halstead quattro gold hw ok no ch

[loafer]

halstead quattro gold located in garage. primary flow and return run up to first floor then t off drop down to downstairs and continue upstairs.
Drain off in garage, no drain off for downstairs section.
Changed rads throughout. Had to drain off using self cutting valve as no drain off cock located on downstairs pipework.
Noticed, whilst under the floor that the downstairs section is on a 1 pipe system??? All downstairs rads are run on 15mm copper and all feeds to and from rads are off the same pipe. Did not alter tails on upstairs so dunno what is going on here!
Repressurised circuit, bled rads topping up to 1 bar b4 firing up.
The section of flow and return pipework in the garage runs to a level higher than the boiler to the first floor landing and the drain off cock is located at boiler height in the garage so wont drain anything other than first floor hence using self cutting wash mac valve for draining down downstairs section of pipework.
hot water ok but no central heating even after setting the programmer in the living room to constant central heating.
boiler isnt locking out. Best running time on central heating is 10 seconds then burner goes off. boiler been off over Easter and central heating only ran yesterday... N problems till now. Am totally bemused.

 

[ChrisR]

It's air and/or sludge. Can be difficult. Make sure boiler (and pump) are bled, and AAV cap is loose.

 

[loafer]

Thanks. Already bled/vented rads b4 firing. aav loosened too. But when I bled the pump the water that trickled out was very dirty and consequently, had a high viscosity. Drained down again and checked the pump. Was very stiff when I tried to rotate it.