Great - an explanation that fits the facts! Thanks for listening and replying. Turning down the heating load control on the boiler improves the situation, but doesn't completely solve it.....
Ground floor flat (5 main rooms) of large and draughty Victorian stone built house with masses of...
OK. The boiler has been off for about 4 hours, but the hot tank is still warm (not hot - does need more heat). Then I flick the timeswitch, and leave it switched on for the remainder of the test.
D40=26, D41=25
Pump on
D41 drops to 16, didn't catch D40
Boiler on
D41 rises to about 46 by...
I can feel the pump vibrating gently when it is on, and not when it isn't. When I switch the central heating on, hot water gets into the radiators. The central heating engineer has checked the valves. I can feel the pipes at various points, and when I start the boiler from cold (not been on...
I'm not a central heating engineer, but neither my central heating installer nor Vaillant have been able to solve my problem.
I had a Vaillant ECOtec plus 428 open vent condensing boiler fitted in November 07, replacing an ancient Servowarm system. Lots of new valves, pump, room thermostat...