Central Heating Room Thermostat Mystery

[wbjimmy1]

Can somebody please advise why my Drayton digital room thermostat (Digistat) still switches on the boiler i.e. "calls for heat" despite my programmer's CH setting ( Drayton LP522) being set to "off". It seems to be either faulty wiring or a faulty programmer. In all other respects the system appears to work fine.

 

[stem]

The Digistat, simply provides a 'switch' that makes and breaks a set of contacts, and that's all it does. The room thermostat should get its power from the programmer, so when the programmer is 'off' there shouldn't be any power going to the thermostat for it to switch, so I feel that the most likely scenario, as you suggest is that the programmer is faulty.

However, there are other things that could cause the boiler to fire if the programmer is 'off'. All are easy to check if you have a good understanding of electrical circuits, and know how to use a multi-meter safely. Only by carrying out electrical testing of the system, would you be able to determine exactly where the problem is.

Here are some things that could cause a boiler to fire, when the programmer is off.

1. Incorrect wiring of the heating controls. However, if this is a fault that has not always been there, and the wiring hasn't been interfered with you can discount this one.
2. The programmer has failed, so that the heating doesn't switch off.
3. If you have a motorised valve (or valves) these are what actually switch the boiler on and off when the central heating is required. It may have failed
4. If you have a frost thermostat that has been turned up or has failed, then this can run the boiler.

If the boiler is actually coming on and off under control of the thermostat (ie and not randomly) then 3 and 4 can be disregarded.

A simple test to check if it is the programmer is to remove the wire from terminal 4 of the LP522, insulate it for safety, and then put the programmer back on its mounting plate. Try to operate the heating, if it doesn't work now, then that would indicate that the programmer isn't switching off when it should.

 

[wbjimmy1]

Thanks for your helpful reply. I will visibly double check again the wiring for errors but I think it might be the programmer which could have shorted when I incorrectly rewired the 3 way port valve head recently. The circuit breaker on the main fuse box tripped as I had mistakenly connected a "redundant" neutral wire (which I think comes from the room digistat). At least my wiring error disclosed the presence of a 13amp fuse (instead of a 3amp one) in the connected fused socket! Is there any way to test the programmer to see if its shorted - all the lights and programme sequencing appear OK?

 

[stem]

but I think it might be the programmer which could have shorted when I incorrectly rewired the 3 way port valve head recently.

Then I think you have found your problem. It's very likely that you have welded the programmers contacts together making them permanently 'on'. It often happens when you create a short circuit and put several times their design current through them. You might have got away with it had you had the correct fuse fitted.

The test of disconnecting the wire from terminal 4 will confirm.

 

[ianmcd]

Cant remember Stem as been ages since I replaced one but does the neon on the LP522 not stay on if the contacts are fused ? agree completely that this is the most likely cause with the wiring fault and the wrong sized fuse

 

[wbjimmy1]

Thanks. I will disconnect wire at terminal 4 and report back.

 

[stem]

Cant remember Stem as been ages since I replaced one but does the neon on the LP522 not stay on if the contacts are fused?

Sorry, I can't say for sure either, but I wouldn't have thought so.

Early programmers that had neon's that were connected to the switching contacts, caused a problem with 'Y Plan' installations, resulting in the Hot Water 'on' light being powered from the 'Hot water off' signal from the programmer, fed back via the cylinder thermostat, even when the hot water wasn't set to be 'on'. Only when the water cooled and the cylinder thermostat switched over to 'call for heat' would the 'on' light go out. So for this reason, the switching contacts were kept isolated from the 'on' indicator.

Also, the LP522 utilises LED's which operate at a few volts, so I wouldn't have expected them to be connected to the mains voltage.

If the OP disconnects the wire from terminal 4, it will confirm if the LP522 is stuck permanently 'on' or not......

Thanks. I will disconnect wire at terminal 4 and report back.

......so we await your report with interest.

 

[wbjimmy1]

I disconnected terminal 4 and the heating didn't come on at all - irrespective of the programmer settings.

 

[stem]

Then your programmer is permanently providing a live supply to the thermostat as we suspected, so should be replaced.

 

[wbjimmy1]

Thank you very much stem for your help in solving this "mystery". I will replace the programmer as you suggest - and remember to check the amparage of my fuses in future.

 

[stem]

Indeed, and don't forget to put the wire back in terminal 4

 

[wbjimmy1]

Fitted a replacement programmer and everything works as it should. Thanks again.

 

[ianmcd]

Thanks for letting us know it is good to hear

 

[dilalio]

Fitted a replacement programmer and everything works as it should. Thanks again.

Might be a good idea to use a sharpie and write "5amp" on the fused spur plate! To help prevent anyone making a similar mistake in the future. We do it on three pin plugs as well, when we have to use those!

 

[stem]

3 amp will be suitable for most boilers. Not many have a load approaching 1 Kw. Having said that, some oil fired boilers may specify 5 amp due to the higher start up current drawn by the oil injection pump. Your boiler manual will advise. In any event 13A is too high.