Central Heating Programmer on Way Out?

[Skyhigh]

Hi guys,

The heating in the house we recently bought sometimes acts a bit strangely - I'm assuming it's the 2-channel Horstmann H21 programmer, but I'm not entirely sure.

Obviously, I'll get someone in to take a good look but if I can just replace the programmer or thermostat to solve the issue that would be handy.

So..

Sometimes when the heating is due to come on, nothing happens. This only happens once or twice a week - and I think it's usually when we use the "boost" button, not the times schedule.
The pump doesn't circulate and the boiler doesn't fire up, but the "central heating" light on the programmer is on. The thermostat can also be set quite high at the same time, but no heating.

The only way to get the heating on is to use the isolation switch to stop mains power to the programmer for a couple of seconds, then put it back on. The pump and boiler then fire up.
(The programmer turns off when power is stopped, but keeps the timer settings when it's back on)

The programmer itself seems to function OK, screen works fine, settings and buttons all work...

'Twiddling' the thermostat to make sure it's not set too low (hence stopping the heating) doesn't help either.

Any idea's?


A couple of times the heating has also turned on "outside" of the programmed schedule and refused to turn *OFF* until power to the programmer is briefly stopped.
I'm aware that this issue could be unrelated as it could be a pump issue or possibly a zone valve sticking as we had this issue in another house. Turned out to be the pump....and then the zone valve..





Cheers!

Drew

 

[jackthom]

The programmer does seem a likely culprit although this is one of those faults where you need to be there at the time armed with a multimeter.

Do you have a 3-port valve in the system or a pair of 2-port valves?

 

[Tipper]

A sticking or sluggish motorised valve could also give these symptoms.

 

[Skyhigh]

There's only one valve - it's a 3 port valve that switches between the hot water and central heating. It as 3 positions I believe.

You could be right that it could be a valve sticking, but this doesn't explain the heating coming on in the early hours when it's not scheduled (?)
(unless the motorised valve moves outside of the programmed schedule for some reason?).

Worst case scenario, it's "a bit of both" - programmer and valve maybe.

 

[jackthom]

If the boiler is firing up when the programmer says both CH and DHW should be OFF it's either a wiring error or a dodgy programmer, because the 3-port valve can only switch on the boiler if the programmer/room thermostat are providing it with CH_ON.

Assuming the system was working correctly and no wiring changes have been made, my money is on the programmer.

If you want to be sure, Screwfix do a very reasonable multimeter for a tenner.

 

[Tipper]

But most common motorised valves have a permanent live feed which via its auxiliary contacts provides the live supply to the pump and boiler...depending on the system wiring of course.

Many boiler firing or not firing or sluggish response problems can be traced to either a faulty valve motorhead or faulty aux contacts in the motorhead.

My money is on the valve motohead!

 

[jackthom]

But most common motorised valves have a permanent live feed which via its auxiliary contacts provides the live supply to the pump and boiler...depending on the system wiring of course.

Many boiler firing or not firing or sluggish response problems can be traced to either a faulty valve motorhead or faulty aux contacts in the motorhead.

My money is on the valve motohead!

That's true of 2-port valves but the switching contacts in a 3-port valve pass on the CH_ON voltage (from the white wire) to the boiler.

http://www.diyfaq.org.uk/plumbing/controls/midpositionvalve.htm

 

[Skyhigh]

Thanks for the input guys!

No wiring changes have been made at all - as far as I know it's the original system from when the house was built (approx 11 years).

I'll change the thermostat and see what happens.

I'm assuming that the only way to use a multimeter to check for the fault would be *when* the fault itself is occurring, but as it's not massively frequent - it's probably more effective to change it out.
(plus we want to be in this house a good 15-20 years after the hassle we had selling our old one & buying this, nevermind all the bodge-jobs with this house, enough said!)






Cheers,

 

[D_Hailsham]

Check out Mid-position valve operation and fault finding