Boiler sending heat, nothing asking for it.

[JazzyTT]

Looking for some help or maybe a common sense suggestion.

We have a Baxi Assure Combi Boiler, with 2 Honeywell T3 thermostats (one upstairs controlling those radiators, one downstairs controlling those).

Had an issue today where the boiler came on without either thermostat asking for heating, but the boiler showing as sending hot water out to the radiators. Both thermostats were set at 19C, with the house temp on both showing as 22C. Only the upstairs radiators were getting hot, and after a little bit upstairs was sitting at 26C with the boiler still sending heating up but the thermostat not asking for anything.

I’ve tried resetting the thermostat, resetting the boiler etc, but every time I switch it back on it just kicks in and starts sending heating to the upstairs zone.

Has anyone had anything similar, or have any common sense suggestions for what the issue could be?

For reference, the house is a new-build, 2.5 years old, thermostats and boiler all the same age, and boiler serviced each year.

Thanks!

 

[Calpol]

Possibly a stuck or failed 2 way / 3 way port. I don't have a combi but most systems have a port that opens to send hot water to the rads when they call for it but closes when you only need hot water for your taps etc. The motor that drives the valves can get stuck or just fail - I had to fit a new one last year and it's a fairly straightforward job.

 

[JazzyTT]

Ah I see, could possibly be that, thanks!

 

[JazzyTT]

Possibly a stuck or failed 2 way / 3 way port. I don't have a combi but most systems have a port that opens to send hot water to the rads when they call for it but closes when you only need hot water for your taps etc. The motor that drives the valves can get stuck or just fail - I had to fit a new one last year and it's a fairly straightforward job.

Ah I see, could possibly be that, thanks!

 

[Toxile]

Yep sounds like the port valve is sticking. Most likely a Danfoss valve.

 

[LuckyBenski]

But regardless of the valve sticking, why is the boiler firing up?

 

[VaderDSL]

But regardless of the valve sticking, why is the boiler firing up?

Our danfoss valve did this, I can only assume since I’m not knowledgeable about these things that normally something asks for heating, which causes the valve to open which completes a circuit which triggers the boiler to come on and send hot water. Again I might be completely wrong, but presumably when the motor in the valve fails or sticks etc it can sometimes cause the boiler circuit to be energized causing heating to come on even though nothing requested the heating?

 

[Gammawolf]

Our danfoss valve did this, I can only assume since I’m not knowledgeable about these things that normally something asks for heating, which causes the valve to open which completes a circuit which triggers the boiler to come on and send hot water. Again I might be completely wrong, but presumably when the motor in the valve fails or sticks etc it can sometimes cause the boiler circuit to be energized causing heating to come on even though nothing requested the heating?

Probably, almost all setups will include the use of the aux circuit. So when the motor turns it'll physically hit a switch that completes the aux circuit, sending power to the boiler/pump to call for heat. If that's stuck or the switch is just closed for some reason it'll constantly be calling for heat. I'd guess the valve is a normally closed valve for a two zone combi system so given the rads get hot the valve must be in the open state. It could be piped diffrently but its likely its just setup in something similar to an S plan, except instead of Hot Water/Heating the hot water is just another heating zone and the cylinder stat is the second thermostat

 

[Smffy]

Mine did something similar after a powercut, it did what things sometimes do and just resolved its self after I had rage quit

 

[K.C. Leblanc]

My Worcester boiler has a setting called preheat. If this is on the boiler will cut in to keep the water in the boiler at temp.

 

[tangey]

But regardless of the valve sticking, why is the boiler firing up?

My understanding is that the thermostat does not directly supply electricity to the boiler, it simply tells the valve to open up and the act of it doing so completes the electrical circuit to power the boiler.

So if the value is stuck open, the boiler will be on regardless of that the thermostat is reading.

 

[ZG002]

Basically the above, the valve will have a live in (grey) and live out (orange) when the valve is open the switch closes and you get your live out on the orange.

The valve can open when heat is asked for or it can manually be opened. They do go wrong fairly often in my experience, either the valve head goes faulty of the valve body gets stuck open or closed. Better stuck open as you can just turn the actual boiler on or off to still get heating while you wait for repair.