Wall thermostat seems to have started working backwards!

JanChris

I have a Honeywell mechanical wall Thermostat with an IdealH15 system boiler and hot water tank. It was working properly until recently.
In the past, if, for instance, it was set at 20C, when I turned the dial clockwise from low, the click  would turn the boiler on at say 18C and if I then whacked it up to max then brought it straight down again for testing, the turn-off click would be about 21C. Fine.

But NOW.....the click-on is at 19C and the off-click is 17.5C!!.   How can it turn off at a lower heat than it turns on??     And TBH it's not actually turning off anyway, it's set at 19 on the wall and the separate thermometer shows its over 20 and it doesn't tell the boiler to stop.   I hear the pump/fan keep stopping then immediately starting again and again and again.

Is it time for a new electronic thermostat?

QrizB

JanChris said:

In the past, if, for instance, it was set at 20C, when I turned the dial clockwise from low, the click  would turn the boiler on at say 18C and if I then whacked it up to max then brought it straight down again for testing, the turn-off click would be about 21C. Fine.

That's back-to-front.

If your room is at 20C, the stat shouldn't turn the boiler on until the stat dial is pointing higher than that (eg. 21C), and it shouldn't turn off again until it's pointing lower (19C).

JanChris said:

But NOW.....the click-on is at 19C and the off-click is 17.5C!!.   How can it turn off at a lower heat than it turns on??

Yes, that's normal.

JanChris said:

And TBH it's not actually turning off anyway, it's set at 19 on the wall and the separate thermometer shows its over 20 and it doesn't tell the boiler to stop.

That could be normal too.

JanChris said:

Is it time for a new electronic thermostat?

I've ordered a new Zigbee thermostat today, but that's not because there's anything wrong with my old dumb one. If you fancy a new toy and have disposable income, I won't try to dissuade you.

Netexporter

I'd turn down the boiler flow temperature first, if the boiler is cycling like that.

The room thermostat will rarely exactly coincide with a room thermometer. It is just a guide.

Thermostats can wear out. My mother's used to arc at times, because the relays had presumably got out of adjustment. A similar thermostat won't cost much, so it may be worth replacing it.

JanChris

Thank you.  I've set it on ECO on the dial which is 69.  Which I assume is the return flow.  It then shows anywhere between 54 and 73 up and down, flame symbol going on and off. 

JanChris

"I've ordered a new Zigbee thermostat today, but that's not because there's anything wrong with my old dumb one. If you fancy a new toy and have disposable income, I won't try to dissuade you."

Sorry, I think my brain dissolved and I forgot what it was meant to do!   My husband of 37 years passed away suddenly in September so I'm doing everything on my own, and being autistic doesn't help!!    I feel a bit stupid now! 😁.       However, I have been contemplating a smart thermostat...but I'm in a council over 60s bungalow, so I may not be allowed!

Netexporter

Do you have thermostatic radiator valves? Have you bled the radiators to make sure they are all working properly ie. hot all over? If you do have TRVs, there should be one radiator without, which must be kept flowing at all times.

JanChris

Yes the system had a power flush in the summer...all new TRVs and the hall radiator no TRV where the wall thermostat is.

Netexporter

If 69 is the flow temperature, that sounds a bit high for best economy. You want it as low as is compatible with comfort. It is more economical to run the radiators at a lower temperature for a longer time.

FreeBear

JanChris said: However, I have been contemplating a smart thermostat...but I'm in a council over 60s bungalow, so I may not be allowed!

Get in contact with the housing department, and tell them you think the thermostat is faulty. If the agree to replace, talk them in to fitting a programmable one (or smart if you really want to go that route). Research has been done that suggests a programmable thermostat will save up to 15% on energy (in reality, the saving is more likely to be 2-5%). Use that as a stick to persuade them to fit a smart thermostat.