Wiring for a Honeywell dial thermostat

DP · 7 Mar 2024

DIYspanner said:
Fault identified!

The 3 core + earth cable run between the boiler and wall thermostat.
Using the blue as neutral, with all wires disconnected at the Honeywell, there's around 120vac between the unconnected wire/neutral and unconnected wire/connected wire at the other end when I connect to the mains supply for the isolator switch. So it seems to be an insulation problem on the 3 core.

There's no other visible device on any wall that could be connected be between the Vaillant and the Honeywell so rather than run a fresh cable, I believe a wifi room thermostat is the solution.

By the way, I removed the two wires from 3 and 4 on the Vaillant. 3 is 240vac and 4 is 0vac so all good there.

Are you using digital multimeter?
These meters have very high input impedence. What you will most likely be seeing is induced voltage from other cables. Use a low Z range or an analogue meter. Or wire a rest circuit with 240 volt bulb as an indicator. Bulb will not light even dimly if what I say above is correct.

This nuisance voltage is known to lock three port motorised valve in place at times

DIYspanner · 8 Mar 2024

CBW said:
Not a bad thermostat, fitted quite a few. Expensive and over engineered imo though.

What is your preference for this type of thermostat product when using it with a Vaillant combi may I ask?

DIYspanner · 8 Mar 2024

DP said:
Are you using digital multimeter?

This one.

If I were to connect only the neutral and test to unconnected (at both ends) live or switch live and still get a voltage, I'd have to agree with you about induced voltage from another cable.

Mister Banks · 8 Mar 2024

DIYspanner said:
What is your preference for this type of thermostat product when using it with a Vaillant combi may I ask?

Vaillant boilers need Vaillant controls for maximum efficiency.

DP · 8 Mar 2024

Mister Banks said:
Vaillant boilers need Vaillant controls for maximum efficiency.

I am using Evohome on my Vaillant 831. Do not see a Vaillant control that can replicate saving I am getting just now

CBW · 8 Mar 2024

DIYspanner said:
What is your preference for this type of thermostat product when using it with a Vaillant combi may I ask?

I don’t have one tbh. Just I fit a few different thermostats where I work, so I get to see certain issues/callbacks. As mentioned ebus controls are good for modulation.

DIYspanner · 9 Mar 2024

I ran fresh cables for the boiler's mains and thermostat, the thermostat temporarily on the kitchen top next to the kitchen cabinet the boiler live in.

Last night the boiler seemed to run better, the gas burning light was on less in my opinion. I sat a greenhouse thermometer next to the Honeywell which is set at 20° and the thermometer read the same.

May just fit a Honeywell T3R. A fella on YT gives a decent appraisal of it and the T4R.

From what I gather, the base unit for the T3R needs to be at least 30cm from any metal enclosures.

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Wiring for a Honeywell dial thermostat

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