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How do you Bypass a Central Heating Room Thermostat

Postby MickWUK1 » Wed Oct 16, 2019 5:31 pm

I want to bypass a room thermostat so that it is effectively permanently "on". I think I just need to jump two wires, but which two? Or am I wrong anyway?

I have four wires labelled NO (normally open), COM (common), L (live) and N (neutral).

Jumping NO and COM seems to do nothing (that was my first attempt - my simplistic view was that this was the thermostat "switch"). Jumping L and NO turns on the CH but it still runs even when the controller is off.

Any advice gratefully received!
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Postby ericmark » Wed Oct 16, 2019 10:09 pm

Com to N/O should have worked.
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Postby MickWUK1 » Thu Oct 17, 2019 7:54 am

Thank you ericmark. I'll try again just in case I made a bad connection and report back.
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Postby MickWUK1 » Thu Oct 17, 2019 12:03 pm

Thank you ericmark. Just tried again - same result. I've confimred the two are connected with a multimeter.

Any suggestions please?
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Postby ericmark » Thu Oct 17, 2019 5:31 pm

N/C means normally closed and N/O means normally open however what is normal can change, so although N/O would be used in most thermostats to power the central heating, with the flowmasta thermostat I fitted there was a small switch so N/C could be used. The idea was N/O could be used to supply an air conditioning unit, so if the batteries become discharged it would not auto start, but it did mean you could use N/C to supply central heating.

In the main we have the programmer that supplies the thermostats, and the thermostats supply the motorised valves, and the motorised valves supply the boiler, however in some cases the tank thermostat supplies the boiler direct without going through the motorised valve.

With some of the Smart thermostats the thermostat supplies the programmer and programmer does switching.

In the main we use plans, there is the Y, S, W and C the latter came in at least three versions, a combi boiler will normally use a W plan inside the boiler, but for us the user it just has a thermostat there is not programmer.

There are three basic connections for the thermostat, one is 230 volt, the other is 24 volt and last is a varying voltage that does not switch boiler off/on but tells the boiler what output to deliver.

So step one is work out how your boiler is controlled, with Com and N/O marked it is unlikely to be connected to the eBus, but could be either 24 or 230 volt.

So what type of boiler? are there motorised valves? if so how many ports? is there a programmer? As said normally Com to N/O would work, and on odd time Com to N/C with some rare systems both N/O and N/C both used but rare.
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Postby MickWUK1 » Fri Oct 25, 2019 2:54 pm

Just to close this off. I replaced the controller and all was well. It seems that the wireless thermostat receiver had blown and perhaps damaged the controller in the process. COM and NO were the correct terminals for the thermostat. All working well now with new controller and (wired) thermostat. Thanks for the various replies.
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