On Sun, 19 Sep 2004 12:55:42 +0100, Malcolm Reeves
wrote:

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.

That isn't true. Non-condensing boilers are designed to operate most
efficiently at 82/70 flow return.


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.

That isn't quite true either.

A condensing boiler's efficiency doesn't suddenly improve as if a Holy
Grail has been found at the dew point.

WHat happens is that the efficiency increases with decrease in
temperature both above and below the dew point. When condensing
commences at the dew point, all that happens is that the *rate* of
increase of efficiency increases with falling temperature.

At a flow of 82 degrees, a condensing boiler will be more efficient
than a conventional one anyway because the heat exchanger is larger.

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.

That is also untrue.

You can replace a conventional boiler with a condensing one. When
required, in the depths of winter the condensing boiler can run up to
the 82 degree level of the conventional one and still be more
efficient. For most of the heating season, the condensing boiler will
modulate down to a lower output and into an even more efficient
working range.

It is true that system efficency can be improved by using larger
radiators, but it isn't essential.

In terms of derating, the manufacturers tables are too high for
typical UK boilers.

The normal derating factors are to multiply by 0.89 for an 82/70
boiler and by 0.6 for a 70/50 design with a condensing boiler.




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.

This is precisely where an Alpha pump is intended to be used. An
automatic bypass is intended to deal with the case where both CH and
DHW demand has ceased altogether and the boiler has been firing at
full tilt. It simply prevents the water from boiling and nothing
more.

The TRVs do not stop the flow completely if adjusted correctly in
relation to the radiator in the non-TRV room. The object is to turn
off the boiler via the room thermostat as all rooms have arrived at
set point, not for the TRVs to have closed completely.
Provided that the boiler thermostat is adjusted correctly, operating
temperatures will also be correctly in range.



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.



..andy

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