Boiler fires on hot water setting but intermittently on heat only - Main 30 HE A (Nest controlled)

[DaveLandy]

Main 30HE A heat only (not combi)


Ok I have a little knowledge on boilers and am trying to establish if I have a boiler issue of a NEST controller issue.

Boiler is connected to heat only via a Nest sensor.

The nest does not control the water.

The boiler has intermittently responded to the call from nest but sometimes the boiler does not ignite. I think there may be a sound coming from the boiler when I turn up the thermostat either by the nest thermostat or via the app. It is not a clicking sound like it’s trying to ignite but more of a whir.

The nest is connected to a Drayton controller which has the heat setting permanently on so that Nest can operate it.

The hot water is on the Drayton Controller and the boiler always fires when I push the hot water button.

My question is that if the boiler only has 1 ignition system is there a possibility that it’s the nest controller that is the issue as it doesn’t seem to be the igniter if it always fires on hot water.

Is it a simple task to disconnect the nest and test the boiler? Or is there something I am missing ?

Is there anything else I could check for to narrow down the issue

Thanks

 

[RandomGrinch]

Is it a simple task to disconnect the nest

Both the Nest and the Drayton controller are basically switches.
Both can easily be bridged out, to help determine what is at fault.
Can you post some pictures of the wiring behind them?
However an intermittent fault may be more difficult to track down.
Two and three port valves are also a likely cause of glitches.

 

[DaveLandy]

Both the Nest and the Drayton controller are basically switches.
Both can easily be bridged out, to help determine what is at fault.
Can you post some pictures of the wiring behind them?
However an intermittent fault may be more difficult to track down.
Two and three port valves are also a likely cause of glitches.

Thanks for replying. I will go and locate the Valve and see what it is. I have this photo of the connections. I think one of the wires( the black one) may lead to the valve. the other connections are Drayton to the right and Nest Link to the left.

 

[ericmark]

This

looks like a cable from a motorised valve. Not all motorised valves are wired the same, but in the main the thermostat tells the motorised valve to open, and a micro switch inside the motorised valve then tells the boiler to run.

There are variations, Evohome, Wiser, Nest, EPH etc, have an option to run the boiler using OpenTherm, so the on/off contacts in the thermostat open the motorised valve, and the up/down controls tell the boiler what output is required, however it seems unlikely you will have that set-up.

It is not a clicking sound like it’s trying to ignite but more of a whir.

That sounds like a motorised valve opening. I have looked at replacing the micro switch on my motorised valve, it does not look like an easy job, think best option is to replace the head.

 

[Stuckinarut]

Could be the Nest (I know there's been some issues with their heat links) could be the motorised valve. Easy enough to test for voltage. If it's intermittent, sounds more likely the motorised valve.