Very likely nothing. It depends on the system we have plans, Y plan, C plan, W plan etc. Each plan is designed for different installation methods. The controller normally has both a physical and a electrical switch inside which can be set to line up with the different plans.
So for example with thermo syphon hot water the controller should be switches so you can't select heating without hot water but this is just a physical stop inside the controller if it is not selected correctly then you can select central heating only but you will never get central heating only.
Today the combi boiler does it all inside the boiler so the few old boilers still around the newer installers have never seen and may not realise the limitations.
So step one is hunt for motorised valves see if these exist. If they do then likely you can get central heating without hot water.
The general idea is the controller works the valve and the valve in turn works the central heating boiler. So the controller can do what it likes but if the valve is stuck the boiler will still not do as it should.
I found one system where the valve had failed and some one had latched the manual operating leaver it is there to allow you to bleed the system before powering up, it moves valve to central position. It also switches the micro switches inside the valve. So in the latched position the central heating and domestic hot water both worked. However in the summer every time it called for hot water the radiators got warm as well. But it did not run for long enough for my daughter to realise the radiators were getting warm.
Thank you. So are you saying we need to get a new system or a programmer? Nothing that can be done by opening the current programmer and resetting it to the system we would need? Sorry for being a bit thick!
I looked and google soon found instructions. It comes in 10 program mode but you can change it to 16 program mode. As I had said there are two switches to move one allows the electronics to assess the extra 6 programs the other allows you to physical select the extra 6 programs.
You simply remove unit from wall and switch the two switches and replace it.
However the extra 6 programs will only work if you have motorised valves. If you have gravity hot water you can select central heating without hot water once moved to 16 programs mode but you will not get central heating without hot water and there is no valve to stop it.
I had this with my fathers central heating and I installed a valve and a tank thermostat to be able to reduce the hot water temperature. It was a lot of work and then he ripped it all out and had new combi-boiler so not sure how well it would have worked.
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