DIY Doctor

Old Style Honeywell Mechanical Room Thermostat Not Working

Postby Pippa2000 » Sun Oct 18, 2020 8:17 am

My old style Honeywell mechanical room thermostat has stopped working... turns fully but no clicking to send signal to the combi boiler. It did work for one day when we kept turning it quickly but has now stopped completely. Can anyone advise whether it’s a simple job to replace this with a programmable thermostat or is it a job for an electrician!

Thanks in advance...
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Postby stoneyboy » Mon Oct 19, 2020 8:03 pm

Hi pippa2000
It is certainly a job for someone with electrical knowledge because depending on the thermostat you choose there may be changes needed to your wiring system.
I would recommend that you fit a wireless thermostat so you can choose it's location. Whether you go programmable is a personal choice but be careful which one you choose because some are very difficult to understand/programme.
Regards S
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Postby ericmark » Wed Oct 21, 2020 8:12 pm

Flomasta 22199SX Wired Digital Programmable Room Thermostat (6259G) at Screwfix £27.56 is what I used. Simple two wire.

I had problems with wireless, the cheap don't have fail safe, so flat batteries can cause heating not to switch off, mid range tend to have a mark/space anti hysteresis software which makes modulating boilers run efficient, so cheapest is likely Hive, but for Hive to work well also need Hive TRV heads which works out expensive.

So jumping from around £30 to £200 to use wireless.

I use Nest gen 3 which works well, but not cheap, and there are better.

Main thing is what boiler? With an old oil boiler which does not modulate a mark/space control is no problem, with modern boiler often better to use OpenTherm so it turns boiler up/down not on/off.

Remember the TRV controls room temperature, the wall thermostat often is only there so on warm days the boiler does not cycle.
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Postby stoneyboy » Wed Oct 21, 2020 9:33 pm

Hi pippa2000
Try searching for wireless thermostat - I’ve fitted Salus in the past with no problems. The programmable one is around £35 but if you already have a time clock on your heating go for the one that is not programmable.
Regards S
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Postby Pippa2000 » Thu Oct 22, 2020 1:12 pm

Wow... thanks so much guys! So much info to work with and you’ve all explained it in a user friendly way.

I do think I’m going to stay will wired and as someone said probably a job for a professional so I’ve put a call in to my boiler guy.

Thanks again... I’ll definitely use the forum again!
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