On Sun, 19 Sep 2004 14:53:59 +0100, Andy Hall
wrote:

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.

Not according to HRM technical department. It also stands to reason
that all other things being equal the lower the boiler average water
temperature the more energy you get from the fuel, i.e. the cooler the
flue gas. SEDBUK cap standard boiler efficiencies specifically to
stop boiler makers designing standard boilers on the edge of
condensing and thus improving their efficiency at the risk of shorter
life (due to condensing causing corrosion). Hence the sedbuk figures
need to be viewed in the light of being capped when comparing
condensing and non-condensing boilers :-/.

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.

So what you are saying is that a larger difference than 82/70 does
improve efficiency. Like I said. The specific latent heat extracted
at the dew point is a lump of energy, either you have it or you don't.
If you are running above the dew point you don't have it and the
boiler is working in standard mode not condensing.

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

True a larger heat exchanger will get more heat out of the fuel and
thus be more efficient. However, condensing boilers are $$$ more
money and if the only benefit is the larger heat exchanger (i.e. not
condensing) would the savings be worth it?

I'm just changing my heating to oil fire standard boiler not a
condensing since on consideration of the extra boiler cost, problems
(smells, plume etc.) vs the potential savings (perhaps GBP20 pa) I
have chosen a standard oil boiler. I'm fitting this now in case they
change the rules in April 2005.

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.

How does a non modulating boiler modulate? The water comes out of the
boiler at 80C goes through the radiators which cools it by 10C (if
they are exactly right for the room loss) and returns to the boiler at
70C. If you want it to return cooler the radiators have to be bigger
since the heat they give up to the room is dependent on their size and
the (fixed) room temperature. You could run with a lower flow
temperature but then you'd have less drop across the boiler which as
we have already agreed is less efficient. In the case of a condensing
boiler the condensing gain would probably exceed the standard heat
exchange section drop in efficiency but it still won't be as good as
if it was running at the design figures.


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.

No. An automatic bypass says it is for TRVs. In any case what will a
bypass do in the situation you have above. If you are trying to get
heat out of boiler after the fuel has cut off you need a pump over
run. That is something that runs the pump for a few minutes after the
thermostats have cut off and the boiler stopped firing. Usually a
thermostat on the boiler I believe rather than a timer but the effect
is the same. Some boilers don't need it as the cut off quick enough
that the water can absorb the extra heat.

A three way valve must be open to at least one way and you must have a
path (non TRV rads or tank). What stops the flow is the pump not
running. If you have flow but not enough then that's where an
automatic bypass comes in. Pump head (pressure) rises and the valve
opens.

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.

If the TRVs are even partially closed then the flow is reduced. An
alpha will keep the head constant-ish resulting in less flow and thus
lower return temperature. A standard + bypass will result in constant
head (the setting the bypass is set to), and a constant-ish flow


--

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
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

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