bank/thermal store
while the boiler operates, using a pump, and increasing efficiency.

yep

They are one of the few eco devices that have a short payback time, along
with rainwater recovery.

Really??

Alpha claim that it'll save *up to* 52% of the gas used to *heat water*
based on 100litres of hot water use a day.

It does nothing to help with heating load - (appart from collecting heat
that then is used to help for hot water I suppose assuming hw demand).

Given they seem to be around 700 quid+ VAT for the Gas saver I'm not
convinced on the payback...

http://www.4ecotips.com/eco/article_...aid=759&id=257 suggests 170
quid saving a year. I'd imagine that's a best case scenario that most
people would never see.

Makes a bit more sense if you can get it in the flowsmart pack at a good
price - but paying full price for it will take a long time to payback.

To warm incoming air you need long flue lengths in concentric flue pipes.

Given the price of flue pipes I'm not all that convinced it'd be a great
saving...

Darren

"D.M.Chapman" dmc@puffin. wrote in message
...
In article ,
Doctor Drivel wrote:

Vague memory of this being mentioned before. And the price-performance
was called into question. It's a preheater for the incoming cold water
supply though, rather than the incoming cold air.

It will heat incoming air.

It will?.. Can't really see how it's much better than a normal flue.

Look harder.

This box is used for combis, so not applicable
for CH. It can also be used to assist in heating a heat bank/thermal
store
while the boiler operates, using a pump, and increasing efficiency.

yep

They are one of the few eco devices that have a short payback time, along
with rainwater recovery.

Really??

Yes really, if on a thermal store and the sums are doen properly.

Alpha claim that it'll save *up to* 52% of the gas used to *heat water*
based on 100litres of hot water use a day.

It does nothing to help with heating load - (appart from collecting heat
that then is used to help for hot water I suppose assuming hw demand).

It can be used heating a thermal store or dedicated heat buffer, to be used
when the burner is on, not just for DHW. Whe used this way it gives a good
payback. The bigger the house and more gas used the greater the payback.
Using one in a one bedroomed flat may never recoup the capital costs. Used
on medium to larger homes using thermal storeage they have a decent payback
and worth buying.

To warm incoming air you need long flue lengths in concentric flue pipes.

Given the price of flue pipes I'm not all that convinced it'd be a great
saving...

The saving was not the point. Long flue pipes will warm incoming air.

On 13 Nov, 10:27, The Natural Philosopher
wrote:

efficiency of a heat engine is pretty much down to the ratio of
combustion temperature to exhaust temperature.

Only if you ignore phase changes (and pressure changes too). Assuming
that temperature is the _only_ significant factor will mislead you.

Admittedly it is the case for a condensing boiler where you draw the
system boundary in the right place, but it won't help you to
understand how it's working internally.

"The Natural Philosopher" wrote in message
...
Tony Bryer wrote:
On Fri, 13 Nov 2009 10:27:24 +0000 The Natural Philosopher wrote :
efficiency of a heat engine is pretty much down to the ratio of
combustion temperature to exhaust temperature.

That's the rationale behind the condensing boiler.

Surely the rationale of a condensing boiler is that the amount of heat
released when the water in flue gases condenses is vastly greater than
any amount of heat gained by just cooling them a bit.


I am not sure there is enough water IN the flue gases to make that a
dominant factor.

There is.

On 12 Nov, 22:06, The Natural Philosopher
wrote:

A balanced flue IS a primitive heat exchanger..;-)

Not since second-generation condensing boilers - they extract all the
sensible heat they can at the condenser, then try to avoid cooling the
flue gas any further, so as to avoid visible pluming. AIUI, there are
even some using double-wall construction to reduce heat transfer in
the flue.

Andy Dingley wrote:
On 12 Nov, 22:06, The Natural Philosopher
wrote:

A balanced flue IS a primitive heat exchanger..;-)

Not since second-generation condensing boilers - they extract all the
sensible heat they can at the condenser, then try to avoid cooling the
flue gas any further, so as to avoid visible pluming. AIUI, there are
even some using double-wall construction to reduce heat transfer in
the flue.

which shows that the heat exchange bit is less important than the
condensing bit.

Or they simply dont like visible plumimg for some reason.

Andy Dingley wrote:
On 13 Nov, 10:27, The Natural Philosopher
wrote:

efficiency of a heat engine is pretty much down to the ratio of
combustion temperature to exhaust temperature.

Only if you ignore phase changes (and pressure changes too). Assuming
that temperature is the _only_ significant factor will mislead you.


one canm usually assume that the working fluid inlet and outlet
pressures eventually become the same, and teh poaahse cahnges are a
function of temeprture anyway.

OK its more true for a heat engine than a boiler, because boilers make
their own gases as it were, as well as simply heating the bulk of the
air (nitrogen) and exchanging that heat with colder water.
but its still the dominant factor.


Admittedly it is the case for a condensing boiler where you draw the
system boundary in the right place, but it won't help you to
understand how it's working internally.


That's the beauty of systems approaches to things. You don't need to
know how its working internally.Calories go in, and calories come out.,
Some in the water that is to be heated, some into the room the boiler
sits in, and some in the flue. They HAVE to add up to a balance.

The less calories come out, the more some have ended up heating the
house in some way.

HOW you get less to come out, is the detail.

Arguably a conventional boiler with a heat pump on the exhaust might be
a complex, but every efficient way to boost efficiency.

Certainly a conventional power station plus heat pump uses less energy
than an in-house boiler. Sadly it tends to cost the same, or more..

"The Natural Philosopher" wrote in message
...

Or they simply dont like visible plumimg for some reason.

In cities condensing boilers have become a nuisance. Cowboy plumbers fit
them in flats and the plumes are a cause of problems. The plumes look
horrible coming out of the sides of buildings.

Andy Dingley wrote:
On 12 Nov, 22:06, The Natural Philosopher
wrote:

A balanced flue IS a primitive heat exchanger..;-)

Not since second-generation condensing boilers - they extract all the
sensible heat they can at the condenser, then try to avoid cooling the
flue gas any further, so as to avoid visible pluming. AIUI, there are
even some using double-wall construction to reduce heat transfer in
the flue.

Trouble is they all quote efficiency based on 50 degree or so return
temp. And mine is a lot more than that - I'd guess 65 at worst case -
which will take off a lot of efficiency.

Andy

wrote:
On 13 Nov,
The Natural Philosopher wrote:

I am not sure there is enough water IN the flue gases to make that a
dominant factor.

There is!


Its more an issue with steam turbines were the steamm is ALL water.

Different issue.
If you think about a typical gas like methane, CH4, then two oxygen
atoms make the CO2 molecule, and two make the two H2O molecules. The
longer chain hydrocarbons (butane, propane, pentane, heptane etc) make
even less water per unit CO2.

Each CH4 produces one CO2 molecule and two H2O molecules.

CH4 + 2O2 = CO2 = 2H2O.


well you didn't even get that right.

CO2 doesn't equal 2H20

AND remember, we are not feeding them a pure oxygen stream anyway, its
mainly nitrogen.

Condensing the water is the equivalent of cooling it through 540 degrees. so
well worth it.

not when its less than 1% of the total mass

wrote:
On 13 Nov,
The Natural Philosopher wrote:

efficiency of a heat engine is pretty much down to the ratio of
combustion temperature to exhaust temperature.

That's the rationale behind the condensing boiler.

But that's completely different to a heat *engine* which is a heat to
mechanical power conversion. In that case it is more to do with how close to
absolute zero, not ambient, you can get the output fluid.

ER, no.

wrote:
On 14 Nov,
The Natural Philosopher wrote:

CH4 + 2O2 = CO2 = 2H2O.

well you didn't even get that right.

CO2 doesn't equal 2H20

Typo. Have you never made one?

CH4 + 2O2 = CO2 + 2H2O.

AND remember, we are not feeding them a pure oxygen stream anyway, its
mainly nitrogen.
Condensing the water is the equivalent of cooling it through 540 degrees.
so well worth it.
not when its less than 1% of the total mass

11.8% more likely! A worthwhile contribution.

cant be even close to that.

21% of air is oxygen. Even if you are exactly at the whatever its called
ratio to use ALL the oxygen, only one third of it, so 7% of the
incoming, will be in the H20.


so that's 7% of the incoming AT BEST. BUT you have added methane as
well, and that essentially dilutes the mix still further. with CO2


The more carbon to hydrogen ratio there is in the gas the less water
and the more CO2, which reduces water content still further. butane and
propane have a much higher carbon to hydrogen ration.


And you never would run a boiler at critical fuel-air tratis. You run it
well over to ensure complete combustion. And full use of teh fuel.

In article ,
The Natural Philosopher writes:
wrote:
On 14 Nov,
The Natural Philosopher wrote:

CH4 + 2O2 = CO2 = 2H2O.

well you didn't even get that right.

CO2 doesn't equal 2H20

Typo. Have you never made one?

CH4 + 2O2 = CO2 + 2H2O.

AND remember, we are not feeding them a pure oxygen stream anyway, its
mainly nitrogen.
Condensing the water is the equivalent of cooling it through 540 degrees.
so well worth it.
not when its less than 1% of the total mass

11.8% more likely! A worthwhile contribution.

cant be even close to that.

21% of air is oxygen. Even if you are exactly at the whatever its called
ratio to use ALL the oxygen, only one third of it, so 7% of the
incoming, will be in the H20.


so that's 7% of the incoming AT BEST. BUT you have added methane as
well, and that essentially dilutes the mix still further. with CO2


The more carbon to hydrogen ratio there is in the gas the less water
and the more CO2, which reduces water content still further. butane and
propane have a much higher carbon to hydrogen ration.


And you never would run a boiler at critical fuel-air tratis. You run it
well over to ensure complete combustion. And full use of teh fuel.

It's not _well_ over as that knocks the efficiency too much.
When my Keston is tuned spot-on perfect, the O2 exhaust is
5.7% and the excess air (over that used for combustion) is
37%, which results in around 40ppm CO. (CO output climbs as
the excess air reduces, because it gets harder to find enough
O2 molecules to oxidise the carbon fully to CO2. You are
balancing CO production against the inefficiency of heating
up excess air.)

A non-condensing boiler doesn't control the air flow, and
they are typically between 100% and 170% excess air.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

"The Natural Philosopher" wrote in message
...
wrote:
On 14 Nov,







21% of air is oxygen. Even if you are exactly at the whatever its called
ratio


stoicheiometric ratio

mark

The Natural Philosopher wrote:

cant be even close to that.

21% of air is oxygen. Even if you are exactly at the whatever its called
ratio to use ALL the oxygen, only one third of it, so 7% of the
incoming, will be in the H20.

21% oxygen by volume. Let's keep with volume, and remember that the
volume is roughly proportional to the number of molecules. Andrew's
Keston burns 15% and leaves 6%. Given Methane fuel you end up with
1xCO2 and 2xH2O for each Methane, and use 2xO2 to make them. (By chance
the burnt mixture has the same number of molecules as the unburnt one).

So: Incoming: 79% inert gasses, 21% O2; add 7.5% methane (it's not
percent anymore but YSWIM)

Outgoing: 79% inert gasses 6% O2 7.5% Co2 15% H2O
(If you want true percent; 73, 6, 7, 14 to the nearest integer). So one
seventh of the outgoing mix is water. Well worth condensing out.



so that's 7% of the incoming AT BEST. BUT you have added methane as
well, and that essentially dilutes the mix still further. with CO2


The more carbon to hydrogen ratio there is in the gas the less water
and the more CO2, which reduces water content still further. butane and
propane have a much higher carbon to hydrogen ration.

I'm on mains. Mostly methane.

And you never would run a boiler at critical fuel-air tratis. You run it
well over to ensure complete combustion. And full use of teh fuel.


I'll admit I was surprised how "lean" a mixture is used. I'm more used
to IC petrol engines and they run very close to stoichiometric mixtures.

It may be that the wet exhaust holds too much energy for the dry intake
to recover a significant amount of energy. Must crunch the numbers some
time.

Andy