Installing a second CH zone

[Kostas Kavoussanakis]

Hi,

My central heating/hot water system involves a 1984 AD boiler, a cold and a hot-water tank with a thermostat and a programmable thermostat in the hall for the heating. The two systems share a motorised valve.

My lounge is open on three sides and has huge windows. Although its radiators can heat it, when the rest of the house has reached the desired temperature, the thermostat limits the output and the lounge starts cooling down disproportionately to the rest of the house.

I would like to have a second zone installed for the 2 lounge radiators, with its own programmable thermostat. The questions I have are as follows:

1. The installer has access to the pipes under the floor within 2m from the boiler; that's far from the lounge. Because of annoying wooden floors, there is no access to the pipes nearer the lounge. Is that a problem?

1. Will the system need a by-pass radiator in the second zone? (Actually, am I right in thinking that I can do away with the TRVs in this small zone?)

2. Will I need to replace the existing, 3-point motorised valve with a more complicated one, or do they just add another one somehow?

Many thanks in advance,

Kostas

[R]

Why not install TRVs on the other radiators elsewhere and adjust as required?

[Kostas Kavoussanakis]

I have TRVs everywhere, except for the one bypass radiator. The problem I see with the TRV-everywhere method is the by-pass radiator, which will be on all the time, plus the lack of ability to have different target temps at different times.

In my current setup I have turned the bypass rad to next-to-a-trickle. I feel I am able to do that because the central thermostat (which is close to the bypass rad) cuts everything out before the need to bypass arises. As a precaution, the bath rads are tuned reasonably high, but that's different to "full on".

So, my qs remain:

Thanks for reading this!

Kostas

[John Rumm]

It may be. You need to be able to access a pipe that feeds just the lounge rads if you want to make them a separate zone really.

Not necessarily.

If you keep the three port and add an additional 2 port in parallel with it for the new zone then there is no need for a bypass since this existing heating loop will provide it.

[Kostas Kavoussanakis]

Thanks. I guess this means one cannot assume that all the pipes start from (and are therefore accessible near) the boiler.

Will there be a bypass rad needed but not available if all of the following are true:

- The room thermostat in the existing system, which hosts the bypass rad, does not call

- The room thermostat in the new zone calls

- The TRVs in the new zone are shut (as a consequence of being set lower than the room thermostat)

Thanks,

Kostas

[Roger Mills]

Yes, but how the hell do you then wire it so that any one (or more) zone(s) calling for heat causes the boiler to fire - bearing in mind the somewhat complex control logic embedded in a 3-port valve?

In my opinion it would be preferable to replace the 3-port valve with two

2-port valves and then add another for the third zone. The wiring is then straightforward since it becomes an S-Plan+ system. Admittedly, it may then need a by-pass circuit (but even the existing Y-Plan system doesn't necessary guarantee an adequate flow path when all the TRVs have shut off).

Any of this is only feasible if the pipework layout is such that the lounge radiators can readily be split off into a new zone. A simpler solution may be to relocate the main stat into the lounge so that the boiler stays on until that (the lounge) is warm - and to rely on the TRVs to stop the other rooms from overheating.

[meow2222]

relay

Indeed, if the OP wants the heat on at the same times all round the house. But rather than rely on TRVs, which are only semi-thermostatic, better to use a thermostat in each room to control each set of rads. IIRC Danfoss RA 2912.

NT

[John Rumm]

Well, you can certainly get to the flow and return, but that is for the whole CH system, not just the rads you want to control independently. So you need to find where they tee off the flow pipe.

Then there is still a path through the three port valve via the hot water cylinder heating coil.

However:

You don't want a TRV on the rad in the room with the prog state - which may mean removing the one on the lounge rad. If you keep it then you could remove its head so that it is uncontrolled and set to full output.

[John Rumm]

Wire "or" the output of the new two port with the existing three port. (or for that matter just use the call for heat from the new prog stat directly).

This would certainly be simpler wiring wise, but a bit more hassle in other ways.

Yup, by far the path of least resistance!

[Roger Mills]

Care to produce a circuit diagram?

I'm far from convinced that what you suggest will work - bearing in mind that the 2 and 3 port valves are very different in the way they operate. The

2-port is simple, and separates the motor drive and boiler switching - which are electrically isolated from each other - and requires a permanent live feed to the auxilially contacts. The 3-port has it's motor drive and boiler switching all mixed up and, in fact, doesn't switch the boiler at all in HW-only mode.

If you mix 2 and 3 port valves in the same system, it starts to get very complicated.

[Roger Mills]

Where does the relay go?

[meow2222]

2 relays, each one has its coil powered by each stat's call for heat. The switch sides of the relays are parallelled to provide a live feed whenever either zone wants heat.

This method can be used in any situation where you need 2 signals to control one line, but to also stay separate so they can still control

2 things separately as well.

NT

[John Rumm]

Yup, I realises they are "inventive" in the way they work, hence I was proposing to leave the 3 port controlled exactly as it currently is. Add a two port (split the feed into the 3 to do this), and drive that from the new room stat. Use the volt free contacts on the two port with a permanent live to provide an additional call for heat to the boiler (must admit I have not checked if the OPs boiler is a system one and hence controls the pump itself). If the existing call for heat is left floating when not on demand then this can simply be wired in, otherwise a relay would be needed to isolate the two demands.

[Kostas Kavoussanakis]

OK, does that differ much from just increasing the target temp of the room thermostat in its current position? Given that I have a programmable thermostat (and know how the heating balances out in the house) that's already available to me to at least experiment.

The initial idea was that I would be heating the lounge fewer hours, but it looks like the 2-zone project is too costly and very doubtful, given the lack of access throughout the house. I hate wooden floors...

This looks like a good idea (though of course my 1-yo TRVs in the lounge are incompatible with this :-)).

may consider that.

Many thanks again!

Kostas

[Roger Mills]

Well ok, but surely two 2-port valves are simpler than a 3-port valve plus two relays? Besides which you would then presumably be running the 3-port valve open loop - so that the relays would switch on the boiler without knowing that the valve had actually done what you told it to (they do tend to seize up!). Using 2-port valves, you don't need relays and the boiler only gets switched on once the valve has actually opened.