secondary return headaches

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I have installed a secondary return on a gravity hot water system and am struggling to get it to circulate. innitially I installed the grunfos bronze pump with the anologue time clock but was informed by tech support that this was used for mains pressure systems and that it wouldn't work on my system.

I was adviced by tech support to get hold of a 1550b or 1560b bronze circulating pump which I have now done and it has been installed as per manufacturers instructions.

It is on the secondary return pipework just beside the cylinder in the vertical plane pumping down into the cylinder and up out of the top and round the hot distribution pipe work.

It seems to work for a while and then stops circulating for no apparent reason. It is also noisier than a usual circulating pump. This I believe to be air and I have tried to eliminate all air by bleed the pump through the slotted screw on the front.

Should I also fit AAV's at the high point on the secondary return loop at the last draw off. If it is air how do I get rid of it. When I went back to the pump after an hour or so the pump is red hot but the pipe work is not ie no circulation.

As it happens I am about to change it over to an unvented system and so will be re piping the secondary return back into the cold feed and replacing the original bronze pump with integral mechanical timer but would appreciate any advice on secondary returns on gravity systems as I have run out of ideas

??
 
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The return should drop down passed the cylinder connection and rise up into the cylinder in a U with the pump being in the up leg
 
Where does the pump outlet go into the cylinder?

How long's the loop, and what size return?

Any AAV has to be in the positive pressure side of the loop or it'll suck air in.
 
I've done this in the past when using an essex flange supplying a monsoon shower pump. Is it the same principle in terms of air elimination?
 
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The taps should take the air out.

Any auto air vent is not recommended with domestic water as they tend to crud up very quickly
 
From pump outlet pipe work runs down 300 mm and bends right into 2nd return tapping on cylinder 1/3 way down from top.

Return side of loop is about 8 - 10 metres. There are however a few ups and downs in pipe work from original plumbing installation.
 
It is on the secondary return pipework just beside the cylinder in the vertical plane pumping down into the cylinder and up out of the top and round the hot distribution pipe work.

It seems to work for a while and then stops circulating for no apparent reason.

??

Pumping down into the cylinder?

The return usually goes into the cylinder at low level, sometimes connected into the cold feed pipe.

Dissolved air comes out of solution in the water as it is heated and usually is vented out of the open vent pipe on a vented cylinder. Does the arrangement of the pipework make it likely that this air is getting into the pump?

What temperature is the water at? High temperature and low static pressure ( pump at high level) makes cavitation more likely. Or maybe dissolved air coming out of solution due to the low pressure in the pump inlet and collecting in the pump casing.
 

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