On timed programe ie.off mid-day to 5p.m. and from 1a.m. to 7.a.m. there is loss of pressure below cut off point. Have had pressure relief valve replaced but no improvement According to Ferroli could be new pressure vessel needed or blocked pipe to latter. The former would require removal of the boiler And £40 cost of part .The latter is a pipe that is available without dismantling but Ferroli would'nt tell me how to identify this nor how to unblock it. The system has been treated with Sentinel 400. Is there some way of repressurising the expansion vessel? Or getting a more accurate diagnosis

clifford dobson, April 2008

The expansion vessel is a device with 2 tanks inside in one unit, separated by a rubber diaphragm. On one side of the diaphragm there is water, on the other side, air. You'll see a car type tyre valve on the air side, and the pressure should be around 10 psi. If you press the valve open with a pencil or something and water comes out, the diaphragm is split. Low air pressure causes the boiler pressure to fluctuate, and it could be the answer.

John, April 2008