W Plan

[Johntheo5]

Is this a typical W plan arrangement?, works fine with both CH & HW programmed on with HW priority but if CH only is programmed on with HW switched off then the boiler will fire up but will still heat the cylinder until its stat is satisfied and then the diverter valve is powered via the satisfied contact (2) on the cylinder stat?, still HW priority even when HW switched off?

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[ericmark]

The valve is powered to turn it off for DHW

there is no other way on that plan to turn it off other than the tank thermostat, in the main the programmer is set to gravity even though not gravity (also called thermo-syphon) so you can't physically select CH without DHW.

In the main W plan is used when the hot water cylinder is very small, often built into the boiler, it seems there are three DHW priority systems, the old W plan, a system using a mid-position valve, and also a system which incorporates the weather sensor. As when DHW is called for, the circulating water needs to be hotter.

The C, Y, and W Plan is still used with old oil boilers, as the boiler often has not cool down system built in, but in the UK most modern gas boilers work on the S plan, but to be able to as default turn off DHW would defeat the whole idea of using the DHW to allow boiler to cool.

I am an electrician, maybe wait for a heating engineer's reply.

 

[ericmark]

I have looked at the problem, and using a double pole change over switch does seem possible, but you need to power to turn off the motorised valve, and also stop power going to the boiler and pump, so moving to Y Plan seems the best way out.

 

[fixitflav]

CH only is programmed on with HW switched off then the boiler will fire up but will still heat the cylinder until its stat is satisfied and then the diverter valve is powered via the satisfied contact (2) on the cylinder stat

Yes, if it's set up that way, so that CH and HW can each be called separately on the timer. But that's wrong, the timer should be set up so that CH is only available when HW is called, i.e. power to CH comes from after the HW contacts. On W-plan CH is only available when HW is called on the timer, and the cylinder stat is satisfied. Edit - very similar timer setup to the old gravity HW, pumped CH systems.