MAINS Eco Elite 30 - Radiators Getting Warm with Heating OFF?

[sentur]

I've got a MAINS Eco Elite 30 combi boiler and I'm experiencing a strange issue that I can't figure out.

Some radiators are getting warm even when the central heating is turned off.

Specifically:

• When the boiler is set to hot water only (or HW + CH mode), certain radiators start heating up
• This happens even when the CH light on the boiler is OFF
• Turning down the TRV on the affected radiators stops them from getting warm
• I can hear the boiler firing up intermittently when there's no demand for heating or hot water (usually late evening or middle of the night after everyone's asleep)
• When this happens, the HW light typically flashes on the boiler - could this be pre-heating?
• The only way to stop the radiators warming up is to turn the boiler off completely (either setting the dial to 0 or switching off at the fused spur)

There's a Tado X smart heating controller relay fitted to the boiler, but I don't think this is causing the issue. The Tado works perfectly when I use it - hitting Boost turns on the CH light on the boiler and all radiators heat up as expected.

I'm completely stumped as to why radiators would get warm when the heating is definitely set to OFF. It's not like any boiler I've had before.

Am I missing something obvious or is this expected behaviour? Or is it time to get a plumber in to find the issue and fix it?

Thanks in advance for any help!

 

[CountryFan]

Passing diverter valve is likely cause along with the preheat.

 

[gas112]

HW light flashing is pre heat
so yep certainly sounds like a passing divertor valve
to turn the preheat off turn the HW thermostat knob fully anti clock wise so little pip is at the top

 

[sentur]

Update from the last 24 hours:

I've noticed a few things that might help diagnose the issue:

• The radiators that were hot yesterday evening were completely cold this morning when I woke up, which suggests the boiler shut off properly overnight.
• However, after I'd been using hot water for 10-20 minutes this morning, some of the radiators started warming up again, even though the central heating wasn't supposed to be active.
• Throughout the morning, I've observed a pattern: the radiators feel cooler when the hot water hasn't been running for a long period of time, but they warm up again shortly after I've used hot water. When this happens, the boiler also becomes noticeably noisier (though the ignition light doesn't always come on, just the HW light flashing).

Does this provide any additional evidence pointing towards a faulty diverter valve, or help narrow down the issue in any way?

 

[CountryFan]

Update from the last 24 hours:

I've noticed a few things that might help diagnose the issue:

• The radiators that were hot yesterday evening were completely cold this morning when I woke up, which suggests the boiler shut off properly overnight.
• However, after I'd been using hot water for 10-20 minutes this morning, some of the radiators started warming up again, even though the central heating wasn't supposed to be active.
• Throughout the morning, I've observed a pattern: the radiators feel cooler when the hot water hasn't been running for a long period of time, but they warm up again shortly after I've used hot water. When this happens, the boiler also becomes noticeably noisier (though the ignition light doesn't always come on, just the HW light flashing).

Does this provide any additional evidence pointing towards a faulty diverter valve, or help narrow down the issue in any way?

Yes, but to be fair, so did the initial evidence as I suggested