Grant Combi Max problem

[MadmanGreg]

I am hoping someone can help with a problem I have with an outdoor Grant Combi Max. The boiler is some 5 years old.

The boiler does not fire up when hot water taps are opened or the central heating is turned on. You can hear the pump start but it does not fire.

However, the boiler does fire on its own, which I am assuming is the internal temparature sensor for the water storage tank triggering this. So we do have some hot water.

There are no warning lights on at all and pressing the Test switch does nothing apart from turning on the pump.

I pulled off the micro switch attached to the diaphragm in the blender valve, that appears to be working OK and the diaphragm rod is pulled out when the central heating is switched on.

Looking inside the pressure switch it appears the top two screws are tarnished and there is also a water leak when you rotate the switch. See picture below. Unsure if this is purely a seal ?? and if the seal is integrated with the pressure switch.

Any thoughts on the problem and what to do next ?

Thanks in advance for any replies

Greg

 

[MadmanGreg]

A small update

Discovered the pressure in the central heating system was zero, probably due to the leaky pressure switch.

Pressurised the system, the boiler kicked in and fired up on its own

But will not fire up when hot water or central heating are turned on, so still have the same problem.

 

[Boilerman2]

When was it last serviced? it sounds like you have air trapped in the top of the boiler, is the auto air vent on top of the boiler working? also while running a hot tap, you say the pump is running, BUT does the plate heat exchanger get hot? (its the silver block in the back ogf the boiler, bellow the pump) if not I would say the impellor has sheared off of the pump.
Which would mean fitting a new pump, a new low water Switch will also be required (Grant now supply a diffrent switch, so you would need to unscrew the fitting from the pipe manifold)

a Service Engineer would be able to do all this whilst carrying out a service so the boiler is ready for the winter!

Good Luck
(A Grant service agent)

 

[MadmanGreg]

it sounds like you have air trapped in the top of the boiler, is the auto air vent on top of the boiler working?

Boilerman2, many thanks for taking time to reply.

I removed the screw from the auto air vent, it was quite waxy, gave it a clean, screwed back in then opened by half a turn. Bled the pump as well.

The DHW now seems to work when the hot taps are turned on. Just got to get a replacement low pressure switch.

On the subject of servicing, we have left this over 2 years, due to finances mainly, but I guess its time to get this done. Will look at the Grant website for service agents close to Boston.

Thanks

Greg