Central heating timer replacement

[oilhead]

Sorry, but you have not got it right. The RWB 30 only has one switched outlet, so that needs to control the boiler and the pump. You've already said that you don't use the boiler for hot water, so with your existing set up, my notes above are the only way you can use the RWB 30. If you want the option of hot water only, and hot water and heating you need a different programmer. If you now want to introduce a cylinder stat into the equation, you will need to convert to either a Y plan or an S plan, which will need a 3 port valve or 2 2 port valves and plumbing changes to a fully pumped system. This will then require a 2 channel programmer and additional wiring.

 

[Ian Molyneux]

Could this go on the cylinder? And wire it like the room stat but giving live to the boiler?

 

[Ian Molyneux]

Ok thank you

 

[stem]

Could this go on the cylinder? And wire it like the room stat but giving live to the boiler?

No, because when the hot water cylinder was hot, the cylinder thermostat would turn the boiler off. Then when heating was required, the room stat would start the pump but it would only circulate cold water around the radiators as the boiler has been turned off by the cylinder stat.

With your type of system there is no control of the flow of hot water to the cylinder. The hot water cylinder is heated by gravity circulation whenever the boiler is 'on'. So, if the radiators are running, the boiler needs to be running and then the hot water will be being heated whether needed or not.

Some part automation can be provided by fitting a thermostat to the hot water cylinder, but it only works fully when the heating isn't on. So during the summer, the cylinder thermostat will turn the boiler off when the cylinder is hot. But it doesn't work when the heating is 'on' for the reason described above.

The diagram below shows how it's done with a two channel programmer. It would work with a single channel where both the heating and water 'live' are connected together to the same output. When both the room stat and cylinder stat are satisfied the pump and boiler will both be switched off.

Although ideally as @oilhead has said; for completely separate control of Central Heating and Hot Water it would need to be converted to fully pumped with motorised valve control.

 

[Ian Molyneux]

Changed it over today to the new timer, worked first time.
Thank you both for the advice and help with the wiring, it had me scratching my head.