Expansion tank

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The central heating keeps discharging into the expansion tank which in turn keeps filling the pipes with air. Have replaced all pipework to and from boiler and radiators, and readiators. Please can anyone suggest a next course of action.
 
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What type of boiler is it?

Where is the pump in relation to the boiler, to the vent pipe, to the feed and expansion pipe?
 
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Potterton profile lpg boiler. The boiler is in the kitchen below the pump. The expansion tank is in the loft above the hot water cystern. There are two pipes sited together that go into the loft from the pump, one small one large bore. Do not know what a vent pipe is.

The water appears to boil over into the loft. Have reduced the effect by turning the boiler down to 1. Hope this helps. Thanks
 
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As I understand it then, there is a pipe which comes from the boiler up to the pump. Does the pipe come from the top of the boiler? Does the arrow on the pump point away from the boiler?

Current "normal" connections are top of boiler to a T where a large (22mm or 28mm) pipe goes off to the tank, (this is the vent). Within a few inches of the T there is another T which is a 15mm pipe going to the tank, this is the feed and expansion. The main pipe then goes into the pump inlet. Is this what you have?

If you turn off the heating system, and drain some water from the system, can you hear the tank filling again? If you can't, there is a blockage in the feed pipe.

Have you bled all of the air from the system? including the pump?

It could be lack of water circulation, what speed is the pump set to? or it could be a faulty boiler thermostat, though unlikely.

How old is the boiler?

It does sound like an airlock somewhere.
 
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Hi
Yes the pipe comes from the top of the boiler. what arrow on the pump points away from the boiler. System as described, and yes fills up ok. Bled all radiators, do not know how to bleed pump. Pump speed is 2 (middle) have varied with no success. Boiler installed aprx 6-8 years ago.
If the expansion tank has both inlet and outlet pipes set to close together would this cause the problem, or could it be that there is insufficient distance between the water level and the inlet.
Its nice to have someone to talk to many thanks Pauline
 
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There is a big screwdriver slot in the middle of the pump, unscrew that and water will dribble out. This is for bleeding. Not much water comes out so it's not dangerous.

The arrow on the pump is between the two pipe connections, moulded into the casting in most cases. What type of pump is it (make/model)?

Was the pump put in the right way round. I have seen one back to front, installed by a plumber in his own house (duh!!)

If the tank was too full, it would overflow when the system gets hot.

Is the inlet you refer to, the pipe with the ballcock and valve on it. Its position will not be the problem here.

The problem I have is working out the answer as you have had the pipework replaced. The most likely place is the position of the vent pipe, but not if the pump is connected correctly.
 

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