[peterbrien]

See attached photo. My neighbour has a central heating system with a new boiler, pump and mid position valve but an oldish hot water cylinder. When the boiler is pumping water around the radiators only it works fine. But when it is pumping water around the primary coil in the cylinder it never manges to heat the cylinder water to 60C, there are sounds of air and gurgling and the boiler keeps switching its gas off, indicating that it is unable to transfer much heat to the cylinder so it is over heating. I am sure there is air trapped in the coil. I get the feeling that it must be quite a lot of air, but perhaps it is only a little. I noticed that the top coil connection is only 10 inches above the bottom coil connection, and one theory is that the top pipe goes up inside the cylinder before it coils downwards. This could trap a lot of air, but it seems unlikely that anyone would make a cylinder like that. The label on the cylinder says the coil is 28mm diameter pipe, and I notice that the plumber has connected to it using 15mm pipe which happens to drop slightly from the side of the cylinder, meaning that some air must be trapped in the top of the coil. Could this be enough of an air lock to cause the problem? Why would he have used 15mm pipe to connect to the coil when he has used 22mm everywhere else? I could change the pipe connected to the top of the coil to 28mm and reduce to 22mm where it rises to the side of the mid position valve, which would certainly get rid of this air lock, but I don't know how old the cylinder is and whether it would be better to just replace it with a new one. How many years should a cylinder last in London before it gets too furred up inside? Many thanks. By the way, the pump is pumping downwards and I fitted an automatic air vent to a tee just below the pump because I thought air was trapped there, but it didn't help. The fed from the tank in the loft and the expansion piper are connected above the pump.

[hvac1000]

((My neighbour has a central heating system with a new boiler, pump and mid position valve but an oldish hot water cylinder))

My first thought is to have the company back that installed the new boiler. I believe it is there responsibility for the problem.

Mid position valve might not be working correctly.

[peterbrien]

hvac1000: The boiler is working fine. The mid position valve is also OK - I can see the indcator on the side points to the correct position (towards the pipe to the radiators when the room thermostat only is calling for heat, towards the cylinder when only the cylinder stat is calling for heat, and in the middle when they are both calling for heat). However, I would like to ask the man who installed the boiler why he used 15mm copper tube to connect to the coil.

[hvac1000]

Ask him ad while you are at it you should ask him if he guarantees his work.