DIY Doctor

How do I Know if an old Honywell T40 wall Thermostat is Still Working?

Postby RickRB » Thu Aug 08, 2019 2:23 pm

I have an old Honywell T40 wall thermostat that I dont use, in fact the dial is broken. I believe it is the original so 42 years old!! We have had 2 new boiler replacements over the years and now have a Glowworm 30 and no longer have hot water cylinder or header tank. We also have individual radiator thermostats that work fine. How do I know if it is still connected to the central heating system and can I remove it when decorating next? TIA.
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Postby ericmark » Tue Aug 13, 2019 2:21 pm

As you have pointed out, the TRV will control each room, and theory is as they close the by-pass valve opens so return water gets hotter and boiler senses this and reduces the output until it reaches minimum.

At this point the boiler starts to cycle off/on to further reduce output, however if it was to switch off there is nothing to tell it when it is required again, so the normal cheap method is a thermostat that is set higher than the TRV in that room, normally a room kept cool, so on warm days the boiler does not cycle.

If not connected you really do need to get it connected, to stop boiler cycling. The best is an old thermostat with a high hysteresis i.e. neutral not connected, as if for example in a room controlled with the TRV at 18 deg C once it hits 19 deg C wall thermostat will switch off and stay off until dropped to 17 deg C so boiler is turned off most of the time in summer months.

The problem I found was the TRV was marked *123456 which really was not much good, don't care if deg F or C but want something I can relate to. So swapped 5 of the heads to electronic bluetooth heads at £15 each so actually setting in deg C and also different temperature night and day.

These will of course only work if the boiler is running, and in the main living area I also want to have varying temperatures night and day, and the problem with the cheap thermostat is it is set to just one temperature, so to be able to vary the TRV to exactly the same temperature as wall thermostat, in living area I am using more expensive TRV heads which link to my wall thermostat, so turning thermostat up/down auto turns TRV's up/down. Not idea but good enough for me.

So as to your unit, likely 24 volt but the boiler can be controlled with either, so it would require one to test live, and clearly are is some danger and you need a meter to test, as to if you want to DIY the testing or get some one in is up to you, however by the fact you have asked the question, I would suggest you get some one in.

Some boilers have the electrical connection box inside the sealed area, so may be better to get some one gas safe to start with so they can continue should it need testing at boiler.
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Postby RickRB » Wed Aug 14, 2019 7:18 pm

Thanks for reply. I have a better understanding of how the system works. Boiler service is at end of month and I will get the thermostat checked out too. Will get him to explain what is going on at the boiler end on my system.
No need to worry on safety aspects. Whilst I am happy to replace light fittings, sockets, cookers, etc. (basic wiring same for same) or fuse wire in the old fusebox fuses, my golden rule for DIY is "don't do electricity or gas as you die if you get it wrong". A rule that has so far worked for 45 years.
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