Boiler fires, goes off after a while, then doesn't re-fire

[Ronald0]

Boiler stat still set to Max, yes?

To find out if the fault is with the boiler or the external controls you need to know whether demand from the room stat is providing a switched live to the boiler via the MV by connecting a voltmeter to boiler SWL terminal.

"Boiler on" light; is that a demand light from room and/or cylinder stats or a light to tell you that the main burner is firing?

 

[T0YB0Y]

Hi, you have what us engineers know as a S - Plan system with 2 x 2 port motorized valves. Sounds like the valve feeding the rads is knackered. You mentioned this is a honeywell valve - on the top silver head is there a small raised lump opposite the end where the cable goes in, if so the head can be changed without draining the system -Good Luck [/img]

 

[T0YB0Y]

As Ronald0 says in his post make sure you have power from the room stat before jumping in and changing the valve

 

[tobers]

Hi all. The latest on this one. Still haven't got a sparky in yet but in the meantime...

With the thermostat on the water tank set to max, and the stat unit removed from the side of the tank so it is hanging free, and with the room thermostat on max, the same problem remains i.e. the boiler fires for a while when the heating controller switches on in the morning or evening, then goes off before the room temp is met.

I'm thinking that as I have removed the water tank stat from the side of the tank, that it should never be satisfied and also should be demanding heat.

So both thermostats should be demanding heat, yet the boiler wont fire for more than 30 mins or so.

Curiously, at some random time during the evening, the boiler may again fire without me having done anything, but will never be on for enough time to heat the house up to the required temp. There's no pattern to this in terms of time or temperatire. I've checked that the room thermostat is quite well calibrated and, when demanding 21 degrees we only get up to about 17 or maybe 18. Same situation when the room stat is set to max - the boiler still goes off prematurely.

 

[tobers]

Further to this:

With both stats demanding heat, the "boiler on" light on the boiler, but the boiler not firing:

1) there is 240v at the switched live on the boiler
2) there is 240v at the permanent live on the boiler
3) there is 240v at the pump live on the boiler
4) there is 240v at the pump itself in the airing cupboard

The Honeywell 2-way valve to the hot water tank has a little slider lever on the top. This has no resistance to it if I try and move it. Same for the other 2-way valve.

The pump (Grundfos Selectric) has a 3-speed switch set to 3. If I change it to 2 or 1 and back to 3 you can hear a change in the noise the pump makes and feel the vibrations change slightly.

 

[tobers]

Diagnosed the problem as a faulty air pressure valve. The plumber replaced it. All working fine now.