How Do I Test a Room Thermostat - Horstman DRT1

Catceefer · 25 May 2013

Our central heating boiler is not turning off, irrespective of the setting on the room thermostat. It was working perfectly, but now the boiler continues to run whether the display on the thermostat display says "heat" or not. The thermostat is only a few months old, so I would like to test it before replacing it again.

It has just two wires: if I use a multi-meter across these, am I correct in thinking that I will get 240v if the thermostat is calling for heat and 0v when it is not?

Thank you.

James.

D_Hailsham · 25 May 2013

Catceefer said:
It has just two wires: if I use a multi-meter across these, am I correct in thinking that I will get 240v if the thermostat is calling for heat and 0v when it is not?

Depending on which boiler you have it may be a lower voltage. In any case, it's the other way round.

A thermostat is just a temperature controlled switch. When the switch is open there will be a voltage difference between the two terminals; when closed there will be a 0 volt difference between the terminals as they are in connected to each other.

It might be safer to turn the heating off, disconnect one of the wires and measure the resistance between the terminals. It should be open circuit when the stat is low and approx 0 ohms when stat is high.

Catceefer · 25 May 2013

Thank you.

Regards,

James

Tipper · 25 May 2013

It's probably a faulty motorised valve and not the thermostat.

Fit a new motorhead.

twgas · 26 May 2013

Tipper said:
It's probably a faulty motorised valve and not the thermostat.

Fit a new motorhead.

Whilst I agree with first sentence, the second may be a little less correct, could just as easily need a valve body as head.

Tipper · 26 May 2013

twgas said:
Tipper said:

It's probably a faulty motorised valve and not the thermostat.

Fit a new motorhead.

Click to expand...

Whilst I agree with first sentence, the second may be a little less correct, could just as easily need a valve body as head.

but at least it's quick easy and relatively cheap compared with the plumbers' Change the whole valve' method which takes ages and costs an arm and a leg!

Catceefer · 26 May 2013

I am puzzled: the thermostat is "off", so surely the boiler should stop heating as well and turn off, which it does not (the hot water was turned off on the timer)? I do not understand why a faulty valve would affect this.

Regards,

James.

denso13 · 27 May 2013

A faulty valve can supply power to the boiler/pump even though the thermostat is off.

D_Hailsham · 27 May 2013

Catceefer said:
I am puzzled: the thermostat is "off", so surely the boiler should stop heating as well and turn off

The problem is that the thermostat just appears to be off, i.e turned to minimum, but it may not be.

You have to check the thermostat before rushing out and buying a valve which may not be required.

marev · 11 Jan 2021

D_Hailsham said:
Depending on which boiler you have it may be a lower voltage. In any case, it's the other way round.

A thermostat is just a temperature controlled switch. When the switch is open there will be a voltage difference between the two terminals; when closed there will be a 0 volt difference between the terminals as they are in connected to each other.

It might be safer to turn the heating off, disconnect one of the wires and measure the resistance between the terminals. It should be open circuit when the stat is low and approx 0 ohms when stat is high.

marev · 11 Jan 2021

D_Hailsham said:
A thermostat is just a temperature controlled switch. When the switch is open there will be a voltage difference between the two terminals; when closed there will be a 0 volt difference between the terminals as they are in connected to each other.

oldbuffer · 11 Jan 2021

Basic heating and hot water controls (S plan, mains voltage) work like this:
1. Programmer reaches an "On" period and the heating and / or hot water outputs go live with 240V.
2. If Heating is ON:
2a. 240V from programmer applied to room thermostat input. If heat is required, room thermostat output is set to 240V.
2b. 240V from room thermostat is applied to central heating motorised valve.
2c. Motorised valve opens. When fully open the valve operates an internal microswitch, and valve electrical output is set to 240V.
2d. 240V from motorised valve is applied to boiler, which fires, starts the pump (possibly internal) and sends hot water round the radiator / heating circuit.
3. If Hot Water is ON:
3a. 240V from programmer applied to cylinder thermostat input. If heat is required, cylinder thermostat output is set to 240V.
3b. 240V from cylinder thermostat is applied to hot water motorised valve.
3c. Motorised valve opens. When fully open the valve operates an internal microswitch, and valve electrical output is set to 240V.
3d. 240V from motorised valve is applied to boiler, which fires, starts the pump (possibly internal) and sends hot water through the hot water cylinder primary coil.
There are endless variations, but essentially this is what happens for system or open vented boilers. Combi boilers are slightly different as valves and pump are internal, but the principles remain the
same.

So, if the valve is faulty, even though the thermostat is saying open, it may or may not open, and may or may not tell the boiler to fire.

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How Do I Test a Room Thermostat - Horstman DRT1

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