Burner turning off on boiler too soon?

[Donkeyfumbler]

Hi,

Odd one that I was hoping I might get some advice on before I have to get out a Gas engineer.

I noticed first thing this morning that house was colder than it should be. Checked and the radiators were slightly warm but not nearly as hot as they should be at 7am.

Further investigation suggests that everything is working OK, except the burner light in the boiler (an Ideal Classic FF250) seems to be off most of the time, even though I can hear the boiler whirring away. It is on some times, at which point the radiators warm up to full heat, but then it turns off again and the radiators quickly cool down. Then, a few minutes later it turns back on again and the radiators warm up. Rinse and repeat.

As a result, the temperature of the house never gets to the temperature of the hall thermostat. The boiler is not overheating - we had this a while back and it trips a switch and everything shuts down when it does - and it certainly doesn't feel as warm as it can get in normal use.

The boiler thermostat was on 4 and I turned it up to 6 (max) but it made no difference.

Any ideas why the burner should be turning off and on like this?

Thanks.

 

[Armo74]

Where is the thermostat?
How high is it?
Are there any other heat sources in the same room as the stat?

 

[Donkeyfumbler]

It isn't the room thermostat turning things off - the whole boiler turns off when that reaches the right point - and it is turned up to fairly high anyway.

The boiler thermostat is turned up as high as it will go (it was on 4 out of 6 before) but that made no difference.

 

[Donkeyfumbler]

Is it possibly the boiler thermostat that is faulty, thinking the water is hotter than it actually is and shutting the burner down while leaving the fan going?

The other thing I have seen suggested is the pump but mine was only just replaced two years ago and seems to be circulating the water fine, whether the boiler is heating the water or not.

 

[jackthom]

Does this boiler show the water temperature while it fires?

If not you should be able to tell if it's getting the water reasonably hot by touching the flow pipe at the boiler. If that is too hot to hold the boiler is probably doing it's job OK and there may be a circulation problem.

 

[Donkeyfumbler]

The water temperature (when the burner is firing) is about as hot as normal I would say (no readout unfortunately). The water is circulating to all of the radiators as they are all heating up equally (again, when the burner is firing) and it is heating the hot water tank too.

The boiler is heating up fine, the question is why does it then stop (but the fan keeps going), let the radiators go cool and then start heating up again rather than maintaining the radiators as constantly hot until the temperature in the house has risen to the level of the hall thermostat and then shutting off completely (as it always has done up until now).

 

[jackthom]

Ah, if the fan is still running there could be an intermittent faulty air pressure switch which won't allow the boiler to fire if it thinks there is a fan problem.

Then again there could actually be a fan problem.

 

[Donkeyfumbler]

Thanks Jackthom.

So if the air pressure switch was faulty (or the fan was on its way out and not pulling enough up the flue) the boiler would stay running but just turn the burner off? It would then wait for the switch to tell it things were OK and the burner would fire back up?

I did a bit of a test and it seems to be all over the place at the moment. For instance, I turned the temp on the room thermostat up slightly and the burner behaved like this:

Burner on: 2.5 mins
Burner off: 8 mins

At which point I turned the thermostat right up

Burner on: 8.5 mins
Burner off: 19 mins
Burner on: 3 mins
Burner off: 5 mins

The fact that it stayed on for quite a while after wacking the heating right up almost makes it look like it works harder when there is a big difference between the current temperature and the temp it has been told to heat up to, but surely it's not that clever? I presume the boiler works just as hard and heats the radiators just as much when it's 20 degrees and the room thermostat is set to 21 as when the thermostat is set to 30?

Am I also right in assuming that there is a temperature sensor in a boiler that checks the temp of the water and turns off the burner when the water gets to a certain temperature, then lets the water fall in temperature a few degrees before re-lighting the burner? I'd really like to know more about how precisely this thing works!

Thanks.

Edit: I tried turning the room thermostat up to 30 degrees again but the burner only fired for a 2-3 minutes this time, so it does just appear to be random how long it fires for and how long it stays unlit.

 

[Donkeyfumbler]

Turned out to be the pilot solenoid not working properly, meaning the gas flow wasn't consistent. Had it replaced and now it all seems to be fine.

Thanks to those who replied.