Airlock or central heating pump problem?

[Darren1971]

Hi, I hope someone can help me diagnose this problem. Yesterday overnight a leak occured in my central heating circuit. I drained down the system and repaired the leak, then refilled and bled everything I could find several times with the CH off - radiators, pump and a vent screw

Now when I switch the boiler on it fires up for between 30s and 60s before shutting off. It is an Ideal Std gas fired boiler (non-combi). During the brief period when the boiler is fired, i have discovered that the flow return pipe is cold and the pumped flow pipe gets scalding. The pump is definitely running upstairs but I'm wondering if there's either a large airlock or whether the pump is damaged and not shifting water - resulting in the boiler heating static water and then shutting down as it goes over-temperature.

Can anyone suggest how i check this or give me an idea what else it could be? Could the pump be operational (it's quiet) but damaged enough not to pump water? Any ideas for purging any remaining airlocks out of the system.

It's either that or shelling out for a new CH pump to discover if that's the problem :o(

Thanks for any advice you can give.

[Peter Dickinson]

Any chance you turned off any valves during the draindown etc and forgot to turn them on again.

Essentially this is an impeded flow problem. That is the water isnt moving. This can be due to gunged pipes, broken pump, massive airlock or .. closed valves.

[Set Square]

It's pretty certainly an air lock, stopping the water from circulating.

I'm assuming it's a vented system - not a sealed one. Is this correct?

Have you got a bleed screw on the main circuit - maybe in the airing cupboard near the hot cylinder? If so, you need to open that with the system running - and be prepared to get quite wet. Also turn up the pump speed while you do this, and turn the boiler stat down quite low to make it less likely that it will trip through over-heating. Eventually, you will be rewarded with a lot of spluttering and the water will start circulating again ok.

I am writing from first-hand experience of this very problem!

[David M]

This is the exact same situation I had last week after the microbore to one radiator got damaged after taking too many knocks from the kids.

An alternative/similar solution is to just drain the system a little while it is running, this made a significant difference, with minimal chance of getting wet :-)

You could also check for a drainage point at the boiler, hook up a hose to it and drain from there, given that the problem is likely right at the boiler then this is a good place to do it.

cheers

David

[John Stumbles]

There's probably a big chrome slot-head screw or a plastic cap in the middle of the pump motor which you can remove (some water will come out, how much and how fast depending broadly on the age of the pump) exposing the shaft of the pump rotor which you can usually spin with a screwdriver and see if it's spinning when the pump is running.

Push comes to shove you can block the vent pipe and feed mains water into the feed pipe at the header tank and open up drain c*ck(s) at low points of the system to blast air (and maybe muck) out. Or feed in mains at a low point and have the f&e tank overflow let off the outflow (with vent pipe blocked off). This has worked for me for a system with severe airlocking and a couple of systems which blocked up with muck from the header tank getting drawn into the pipework.