[MattB]

Originally Posted by meldrewreborn

Just to explain in plain English, when a thermostat calls for heat a live feed goes to the motorised valve, the synchron motor opens the valve and then trips a micro switch sending a different live feed to the pump and boiler. When the demand for heat ceases power to the synchron motor ceases and the valve closes under a spring return. The micro switch will be released and the feed to pump and boiler will cease too.

but if the micro switch contacts have fused shut due to sparking over time then the boiler will still be powered together with the pump. With all 3 valves shut the boiler will fire, get to temperature and shut down, water will circulate though the by pass units, and gradually cool the return flow back to the boiler, where eventually the boiler will fire again and the cycle will then happen again and again.

this would explain the constant gas use from midnight onwards, but not the peak at 03:00.

Posted just for the lay forum members to aid understanding.

Replacing the zone valves would sort this if that were the cause?

[retrotecchie]

@MattB

Main heating flow pipe. Towel rails tapped off that, with the rest of the zones controlled by motorised valves.

Burned open or burned shut microswitches on motorised valves is why any decent commercial heating system doesn't use mains 'signalling' or 'calling' but low voltage DC logic levels, with diode blocking to avoid back-feeding anything that shouldn't be back-fed. It also makes it far easier to interface to a monitoring or BMS system. But not so common in domestic settings, unfortunately.

I did once have a gas fired system in the house that used completely isolated 24v motorised valves and a DC logic based system. The valves were also proportional rather than just open, closed or midway.

[retrotecchie]

@MattB

Depends how old they are. A lot of newer kit, you can just change the motor/switch assembly without having to touch the valve body itself. Saves faffing about with the plumbing side of things.

[Plantagenet]

Originally Posted by MattB

Agree and thanks for your expert advice. I find it interesting in that it was not until I could monitor with a smart meter that I realised there was a problem.

Of course there is the possibility a wiring fault has been present since the installation of the system.

[Plantagenet]

Originally Posted by retrotecchie

@MattB

Main heating flow pipe. Towel rails tapped off that, with the rest of the zones controlled by motorised valves.

Burned open or burned shut microswitches on motorised valves is why any decent commercial heating system doesn't use mains 'signalling' or 'calling' but low voltage DC logic levels, with diode blocking to avoid back-feeding anything that shouldn't be back-fed. It also makes it far easier to interface to a monitoring or BMS system. But not so common in domestic settings, unfortunately.

I did once have a gas fired system in the house that used completely isolated 24v motorised valves and a DC logic based system. The valves were also proportional rather than just open, closed or midway.

Honeywell smart fit was one such set up .Obsolete now I think , came as a package with some pre plumbed unvented cylinders. Very much plug and play when replacing components!

[meldrewreborn]

@MattB

my heating thermostat goes to 17 overnight which effectively means off! If the family is tucked up in bed there is no need for any heating overnight. The system soon picks up in the morning. Others will think I’m being extravagant doing this even.
if you are happy to pay the bills then you can have whatever temperature you like.

[retrotecchie]

Originally Posted by meldrewreborn

@MattB

my heating thermostat goes to 17 overnight which effectively means off! If the family is tucked up in bed there is no need for any heating overnight. The system soon picks up in the morning. Others will think I’m being extravagant doing this even.
if you are happy to pay the bills then you can have whatever temperature you like.

My stat never goes above 18°C. With an outdoor temperature currently at 1°C, the system is maintaining a pleasant 11°C in the house. Trust me, that's plenty warm enough when you come in from walking the dogs. It will go back up to 15°C once the missus gets back from her craft group.

[MattB]

@meldrewreborn

Most of the rooms are (even currently) set to 6. Two of the colder bedrooms are set to 17 overnight.

[MattB]

I guess an alternative might be the main tado thermostat controlling the calls for heat might be faulty? If it continually called for heat when the TRVs were closed the boiler would also cycle in the manner described above? If I can still find the thermostat it replaced, and put that back on it would clarify it was the thermostat which was faulty if the constant gas use stopped?

[Plantagenet]

Originally Posted by MattB

I guess an alternative might be the main tado thermostat controlling the calls for heat might be faulty? If it continually called for heat when the TRVs were closed the boiler would also cycle in the manner described above? If I can still find the thermostat it replaced, and put that back on it would clarify it was the thermostat which was faulty if the constant gas use stopped?

Are any TRV valves that you fitted the Tado's to, jammed / seized up ?