Intermittent boiler firing

When I turn on my central heating the boiler fires up but doesn't stay fired up for long, the burner goes on for maybe 20 seconds before extinguishing. It will fire up again after 2 or so minutes for another 20 seconds and this pattern repeats. The output pipe for the radiator circuit goes cold a good time before the boiler fires again. I do not have a room thermostat so it's just the internal thermostat controlling the heating.

When the boiler does fire it heats the water up well. With the system totally cold it takes a bit less than 20 seconds til the input side of a radiator close to the boiler gets scalding hot. I don't know the precise pipework layout but there will be at least 2 or 3m if not more worth of piping between the boiler and this radiator.

I am planning to give the system a clean with some x800. I was wondering if the pump is also a bit dodgy (sadly it has no speed setting to fiddle with), though given the hot water reaches the the first rad fairly quickly, perhaps the flow rate is ok?

Any other ideas what could be causing this behaviour?

Knowing the boiler make and model would somewhat aid my guesses!

Tony

It's an Ideal C80FF

It sounds as if the flow rate is too low.

Possibly a blocked pump impeller.

Could also be an iso valve under the boiler not fully open if you have been fiddling with them.

Tony

Is it possible to clear out a pump impeller?

My current plan was to see what the clean did and then potentially replace the pump if I was still having issues, does that sound sensible?

I did wonder if I had a partially closed valve causing issues. The two pipes that make up the radiator circuit don't have any taps on them but do have fitting with a small screw. The screw on the out side seemed fully open when I tried it, though potentially just stuck. The screw on the return was surprisingly loose, experimentally I tried loosening it further and it turns out the screw was just holding metal cover in place. With the screw totally taken off and the cover removed it looked like I'd opened up the fitting however no water came out. I put it back in place and tightened the screw again (maybe to a little tighter than what it was) and there's no sign of a leak. Any idea what these screws/covers are? Doesn't seem like a valve. Picture below (the fittings I'm talking about are on the far left and far right, the square bit with the slot through is the cover that I could remove, the screw is in the middle, taps are cold water in and I think gas):

Boiler valves look fine.

Drain boiler and remove pump head and inspect impeller carefully.

If its clean the do the "finger test" see FAQ on this site.

Tony

You don't remove the screw and cover but rather turn the whole thing a quarter turn - with the slot vertical the valve is open, horizontal and the valve is closed.
Designed to be turned with a spanner.
Also worth checking - do you have thermostatic valves on all your radiators? Are they turned up?

Also worth checking - do you have thermostatic valves on all your radiators? Are they turned up?

Click to expand...

Yes and yes. The reason I started fiddling with this in the first place is I've been finding it very tricky to balance a couple of the radiators in my flat. They remain luke warm whilst everything else heats up relatively well. If I shut off all of the radiators (via closing the TRVs) other than the two problem ones they heat up very nicely. As you add more radiators back in (via opening TRVs) the two problem radiators get worse. However if I start closing down the lockshields on the good radiators in an attempt to get the bad ones heating better the good radiators stop getting as warm also.

So what I reckoned was whilst the bad radiators are getting some flow off the heating circuit the others get the lion's share and with the boiler seemingly failing to properly heat all the water they don't end up getting much. Hence the hope a clean might help (maybe the bad radiators will see better flow after the clean). With the boiler firing 'shots' of hot water into the circuit rather then heating the whole lot up the bad radiators aren't getting a chance to fully heat up. Were the boiler to keep heating properly then I'd hope the bad radiators would warm properly.

On removing the pump head how easy/likely is it that I would damage the seals in doing so so it leaks when I put it back together again? The pump is mounted in the boiler itself just above a bunch of wiring (it's right over the front control panel) so even a slight leak wouldn't be good (it's going to be hard enough as is to try and put something to catch any water that does come out if I do take the head off as there's so much wiring straight underneath it).