Boiler firing 30 seconds, off 4 minutes, house not heating..

[mooky]

Ive done a bunch of reading, poking at the heating and scratching my head (not my area of expertise at all - but Ive learned a bit reading here).

Fundamentally, the problem is that the heating is only half-working - radiators are only just warm - and the house is cold. However, the twist is that now and then, it works fine! (occasionally, the hot water isn't so hot either, so I don't think its just the heating)

The boiler is a Potterton Suprima (70, I think). I think its 2007 vintage...
From what Ive observed of the boiler, it comes on for only 30 seconds (i.e. flames for 30 seconds, plus the 10-second verification the fan is on). It then goes off for about 4 minutes. If I understand the diagnostic LED correctly, it thinks its reached the desired temp. (slow flashing green).

The boiler thermostat is set to max.
The heating thermostat is set to max.
The pump (grundfos) appears to be working ok...
The motorised valves appear to be working ok...
Its a sealed system. The pressure might have been a bit low - but its ok now.

When the boiler is not firing, the in/out water pipes are luke-warm. When the boiler fires, the outflow pipe gets hot - as you would expect.
The inflow pipe is luke-warm - and hasn't really changed in temp before the boiler stops firing. When the boiler stops firing, the outflow pipe quickly cools to luke-warm again.

When the boiler is working properly - i.e. heating working, and boiler staying on more than 30 seconds, the in/out pipes are hot.

I can't think of any other relevant info to add...

Any suggestions?
Is the pump faulty - i.e. not shifting the heated water quickly enough to prevent the boiler reaching temp? After what time/temp should the boiler re-fire after it shuts down.
Is the temp sensor on the boiler not working properly?

Any ideas greatly appreciated.
Cheers.

 

[Agile]

All the indications are that the pump is not working or there is a blockage.

Why do you think the pump is working?

I saw one today that was doing about 30 rpm.

Tony

 

[Dan Robinson]

Just reminded me of a puzzler.... cheers bud.

 

[mooky]

All the indications are that the pump is not working or there is a blockage.

Why do you think the pump is working?

I saw one today that was doing about 30 rpm.

Tony

Thanks Tony.
The things that would lead me to believe the pump is working are:
1. The pipes cool pretty quickly after the boiler turns off. (and the boiler stays off for 4 minutes despite the fact the pipes are luke-warm)
2. The heating works sometimes - and I'd have thought a pump problem would not be intermittent. If it was faulty, it wouldn't work at all?
3. The pump sounds like its working ok - no whine or disturbing noises...

But that's all. I've got nothing concrete that would suggest its working 100%.

 

[mooky]

All the indications are that the pump is not working or there is a blockage.

Why do you think the pump is working?

I saw one today that was doing about 30 rpm.

Tony

How can I check the pump is working/notworking?

 

[BlueMoonMole]

Continuity across the coil?
230v to L and N?
Remove vent cap and see if a lot of water comes out or a dribble... More water can indicate the impeller is becoming detatched
Alternatively remove vent cap and stick a screwdriver in... does it spin easily? Can you stop it spinning when on?

 

[mooky]

Continuity across the coil?
230v to L and N?
Remove vent cap and see if a lot of water comes out or a dribble... More water can indicate the impeller is becoming detatched
Alternatively remove vent cap and stick a screwdriver in... does it spin easily? Can you stop it spinning when on?

Will have a look and get back to you. Thanks.

 

[mooky]

Continuity across the coil?
230v to L and N?
Remove vent cap and see if a lot of water comes out or a dribble... More water can indicate the impeller is becoming detatched
Alternatively remove vent cap and stick a screwdriver in... does it spin easily? Can you stop it spinning when on?

Took cap off, just a dribble of water comes out.
Seems to be spinning fast/strong. Can't stop it with screwdriver when it's on.
Turned pump off. Shaft moves freely, but doesn't continue spinning on its own if given a spin - that's normal?

Does it sound like the pump is ok?

Cheers

 

[mooky]

Result!

Well, partial result...

Pulled the boiler's temp sensor (thermistor) from its location and the boiler ran happily for over an hour.
Eventually it tripped out on overtemp - but I expected it would eventually.

So, I guess it narrows it down to the boiler temp control.
Suspect 1 is the thermistor.
Suspect 2 is the temp control input? (the controller permanently sees a low temp requested)

Any further suggestions on what to test are welcome.

In the meantime I'm going to see about getting a replacement thermistor. Any tips on where to buy most welcome.

 

[mooky]

Do you expect all the heating engineers to rush out and help you? you maybe putting people have trained for years, out of work, youv'e had enough info, now push off.....agile.

 

[mooky]

Celebrated too soon....

Replaced thermistor ... still runs for only 30 sec...

Any further suggestions?

 

[Hugh Jaleak]

Still seems suspiciously like a circulation problem. Boiler is getting the water up to temp then cutting out as the water isn't leaving the boiler....

Its not unknown for the impeller to part company with the shaft in the pump, whilst pump is spinning it isn't actually moving any water.....

 

[mooky]

Still seems suspiciously like a circulation problem. Boiler is getting the water up to temp then cutting out as the water isn't leaving the boiler....

Its not unknown for the impeller to part company with the shaft in the pump, whilst pump is spinning it isn't actually moving any water.....

But if I remove the temp sensor in the boiler, it runs for ages before going overtemp (over an hour or more of constant firing).
When its running, all the radiators are nice and hot & the house warms up. The hot water is nice and hot...
If the pump was not working, I'd have thought that it would trip on overtemp pretty quickly or at the very least it wouldnt be circulating water through the radiators enough to make them hot... ?

Interestingly enough, with the temp sensor back in on Friday, it was working perfectly on Friday evening when I got home . It worked all night friday and into saturday. Sadly, it wasnt working when I got home Sunday evening - radiators were only luke-warm and boiler only firing for 30 seconds....

So I pulled the temp sensor again to get the house warmed up.
Interestingly enough, with the temp sensor out, the boiler still stops firing occasionally - in the same manner as if it had reached desired water temperature...
Not sure what is getting it to stop firing - whether the temp sensor is still picking up some radiated heat - or whether its something external?
But with the sensor out, it worked all night without tripping. All heating radiators were nice and hot this morning....

Its quite a puzzle....

 

[warmadmax]

Sounds like an issue with the thermostat selection on/in the boiler.


You've said the boiler flashes green to say it's reached it's set temperature,
it obviously thinks it has, even if the boiler thinks the desired temp is 35C ??
even though the thermostats are at "Max", are they actually at maximum??

what exactly is flashing / lit when it's not working as expected?

removing the thermo-resistor mean the boiler has no indication when it gets to your desired operating temp and the backup systems to prevent overheat are kicking in to stop it overheating/superheating the water above 90c.

I Don't know what's causing it to stop cycling before overheat protection (being a "layman" as well)

Suggest you get someone knowledgeable with potterton's out to look at it and let him know what symptoms you're getting.

 

[mooky]

Sounds like an issue with the thermostat selection on/in the boiler.


You've said the boiler flashes green to say it's reached it's set temperature,
it obviously thinks it has, even if the boiler thinks the desired temp is 35C ??
even though the thermostats are at "Max", are they actually at maximum??

what exactly is flashing / lit when it's not working as expected?

removing the thermo-resistor mean the boiler has no indication when it gets to your desired operating temp and the backup systems to prevent overheat are kicking in to stop it overheating/superheating the water above 90c.

I Don't know what's causing it to stop cycling before overheat protection (being a "layman" as well)

Suggest you get someone knowledgeable with potterton's out to look at it and let him know what symptoms you're getting.

When it's not working as expected, the led flashes green slowly after it stops firing. As far as I can gather this means it has reached the designated temp.
Since the thermistor was cleared as a suspect, it has to be something wrong with the control circuit. The pot for the temp control looks to be affixed to the PCB itself - so its a bit of open-boiler surgery to check that out.
New controller is about £200 I'm told...