Central heating

[Corporal Jones]

I have an old style boiler installed over 20 years ago, vented system, Wickes (Halstead) 40/50. The boiler works fine but every time it starts up it has air in the system, lots of bangs, cracks and whooshes as it vents into the header tank. Sometimes this will go on for a few minutes before the air is expelled then it works fine and is quiet. Other times the air will not clear and I have to vent it with the bleed screw on the pump, bit of a bugger getting up at 06:30 to bleed it. The pump was replaced last November as the old one was blowing the fuse, Grundfos, but only replaced the head as the large nuts where impossible to undo. I have one radiator that needs bleeding occasionally in a bedroom upstairs but it has always been a problem, it is second in-line after the towel rad from the pump. The header tank is above the HW tank in the bathroom airing cupboard where the pump is situated, distance from pump to vent is about 4'. I have been unable to find any leaks but a several days ago I put a sealant into the system. The header tank is not losing water, I can vouch for that as when I drained down I turned off the water with its supply tap which has now seized so had to fill the tank by hand, another job to do. The only thing I can think of is that air is being drawn in via the vent!

Barry

[Andy Cap]

Has the pump several speed settings. My system did exactly this after a new boiler was fitted and it was pumping over. Since I backed off the pump - which was changed at the same time - the system has been just fine.

Andy C

[Roger Mills]

Does the system have inhibitor in it, and has this been replaced recently. If not, the air might not be "air" but might be hydrogen caused by corrosion in the system. If you bleed some out of a radiator, and hold a lighted taper in the "air" stream, does it burn with a blue flame?

Failing that, the system could be pumping over. Is there any evidence of water flowing from the vent pipe back into the F&E tank when the pump, is running? Are you sure that the pump still rotates the same way, following replacement of its guts? If you reverse the flow direction in the system, that can sometimes cause pumping over.

[Corporal Jones]

It has 3 settings, tried it on all of them

[harry]

Air is likely being drawn into the system. Only one place it can come from, the header tank. You need to get up there when the system is running & see what's happening. Make sure water is not coming from the vent pipe which may let air get sucked into the cold water feed pipe. (Pumping over)

Air can also get sucked down the vent pipe if the pump is incorrectly installed or running backwards (latter not normally possible).

Check the water level in the tank is 3"-4" deep.

Also check isolating valves (if any) on the boiler are fully open. (Not fully open can cause pumping over)

The system may have a partial blockage (Sludge) causing pumping over.

[harry]

Also. Check the float valve in the header tank is functioning, ie not letting tank water level fall.

Slight possibility the heat exchanger in the hot water cylinder has a leak. In combination with above could cause a problem.