Boiler wont kick in

[DNC]

Hi all - i'm a newbie to the forum and have a problem with my central heating that i really hope someone can help me with:

About three weeks ago i drained down the CH system to move a radiator. When i fired it back up the pump made a real racket and the boiler (Potterton Netaheat?) kept turning itself off. I bled the rads and eventually called a local plumber who replaced both the pump and air vent which had been leaking for years and was corroded shut. It took a few hours before the pump settled down and the boiler stopped going out but it has worked fine and silent for 3 weeks (cost £220 inc VAT)

This weekend i drained the system again for more building work and the same thing is happening, the pump makes a rattling sound and runs all the time and the boiler kicks out after 3 or four minutes - overheating because the pump is not performing??

I can call the plumber back but would like to know what the underlying problem is before incurring any more cost - i have a lot more work to do over the summer which will mean draining the system at least a couple more times.

Any ideas would be very gratefully received.

 

[MANDATE]

How about bleeding the pump after refilling, when the system is refilling it is likely air is trapped in pipes and most of it finds its way to a radiator where you can bleed it off. Large pockets of air in pipe work are relucltant to move and therefore get compressed bringing the system to a standstill. Unfortunatley bleeding a radiator only allows the air in the radiator to escape unless you want to stand there for hours, so it is essential you also bleed the pump.
Not sure why you drained the system to remove a radiator, however It would have been better to re-fill and run the system without the radiator and if there is no circulation you could open each radiator valve (using a fitting,hose and bucket) to run off a little water (not too quickly ) and allow any large pockets of air to escape. Then fit the radiator.

 

[kevplumb]

totally agree with mandate

have you thought of freezing the pipework to be altered and fit a couple of valves

you can get cans from most merchants

i do it all the time although i have an electric freezer ,but you can hire them

 

[DNC]

Thanks Mandate - I needed to change a corroded valve on the rad and as the room was fully decorated it would have been messy to take off skirting/boxing to freeze pipes.
There is an automatic air vent near the pump so i thought that would handle any air at pump end; hot water is not getting out of the boiler although the rads are full.
OK so if i understand right i can vent the pump through the pump itself, is that by opening the nut in the centre of the pump?

 

[MANDATE]

Should be a screw in the centre of the nut for bleeding, slacken using screwdriver.