Persistent air in central heating pump

[redharry123]

Hi,
I have a 25 year old Potterton boiler with a Grundfos pump of same age. When the heating/hot water switches on the boiler fires up for a short while before shutting down, presumably because of the thermostat tripping on the boiler. This process continues with no hot water circulating.
However if I bleed the central heating pump air will come out for sometimes a couple of minutes. Sometimes this cures the problem and the heating will come back on and work O.K. Sometimes it doesn't work and the process is repeated. I know when the process works because there is a whooshing sound of trapped air being pushed through the pump area. Sometimes a few clouts with the hammer on the pump seems to speed things up! The pump setting is on 3. The pump does get very hot. The heating once on continues to work until the next timer episode, in other words at least every day !! Room thermostats & motorised valve all seem O.K. all rads get hot when it works.
Clearly something is persistently drawing air into the system, can anyone point me in the right direction as to resolving this. I have also noticed the expansion tank overflow sometimes vents water while the boiler is on. Any help would be gratefully accepted .
Thanks.

 

[igneous]

Is the system open vent? I guess it is

 

[igneous]

Lol I should read post throughly 😁

 

[igneous]

Your system is pumping over and drawing air in, a badly designed system, you need to look at the feed and expansion connections to the system, possibly the vent pipe does not rise high enough before going back to cistern. If this has been like it for a long time then your system will probably be quite corroded

 

[igneous]

Solutions are.... Fitting an air separator or changing over to a sealed system, if the boiler allows. Both will need a flush of the system. The pump may need replacing too

 

[regsmyth]

Could the feed from the tank be blocked? If so, when you bleed air from the pump, could you be sucking the water down the expansion pipe and then a new whole load of air?

 

[redharry123]

Your system is pumping over and drawing air in, a badly designed system, you need to look at the feed and expansion connections to the system, possibly the vent pipe does not rise high enough before going back to cistern. If this has been like it for a long time then your system will probably be quite corroded

Thanks, the system has been the same since I moved in 25 years ago but problem has only occurred this winter. Any idea what would cause it to start drawing air in now. the vent pipe is in the loft while the pump is on the first floor, the pipe is a few inches above the water level in the tank.

 

[igneous]

Are you bleeding the air when the system is off? ( pump not running) the feed could be blocked but that would seem to be a symptom not a cause

 

[igneous]

A poorly designed system can take a while to become evident, as pipe work becomes restricted by sludge etc it can create restrictions. If the vent is just above the water level, as the system heats up it expands the water level in the vent pipe is the same as in the tank, so as it expands it can siphon over, sometimes compounded by the positive pressure caused by the pump

 

[redharry123]

Are you bleeding the air when the system is off? ( pump not running) the feed could be blocked but that would seem to be a symptom not a cause

I have been bleeding the system running. When it is not running there doesn't usually appear to be air in the pump.

 

[igneous]

You may be drawing air into the system by opening the air vents whilst the pump is active

 

[redharry123]

A poorly designed system can take a while to become evident, as pipe work becomes restricted by sludge etc it can create restrictions. If the vent is just above the water level, as the system heats up it expands the water level in the vent pipe is the same as in the tank, so as it expands it can siphon over, sometimes compounded by the positive pressure caused by the pump

Thanks for explanation. Will have a look up in the loft tomorrow with system running. Will come back with any more questions tomorrow.
Cheers

 

[redharry123]

You may be drawing air into the system by opening the air vents whilst the pump is active

O.K. will try and avoid that.