Having trouble with boiler not turning off even though room stats turned down. Has been like this since we moved in.

Underfloor heating with combi bolier. Two zones each with pump and 2 port motorised values (Honeywell).

Have checked for stiction in the valves but seem fine. They open and close with room stats. So I’m thinking microswitch or wiring (internet plumber…)

When I disconnect orange (and grey) wires from the valves to junction box the boiler is still running. Does this mean the boiler could be wired incorrectly?

Any help much appreciated.

How long does it run for? Is it heating or just the pump going?

With old cast iron heat exchanger boilers, the thermostats / valves switched the boiler and pump on and off.

With modern thin walled Alu heat exchangers, if the pump was switched off the heat exchange would probably melt, so they the boiler is independently powered and will run the pump till it’s cooled down after the demand has gone.

Footflaps – pump overun is what you are describing. Should only be pump but sometimes the fan too. No flame though.

If you have flame with no demand then most probably a wiring issue or possibly faulty valve.

Also, if you have an outside thermostat you might find that the system is just running a little to stop pipes from freezing even when your room thermostats are down. I think when it gets below 6 ours likes to keep the water moving and heats occasionally as well

Thanks for replies. I don’t think it’s pump overun or frost protection.

I think the wiring is a bit of a DIY job so maybe they got something a bit wrong. Not the easiest thing to work out though with so many wires…

ex honeywell sales here! I’d guess it’s a faulty stat or incorrectly wired stat. On a combi the stat is basically interrupting a loop, try checking the wiring of the stat, it’s common for people to replace stats that require two wires and the wiring is three and park the third wire thus creating a permanent live, OR, try a new stat.

thanks that sounds like something worth trying. I was a bit confused as from what i remenber there only seemed to be one lead coming in but 2 room stats (3 wires in it). Does this sound ok?

Depends on the thermostat. Mechanical ones (or battery powered units) only close a contact, so need two wires. Some non battery units also need power, so they have L, N and switched L for the boiler.

What boiler is it?

It’s possible for the micro switch in the zone valve head to stick, sending 240vac on the orange to the switch live and fire the boiler.
If that’s been ruled out, do you have external frost protection?

Boiler could be faulty too.

Check existing wiring first before you spend any money on parts.

Sounds like an S Plan. Are you getting a signal on the orange wires of the valves with no demand?

Trace it back from that point, through thermostats, time clocks etc.

i don’t have a current meter. I disconnected the orange wires from each valve thinking that should kill the boiler just like the microswitch being off. So i was surprised when it didn’t….

I think there may be something in what geordiemick says as there are 3 wires in the stat lead but it is battery powered.

However i’m still confused why it only looks like one stat is wired to junction box….

Thanks for all suggestions much appreciated

Did you have both oranges disconnected at once?

You’ll struggle to fault find it without a multi meter and the knowledge of how to use one.

What Paul said, unless you’ve got a multimeter and a reasonable understanding of heating wiring (from experience this rules out a lot of plumbers and electricians!) then you are not going to find the fault very easily.