Heating stuck on

[Alex DV]

I have a had a problem for a while now that my heating is constantly on, I cant turn it off. Setting the t-stat down to min, the programmer to "off" (for heating and h/w) makes no difference. It can stay on for days, i usually have to power off the pump to turn off the heating. What is strange is that in general the boiler does seem to respond well the the t-stat and programmer - in that when i turn the t-stat all the way up, the boiler ignites, turn down it goes out. Same for switching between "off" and "constant" on the programmer. The problem is it never stays off for long, it seems to turn itself on periodically (like every 5/6 mins), stays on for a minute or two, then off again.

Does anyone have any idea what is going on?

Dont have any experience in this area, so dont know what info you need, i'll stab at:

Boiler - hmm says "Apollo Fanfare" on it Pump - Celsia Valve - SwitchMaster (i *think* its a 3-way, well there are 3 pipes going into it) Programmer - SwitchMaster 900, mechanical (has both CH and HW switches)

All help very much appreciated!

Alex.

[Nigel Molesworth]

Do you have a frost stat?

Now I'm confused. You first said it doesn't go off, now you say it is cycling.

[Alex DV]

What's a frost stat?

Sorry for the confusion. The heating is always on - ie the radiators are always hot, but the boiler is on and off. So, to be clear. Right now the programmer is set to off, and the therm is set to min. The boiler is off. In a few mins the boiler will ignite by itself and stay on for a few mins. Since the programmer is already "off" and the therm already on "min" I cant "turn off" the boiler using those mechanisms (I can hit the switch on the boiler itself which will turn it off, but doesnt really get me anywhere). After a few mins the boiler will swtich itself off. When in this state, if I flick the programmer to constant, the boiler will come on. Or, if I set the therm to max, the boiler will come on. If I turn the therm down, or switch programmer off, the boiler will go off. However, after another 5 mins, whatever I do, the boiler will come on.

So, Once its on of its own accord, i cant switch it off (the programmer is already set to off, and therm to min).

If I've manually switched it on, then I can turn it off again.

Hope that makes things clearer.

[Nigel Molesworth]

A thermostat that overrides the whole system if there is a danger of freezing. If so, it does this ...

They are often mounted in a boiler room, and rely on being heated by the boiler. So if you have recently put in a new boiler or flue, it could be confused.

I've got a frost stat on my "landing" (it's actually a bungalow, but YSWIM) set to 15 deg. It turns on the heating on cold nights.

[Ed Sirett]

The main FAQ has stuff on heating controls. This could be a stuck 3 port valve (with just enough going to the HW to keep it warm enough).

If you are not up to using a circuit tester you'll need some help.

First thing is to see if the programmer gives the correct output.

[Alex DV]

The only other stat that I know of, apart from the room stat, is on the hot water tank (currently set to 50 degrees).

I hadnt changed/installed anything until the problem occurred. Then the one component i did think might be the problem was the programmer, only because it was sooo old and grimey, so I have replaced that with a new one. Although I followed the wiring diagrams there is a chance I have wired it up wrong. But, as mentioned, I have tested and that does appear to ignite/switch off the boiler in the expected way.

Thanks for the input thus far.

[Ed Sirett]

Well the next thing is to see if the tank and room stats give the right signals.

Then proceed to see if the valve head moves in the right manner. My guess is that the 3-port valve will be the most likely culprit.

Occasionally a boiler will fire when un-demanded the only one I'm aware that do this are Vaillant VCWs combis, doubtless there are other models which which could display this fault.

[Alex DV]

Turning both room and tank stats seem to ignite/turn off the boiler as expected.

As for the valve, I have tried manually turning it, and it does turn all the way round to the correct position (HW when HW is on Constant, CH when CH is on Constant, CH when HW and CH are both on Constant).

An extra bit of information, I think the pump is always on - not sure this is significant.

As ever, thanks for the responses so far, any further help very much appreciated!

[Ed Sirett]

The pump may be controlled by the boiler? I'm not familiar with the Apollo Fanfare (from Thorn?).

As you mention that this valve move 'all the way round' it might be something different form the 'standard' honeywell mid-position type.

Since moving the thermostats turned the boiler off maybe the problem is that the wall thermostat is electrically OK but has become insensitive to temperature?

[Alex DV]

insensitive to temperature?

Thats a very good point. How would I check that?

[Dave Fawthrop]

|>maybe the problem is that the wall thermostat is electrically OK but has become insensitive to temperature? | |Thats a very good point. How would I check that?

Twiddle the knob and see if it switches the heating on and off.

[Guy King]

The message from Dave Fawthrop contains these words:

That would test its sensitivity to twiddling - but not to temperature.

[Dave Fawthrop]

|The message |from Dave Fawthrop contains these words: | |> |>maybe the problem is that the wall thermostat is electrically OK but |> has become insensitive to temperature? |> | |> |Thats a very good point. How would I check that? | |> Twiddle the knob and see if it switches the heating on and off.

Perhaps I should have added "at about the temperature of the room"

|That would test its sensitivity to twiddling - but not to temperature.

That always works for me!

[Guy King]

The message from Dave Fawthrop contains these words:

Twiddling a cold knob can be futile.

[Ed Sirett]

become insensitive to temperature?

1) Is there a large variation between the on click and off click? 2) If you set the heating on and then turn the thermostat down till it nearly clicks off - what happens then?

[Alex DV]

2) If you set the heating on and then turn the thermostat down till it nearly clicks off - what happens then? Boiler stays on until stat reaches stat's temp.

Ok, so after some days of investigations, I have more information.

I tried disconnecting the room stat, the programmer and the tank stat (removing the L and N wire in each case). The result is that the pump remains ON and the boiler continues to cycle, 5 mins off, 5 mins on. My understanding was that the programmer feeds the power to the rest of the system, so how did the pump continue ? Could this be a short circuit? How did the boiler contine - is it on the same circuit as the pump?

[Ed Sirett]

OK. Perhaps we should really have started with the boiler.

Either the boiler has just line,neutral & earth connnections and the pump is in parallel with it. Or the boiler has Line, Switch Line, Neutral & earth in which case it the boiler will control the pump or the pump is in parallel with the switched live.

Trace back from the boiler to find out why it is getting a demand signal.

[The Natural Philosopher]

Sounds like you have a stuck motorized valve.

The way it goes in my system is

overall switched fused spur feeds timer.

Timer feeds overall 'stats.

Overall 'stats feed motorized valves..

...these have switches in them that feed the boiler/pump.

Boilers own internal stats switches it on and off (but leaves pump running anyway) to circulate hot water, but not TOO hot water :-).

One of mine stuck ON..the motorized bit seized up. Fortunately I was able to replace just the motor and gear and switch assembly. No plumbing required.

[Roger Mills]

Can you remind us how many motorised valves you have, and what type it/they are (2-port or 3-port)? If you have more than one 2-port valves, it is likely that you have an S-plan system.

The way in which this works is that the programmer and room/tank thermostats purely control the valves. The valves have auxilliary contacts which close when the valve opens - and it is these contacts which control the boiler and pump. So if a valve is stuck open, the boiler and pump will operate regardless of what the programmer and stats are doing.

[Alex DV]

There is only 1 3-way valve.

When I switch the programmer to HW on (CH off), the valve correctly "turns" to "Hot Water". When I switch the programmer to CH on (HW off) the programmer once again turns correctly to "Central Heating". When I switch the programmer completely off (CH off, HW off), the valve stays on whichever was the last position it was pointing to. Then the bolier cycling behaviour starts, which either heats the radiators (if valve points to CH), or heats the hot water tank (if valve points to HW). Whatever the position of the valve, and whatever the setting on the programmer (including fully off), the pump stays on.

Alex.