Central Heating again - does Combi need at least one rad on all the time

[dave]

Re Meeting with BGas chap about Combi condensing boiler CH system. I thought there had to be at least one rad left one as a load for the combi. He said this was not needed with modern boilers. Fine, except when it came to the bathroom towel rail, for some reason that had to be on all the time! The towel rail is a radiator surely. But no - he said no thermo valve would be attached to that. So the towel rail would only be off when the heating was off. I should have ask "what if I fit a radiator in the bathroom instead of a towel rail". But other things got talked about so I still don't have an answer. Can someone tell me the fatcs about this? tvm

[The Natural Philosopher]

pumps don't like running against a shut down circuit.

So you need at least a short circuit somewhere if there is an all TRV setup.

[spuorgelgoog]

If all the rads have TRVs, then they can shut and the water will continue r unning round the circuit until the boiler stat opens. That's no longer cons idered acceptable due to energy inefficiency. You now need a radiator that' s continuously on, so the room that's in continues getting warmer until the room stat opens and turns off the boiler.

There are other ways of achieving boiler interlock to prevent cycling but t hat's the most common one.

However note that the room stat and the un-statted radiator have to be in t he same room. A (wireless) room stat would be electrically compliant in the bathroom but would not be a good idea as the temp may fluctuate rapidly du e to external sources like steamy baths and/or opening the window after sam e.

Owain

[Robin]

What you were told makes no sense to me. Perhaps he just wanted to maintain BG's reputation :)

I suggest you look at the manufacturer's instructions for any boiler you are thinking about. If it has an internal by-pass (as is indeed common) the advice is usually that TRV's must be used on all radiators except where a room thermostat is sited - if there is a room thermostat.

[dennis]

At the price they were quoting I would want zone valves on each room with a stat in each room. A stat and zone valve costs about £100 and if you use plastic pipes it makes little difference to the plumbing.

[John Rumm]

Many modern boilers have an internal adjustable bypass that will allow them to run into a fully occluded system (i.e. all TRVs shut). However its a non optimal way of doing it, since you can't then easily provide a proper interlock to shutdown the boiler when the whole place is up to temperature.