Central Heating on despite room thermostat being satisfied

8 May 2013
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United Kingdom
The central heating comes on when the timer switches it despite the room temperature. Today a thermometer showed the air temperature around the room thermostat to being 20C. The dial on the room thermostat is turned right down to around 10C and still the heating comes on.

The boiler A Gloworm Ultracom hxi is just on a year old, but the thermostat (Randall) could be anything up to 20 years old.

It has an accelerator Resistance in it connected between a red cable and blue.

If it is felt that this is faulty, Could I replace the same with A Danfoss Randall RT51 digital?
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You need to check if it is the thermostat before running off and buying one.

Turn the power off.
Disconnect the red wire
Turn power on

If the problem goes away - buy a new stat
If the problem remains - you need to investigate further.

How many motorized valves do you have and which type (see pics)?

View media item 5946 View media item 11762
There is only one such valve in the system and it was playing up, as we could only get the CH working if the hot water was on demand as well. However, this was cured by fitting new top ( electrics)

Valve is same as LH picture
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Have you checked the stat, as in the first part of my previous reply?
It looks as if the thermostat is faulty. However your diagram is suspicious.

Are you sure the Anticipator connects to T1? I would have expected T3.
Are you assuming that the Red is the live and Yellow the switched live?

If you have a multimeter you can check the wiring as follows:

Turn heating on at programmer
Set meter to AC volts
Measure voltage between Yellow and Blue
it should read 240Vac whether stat is turned up or down.
Measure voltage between Red and Blue
It should read 0Vac with stat turned down and 240Vac with stat turned up.

If you get the opposite (voltage constant on red, varies on yellow) swap the red and yellow leads.

If necessary, post a pic of the thermostat showing the wiring.

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