Boiler cuts out but only on coldest days

[AndyBr]

There are hundreds of posts about boilers cutting out when they should be on but my problem seems fairly unique. I have a conventional boiler which works perfectly most days - until the weather turns abnormally cold. Then the boiler will simply go off and not come back on even though timer and room thermostat says it should (I have red on light on the timer and a flame symbol on the thermostat.

Some suggestions to the general problem are that the boiler is overheating and the safety cut out has been activated but I can feel the reset button and it has not popped out. But it sort of makes sense that it is something to do with when the boiler has more work to do without a rest (i.e. on coldest days).

It was very cold outside yesterday (-5c) and the boiler was ok for the morning (fired up for 2hrs) and then at lunchtime (about same period). But it came on for its evening period and then cut out after 50 mins. About an hour later it came back on but then lasted another 50 mins. Then it remained off for at least 2 hours before coming back on. Room stat never got to its cut out temperature.

If it was a faulty pump, timer, thermostat, motorised valve, boiler PCB, low water pressure, blocked vent, sludge, etc all of which are suggested as possible causes to the general boiler cut out problem - it would not explain why it has worked perfectly today when outside temp is +5c but could not keep running yesterday when it was 10 degrees colder outside. Also most cut out problems reported here seem to occur much quicker and/or report the boiler constantly cycling off/on every few minutes.

Any bright ideas?

 

[Agile]

Andy, if you think that the boiler may have a fault then don't you think that you should at least tell us the make and model?

Tony

 

[DaveHerns]

Condensing boiler and the drain pipe is freezing?

 

[AndyBr]

The boiler is a Potterton PrimaF. Sorry I don't have the product manual to hand to see if there is a more detailed reference but that is what it says on the front. The only user accessible controls are a numbered dial on the lower panel (presumably another thermostat but not sure what purpose it serves if I have a room thermostat and HW thermostat on the tank) and a hole on the underside where a reset button can be reached. It is an old boiler (20 yrs) but my plumber who serviced the boiler a couple of months back seems to like it and quickly dismissed the idea of whether it was time to replace the boiler.

It is back to being a very cold day today and so will see what happens later. CH happily fired up for 6am and ran for 2.5hrs on timer (did not reach 20C on room thermostat and so ran continuous in that period). Now off (on timer).

I have observed the loss of CH when this cut out happens (easy to check) but I have not yet established if I have a HW problem too and so whether running a hot bath would fire up the boiler for HW purposes (and whether the rads then heat up too). I suppose I have been reluctant to deplete my HW in case it does not fire up either but I guess it would help the diagnosis.

 

[bernardgreen]

Condensing boiler and the drain pipe is freezing?

Good call.

 

[AndyBr]

No it is not a condensing boiler - which is what I guess most people have these days as I gather these were mandated in 2005. My Potterton predates this. So a lot of other threads seem to be for condensing boilers and offers up the condensate freeze as the problem

 

[Agile]

Your boiler is very simple indeed. But the most likely faults will be those we cannot give advice on here.

Any competent boiler engineer should be able to quickly find the fault when it is in the fault mode.

Apart from possibly checking that the boiler does indeed have a call for heat in the fault mode, there is little that you can do.

It might be something very unusual. For example a wrongly wired frost stat somewhere!

Tony

 

[Agile]

It is back to being a very cold day today and so will see what happens later. CH happily fired up for 6am and ran for 2.5hrs on timer (did not reach 20C on room thermostat and so ran continuous in that period).

Your rads may be undersized or the flow temperature may be too low. The design flow temperature in your case is 80 C.

BUT the design is only to provide the 21 C when it is -1 C outside. So it could be just that it is colder outside than the design temperature.

It would be interesting to know what the flow temp is from a contact thermometer ( Not an IR one! ).

If you look at the FAQ on this site you could measure the power input of the boiler and compare that with the specs.

What is the thickness of your loft insulation?

Tony

 

[D_Hailsham]

The only user accessible controls are a numbered dial on the lower panel (presumably another thermostat but not sure what purpose it serves if I have a room thermostat and HW thermostat on the tank)

The numbered dial is a thermostat which controls the temperature of the water circulating through the rads. In the winter it should be set to a high number, but in the summer a lower number is OK as it is only heating the HW cylinder. What number is it set to at the moment?

 

[Agile]

Your rads may be undersized or the flow temperature may be too low. The design flow temperature in your case is 80 C.

BUT the design is only to provide the 21 C when it is -1 C outside. So it could be just that it is colder outside than the design temperature.

It would be interesting to know what the flow temp is from a contact thermometer ( Not an IR one! ).

If you look at the FAQ on this site you could measure the power input of the boiler and compare that with the specs.

What is the thickness of your loft insulation?

Tony

 

[ianmcd]

Do you have another gas appliance cooker or hob , if so have you tried to see if these work when the boiler locks out , if your gas meter is outside it could be the meter governor freezing up

 

[Agile]

The meter governor sticking which Ian is referring to, is most prominent when first lighting boiler after a while off, particularly over night. But that is more likely on a governor located outdoors and not on one indoors which does not get so cold.

But that will not usually happen if you have a demand for gas such as a hob burner on. If you get up early before the boiler starts and run the hob then it is unlikely to happen if it is that at fault. Most likely with an old type with a horizontal cylinder shape.

An indication of the performance of the governor can be given by running a small hob burner and see if the flame dips when the boiler lights.

But what you are describing seems to me to be much more likely to be associated with the boiler's internal temperature control.

Tony

 

[ianmcd]

very common for meter governors to feezi in Scotland Tony OP doesnt say where he lives but it is a lot colder up here than where you are we also get ots of water in service pipes which when it gets into the regulater will freeze as soon as it gets cold

 

[Agile]

Yes Ian, I appreciate that its much colder in the north.

However, that mostly affects outdoor meters. The OP is not being very communicative.

But the problem from his description is that the boiler fires for an hour or two and then stays off for an hour or two.

That would most often be associated with the temperature control within the boiler. But the odd aspect is that it seems to be associated with a low outside temperature!

But for all we know the boiler could be in an outhouse!

Tony

 

[ollski]

Is your pump running?, if not push the lever over on the mid position valve with heating demand on.
Does the fan sometimes repeatedly come on / off before it fires?