Air circulating after 40 minutes

[Diyplumb]

I have a problem that try as I might I can't solve. It all started when I changed a rumbling UPS for a new UPS2 pump. After 40 minutes I can hear some gurgles of air / gas entering two radiators. This carries on for a few minutes and then stops. I bleed this off and the same thing happens the next time the heating is on. We have an S plan system, independant heating and hot water valves so the boiler is a system one. I do see minor air bubbles appearing from the cold feed in the f/e tank but they are really small, covering only a patch of one squire inch. I think the 40 minutes timescale might be related to this happening when the hot water cylinder gets hot enough and the hot water value closes. This then cause any trapped air to be more easily distributed to the radiators. I've been bleeding the system many times. So I have tried:
All radiators taken off an manually flushed - very little sediment.
I have circulated x400 for 3 days.
I have attempted to descale the Baxi Platinum He24 boiler with X800 left overnight in the heat exchanger. I didn't want to do this hot as it's an aluminium heat exchanger. The boiler does not sound very noisy so I have doubts about kettling.
The system is definitely not pumping over into the open f/e expansion tank.
I have changed the pump speed which makes no difference.
I have pressure tested the system for leaks and have maintained a steady 1 bar for 12 hours. This was done with part water in the system.
The water leaves the boiler at 65c and returns at 50c.
The pump is definitely round the correct way as the boiler has a flow switch which is working.
The pump is on the flow, 3 metres from the boiler with the cold feed and vent just before it on the inlet side. The cold feed / vent pipes are combined via an air separator, vent on top, cold feed 1 inch from vent.
The system is dosed with 1.5 litres of X100. The system has 10 radiators.
All radiators get hot ok so I can't think that there is any blockage around the pipe work.
The f/e tank has a head of about 3 metres to the top of the boiler so I guess there is about 0.3 bar in the system at this point.

I have thought of increasing the system pressure by raising the f/e tank but I can only achieve 1 metre of height which will only change the pressure slightly.

Is there any way of definitely checking whether my system is kettling and therefore creating air?

Any suggesting would be extremely gratefully received a I have run out of all ideas apart from converting to a sealed system.

 

[Chuck]

What's the height difference between the water level in the expansion tank and the pump?

 

[Diyplumb]

There is 2.8 metres between the top of the pump and the bottom of the f/e tank. There is 200mm of water in the tank. The vent pipe goes above the f/e tank by 0.5 metres.

 

[Chuck]

Strictly speaking kettling doesn't generate air, it's bubbles of water vapour forming at hotspots under or between patches of scale on the heat exchanger surface where the temperature is locally close to the boiling point. As these bubbles move into the cooler water in the body of the heat exchanger they collapse. If it's happening you'll be able to hear it.

You say the system was fine before you changed the pump and the replacement is a good quality pump. Assuming you were reasonably careful when you installed the new one I think you may be worrying unncessarily. You've cleaned the system and put new water in it, I'd expect it to need bleeding for a while. The amount of gas will decrease with time and it would be several weeks perhaps three months before I started thinking there was a problem.

When you put fresh water into a system it will normally have air dissolved in it. A few percent by volume is a reasonable rule of thumb. So, on a system containing say 100 litres of water there are a few litres of air to come out even if there are no trapped volumes in the pipework.

That said, I greatly prefer sealed systems to open ones.

 

[Harvest Fields]

Also you can use x800 on Aluminium heat exchangers.

 

[zzzjim]

When cold check for slightest weep near pump
(or anthing else you disturbed !)
Is pump on highest speed ?

 

[Diyplumb]

Thanks all for the thoughts. The pump speed makes no difference, I have had it on all three speeds. It's currently on speed 2 which gives the above temperature readings across the boiler. I can't see any weeps and have pressure tested the system which does not show any leaks.

 

[Last Plumber]

Can you post any photo's of the pipework around the pump and the valves, possibly showing where the cold feed enters your system ?

 

[Diyplumb]

Here is the best photo I can get. The flow from the boiler comes in from the right through the air separator. The vent is the 22mm pipe from the top of the separator, with the 15mm on the left of this being the cold feed from the f/e tank. The valve sitting under the pump is the heating valve and the one to the left is for the water flow into the hot water tank. The pump is pumping from right to left in the photo.

 

[Last Plumber]

I can't see on the photo but I think there's a bypass after the second tee ( where the heating valve comes off).

Where does that pipe tee back into and is there a control valve or automatic bypass valve on that pipe?

What size is the pipework to your radiators ?