Air in gravity heating system

[Spr0cket]

I've recently moved in to a new (to me) property - circa early 80's build.

It has in the last 18 months had the hot and cold taps converted from gravity to sealed system The heating circuits have remained on gravity feed from the remaining small F&E tank in the loft.

The system has a lot of air being induced into it - which sounds like cavitation at the pump every couple of minutes when the system is running. It's not as severe when heating water only but more evident when rads are in use. The air is being captured at top of the magnetic filter assembly and two or 3 of the rads that seem most acute to it. Pump is set to level 1 to minimise the symptom and noise - it just worsens as pump flow is increased.

On looking at the pipework, I wonder if the feed and vent being joined above the pump flow could be responsible (circled in red in attached image)? Had a look at the feed tank and can see air bubbles being passed back up the feed pipe when the pump goes into air pushing mode and the end of the vent pipe is also very wet.
Layout seems to be boiler, pump, single pipe for feed (on left of the t in pic) vent (right of t) and then on to the motorised valves.

What's the general view, could the combined pipe being after the pump be a combination that'll be problem bound... or should I be looking elsewhere?

 

[Johntheo5]

The combined Vent& cold feed generally works very well, I have that system for over 50 years but the vent just goes straight up and the cold feed is teed in on a stub pipe, you may have a restriction/blockage where they combine but I feel it would work far better if combined something like mine, ie, up high just at or under the F&E cistern, but I have seen systems like yours that work very well. A magnet might show up any blockages.
If that's a UPS3 pump then even at speed1 will still pump at ~ 4M which, although it shouldn't cause any problems, might.

How may rads??.
What level is the pump & cylinder on?.
Check model of pump,
And just to be sure, to be sure, check that the arrow on the pump body is pointing upwards!.
Post a photo of the F&E cistern showing where the cold feed is attached and the vent if not extending well up and over the cistern.

 

[Spr0cket]

I've checked pipework with magnet - all seems clear from that inspection.

It is a Grundfoss UPS3 pump. It's sat in airing cupboard on 1st floor, with the F&E tank located directly above in the loft area. The vent pipe extends maybe 8 inches above the top of the cistern - so approximately 12 inches above the water level in the tank.

Heating system comprises 6 rads down, 4 rads up, along with 1 towel rail down and 2 up that are not valved so heat whenever pump runs.

For the life of me, I cannot fathom anywhere else that could be sucking air in to the system.

 

[Johntheo5]

I've checked pipework with magnet - all seems clear from that inspection.

It is a Grundfoss UPS3 pump. It's sat in airing cupboard on 1st floor, with the F&E tank located directly above in the loft area. The vent pipe extends maybe 8 inches above the top of the cistern - so approximately 12 inches above the water level in the tank.

Heating system comprises 6 rads down, 4 rads up, along with 1 towel rail down and 2 up that are not valved so heat whenever pump runs.

For the life of me, I cannot fathom anywhere else that could be sucking air in to the system.

If you mean by the 1st floor that its not the ground floor and is upstairs on the "landing" then I think I can see your problem, the pump is running at 4M so if the F&E cistern water level is not 4M or more above the pump then you might get air ingress because the combined Vent/cold feed are on the discharge side of the pump, so the pump inlet/suction head will be that "height" minus the pump head, if the height is only say 2M then the pump inlet will be 2M-4M which is a negative inlet pressure of -2M. The combined vent/cold feed should be relocated to the pump inlet, it will help if you change the pump mode from fixed speed1 to constant pressure, CP1 which is 3M.
Get a rough idea of height between the F&E cistern water level and the pump and might be able to suggest a pump PP setting which might help without having to change the combined vent/cold feed.

 

[Spr0cket]

That makes sense. I'd thought it less than ideal to have the vent straight after the pump output.
I'll get an accurate measure of the tank height above pump and circle back.

Thank you!

 

[Johntheo5]

Remember its the water level height above the pump you require.