On 10 Mar 2006 23:45:17 -0800, "legin"
wrote:
It is the return temperature that determines condensing efficiency, not the
flow temperature. The temperature at the bottom of the heatbank, provided
the thermostatic control is good, should actually be quite low.
Christian.
If that is the case, what will be the flow temp. Assuming that the
boiler will lift the temp say 20 degrees. If the return temp is say 45
then the flow temp will be 65. Going through a heat bank previously
heated to say 75, then it will have a cooling effect on the heatbank.
Thanks to Andy for the detailed response. I understand what you are
saying about the knee in the curve. However at this stage it would be
nice to get it right and have the most efficient system. That to me
means making sure that the condensing boiler actually condenses. The
heat bank technology is completely new to me. Is there not a boiler out
there that could supply heating to the heatbank at say 75 degrees until
it is totally satisfied then reduce its temp down to say 65, to ensure
that the return is well within the condensing range. I know that this
would mean using a S plan to supply the heatbank and not actually use
it to feed the rad circuits and underfloor circuits. Or could it be
plumbed as a heatbank supplying the rad/ ufh circuits, with a low
return as Christian suggests. Would the boiler then be capable of
lifting the temp say 25 degrees. Also dare i say thanks to drivel. I
know his ramblings have been the source of a lot of banter, but I have
got to admit that it does amuse me. More importantly he has raised my
awareness of heatbanks to the point where I am now seriously
considering using one.
Legin
Yes.
I have the MAN (now MHG) Micromat EC and it will do precisely that.
http://www.mhg.de/en/products/gas_un...cromat_ec.html
In the UK, this unit is sold by Eco Hometec and targetted (among other
places) for the homebuild market. MHS boilers sell it as the Strata
1 more for commercial applications where several can be clustered for
more output.
http://www.mhsboilers.com/boilers/premix/strata1.htm
http://www.eco-hometec.co.uk/EC30.htm
I mention the MHS site because it has much better brochures and
technical manuals and the product is the same.
This boiler has a wide modulating range - down to 3kW on some models -
and a very good build quality. Servicability is also good in that all
major components push fit and clip onto the back plate and can be
swapped very quickly.
The internal controller has
- Weather compensation. A sensor located outside provides the
temperature and the boiler will adjust output as that changes. For
UFH this can be helpful because heat output from the floor can't be
quickly adjusted. Because of the insulating effect of the house an
inside sensor knows about outside temperature change much later than
if it is measured directly
- Analogue sensing of HW cylinder/store temperature. There is a
sensor which fits into a pocket in the cylinder which places it in the
middle rather than the surface. The boiler knows the store
temperature and a reheat cycle can be triggered at a programmable
temperature.
- Optionally, analogue sensing of room temperature. There is a
programmer made by Siemens for this boiler which gives it the actual
room temperature to use rather than just a controller telling the
boiler to switch on and off. This provides better results when
combined with the weather compensator.
- Analogue control of the pump. Water flow is optimised to match the
boiler output and thus the temperature difference across the heat
exchanger is optimised.
- Control of motorised valves. There is an option on the boiler to
have an internal diverter valve, but a better solution is to use
either an external diverter valve or zone valves (S plan). There
are installer settings on the controller which allow the boiler to
control either scenario directly - i.e. you don't need an external
controller.
The controller has about 40 different combinations of operational
settings for different max flow temperatures, valve operations,
external controls. You can set the weather compensation curve,
which basically means the weather compensated flow temperatures at 20
and -1 degrees outside.
Max. flow temperatures in CH mode can be set for systems with radiator
outputs designed for conventional boilers (85), condensing (70) and
UFH (55). My system had a conventional boiler originally and I
changed radiators where needed to give enough output at 70.
If you want to run radiators and UFH, then the normal way is to run
either a separate zone or one derived from the main CH with a separate
pump and mixing valve which will blend some of the UFH return water
with flow at 70 degrees to provide a low flow temperature for the UFH.
In CH operation, I find that the boiler will modulate down to a flow
temperature of about 40 degrees or so when the weather is relatively
warm and will sit all day doing that. In colder weather, it's rare
that it goes over 60 degrees. There can be as much as a 25 degree
difference across the heat exchanger. The pump speed drops to as low
as 35% of max output.
HW reheat operation depends on one of two things. The cylinder
temperature has dropped below the low set point (in my case 55 degrees
and a storage temperature of 60 degrees) - it would probably be 85 and
80 or possibly 85 and 75 for a store depending on size. If the boiler
detects the cylinder temperature dropping rapidly - i.e you just
started running a bath and a shower, then it will start much earlier.
This prevents regular reheats when small amounts of water are being
used, but begins reheat earlier when there are large amounts.
The boiler controller switches over the zone valves and winds the
boiler and pump up to full power. The return temperature is low
until the last couple of minutes of the cycle, whereupon the power
level and pump speeds are gradually reduced. This prevents the
cylinder temperature from overshooting and maintains the boiler return
temperature as low as possible.
In practice all of this works very well. However the boiler does
cost around 60-100% more than other good quality boilers.
--
..andy