Radiators are on when heating is switched off!

[Pillowcovers]

Does anyone know why this is happening? Our old Hive unit is obsolete now (new version came out) so can't be controlled with the app or website. However you can still control it manually with the unit itself. But it has a mind of its own! We are boiling today, even though I've switched both heating and hot water completely off now. How on earth could that happen? Surely if the unit isn't working then the heating would be consistently on all day and every day, not just at random times.

@pigletjohn may be able to help?

Had this last year. Turned out there were batteries in the wall thermostats (very cleverly hidden, I looked and couldn't find them) that were used to send a wireless signal to the boiler or something; the battery was dead.

Same thing happened exactly 2 months later in another heating zone with another dead battery.

Poke and prod your wall thermostats to see if there's a hidden battery compartment.

Ooh thank you, I will try this!

When I had issues it was the diverter valve that was the problem

Diverter valve when I had the same problem too. Needed a boiler person to come and replace it, quick job & not too pricey if I remember correctly

Have you got a combi? Or a hot-water cylinder?

Diverter valves only last a year or two in my experience. I've been through loads over the years. If you know where it is (mine is immediately under the boiler, small metal cube) you'll be able to manually switch it off as they have a little lever on the top.

My Mum had this recently, and I think the engineer installed a diverter valve as mentioned above.

Hot water cylinder

Also, weirdly enough the radiators come on only upstairs. Downstairs stay off. But we don't have zones, just the one thermostat

As above its probably the valve. It probably isnt closing properly and will let a bit of hot water through, enough to heat some radiators but not all. It should only happen when the hot water is on.
You need one of these, quite easy to change

https://www.screwfix.com/p/flomasta-3-port-motorised-valve-22mm-22mm-compression/7091g

Hot water leaking into radiator system, diverter valve failed, I bought one on line and did it myself. The worst problem was reaching it, tucked away in an awkward corner beside the hot water tank. You don't need the pipe work bits (shown in the pic above) you can just change the bit in the box on top. You can check if it's working with the lever on the side. I don't remember the details but google reveals all. No need to turn the water off (as I remember, a few years ago now) or to drain the system.

We have this problem and it's just finding time to get a plumber that we haven't sorted it yet. Very encouraging that it's easy enough to fix yourself. @RubberyChicken does that for all brand of boiler?

If it is leaking through the valve, I think the rubber ball (plumbing part) is worn out. They commonly last around 20 years. If you want to verify that, you can take the upper box containing the motor and switch off, and rotate the spindle of the valve with pliers.

A worn out ball will not start the boiler up. The switch that does that is inside the motor box.

So the question is, does the boiler fire up even if there is no call for heat from either thermostat? If so, the motor switch is not working correctly.

If heat leaks into the radiators when only the cylinder should be heated (and vice versa, leading to surprisingly hot tapwater in winter) then the ball is worn out.

The cost of the entire valve is about the same as the cost of the motor. It is normally sold as a complete part. If one of the components is worn out IMO it is worthwhile replacing complete. Certainly, if the valve is worn out, there is no extra effort or time is replacing the motor as well.

It is less work to change just the motor so some plumbers do that for a quick fix. The valve is a water part.

If you change the two components at separate times it will end up costing you double.

Incidentally, I prefer Honeywell or Drayton brands. They are a bit dearer. You can probably recognise what make you have from the photos.
https://www.screwfix.com/c/heating-plumbing/motorised-valves/cat831028

For a combi boiler, you'd need a gas engineer and a boiler branded part.

@PigletJohn can I ask you if my boiler engineer cheated me? He charged £320 for this. Said the replacement diverter valve or whatever is £150 (the link @RubberyChicken posted is £65) and the rest was him messing around cos he couldn't get the hot water or heating working after that. I had to call Worcester Bosch the next day and they came out and said there was a big airlock in the system and the engineer seemed a bit inept. I guess I should just look for a new one!

There is more work changing a valve inside a combi boiler, and the boiler branded part is more expensive than the separate valve commonly used with a hot water cylinder.

Drayton and Honeywell are more expensive, and I think better.

It's not a combi boiler. It's the other thing, with a big hot water tank etc. Maybe it was OK then. I've been satisfied with his previous work before this

Pillowcovers - funnily enough that's exactly what my DM was charged about two weeks ago. It did include an annual service though.

Thank you @Sunnyside4 This didn't include a service but the price doesn't seem TOO far out then