In article , Malcolm Reeves
wrote:

On Sun, 19 Sep 2004 15:45:12 +0100, Mike Clarke
wrote:


To summarize my original post, we've just bought a house where there is
no room stat and only the bedrooms have TRVs. The "standard" approach of
a room stat in the living room and TRVs everywhere else doesn't appeal
since there will be times when we'll want to heat parts of the house
without heating the living room. Separate zones would obviously be the
best solution but I don't fancy the task of ripping up floors and
modifying the pipework layout to achieve this so I was considering
fitting TRVs to all the radiators and relying on the boiler's internal
bypass.

That won't work and probably is against building regs which now cover
heating systems. If you had a modulating boiler it could work but you
would need a bypass or an alpha pump since when all the TRVs closed
the pressure would rocket and most likely the TRVs make an awful
racket.

It is a modulating boiler and has an internal bypass so it works OK but
I am conscious of the energy wasted when all the TRVs are shut as the
boiler cuts in from time to time to keep the water circulating inside
itself up to temperature. Since this is an existing system it doesn't
have to be modified to comply with the more recent regs. OTOH I do need
to make some improvements over the current situation where 7 of the 12
radiators have no thermostatic control at all.

Have you thought about a radio thermostat? Not cheap but you could
take it from room to room. Then in the room that you want to keep
constant turn the TRVs to full and in the old thermostat room down to
some level.

That looks like a possible option to go on the list.

I was back at the "new" house last week and picked up the manual for the
boiler. It's a Worcester Bosch "350 Combi" (HC350.FSN), probably about 4
years old. The boiler has a single heat exchanger in the combustion
chamber, this normally feeds the CH circuit unless there's demand for
DHW when the CH pump stops and a separate pump starts up to divert the
flow to a water to water heat exchanger to provide DHW. In case my
description isn't clear I've put a copy of the boiler water flow diagram
on http://milibyte.co.uk/boiler.gif. As I said I'm a newbie to
radiator systems so I don't know if this is an unusual design or not.

Are you sure the CH stops? Usually both pumps run and the flow is
shared. It's the same as one pump + 3 port valve only with 2 pumps
instead. Boiler drives DHW, CH or CH+DHW.

Well, the manual states that demand for DHW will override the CH
requirement and when DHW is no longer required the burner will
extinguish for a waiting period of about 3 minutes before returning to
CH state. I can certainly confirm that the burners always shut down for
a few minutes after drawing hot water.

[Snip - explanation of room stat operation]

If the boiler flow water exceeds its set temp before the room stat
clicks off then the boiler short cycles. But as you can see from
above this is function of the boiler output, rads etc. You can't
really stop it with controls unless you vary something above (i.e.
increase rad size etc.).

Yes I appreciate that the boiler will cycle in this way when it can't
modulate down low enough. This form of cycling is OK, it's saving
energy. I was more concerned about the wasteful cycling when all the rad
valves are shut and it's just circulating through the bypass. Perhaps
I'm just being paranoid and the total heat wasted will be very low
compared to the heat used in warming the house at other times.

--
Mike Clarke