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Issues with microbore heating

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Hi everyone.
I'm currently working on a job for a customer. their radiators are not all heating up. in total there is 12 radiators. 4 of these are on 15mm pipework, the rest are on 10mm.
its powered by a Grundfos ups3 pump. the boiler is an 18kw heat only boiler.

There are two radiators mainly causing problems, they are not heating up enough and causing damp to come on the walls, the house is very old so I imagine the insulation isn't great. I have tried balancing already, i managed to get the 2 radiators warm but not hot enough. the only way I have managed to get them to heat up is turning all other rads off.
its worth noting here that changing them all to 15mm is an option but the rooms that are not working are part of an extension with no access to roof or floor space so would involve digging out his wall (last resort)

Im going to chemical flush the system soon, to make sure the microbore isnt blocked. but incase this doesent work i was hoping someone could give me some pointers with the pump. i dont have to much experience with heating pumps but i understand they slow themselves down based on resistance and flow, I'm wondering if the microbore is causing the pump to think its meeting resistance(closed trv's) and not pumping as hard as it needs to.
someone recommended i fit a light commercial pump, which would have a lot more power and could muscle past through the issue, does anyone have experience with this?

any help would be greatly appreciated
cheers!
 
I would check the capacitor in the pump.
There are 2 way to check this.
1/ with a multimeter that checks for capitance...or
2/ with a screwdriver.
undo the screw at the motor end of the pump, you will see the spindle spinning when the pump is on.
Put the same screwdriver onto the spindle and see how easy it is to stop the spindle spinning.
If the spindle stops with little force - capacitor stuffed. - replace pump or capictor
If the spindle requires a lot of force to stop, or doesn't stop, then the pump isn't the issue.
- Then go to Plan B,C,D,E,F..........and so on
 

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