hot water OK, central heating not -- intermittent fault

[Thomas90210]

i wonder if anyone could help out with an extremely annoying intermittent fault that has been on our central heating system for a few months.

basically, every so often, we go through a period of hot water working fine but central heating not. we've got an S-plan system with two 2-port valves. when our CH/hot water comes on, both valves open and the boiler comes on -- therefore the radiators start to heat up. but then, once the hot water has reached temperature, the boiler switches off even if the thermostat isn't at temperature, but the pump seems to keep going. (boiler is a baxi solo HE -- when the fault is present, not even the power light on the boiler is on).

i'm guessing there is some issue with feedback: that the 2-port valve for the CH is feeding back to the boiler that it is closed even when it is open. any ideas what part of the system could be the problem? as it's intermittent, we haven't been able to show the fault a plumber when the fault is actually happening. someone came out a couple of days ago and changed the motor in the CH 2-port valve, but fault was back this morning.

thanks in advance!

 

[MANDATE]

I would think you're on the right lines with the valve, but I'd be more inclined to suspect the micro switch. either not being triggered fully or the contacts being burnt/dirty.

 

[Thomas90210]

Thanks!

Can the microswitch be cleaned/replaced or does the whole valve need replacing (it was only about 18 months old when the fault first started)?

 

[MANDATE]

Can't help on that. I dismantled a 3 port only to find the micro switch was 'soldered in type' and was fitted alongside another.
I did actually manage to split it open and sure enough the contacts were dirty/burnt, I also managed to reassemble after cleaning and it has been used since on a temporary basis.
I would think you may be able to just change the head without the valve body. suppose it depends what make they are.

 

[namsag]

Heating valve is not motoring far enough over to hit microswitch to bring boiler on complete head req or whole new valve

 

[D_Hailsham]

If you have a meter, check the voltages on the valve. Measure between each wire below and the blue wire. If the valve is working properly you will get the following:

When valve closed

brown wire = 0V
grey wire = 230V
orange wire = 0V

NB the grey and orange may be the other way round.

When valve open

brown wire = 230V
grey wire = 230V
orange wire - 230V