Boiler cuts out after 2 minutes Help and information on all topics relating to your central heating, air conditioning and ventilation issues.

[fjd@onetel.com]

My boiler starts up from cold, the flames are nice and blue but as soon as the water in the boiler gets hot it cuts out (Usually 1.5 minutes to 2 minutes).
I think there was an air lock above my boiler, as the pipes didn't heat up, even though the boiler did. However, yesterday I reversed flushed the system, by putting the cold pressure water onto the hot pipe at my washing machine. I checked the loft until the water was flowing with no air in it. Today when it started up, the pipes did heat up over the boiler but it still cut out after 2 minutes.
I was informed last week by the plumber that my PCB is probably faulty.
I removed it during the week and found one of the 2 relays had melted.
I replaced this with a new component but the same problem exists.
Another engineering colleague suggested it might be the diverter valve.
I did check today that with the system on (i.e. boiler working) current was going to this valve. I manually opened the valve but it didn't change anything in terms of the boiler cutting out. Any other ideas?

[property man]

Do you get hot water when the heating is on?

[marrtin]

From your description, it is reasonable to suggest the valve is working correctly.

You need to confirm that the pump is circulating the water. Can you hear it running?

[fjd@onetel.com]

The Boiler control unit has separate heating & water switches but I have only been checking the heating function. I have a separate electrical heating element control for the water cylinder, so we can always have water.

I can clearly hear the water pump and feel water being pumped around the system, Its a 3 speed Grundfos pump (about 3 year old).
Although the bolier goes off, this remains on. By the way, the big stainless steel screw in the front of the pump is for what? This was loose so I tightened it up yesterday. I was thinking it could be drawing in air at the screw but there was no water leaking, so I dismissed the idea.

[marrtin]

The screw give access to the pump rotor shaft that has a screwdriver slot so it can be rotated manually for testing. Usually water leaks out if the screw is loose.

Generally, if the pump is running, the boiler has power. If it does, it should be heating the water.

Feel the unions where the pipes leave the boiler, If the flow is restricted they are likely to be very hot. If they are cool or cold, it points to a fault in the boiler circuit still.

[property man]

Sounds like a big air lock therefore the boiler stat will cut the boiler heat of but still allow the pump to turn you will have to force the air out of a rad with a drain of valve on it as it may take a long time