On Sun, 19 Sep 2004 08:32:57 +0100, "Tom"
wrote:

I'm attracted towards using TRVs on the rads in the 4 rooms which make
up approx 80% of the total heat load.
I'm further minded to install a Grundfos Alpha pump, however, I'm undecided
as to the control mechanism i.e. a flow switch as advocated by IMM in a
previous thread or to rely on the boiler stat set to 82degC as you stated.
I'm anxious to avoid short cycling of the boiler.


You should have TRV on at least all the bedroom rads which is the new
rules AFAIR. Ideally all rooms should have TRVs except one (the
lounge?) and that room should have the thermostat that turns the
system on/off.

The optimum system is where the water comes back to the boiler at just
above the condensing level for non-condensing boilers or below for
condensing. If condensing boiler return temperature is above then it
doesn't condense and so the extra money you paid for it is wasted as
it's no more efficient than a standard boiler. If the temperature is
too low on a non-condensing boiler then you get condensing which
corrodes the heat exchanger. I have been told that 56C is the
condensing threshold.

The normal flow and return is 80C out, 70C back (in round numbers,
82C, 71C if you want, in F it was 180F out, 160F back, rads 100F above
room temperature). So rads will be at 75C, about 55C above room
temperature. If you run which a larger drop, say 80C out, 60C back,
then the rads are at 70C. That 5C difference is significant (see rad
makers for curves). The rad could be 10% down on output. Hence for a
condensing boiler system you need larger radiators.

I wouldn't go with an alpha pump with a standard boiler. Think about
it. An alpha pump slows down the flow so as the TRVs cut in the flow
back to the boiler is reduced. Water spends more time in the rads so
comes back colder. The risk is you get condensing. What I would go
for is a standard pump + an automatic bypass (which is cheaper
anyway). Then as the TRVs cut in the hot water gets fed back with the
cold from the rads and the return temperature flow goes up - no risk
of condensing.

Of course as the return flow temperature rises the efficiency goes
down but the trade off is that you can set the normal flow for a
larger drop (assuming you have the rad area to cope) and know that the
return temperature can only go up.

The effect for short cycling is the same. For a bypass the flow is
constant so temperature rise across the boiler is constant. It's the
return temperature that rises. Exceed the limit before the thermostat
cuts in and the boiler short cycles. For an alpha the flow drops so
the rise across the boiler increases. Again exceed the limit and the
boiler short cycles. It's all down to the boiler outputting X kW and
the house only needing X kW. The way to lengthen the cycle time is
to have enough heat capacity (water and rad metal) to absorb the X kW
until the main thermostat cuts in. That's all about having a balanced
system.

BTW this assumes that you don't have a modulating boiler (i.e. one
that can vary the output power and so doesn't short cycle). I think
some as boilers are as it is easier to do on gas. Most oil boilers
aren't.


--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
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