Airlock in HW Cylinder Gravity Fed Open Vented HW/CH system

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As a professional plumber with the bill payer's permission this is what I would propose to do for the fix.
1, Move that h/w mv to the primary return.
2, Take that high drop down out of the primary flow so that the flow pipe is level to the top primary flow connection to the cylinder.
3, Fit an air separator to a high point just above the primary flow to the cylinder and connect the cold feed and expansion into this (a little bit 80s I know).
4, Turn the pump down to its lowest setting.

The system will still be running with a very low circulating head, however, carrying out the above work should get the system working.
servotech,
 
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I feel the vibrations of the pump and when I turned on the hot water I could feel the return pipe get warmer from the cylinder water that was already warm from the morning. At this point the feed to the cylinder was still cold as the boiler had just started up.

I also noticed when the heating and water were on the temperature of the top part of the feed pipe going to the cylinder slight cooled as the boiler was working for both heating.
Just to be clear - is the HW cylinder not being heated? Is the CH working normally?
 
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Just to be clear - is the HW cylinder not being heated? Is the CH working normally?
The hot water only gets lukewarm whereas it was cold before. The feed is baking hot to the cylinder, the cylinder gets lukewarm, the return is a little less than lukewarm. The CH is hot and working.
 
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The hot water only gets lukewarm whereas it was cold before. The feed is baking hot to the cylinder, the cylinder gets lukewarm, the return is a little less than lukewarm. The CH is hot and working.
It's a puzzle! Have you had a look inside the pump? It seems unlikely to be the problem if the CH works OK, but you wouldn't expect speed 3 to be necessary (maybe still OK at lower speed). It's not a big job and if the impeller is partially clogged cleaning would increase the flow, so giving a good splurge (on max speed) which might do the biz. Eliminates it anyway.
 
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I haven’t looked in the pump and nor have any of the plumbers. I suppose as it is a large 1 bedroom flat all the 5 radiators are floor level to about a metre tall. The last one in the chain which is the furthest away doesn’t heat up as well as the others. The closest one in the bathroom is baking hot within a minute or two. The hot water cylinder is probably about 2m or more tall and starts above the height of the radiators. I’m wondering if it is a combination of sludge and inefficient pump that maybe causing a circulation problem? The boiler is about 17 years old and most of the radiators and pipes are well over 20+ years old.

We are contemplating replacing the whole system with a combi and the radiators/pipe work.
 
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I haven’t looked in the pump and nor have any of the plumbers. I suppose as it is a large 1 bedroom flat all the 5 radiators are floor level to about a metre tall. The last one in the chain which is the furthest away doesn’t heat up as well as the others. The closest one in the bathroom is baking hot within a minute or two. The hot water cylinder is probably about 2m or more tall and starts above the height of the radiators. I’m wondering if it is a combination of sludge and inefficient pump that maybe causing a circulation problem? The boiler is about 17 years old and most of the radiators and pipes are well over 20+ years old.

We are contemplating replacing the whole system with a combi and the radiators/pipe work.
If you only have 5 rads and the pump needs to be on speed 3, some blockage is a possibility. I have 10 rads, and the pump is on speed 1
 
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I think you might be right… although I would have thought that the issue would be the other way. Typically sludge/blockages I’d expect with central heating at floor level, but not necessarily with only a hot water cylinder.
 
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I would seriously think of investing - £100/£120 in a A rated 6M circ pump that displays the power in watts to tell you exactly what's what. Some like the DAB Evosta 3 display the power in watts, the head in meters and the flowrate in m3/hr.
 
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I think you might be right… although I would have thought that the issue would be the other way. Typically sludge/blockages I’d expect with central heating at floor level, but not necessarily with only a hot water cylinder.
If the pump impeller is blocked with crud, like mine was, that will give poor flow to CH and HW.
 
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If you only have 5 rads and the pump needs to be on speed 3, some blockage is a possibility. I have 10 rads, and the pump is on speed 1
I have 12 rads, 10 with TRVs and use a 6M Wilo Yonos Pico set in PP mode to 4.6M which results in pump head of around 3.6M and 22/24 watts, when the TRVs throttle in the power falls to - 16/18 watts to give a head of around 2.8M.
 
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So the boiler and radiators are being replaced today with a Vaillant Combi.
 

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