Radiators too hot, not responding to room thermostat position

Skylark NW · 2 Apr 2020

The conventional central heating system was installed in 1987 when the house was built and has been largely untouched since then (pump and Honeywell valve changed over the years)

Over the last couple of days we seem unable to reduce the temperature of the radiators (there are no TRVs fitted). The eye level thermostat, made by acl, is located in the hall. I can hear it "click" when the knob is rotated to both reduce and increase temperature but it's not now affecting radiator temperature.

Could anyone kindly help with some form of fault diagnosis process to follow?

I'm happy to call-in an engineer if needed but, mindful of Coronvirus social distancing, I'd be equally happy to take a look myself if it's a basic/easy fix fault.

Many thanks in advance.

stem · 2 Apr 2020

Skylark NW said:
I can hear it "click" when the knob is rotated to both reduce and increase temperature but it's not now affecting radiator temperature.

The room thermostat will never have controlled the radiator temperature.

The boiler thermostat is what controls the radiator temperature

The ACL thermostat controls the room temperature. It's just a simple on/off switch that turns the radiators 'off' when the room reaches the set temperature and back 'on' when the temperature in the room falls.

oilhead · 2 Apr 2020

A few details needed, Boiler? Motorised Valves? Make and model numbers would help. First guess would be a stuck motorised valve.

Hugh Jaleak · 2 Apr 2020

+1, if heating valve is stuck open, then radiators will continue to heat until Hot Water is satisfied and boiler switches off. First check for me would be the motorised valve(s).

Skylark NW · 2 Apr 2020

Many thanks for the quick replies, much appreciated.

To stem, thanks for clarifying the working principles. I think that I didn't make a good job of describing the problem. The rooms are reaching and exceeding the thermostat set temperature, the on-off nature of the thermostat doesn't seem to be working.

The boiler is a Baxi WM 51/3RS
The motorsied valve is a Honeywell F5-9644

(Happy to take an post any pictures, if they may help)

If the motorised valve is stuck open, is there an easy test for this?

Alec_t · 2 Apr 2020

Possibly the roomstat contacts aren't breaking when they should. Switching off the electrics and doing a continuity/resistance check of the stat with a multimeter would verify this.

Hugh Jaleak · 2 Apr 2020

Does valve look like this?

Should be a small lever on the side, as shown in pic above, should spring return to HW position, blocking off CH port. See if their is any resistance on it, is there is, its returned to rest position, if it's floppy, it'll be stuck in mid position. If stuck, give it a gentle thump with your fist, you may here it whirr back on the spring to HW position.

Skylark NW · 3 Apr 2020

Many thanks to all for taking the time to respond. The lever was limp. I did a bit of YouTube research, then bought a replacement Honeywell Powerhead from Screwfix and I've changed it this morning.........now all working fine again.

Great resource, much appreciated.

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Radiators too hot, not responding to room thermostat position

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