How to replace a Room Thermostat

[ericmark]

The way forward is to work out how it is controlled, in the main we follow plans C, S, W, Y etc. Most likely is the C plan which has been modified over time, so one can look at it as three stages. this is the really old system there was no control over DHW other than the programmer, then this one added a thermostat, so in the summer you can switch on DHW 24/7 and it will only fire when required and then this where a motorised valve has been added to DHW so it can be switched off and the temperature controlled independent from the CH.

Although the latter looks good I was advised against it as it puts extra strain on the boiler because when it switches off there is no where for the get to go, with latter boilers there are fans that continue to run after boiler has turned off to cool it through the flue, how true this is I don't know, I am not a plumber, I am an electrician.

With all you should be able to turn off domestic hot water when central heating not running so I am trying to work out why you can't. I know with Nest I had a problem with the boiler firing up, and it turned out to be anti legionnaires software which could be disabled, however nothing leads me to think you has a complex system like Nest installed. There may be a wiring centre under my Nest heat link there is a Nest of wires, the problem with Nest is the space for the wires is limited, so it made sense to have a wiring centre below it, to allow all the old wires to be reused, also by the pumps I have a second wiring centre at one point part of my heating was controlled by a wifi socket, when I moved in I found granny flat below house central heating worked OK, but the house had very poor control, I could switch it on/off from house, seems one wire of three and earth was faulty, so could turn heating on/off but to select if DHW or CH + DHW I had to walk outside down a set of steps and into the granny flat under the main house and plug in the pump. I still have a relay to add, but the house now works OK, the whole reason for Nest was two wires from heat link to thermostat charge the battery in wall thermostat and take all the commands to the heat link, and I only had two wires, so it enabled an easy fix.

So your not alone having a messed up system. Mine nearly fixed as I say one relay to add and since not using flat under house does not matter at the moment. So pictures as you can see above do help, and @stem is very good at working out what people have. I have been known to make errors. The way to stop making an error is to draw a plan, this is how I worked out how to wire my house so my house plan is and it was drawing out this plan which alerted me to need for second relay. Yes it took some time to draw out, but any fault in the future I can get any electrician to fix as he can follow the plan.

 

[Steven Ferguson]

The way forward is to work out how it is controlled, in the main we follow plans C, S, W, Y etc. Most likely is the C plan which has been modified over time, so one can look at it as three stages.View attachment 180821 this is the really old system there was no control over DHW other than the programmer, then View attachment 180820 this one added a thermostat, so in the summer you can switch on DHW 24/7 and it will only fire when required and then this View attachment 180819 where a motorised valve has been added to DHW so it can be switched off and the temperature controlled independent from the CH.

Although the latter looks good I was advised against it as it puts extra strain on the boiler because when it switches off there is no where for the get to go, with latter boilers there are fans that continue to run after boiler has turned off to cool it through the flue, how true this is I don't know, I am not a plumber, I am an electrician.

With all you should be able to turn off domestic hot water when central heating not running so I am trying to work out why you can't. I know with Nest View attachment 180823 I had a problem with the boiler firing up, and it turned out to be anti legionnaires software which could be disabled, however nothing leads me to think you has a complex system like Nest installed. There may be a wiring centre View attachment 180824 under my Nest heat link there is a Nest of wires, the problem with Nest is the space for the wires is limited, so it made sense to have a wiring centre below it, to allow all the old wires to be reused, also by the pumps I have a second wiring centre View attachment 180825 at one point part of my heating was controlled by a wifi socket, when I moved in I found granny flat below house central heating worked OK, but the house had very poor control, I could switch it on/off from house, seems one wire of three and earth was faulty, so could turn heating on/off but to select if DHW or CH + DHW I had to walk outside down a set of steps and into the granny flat under the main house and plug in the pump. I still have a relay to add, but the house now works OK, the whole reason for Nest was two wires from heat link to thermostat charge the battery in wall thermostat and take all the commands to the heat link, and I only had two wires, so it enabled an easy fix.

So your not alone having a messed up system. Mine nearly fixed as I say one relay to add and since not using flat under house does not matter at the moment. So pictures as you can see above do help, and @stem is very good at working out what people have. I have been known to make errors. The way to stop making an error is to draw a plan, this is how I worked out how to wire my house so my house plan is View attachment 180826 and it was drawing out this plan which alerted me to need for second relay. Yes it took some time to draw out, but any fault in the future I can get any electrician to fix as he can follow the plan.

Thanks for the very impressive reply I will take time to digest and investigate my setup.
Regards
Steven

 

[ianmcd]

i will ask again, when the HW only is selected does the pump run ? if the pump does not run and the boiler fires you have Gravity HW, which I think you do, and if you do you will not get CH without HW, most controls can be set to gravity HW, but if you want CH without HW you need plumbing alterations and additional controls

 

[Steven Ferguson]

i will ask again, when the HW only is selected does the pump run ? if the pump does not run and the boiler fires you have Gravity HW, which I think you do, and if you do you will not get CH without HW, most controls can be set to gravity HW, but if you want CH without HW you need plumbing alterations and additional controls

Yeah I'd say its definitely gravity HW as no pump runs when on hot water only.

Regards

Steven

 

[Steven Ferguson]

Picture of dhw tank.
Regards
Steven

 

[stem]

Normally heating systems with stored hot water cylinders have a two channel programmer, with separate time controls for Central Heating and Hot Water, as per the examples below.

When a traditional thermostat is replaced with a programmable thermostat, only the heating channel at the existing programmer is set to be permanently 'on'. The hot water control channel can still be used to control the hot water as before.

However, with some older systems, because of plumbing restrictions it is not possible to have the central heating 'on' unless the hot water is also 'on' at the same time. If this is the case with your system, then it's a bit of a faff, but provided the hot water is set to be 'on at' the same time as the heating is set to be 'on' at the new thermostat, it will work. And the hot water can be set to be off when neither heating or hot water is required. But here's the thing....

....the existing programmer may be set to 'gravity' mode. So what happens is that the hot water automatically comes 'on' when the heating is selected 'on'. So, if yours is like this, now that the heating is set to be permanently 'on' the hot water will also be permanently 'on'. This can be overcome by setting the existing programmer to 'fully pumped' mode and that will prevent the heating also selecting the hot water. But you will have to make sure that the set hot water times mirror the set heating times at the new programmable thermostat....like I said it is a faff, but doable.

On the other hand, if you have a single channel programmer and the heating and hot water share the same on/off time control. Then you will need to change the programmer for dual channel one, and if you have the old plumbing arrangement, consider updating that too.

 

[Steven Ferguson]

Thanks for the reply Stem I move the jumper over to pumped I could hear the pump inside the boiler going but the boiler wouldn't fire for the ch. So I reverted back to gravity.
Dont know if I did something wrong.
Regards
Steven

 

[Steven Ferguson]

This is the programmer on front of boiler.

 

[stem]

I moved the jumper over to pumped I could hear the pump inside the boiler going but the boiler wouldn't fire for the ch.

Was the hot water time control set to be 'on' at the same time that you did this?

If your system has gravity circulation for hot water, and pumped central heating, how it works, is that the hot water when set 'on' operates the boiler only. The boiler fires up. and water from the boiler circulates by natural convection around the the hot water cylinder.

Then, when the heating is set 'on' all that does is start the pump to circulate water around the radiators, nothing else.

This is why the hot water has to be set to be 'on' at the same time as the heating is required, otherwise the boiler won't fire up.

 

[ericmark]

Instructions for programmer not a good manual, however it would seem you have a gravity system without any tank thermostat, and the stand alone programmers would have both electrical and mechanical switches, the program was called 10/16 programs, I question that description seems they called off as a program, but idea was in gravity mode the mechanical interlock stopped you being able to switch DHW off when CH was on. So you have it seems 10 options.

Although it shows the electrical connections it does not show mechanical interlock. So I would have to test, to know what is required, likely it will need removing.

 

[stem]

So in summary:

1. The programmer should be set to 'fully pumped' and the heating set to be permanently 'on' and left on this setting.

2. The central heating 'on' times would then be set as per your preferences at the new Salus programmable thermostat.

3. The hot water must then be set at the programmer to be 'on' to match the same times as the heating is set at 'on' the Salus, as per 2 above. [The hot water can be set to be 'on' at additional times if required, but as a minimum it should match the Salus 'on' times]

 

[Steven Ferguson]

Thanks guys for the responses I will try them again tomorrow.
Regards
Steven