Combi Boiler and Room Thermostats

[Daern's Instant Fortress]

I'm after a little bit of impartial advice on my recent CH upgrade. It's a bit of along tale, but quite simple in truth...

Our old CH system used a single room (house?) thermostat, located in the hall. The radiators did not have thermostats fitted. Our house has a very large conservatory, which was heated by a CH fed convector. Unfortunately, because the room 'stat at the other end of the house would always turn off before the conservatory was warm, we would be left with either a warm house and cold conservatory or, after turning the 'stat up, a warm conservatory and a _very_ warm house :-(

I've now had a Worscester Bosch 35CDi II combi fitted to replace my (somewhat ageing) gravity fed boiler as part of some work we are having done on the house.

Because of the conservatory problem, we have decided to remove the room 'stat and use radiator thermostats on all of the radiators, bar a single one near the boiler. This should (apparently) allow each room to maintain its own temperature and once the whole system reaches temperature, the system will shut off because the returned water feed is at the same temperature as the outgoing feed (apparently this is because it's heating only one radiator).

I suppose my question is - is this a good plan, or am I going down the totally wrong road? What considerations should I be making here? Can I run a combi CH system without a house 'stat?

Thanks in advance for any help you can offer me - as you have probably gathered, I know very little about plumbing ;-)

DIF

[Ed Sirett]

The very best way would be to zone the conservatory independantly fromt the rest of the house heating.

TRVs on cooler rooms is no bad thing.

However the arrangement you outline assumes that you want to heat the conservatory whenever you heat the house and to much the same sort of level. If you can arrange for the conservatory to be independantly heated you can opt for such things as having it mildly heated for plants even when you are out or away. Alternatively you might want to avoid heating the conservatory entirely in deep-winter? Again you will probably want your heating system to stop heating the conservatory when the sun shines as such locations are apt to have a large solar gain.

Depending on how the pipes are laid and the state of decor the ease of doing what you want will vary. In essence you would block off the existing feed to the conservatory then connect a new feed pipe from before the first T to feed any house radiators. Both the existing pipework and the new pipe would then have there own valves to control them independantly. A programmable thermostat would be used to control the conservatory.

I guess others will want to give you their own 2ps.

[Owain]

"Daern's Instant Fortress" wrote | Our house has a very large conservatory, which was heated | by a CH fed convector. ...

Moving to TRVs is generally regarded as a Good Thing, and may actually be required under Building Regulations (when you replace your boiler you may be required to bring the whole system up to current standards).

However what you really should consider is putting your conservatory on a separate 'zone' on the heating system, so you can programme it with its own time and temp settings completely separately from the house. You will be using a lot of heat to keep it warm in the depths of winter. Depending on how the plumbing is arranged this may be a fairly simple job adding a zone valve and an appropriate controller, or it may be a complete pain.

Owain

[Daern's Instant Fortress]

I'll reply once for Ed and Owain's replys, as you both say more or less the same thing:

I see where you are coming from with the seperate zones, but surely turning the TRVs down in the winter (or if we are not using the room) amounts to the same thing. The conservatory will be out of circuit and not using any energy. Likewise, when the sun shines, the room is warm, the TRVs are off and I'm not spending money. Am I missing some subtlety here?

As a general system question, when using TRVs, is it normal to *not* have any room thermostats at all and what is the best practice for managing this? Should I have all of my rads with TRVs or just leave one without?

Thanks for your replies on this subject. They're much appreciated!

DIF

[John Stumbles]

Yes: the boiler is still on demand, and producing hot water (even though it's not doing a lot since it'd only trying to heat the one un-TRVed rad.

Leave one without and have a room thermostat sensing the temperature in the ('master') space that it heats. Then that 'stat can turn off the entire system when that space is warm enough. It means you have to adjust the valves on that rad so that it puts out little enough heat that the rest of the house gets enough heat to satisfy their TRVs before the master space gets too hot and the stat turns off, but hey! this is plumbing & heating, not rocket science :-)

For a better system you'd have each space monitored and heating controlled by its own thermostat, with the boiler told to provide heat when any of the room 'stats demanded. Having 2 or more zones rather than treating the whole house as one space is a step towards this ideal (and therefore A Good Thing (tm) :-).

[Christian McArdle]

It is permitted to use TRVs on all radiators and with no room thermostat. However, you must still achieve boiler interlock if you wish to comply with the Part L1 approved document.

One way of doing this is the following:

1. Install automatic bypass valve.
2. Install flow switch in heating circuit (*AFTER* ABV, *BEFORE* radiators)
3. Use flow switch output as a call for heat (simple on S-Plan, more complex wiring for Y-Plan)
4. Wire pump permanently (or for a more sophisticated system, run the pump on an 'OR'ed combination of the boiler pump output and a timer which produces 5 second pulses every 10-15 minutes when the programmer CH On is active).

Either. TRVs on all radiators is preferred. You can use a permanently open radiator (lockshield both ends) instead of an automatic bypass valve, but it must still be the only radiator BEFORE the flow switch. Putting it after the flow switch will result in the interlock being ineffective. Putting any other radiator before the flow switch will result in that radiator's call for heat being ignored (although it will still heat up if other radiators do call for heat).

Christian.

P.S.

You should still subzone the conservatory if at all possible, as this would allow different timings to the rest of the house. It is possible, if you want reduced timing periods for the conservatory but don't want the disruption of running new zoned pipework, to do a "subzone lite". To do this:

1. Implement the flow switch interlock above.
2. Do not install TRV in conservatory.
3. Install 2 port 22mm zone valve on conservatory radiator/convector.
4. Install programmable room stat in conservatory.
5. Wire zone valve to run when room stat activates.
6. Ignore zone valve switch outputs.

This will result in the conservatory system operating as a separate zone, provided that the house programmer is set to heating on. This is a very good idea, as it allows the conservatory to be heated only when required. If you use it as a dining room, for example, you can set it to come on from 7pm to

9pm, for example, whilst being cold the rest of the day.