CH fault, the programmer won't turn it off

[cl]

My daughter has a fairly old fashioned but pretty standard gas CH system which is controlled by a Horstmann H37 controller.

A couple of months ago it stopped working completely, it was blowing the fuse in the wiring box. After a bit of diagnosis with a meter I discovered that the suppresor capacitor in the control box on the boiler had failed. I fixed that (with a proper X rated mains capacitor) and everything worked perfectly again - until now.

Now the controller doesn't turn the boiler off at all, the only way to stop the boiler is to remove power from it (there is a switch to do this).

Is this likely to be simply a programmer fault/failure or are there other possible explanations? The programmer is obsolute so if it is faulty will have to be replaced.

[Graham.]

Another explanation could be a mictoswitch in a motorised valve permanently closed. Does the controller operate make the motors open and close the valve(s)?

[Roger Mills]

You haven't told us what other controls there are. Are there any motorised valves? If there are two 2-port valves, it will be an S-Plan system. In such as system, the programmer and room/cylinder stats drive the valves and, once the valves are open, auxiliary switches within the valves turn on the boiler and pump.

If one of these switches fails closed or, in some cases, if one of the valves fails open, the boiler will run even if there is no demand from the programmer and stats. Could this be your daughter's problem?

[meow2222]

Get your multimeter out & find out. Does the programmer's 'CH on' wire stay live or not?

NT

[cl]

That's a thought, thanks, I didn't really have time to see if the valves were operating the last time I was there.

[cl]

Yes, there are motorised valves, three I think (the H37 has two heating programs and a hot water program).

It could very well be this, thanks, I will check the switches associated with the motorised valves.

[ARW]

And how do you do that?

Remove the programmer from the base plate and then there is no permanent live to test the switched live.

[Peter Parry]

The failed capacitor may have damaged relay contacts in the controller and they have now decided to weld themselves together. Even if the controller is obsolete relays are pretty standard and you may well be able to fix. If not speak to Geoff at CET

who may be able to help if you say you came from here :-)

[Chris French]

check the other end of the CH on wire to see if it live?

[John Rumm]

Pick the appropriate circus diagram from :

and poke about with your DVM in the wiring centre ;-)

[meow2222]

so probe the wiring centre end of the cable, or attach cable and put programmer back. Its not rocket surgery

NT

[Dave Liquorice]

Or measure between the C and NO heating terminals on the back of programmer.