Yet another boiler woe

[Roger Chapman]

I was woken up from a deep sleep in the early hours of the morning by the boiler boiling.

By the time I got into the kitchen everything was quiet and none of the thermostats were calling for heat so I pressed the reset button an was more than a little surprised when the boiler immediately fired up and in due course boiled again. The bathroom radiator and the bypass circuit is just not big enough to dissipate the heat when the boiler carries on firing after with only that circuit connected (or so I am assuming for the moment).

The heating was on all evening without the set temperature being reached and turned off normally when the downstairs thermostat switched to night mode. The trouble occurred after the night time set temperature was reached (which I have only just raised to a point where it is actually effective).

Now I am not at my best trying to think things through in the middle of the night but all I could think of was that the micro switch on the downstairs two port valve must have stuck in the closed position when the valve closed thus continuing to fire the boiler after the thermostat stopped calling for heat.

So is there anything wrong with my conclusion and does anyone have any other suggestions as to what might have happened?

The boiler in question is a fully pumped Potterton Prima Profile now almost 20 years old. However various bits have been replaced over the years including actuator heads and control board so some of it isn't anywhere near that old.

[Roger Chapman]

Additional Information

This is an intermittent problem.

In order to get back to sleep secure in the knowledge that the boiler wouldn't keep setting off the overheat stat I switched to constant temperature. Looking at the room temperature after breakfast I thought it was a bit close to set temp so pushed set temp up a degree overlooking the fact that this re-engaged the variable sequence and the thermostat duly switched to day time set temp at 9.30. However all is so far quiet on the boiling front so I am in two minds whether to let the temperature drop to the next set temp and see what happens or put the potential problem off till I am certain to be around if it does start to boil again.

[The Other Mike]

I don't know the model but surely the boiler thermostat is a prime suspect.

Maybe not the thermostat then.

[Ronald Raygun]

Sounds reasonable.

If you look at the S-plan wiring diagram in

will see what conventional practice is for wiring up this switch. One end obviously goes to the boiler and pump (terminal 10), the other (i.e. where it's fed from) comes from constant live mains (terminal 1).

As an experiment and temporary fix you could wire the switch to the motor input instead of to constant mains (move the switch feed from terminal 1 to terminal 5). If that fixes the symptoms, it would confirm your diagnosis of a stuck switch.

[Ronald Raygun]

It should be easy enough to trace where the switch wires go without needing to consult your temporarily mislaid diagram.

I recommend storing it next to the wiring boxes instead of in your, erm, general-purpose black hole of a filing system. :-)

In that case I would recommend sticking with the hand-drawing.

[Roger Chapman]

Thanks.

I have a rather complicated wiring set-up involving the original two channel controller which is now set to constant central heating and intermittent DHW, two programmable thermostats (upstairs and downstairs) and two wiring boxes with the common wires linked. Every time I mess with it I have to go back to scratch to work out what does what. Somewhere I have my wiring diagram but whenever I need it I can't find it. And somewhere at the bottom of a big pile is a round tuit for drawing out the diagram properly using Turbocad, a program I now use so infrequently that with that too I have to start all over again.:-(

[Roger Mills]

It sounds to me as if you might have several problems rather than just one!

If the boiler fires when there is no demand for heat from any stat, it does indeed suggest that the microswitch in one of the zone valves has stuck in the closed position.

However, that in itself, shouldn't make the boiler overheat - because the boiler should cut out on its own stat, and should be able to dump any residual heat through the by-pass circuit without tripping the over-heat device.

You mention that, during the daytime, the set room temperature is never achieved. Is this always the case, or only so during this cold snap? Is the boiler running continuously, or cycling on its own stat? If the boiler and rads are adequately sized for the job in hand, it may be that the pump is playing up, and not circulating the water adequately - and not able to dump the residual heat, to boot.

I would investigate all of these things - zone valve microswitch, boiler stat, and pump - in order to try to work out what's happening.

[dennis]

About the only things that should make the boiler get near to boiling are pump failure, auto bypass failing closed, boiler limit failing. If the heating is working then its probably not the pump. You can tell if the bypass is working by seeing if the bypass pipes are getting hot.

That leads to the boiler temp limit. On mine its a simple thermostat, yours may be a stat or an electronic system. It should stop the boiler boiling even if there is a stuck micro switch calling for heat as long as the pump is pumping water through the bypass, which it should as long as something is calling for heat.

[Roger Chapman]

The heating system is not up to the job when it is this cold although the shortfall has been less than one degree once the boiler has been running all day. During the overnight incident the pump was still running (I could feel the slight vibration).

The boiler stat has been working satisfactorily with the burner cycling off and on as the stat continues to call for heat. There is (and has been for years) some sort of off set problem as the boiler temperature control has to be set to less than 3 (out of 6) to avoid the occasional overheat problem but replacing that control (year or more back) had no effect on the problem.

Up to now I have put the occasional overheat incidents down to an inadequate bypass loop. Originally 15mm copper with the bath room rad conveniently plumbed off it. That rad no longer has a thermostatic valve head fitted which led me to conclude, perhaps erroneously, that as a heat sink the circuit was borderline even with the bathroom rad on all the time and having the boiler continuing to fire after the motorised valve closed pushed the temperature within the boiler over the edge.

The central heating circuitry is not ideal but I am not sure now whether that is a fault that existed before I replaced the previous boiler or one I introduced as I replumbed some of the pipework as I changed from a floor standing 4 pipe boiler to a wall hung 2 pipe. I have known about that for a long time and was going to do something about it last summer but the round tuit was still buried in the pile when winter arrived. I still haven't even bought the extra fittings required and being snowed in now unlikely to do so anytime soon even if I could stand the cold that would surely accompany having the boiler off for an appreciable period.

[Roger Chapman]

Bypass circuit includes the bathroom radiator and is always hot.

Simple thermostat with the bulb clipped to the flow pipe.

There is pump overrun as well if the boiler temperature is high enough but that wouldn't be an issue if the closed motorised valve is still providing power to the boiler.

[Roger Chapman]

A bit of an update just in case anyone else is still interested.

snip

My memory is at fault. I never did get round to drawing out a full wiring diagram. (That roundtuit is still buried). What I do have is a list of the wires at each accessory so I can, eventually, draw the diagram. And where was this list stored - on the computer of course.

Well I do have a full size drawing board and a 3 foot t-square somewhere.

Meanwhile the fault has not occurred again. I checked the valve head I thought was responsible (Danflos - warranty expired 4/10) only to find that that was working perfectly and is by far the newest of my 3 motorised valves. Unfortunately the upstairs valve is far less accessible and, as that is one of the original (Honeywell) valves from

19 years ago which may, or may not be the one that got a new motor long ago. The Honeywell is now the prime suspect and I am still looking for the discarded head from the other Honeywell valve which was still working when the valve body failed.

And I have another roundtuit. Fit neons across the various accessories so I can tell at a glance whether the pump is still working and the boiler is still firing after the thermostats have ceased calling for heat.