On Sun, 19 Sep 2004 13:59:34 +0100, "Tom"
wrote:
"Malcolm Reeves" wrote in message
.. .
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.
Fine, Ok to that, existing thermostat in hallway, a non-TRV area.
Lounge has additional heat source(SWMBO requirement) + high Solar Gain
,hence TRV.
You might find that you get smoother results if you put it on the
upstairs landing. It will be less suscepitble to the effects of
opening the front door.
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.
Agreed, I remember in the dim and distant past a HVAC consultant quoting
that figure at me in work.
It's around 52-54 degrees.
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.
Agreed, no problem with that, makes sense.
See separate note on derating of radiator specs.
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.
Yessss, I hadn't thought about that, getting too old in the tooth I think!!!
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.
OK.--The automatic bypass, do these work in an analogue fashion, do they
have differing pressure/resistance curves or are they on/off devices.
They are pretty much on/off and are intended to deal with the case
where DHW and CH both stop demanding and the boiler is in full burn.
The setting should be arranged so that the valve is closed in all
cases exept where this happens. The idea is *not* to allow any
feedback of flow water to the boiler except under these conditions
when the only purpose is to keep a flow going and to prevent boiling.
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
Implying analogue characteristicts?
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.
Agreed again.
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.
Unfortunately I don't have a modulating or condensing boiler.
Much obliged to you for your detailed critique, I will be looking at bypass
systems I think. I inherited the boiler (Gloworm Hideaway 60B FF) with the
bungalow I moved into 6 Months ago, I shall use the existing pump which will
save me a bit.
You can do that provided that the pump is set to compromise with the
requirements of full heat demand and at the point when the TRVs are
closing. You also need to arrange that the room thermostat is
turning off the boiler when there is still some flow left through the
TRVs.
Best Regards
Tom
..andy
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