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.

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.

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.

I suppose it all boils down to whether the cost of the fuel wasted in
cycling the boiler is sufficient to justify the cost of installing a
more complex control system.

Don't forget that the entire house is a system. Consider what happens
when the thermostat switches on. The water in the rads is at X C.
This is pumped through the boiler raising its temperature, heating the
rads up, heating the room air up. Eventually room stat clicks off.
Water in rads cools, air cools, room stat clicks on. Cycle repeats.
So the cycle time is a function of boiler output, amount of water in
the rads, amount of air in the room, etc.

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.).

Basically the boiler is stuffing out X kW. It has to go somewhere,
the water, the house fabric, etc. If the boiler output is too high
then it will exceed temperature before the room has warmed up and the
stat turned off. The solution is to increase the output in the stat
room so that the stat turns off first (or increase the power sinking,
more rads say). Clearly balancing the system so each rad gets its
share of the power is a must.



--

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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