Is My Honeywell CM907 Thermostat Faulty?

[washcroft]

Hello,
I have just replaced an old bimetallic thermostat with this new digital thermostat however I think it may be faulty - the temperature readout is about 10 degrees lower than the room actually is, verified with a digital thermometer placed next to it.

Have I missed any settings or does it need calibrating somehow?

Cheers,
Warren

 

[gaswizzard]

This is normally an issue when the stat has just been pulled out the van.

 

[washcroft]

No, it's had a chance to settle as its been installed for over a week now

 

[steve32]

This is normally an issue when the stat has just been pulled out the van.

Cold

 

[washcroft]

Any ideas before I go out and buy another?

Cheers

 

[D_Hailsham]

Was it brand new in original box or second hand?

 

[washcroft]

Was it brand new in original box or second hand?

It was brand new, boxed.

 

[D_Hailsham]

It was brand new, boxed.

Take it back to the supplier and get it replaced under guarantee.

PS It's very unusual for these stats to be faulty out of the box. Are you sure that the digital thermometer is accurate? Borrow an old fashioned mercury/liquid thermometer and check it.

 

[washcroft]

It was brand new, boxed.

Take it back to the supplier and get it replaced under guarantee.

PS It's very unusual for these stats to be faulty out of the box. Are you sure that the digital thermometer is accurate? Borrow an old fashioned mercury/liquid thermometer and check it.

Yer I'm going to, it's definetly wrong its reporting 10 degrees in a warm house with the digital thermometer showing above 20.

 

[washcroft]

...took it off, had a fiddle with the PCB, put it back and now it works and is showing the correct temperature Anyway, I'm happy, thanks to all who replied

One other thing...

My system is open, gravity fed and has no motorised valves anywhere for separate control of hot water/central heating. When hot water is on, the boiler fires and the pump does not run and when central heating is on, the pump kicks in also.

Now that I have this new digital thermostat I don't want to use the old clock programmer next to the boiler so that is now set at permanently on to let the CM907 decide everything. Is this OK, can I rely on the boiler thermostat to keep the hot water under control when central heating isn't required by the thermostat? Or is there a better way of doing things?

 

[gaswizzard]

Gas consumption is going to be high leaving the programmer on 24/7 for hot water , maybe at least convert the system to a C plan (full temp control).

 

[washcroft]

Gas consumption is going to be high leaving the programmer on 24/7 for hot water , maybe at least convert the system to a C plan (full temp control).

I thought so, I'll have to look at a C plan conversion when I have a bit more free time.

In the meantime I'll tweak the wiring at the programmer so that it can control hot water on its clock timer and the thermostat can control central heating - that way assuming thermostat doesn't demand heat the boiler won't be fired through the night.

 

[mikely]

If you fit a tank thermostat, you can wire the controls as shown below which will prevent the boiler cycling when there is no heating demand and the tank is up to temperature.

Gravity Interlock

• mikely
• 7 Jun 2010
• 3

When using HW only, the tank thermostat it will control the tank temperature.

This obviously isn’t as good as C plan or a fully pumped system, but should be better than what you have now.

 

[washcroft]

If you fit a tank thermostat, you can wire the controls as shown below which will prevent the boiler cycling when there is no heating demand and the tank is up to temperature.

Gravity Interlock

• mikely
• 7 Jun 2010
• 3

When using HW only, the tank thermostat it will control the tank temperature.

This obviously isn’t as good as C plan or a fully pumped system, but should be better than what you have now.

Cheers for that, the hardest bit is working out how to get a cable from the cylinder stat to the programmer on the other side of the house in the garage.

Where would the motorised valve best be fitted, nearer the boiler or nearer the cylinder? If the latter I'll run a cable for that at the same time.

Cheers,
Warren