No call for central heating - problem solving

[dishman]

System suddenly decided to stop calling for central heating.

Just wanted to go through my through process and get any other ideas where to check.

It's a very simple open vent non condensing set-up with an old dial thermostat and a basic central heating/hot water control switch.

1) Central heating suddenly did not come on when time to
2) Hot water calls fine, the gas valve opens fine and the boiler fires up for hot water.

The hot water is gravity circulated so no pump is triggered for that.

3) My initial suspicion lies with the hot water/central heating control. It was being a bit dicky recently. The buttons could be a bit temperamental, but if left alone it seemed fine.



4) Then I checked the thermostat and rotated it and realised there was no clicking at all, so then my attention turned to that.
This is the first time I noticed anything amiss with it, as it was so inaccurate, it was more or less defunct, and permanently called for heat. But it did click if you rotated it enough.

Inside the thermostat it seems to be permanently making contact with the lower connection, no matter how the dial is turned.



Edit: I have confirmed that while the thermostat is old, it is functioning fine, I tested it with a hairdryer and it moves to the top contact when warm enough, and drops when it has cooled back down.

So my suspicion is back centred on the control unit just not calling for the pump.

Or, if I end up excluding that, the pump?


Edit: I suppose I could bypass the control unit to call for heat and that would test that.

 

[Echo the husky]

You seem to be most of the way there with fault finding. Yes you can bypass the CH part of the control unit to test, this is quick (and possibly the safest option as it doesn't rely on prodding live parts with a test meter.) Just link whatever is in the CH on terminal to live. You haven't shown what other parts are wired to the system, some systems rely on the hot water to be up to temperature before the heating can be allowed to operate.

There are a couple of ways to wire gravity hot water systems depending what valve is fitted for the cylinder, motorised or thermo mechanical or nothing. If you have a meter you can test if mains is appearing at the heating terminal of the timer when CH is on if this is accessible . You might be able to test it at the room stat if you test between live in and neutral (if present) or live in and earth. The terminals are under the plastic cover shown in your photo. Turn off the mains before opening the cover if you test at the room stat as there is a lot of dangerous exposed live parts in these.

With the heating timer on and the room thermostat calling there should be 240v measured across terminals 2 and 1 as well as 2 and 3. If that is good test for power at the pump.

 

[dishman]

The system has no cylinder valve, so the hot water is always on if the central heating is on.

So the stat pretty much goes to the pump via the timer/programmer, I think.



So based on the diagram on the back of the timer, I should just bridge L and number 4?

Could I also do some kind a continuity test on the programmer? Should I not get some kind of continuity from the L prong and CH prong if it is switch to 24hr or something. Although I would expect the unit needs to be energised to get any kind of continuity.

 

[CountryFan]

Maybe do some voltage testing? Heating on to room stat should be 240vac, room stat to motorised valve and or pump should be 240vac

 

[ericmark]

The basic C plan is very simple, I also have C Plan, and I have modified mine to increase control, but it is rather simple.