poor central heating flow?

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Hi -testing my sanity here -
Potterton Profile 80E - Circa 16 years
Three floor house
11 Rads
Most Rads with TRV
Fully Pumped S plan with cold feed close coupled on air separator behind the pump

About 2 weeks ago, central heating downstairs started to be noticeably cooler - getting worse by the day - upstairs OK. Boiler cycling more than it should and some kettling.
My thoughts
Is it sludge - There is a Magnaclean installed but its a three floor house and the Pump and Magnaclean are on the 1st floor so. Loaded the system with Sentinel X800 for a couple of days and hired a Power flush. - Result boiler stopped cycling so much but still poor heating - some days none at all until all the rads turned off except one and then balance everything up again - even then sometimes I can only get ANY 8 of the 11 rads hot. I had a spare pump so changed that - no impact but the heating system seems to be making a lot of air - don't understand why as heating circuit is positive except for about 6 inches between air separator and the cold feed. With the close coupled expansion and cold feed I suppose the expansion could be negative? but its got about 10 feet of head and I can't believe its pulling air. Its London so hard water, could the problem be poor flow because of limescale in the heat exchanger or is is more likely a blockage in the return. If so is it worth trying to strip the heat exchanger out and chemical clean. What am I missing?
 
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Have you done the finger test on the pump? See FAQ.

Are you quite sure the pump valves are fully open?

How did the spare one arise?

I dont find London water particularly hard from a boiler point of view!

Tony
 
Air sep blocked, X800 not very "strong", 2 x knackered pumps... Probssibly needs a 6m head pump...
 
thanks guys - new pump is straight out of the box (mate bought it and never fitted) and has the head. Flow to each rad is red hot but not a lot getting away from any of them. Thinking of disconnecting the flow and return from the boiler to see what the HE looks like. as usual pipes boxed in so will be mess if I do so trying to think of other ways to check 1st
 
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You could do the bucket test on the pump to ensure they are opening properly and the valves and veloute are not blocked!

Tony
 
I've seen the finger and bucket test mentioned before but never maged to locate either in the FAQ (assuming they are listed with appropriate names), could you post a link to them here please?

The closest I've found to testing a pump is this but it does not mention a bucket (I assume the mention of trying to stop the pump with your finger is the finger test)

//www.diynot.com/wiki/plumbing:faq:faq19

TIA
 

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