Changing a Central Heating Thermostat

[MattMunro65]

My old CH thermostat seems to have had it's day with at least a 5C difference between the set temp and the actual temp. I'm thinking of replacing it with a new one and have a couple of questions.

What is the best one to get? I'm thinking of fitting it myself so this is both for how well it works and how easy it is to fit ?

Will it cause any problems with the programmer/boiler ? I think they are both about 15 years old so pre-digital I would guess.

What I have at the moment is a PRT100 thermostat in the hall, in the kitchen a Potterton EP2002 programmer and a Prima F40 boiler.

On a related point, is it an urban myth that leaving your heating on a low setting 24/7 uses less gas for the same heat as compared to having it switch on/off with a timer ?

Any advice much appreciated

 

[1983]

How many wires are connected to existing thermostat??
Can you have your heating on without hot water?

 

[MattMunro65]

How many wires are connected to existing thermostat??
Can you have your heating on without hot water?

I haven't taken it off the wall yet. Was planning to do the job at weekend

The answer to the second question isn't straightforward: I had to replace the programmer a few years ago and bought an identical one but when I fitted it there was a plastic widget which obviously locks the two switches together, you could set it to either locked or independent. But, I couldn't remember which way it went on the old one so I set it to lock, as in you can have HW only, or CH+HW but not CH without HW. I don't know if that was right or not but everything seems to work ok. In the loft there's a big cold water tank and a smaller gungy one next to it if that helps. Underneath that is a hot water cylinder in a cupboard.

 

[1983]

Ok, was just asking to see what options you have.
The thermostat should have a neutral but t was often missed out. It needs this to operate properly. If it has one just change it for a standard 240v one.
If the neutral is missing buy a battery operated 2wire thermostat that's ok with 240v.
As for leaving heating on it depends. I wouldn't do it if you don't ave full thermostatic control over hot water as well. It's not quite an urban myth about leaving it on. A house will loose its thermal mass if heating off. Then you have to heat it all back up again.
Modern systems use a programmable room thermostat amongst other things to warm the house at different temps at different times of day. The heating is never completely off but just reduced at traditional off times.

 

[MattMunro65]

Ok, was just asking to see what options you have.
The thermostat should have a neutral but t was often missed out. It needs this to operate properly. If it has one just change it for a standard 240v one.
If the neutral is missing buy a battery operated 2wire thermostat that's ok with 240v.
As for leaving heating on it depends. I wouldn't do it if you don't ave full thermostatic control over hot water as well. It's not quite an urban myth about leaving it on. A house will loose its thermal mass if heating off. Then you have to heat it all back up again.
Modern systems use a programmable room thermostat amongst other things to warm the house at different temps at different times of day. The heating is never completely off but just reduced at traditional off times.

Thanks, I was thinking of getting a digital one but a bit confused about the types and compatibility etc. Seems like some of them basically replace the programmer as well. I'll take the thermostat off the wall and have a look before I do anything else

 

[MattMunro65]

Finally arrived, taken the cover from the old thermostat off. There are 3 wires connected to it: blue, red, earth. They go into a terminal as follows

Red - 1
Earth - 2
Blue - 3

problem the CM907 has terminals marked A,B,C. What goes where ? I took a picture if it helps

 

[MattMunro65]

Correction - it isn't earth, its just a yellow wire