Radiators intermittently not firing up

[Andrew19]

Perhaps some kind experts can help with a diagnosis

I have gas central heating with a conventional open vent boiler.

Heating and hot water are controlled via an old UP1 British Gas timer unit.
The heating side is left on continuous, and the roomstat is replaced by a Honeywell 4 period thermostat set at 19C during the day at 16C at night.

Every three or four days, the central heating stops circulating. Then a day or so later
it spontaneously restarts. Assume that both controllers are set correctly, and both are telling the system to "fire" at the ambient temperate (which falls to 10 degrees..... until I get out the old electric blowing heaters). The hot water side is fine. No amount of fiddling with the two controllers will get it to restart (heating off/on etc).

It was hard for the engineer to figure it out the first time because it was working at the time.

I am fairly ignorant, but I'm I am guessing it could be any of
a) A sticking pump
b) A sticking valve
c) The relay (or something else) in the Honeywell
d) the relay (or something else) in the old UP1 which is on continuous.

What I have now figured out is that when it is not working, if I switch off the MAIN power switch on the wall near the UP1 (I am not sure what that serves, but it at least it serves the UP1) wait a minute and then switch it back on then heating circulation is restored.... Does that help locate the cause? If it is just one of the controllers that is faulty I can probably replace that myself.

 

[stem]

All of the above are possible. Unfortunately the only way to find out for certain is by troubleshooting when the fault is present. If you have a fully pumped system and the hot water is still heating up but the radiators aren't then the pump is working. [As you mention a 'sticking valve' I assume it is a fully pumped system with motorised valve control, and doesn't have gravity circulation for the hot water]

So the next suspect would be a 3-Port valve / 2-Port valve depending on what's fitted. These can be tested with a multimeter by someone competent to see if the correct control signals are present. (and a neutral of course) If they are all correct but the valve isn't moving to the heating position, or it is moving, but it's internal microswitch isn't operating the boiler, then the valve is faulty.

If the appropriate live signals are not present at the motorised valve, the then the thermostat and programmer would be checked to make sure they are providing them.

You don't give details of the model of Honeywell roomstat, but if it's battery operated have you checked the batteries?

 

[Andrew19]

Many thanks for taking time to respond.
Yes fully pumped. And yes fresh battery.

Today the trick of flashing the power did not work. But 8 hours later it pumped again for an hour, then stopped....

One of these occasions where I wish it would break completely so can figure out what is happening when the engineer visits again.