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Have had a new Exterior GRANT Combi Vortex Condensing 36 fitted. Getting hot water no problems. Have 6 Radiators of various sizes, though nothing remotely 'Big', and two towel rads. Boiler is ample big enough and should handle this amount of Central Heating no probs. Plumber has managed to balance system appart from last two which will not heat up at all despite lock-shields been fully open. Pump is at maximum, system is coming off boiler in 22mm and splitting down to rads via 10mm Microbore. All rads have been vented and contain no air, so at a bit of a loss as to why these last two will not heat up. Have even tried balancing system in reverse i.e. Getting last two rads hot first and working back. By time I get to the first rad the last two are cold again. Any ideas coz it's doing my head in??

The fact is that water always finds the easiest way back to the circulator/pump.
The pump has to pull the water through the system overcoming the resistance of the bends and the pipe friction.
Obviously it feeds through the closest radiators first.
Do you have a typical home? Or is it very high or long?
Does the 22mm pipe extend to the last radiator or is it short, with long 10mm runs?
May I ask where the room thermostat is?
Is it possible that the room thermostat turns off before the hot water has an effect on the last two radiators
Do you have a twin pipe system, do all the radiators come off the same hot water pipe?
If they do, then the ones at the end probably need to be on full for some time before the hot water has an effect.
You have proven that there is no blockage.
But, the pump is unable to pull the water from the furthest radiators, therefore, the balance must be wrong or the pump is not powerful enough.
There are four ways to deal with this.
One is to turn the hot pipe into a circle by fitting a T close to the boiler and joining it to the far end of the 22mm pipe so the hot water goes both ways to the cold radiators.
Or fit thermostatic valves to all radiators. As the rooms close to the boiler reach their set temperature the radiators turn off and route the hot water onwards.
Have two zones, where the bedrooms are separate and once the living area has reached its temperature and turned off, all the hot water will be directed to the bedrooms. Or the cold area however described.
Buy a more powerful circulator/pump.

"10mm Microbore."

I would suggest thats where your problem lies. Sludged up pipework OR too much resistance to circulation with small bore pipework.

Cheers

I have a microbore system with 12 rads and a big old fashioned floor 90,000BTU boiler ( great).
I have had problems with the furthest away rad not getting full heat.
Obviosly flush and check for sludge etc. But then I just drain water out of farthest rad - with boiler on and pump running- until the water is coming through at correct temp. This seems to do the trick and then the system is OK. Only have to do this when been meddling wth system though, for decorating , cleaning tank out or moving rads.
Microbore is easy to instal but a pain to manaintain, and the pipe coming out the wall are ugly . I have TRVs, as I replaced the' in and out' valves years ago with proper ones on all rads. Most is 8mm , some 10mm with two manifolds off 22mm pipes. I even installed a 'Cyltro' on the gravity pipes to the Hot Water Tank. One day it will all have to go!