potterton prt2 thermostat not responding to temperature

[MilaGB]

Hi, I have an old style back boiler and can't get my ch work. The dial on the thermostat is on maximum but heating comes on only when it's extremely cold. Hot water works ok. I'm thinking the thermostat could get dusty inside as we did some plastering but no changes after I cleaned it. I unscrewed the top bit of the thermostat but it's made of 2 parts and I couldn't separate them as I was afraid the break it. Perhaps the part that needed cleaning was inside? I heard these thermostats could have sticky valves, where are they exactly?
Is it easier to replace the thermostat? Would any new one be ok?
Thank you so much for your help.

 

[oldbuffer]

1. Thermostats don't have valves inside them.
2. What thermostat is it (make and model)?

 

[MilaGB]

Oh ok re valves. It's potterton prt2, really old one it seems Thank you.

 

[JohnD]

is the hot-water cylinder surprising hot?

Do you have any hot radiators? For example upstairs?

 

[MilaGB]

I'll check the hot water cylinder in a few hours but I don't thing it's extremely hot.
The radiators are only hot when the heating is on, otherwise they're cold.
It just looks like the thermostat misreads the temperature but I might be wrong here.

 

[CBW]

What do you mean they only come on when it’s extremely hot?

 

[MilaGB]

What do you mean they only come on when it’s extremely hot?

Oh sorry, I meant cold, will edit

 

[CBW]

Do you have a motorised valve?

 

[JohnD]

I think this will be much easier if you can lay your hands on a multimter.

your thermostat is just an electric switch, so it will be easy to check if it is turning on and off according to the temperature.

If you post a a photo of it we can probably describe how to test, and clean or replace, but the fault may be elsewhere. Is it one of these?

You can copy and paste photos into your reply from your phone or a camera conected to the PC, or a tablet will have its own camera.

You can pick up a cheap one for around £12 in a DIY or hardware shop.

https://www.wilko.com/en-uk/rapitest-digital-mini-electrical-test-meter/p/0202332

https://www.wickes.co.uk/search?text=multimeter

https://www.screwfix.com/c/tools/mu..._sp=managedredirect-_-electrical-_-multimeter

better ones are sturdier, but more expensive

 

[MeldrewsMate]

Do the lights still work on your PRT2 thermostat? They have the legends 'BELOW' and 'REACHED'. If they work and there's an audible click when turning it up and down then, despite the plastering, it is probably OK. 'BELOW' should be illuminated when the dial is turned high.

Let's look at the symptoms:

Hi, I have an old style back boiler and can't get my ch work. The dial on the thermostat is on maximum but heating comes on only when it's extremely cold.

Which dial, the boiler or the room thermostat? For the purpose of diagnosis set them both high.

heating comes on only when it's extremely cold.

Do you have a frost protection thermostat? It looks like a standard dial thermostat, but usually with a cover over the dial.

The radiators are only hot when the heating is on, otherwise they're cold.

That is quite normal. ???

 

[MilaGB]

If you post a a photo of it we can probably describe how to test, and clean or replace, but the fault may be elsewhere. Is it one of these?

You can copy and paste photos into your reply from your phone or a camera conected to the PC, or a tablet will have its own camera.

Sorry, wasn't online, have covid and wasn't feeling great for a few days.
Anyway, here's the photo, I can also take the photo inside if needed?
Will try to get a multimeter today.

 

[MilaGB]

Here's the photo

 

[MilaGB]

Do the lights still work on your PRT2 thermostat? They have the legends 'BELOW' and 'REACHED'. If they work and there's an audible click when turning it up and down then, despite the plastering, it is probably OK. 'BELOW' should be illuminated when the dial is turned high.

Let's look at the symptoms:

Which dial, the boiler or the room thermostat? For the purpose of diagnosis set them both high.


Do you have a frost protection thermostat? It looks like a standard dial thermostat, but usually with a cover over the dial.

That is quite normal. ???

The below light gets illuminated/green when the heating comes on, it also clicks then.
The problem is that it only comes on when it's let's say 12-15 degrees in the hall while the room thermostat dial is set on maximum 30.

Not sure about the frost protection...

Yeah, it's normal, I was answering someone's question about it.

Thank you for trying to help.

Will go and look for the dial where the boiler is.

 

[MilaGB]

Just checked the dial next to the boiler, it showed 75, I turned it up to 85. But I suspect it's the dial for hot water because the lights aren't on, and hot water heating us turned off at the moment.

 

[JohnD]

That's unusual, it sounds like the room stat is out of adjustment. It is mechanical so there might be some kind of adjustment inside.

I don't know that model, but the easiest fix would be to fit a new one. There are a number at around £20 or less. Your current one has indicator lights which are quite handy. Yours may have two, three or more wires inside and you can get one with the same. Examples (sort it to "price low to high," though Drayton and Honeywell are quality brands)
https://www.screwfix.com/c/heating-...s/cat831444#category=cat4600002&sort_by=price

There are also programmable stats and smart stats at higher cost. I favour the programmable. The more sophisticated ones are less intuitive and harder to set.

Wireless ones go wrong more often.