Poor heating with semi microbore system

[nocashleft]

Hi Folks
I bought the house last year, 4 bed detached, built 1980.
Boiler is Concorde WRS 55,000 btu, seems to work fine. Have checked the pump, also fine.
The system was originally 1/2" pipework. The main pipes under the floorboards are still 1/2", but at some stage somebody has changed the feed and return pipes to the radiators to microbore.
The system has 8 old single panel rads, but we are unable to get more than three working (any three) at one time, and not very warm at that. Have removed one radiator and flushed to test, which as expected has not resolved the problem.
I suspect that the microbore valves may be the problem - these connect feed and return at the same location, bottom right of the rads, the only area of the larger rads which actually get hot - eventually the rest of the rad gets warm, but I it seems the water doesn't actually flow through the whole of the rad.
I am considering replacing the rads (large & inefficient so need doing anyway), and probably while I am at it returning to 1/2" pipework throughout.
Your comments appreciated before I waste buckets of time * money

 

[taydo]

this type of rad if memory serves me correct have a long tube in them connected to the return it is this tube that either corrodes or becomes detached this is why the only part of the rad that gets warm is the bottom corner.
as for changing the pipework and or the valves check them out first, disconnect the rad and open the valves to see if you have a good flow.

 

[nocashleft]

There is a tube about 1 foot long on the return side of the valve. Not sure about the rad. The flow looks Ok ish - considering it is microbore.