Using two thermostats to control a boiler

[will0787]

Hi all,

I am looking for a bit of advice and would really appreciate some help here. I have a Baxi Platinum 40 HE boiler. This uses a Towerstat RF/BM wireless thermostat. The boiler and wireless thermostat work fine. I have now installed a wet under floor heating kit in a conservatory. This kit uses a 2 circuit manifold with zone valve and pump and is wired to an Eco ET2 thermostat. Both the house circuit and the conservatory circuit are on the same pipe circuit and are both fed from the Baxi boiler. My question relates to the control (call for heat signals) of the boiler using the thermostats. As far as I can tell the boiler stops providing heat when either of the thermostats reach their set point? For example If I set the main thermostat set point to 21 degrees and the wired (conservatory thermostat) to 25 degrees the boiler will stop supplying heat when the 21 degrees temp is reached on the main thermostat. I realise that they share the same water circuit but should this be the case? Should it not be the case that boiler should supply heat until all thermostats reach their set point temp? Also, is there a slave and master thermostat relationship going on here whereby one thermostat (presumably the Towerstat RF/BM) has priority over the other.
Apologies for the lengthy question but I am trying to give as much info as possible.

Regards
Bill

 

[Terrywookfit]

All circuits require a zone valve.
Look at Honeywell S Plan +.
You will then get independant control.

 

[will0787]

All circuits require a zone valve.
Look at Honeywell S Plan +.
You will then get independant control.

Thanks for replying Terry. I already have a 22mm zone valve on the manifold connected to the conservatory piping. I am not wanting individual control in two separate areas what I would like to know is if the thermostats are working correctly given my current configuration.

Cheers
Bill

 

[Richard C]

I am not wanting individual control in two separate areas what I would like to know is if the thermostats are working correctly given my current configuration.

Must admit I thought that was what you were after & is what you should have.

Just so you’re aware, a conservatory heating system must be capable of independent control & isolation from the main system to comply with Building Regs.

 

[D_Hailsham]

My question relates to the control (call for heat signals) of the boiler using the thermostats. As far as I can tell the boiler stops providing heat when either of the thermostats reach their set point? For example If I set the main thermostat set point to 21 degrees and the wired (conservatory thermostat) to 25 degrees the boiler will stop supplying heat when the 21 degrees temp is reached on the main thermostat. I realise that they share the same water circuit but should this be the case? Should it not be the case that boiler should supply heat until all thermostats reach their set point temp?

So you need the thermostat to shut the zone valve of zone 1 when the zone reaches temperature, but not to turn of the boiler until zone 2 reaches temperature; and vice versa.

The first thing is that this must be physically possible, i.e shutting one zone valve does not prevent water flowing through the other zone; i.e the the two zones are completely separate branches.

Secondly, both zone valves must have auxiliary switches to control the boiler. You then wire up as an S Plan.

View media item 72

 

[will0787]

My question relates to the control (call for heat signals) of the boiler using the thermostats. As far as I can tell the boiler stops providing heat when either of the thermostats reach their set point? For example If I set the main thermostat set point to 21 degrees and the wired (conservatory thermostat) to 25 degrees the boiler will stop supplying heat when the 21 degrees temp is reached on the main thermostat. I realise that they share the same water circuit but should this be the case? Should it not be the case that boiler should supply heat until all thermostats reach their set point temp?

So you need the thermostat to shut the zone valve of zone 1 when the zone reaches temperature, but not to turn of the boiler until zone 2 reaches temperature; and vice versa.

The first thing is that this must be physically possible, i.e shutting one zone valve does not prevent water flowing through the other zone; i.e the the two zones are completely separate branches.

Secondly, both zone valves must have auxiliary switches to control the boiler. You then wire up as an S Plan.

View media item 72

Hi, thanks for the repsonses. I have the following http://www.underfloorheatingdiy.co.uk/underfloor_heating_prices.html underfloor heating kit. My understanding was that the isolation valve of the UFH manifold opens when the thermostat makes a call for heat - the conservatory is thus isolated from the main supply. If I turn off the conservatory thermostat water does not flow into the conservatory circuit. I am not wanting individual temp control across two zones - what I would like to know is if the thermostats are working as expected - should the boiler stop providing heat as soon as the first thermostat reaches it's programmed temperature? Cheers.[/url]

 

[D_Hailsham]

If I turn off the conservatory thermostat water does not flow into the conservatory circuit. I am not wanting individual temp control across two zones - what I would like to know is if the thermostats are working as expected - should the boiler stop providing heat as soon as the first thermostat reaches it's programmed temperature?

What do you want the thermostats to do?

What happens will depend on how the system is wired up.

Are you saying that you do not want to control the conservatory separately from the rest of the house? If the only boiler control is via the main house thermostat then, when the main thermostat reaches temperature, the boiler will shut off, irrespective of what the other thermostat says.

It seems a bit pointless for the boiler to be turned off by zone 1 valve if zone 2 has not reached temperature; but if this is what you want, then it's wired correctly.

Or do you have a problem which you have not told us about?

 

[will0787]

If I turn off the conservatory thermostat water does not flow into the conservatory circuit. I am not wanting individual temp control across two zones - what I would like to know is if the thermostats are working as expected - should the boiler stop providing heat as soon as the first thermostat reaches it's programmed temperature?

What do you want the thermostats to do?

What happens will depend on how the system is wired up.

Are you saying that you do not want to control the conservatory separately from the rest of the house? If the only boiler control is via the main house thermostat then, when the main thermostat reaches temperature, the boiler will shut off, irrespective of what the other thermostat says.

It seems a bit pointless for the boiler to be turned off by zone 1 valve if zone 2 has not reached temperature; but if this is what you want, then it's wired correctly.

Or do you have a problem which you have not told us about?

Hi, initally I thought the setup would have allowed me to have the conservatory at a different temperature than the rest of the house. I have since found out that the isolation valve on the conservatory manifold opens to allow hot water into the circuits when the temp on the conservatory thermostat has not been reached. The pump on the manifold relieves the boiler pump form pumping water round the UFH pipework. The thing is that if the conservatory thermostat reaches it's set point then the boiler turns off. So I am trying to work out a way whereby I can have the same temp across the house - maybe I should move the wireless thermostat to the conservatory as well.
Cheers

 

[D_Hailsham]

Hi, initally I thought the setup would have allowed me to have the conservatory at a different temperature than the rest of the house. I have since found out that the isolation valve on the conservatory manifold opens to allow hot water into the circuits when the temp on the conservatory thermostat has not been reached.

Does the isolation valve on the manifold look something like this?

View media item 11762
That is normally called a zone valve.

If the conservatory zone valve is turning the boiler off when the conservatory reaches temperature, it's because the auxiliary switch in the valve is connected to the boiler.

The pump on the manifold relieves the boiler pump from pumping water round the UFH pipework.

Not strictly true. The main pump would not be suitable for the underfloor circuit as the water circulating the underfloor is at a different temperature to the rest of the house. The underfloor water is maintained at the correct temperature by the thermostatic valve, which controls the mixer valvin the manifold and allows water to enter and leave the underfloor circuit.

The thing is that if the conservatory thermostat reaches it's set point then the boiler turns off.

That's due to the way the underfloor zone valve is wired

So I am trying to work out a way whereby I can have the same temp across the house - maybe I should move the wireless thermostat to the conservatory as well.

You haven't told us about the rest of the system.

If you have a combi boiler, is there a zone valve in the radiator circuit?

If you do not have a combi, is there a mid position valve or separate zone valves for the hot water and radiator circuits?

Look at the S Plan circuit I posted earlier, the two heating zones shown are, in your case the rad zone and the underfloor zone.

 

[mikethomas30]

dont know if you ever got this sorted i am also looking in to this same conservatory but with a combi and a zone valve from what i can work out if i use the froststat input, it will override the roomstat or in my case a digistat just wondered if this was of any use to you.

 

[Dan Robinson]

You should use a standard S-Plan.

 

[mikethomas30]

i should just move the digistat to the conservatory but because we have dogs and cats and conservatory gets very hot or very cold,we shut all patio doors and signal is lost. Would two stats and a zone valve not work ?.

 

[Dan Robinson]

What use would two thermostats on one zone valve do?

 

[SimonH2]

Would two stats and a zone valve not work ?.

Ideally you want two stats and two zone valves, though the mixer on the manifold for the UFH may well act to shut off the flow completely if it's set up right.

Motorised zone valves include a switch which can be wired to the boiler. Standard S plan wiring applies as mentioned earlier - you wire all the switches in parallel and across the demand connections on the boiler. Then you wire each valve to the thermostat for the zone it's controlling.

When any stat calls for heat, the valve for that zone motors open, the switch closes, and the boiler turns on. Only when all stats are satisfied, and the valves sprung shut, does the boiler turn off.