Water Heating Wont Work Unless Tank Thermostat Is Turned up and Down Manually!

Have a sytem boiler with unvented cylinder with 2 heating zones plus hot water in a house we moved into recently.

To start with we were always amazed at the water pressure so on the odd occasion that the water was only luke warm it was easily forgiven, but over time we have come to realise that for some reason the water heating wasn't working at random times.

Having looked back on the Octopus app at gas usage, it was clear that randomly on some days the water heating wasn't coming on once or twice a week. This then progressed to being missing at least one scheduled on time per day and now doesn't come on a schedule or boost at all.

It appears that on each occasion a schedule for hot water starts or we boost it from the controller, the motorised valve opens but the pump doesn't start and the boiler doesn't fire up. As soon as one turns the thermostat mounted on the side of the cylinder down past the current temp and back up to 65 so it clicks both ways, the system springs into life.

I spoke to Range who manufacture both the tank and thermostat and was told it was worth replacing the thermostat as they have heard of cases of the switching sticking inside, but that it could be overridden with a turn down and back up of the temp dial.

So I have now changed the thermostat but the same behavious continues.

Oddly, it seems that if one activates the Nest Heatlink manually (which puts central and water heating on), the boiler fires up and the pump etc. starts up and the water heats as it should.

What else might be the cause of the boiler and pump not firing up when the hot water heating is activated, unless the stat is turned up and down past the clicks?

Thank you

You say the M.valve opens, auxiliary (end) contacts in this normally gives a run signal to the boiler, a system boiler then normally controls the pump so something strange because the normal wiring route is controller, cylinder stat then boiler/pump, can you locate the terminal box where the 4 wires from that M.valve are in.

John.g said:

You say the M.valve opens, auxiliary (end) contacts in this normally gives a run signal to the boiler, a system boiler then normally controls the pump so something strange because the normal wiring route is controller, cylinder stat then boiler/pump, can you locate the terminal box where the 4 wires from that M.valve are in.

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Thank you for taking the time to reply.

Yes I can access that box and look at it (incidentally the actual valve motor needs replacing anyway as it is very noisey so happy to do that if it may be part of the problem).

In terms of the connections, should I then look at anything in particular or do I need to map out all of the connections in the wiring centre and try to understand it all to ensure it's all wired as it should be?

I have looked for a basic wiring plan as to how things are wired, and what part relies on which etc. in the process of firing the boiler/pump for hot water heating but don't seem to be able to find what I am looking for the completely understand it all.

As per John.g. I think the 3-port motorised valve is 'sticky' towards the (HW) end of its travel, and sometimes doesn't activate its end microswitch. So when you 'flick' the cylinder stat, the motor stops receiving power, begins to run back to the CH position pulled by the internal spring, then when the stat makes contact again, the motor is activated and has a second go at reaching the microswitch/end stop, which it then does (and the boiler springs to life!)
Well, that's my visualisation anyway!
I think your problem is the valve or (noisy!) motor (most likely) or both!

Basher said:

As per John.g. I think the 3-port motorised valve is 'sticky' towards the (HW) end of its travel, and sometimes doesn't activate its end microswitch. So when you 'flick' the cylinder stat, the motor stops receiving power, begins to run back to the CH position pulled by the internal spring, then when the stat makes contact again, the motor is activated and has a second go at reaching the microswitch/end stop, which it then does (and the boiler springs to life!)
Well, that's my visualisation anyway!
I think your problem is the valve or (noisy!) motor (most likely) or both!

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Thank you.

Not something I had taken into account in terms of the valve making full travel until. Like you say when the valve is switched off again by the action of turning the stat down (valve starts to close), then stat up (valve starts to open again) might be the issue so I will get a new motor ordered and see if that sorts it in the first instance.

Thank you for the suggestion both of you.

You could just remove the motor and open close the valve hard over each way with a adjustable spanner/pliers, replace the motor and see how it goes.

If the motor is noisy then probably the gears are worn so maybe best to renew it anyway, you can see the wiring colours, below., just change one at the time.

John.g said:

You could just remove the motor and open close the valve hard over each way with a adjustable spanner/pliers, replace the motor and see how it goes.

If the motor is noisy then probably the gears are worn so maybe best to renew it anyway, you can see the wiring colours, below., just change one at the time.
View attachment 87227

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That's brilliant thank you.

The motor is a Sunvik SZM 1801 which is discontinued so next challenge will be finding a compatible replacement. I suspect a replacement of the valve and motor is going to be needed, which was planned anyway so will bring that forward.

Thanks again.

I don’t think there is an alternative replacement head you may need to change the full unit

Normski said:

The motor is a Sunvik SZM 1801 which is discontinued so next challenge will be finding a compatible replacement. I suspect a replacement of the valve and motor is going to be needed, which was planned anyway so will bring that forward.

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I had a quick look for a possible alternative head.
I think I've come across a possible option, assuming what is being said is true, but it relies on your model having the motorised head fixed to the valve by two screws (in opposite corners, screwing upwards from the valve metal plate into the head.
If yours is an older version where I gather the head pulls off, there is apparently no alternative available.
There was a Sunvic SZM1801 advertised on eBay in December last year for £249.99p and it sold!

If yours is fixed with screws, the one below might work as a replacement head (see "Note" at end of the listing referring to the Sunvic SZM1801)