Wiring diagram for fully pumped Honeywell S plan

[Keefiedee]

I am upgrading my very antiquated central heating. I am fitting a two channel programmer, two 2 port zone valves for HW and CH, and an extra pump for the HW which is gravity fed at present.

As I understand it, this constitutes a Honeywell S plan, and, except for the extra HW pump, is shown at:

and:

but these diagrams only show a pump for CH which seems to be wired to run all the time whether there is a call for heat for HW or a call for heat for CH.

Is there any reason why I cannot wire the CH and HW pumps respectively between the room thermostat and the CH zone valve, and the cylinder thermostat and the HW zone valve? Or can anyone give me a link to a wiring diagram for a fully pumped S plan system?

TIA

Keith

[Keefiedee]

I think I've got it. The pumps don't go between the thermostats and the motorised valves, but are fed from the thermostats rather than fed from the switches in the motorised valves.

Will someone please confirm?

Keith

[dennis]

On my system the end switches on all the valves connect to boiler on. The boiler has a pump over run stat and connects to the pump.

You don't say what boiler you have or if you are changing it. It may need an overrun stat if you remove the gravity loop.

[Keefiedee]

The boiler is oil - about 5 years old. Not sure what you mean by overrun stat. Please clarify.

Keith

[dennis]

Runs for a period after the boiler has finished to allow the heat to be removed from the boiler. You don't need one with a gravity loop but the boiler may need one if you remove the gravity loop.

[PM]

Why do you want two pumps?

You have two valves which are opened /closed by the room stat and tank stat. If either valve is open, the boiler fires and the pump runs.

[Keefiedee]

Because the HW and CH have separate pipework coming out of the boiler, and the HW was only heated by gravity before I fitted the pump. I know a pump isn't absolutely necessary, but I wanted a more efficient system.

[PM]

That makes sense then!

[Roger Mills]

So what you have *isn't* an S-Plan system - because that would only have

*one* pump.

With two pumps, you don't actually need any motorised valves. You simply control each pump with its (room or cylinder) stat - and you then need some relay logic to fire up the boiler when either (or both) pump is running. [You may then find that you still get gravity circulation in the HW circuit when only the CH is supposed to be on - in which case a simple check valve, which requires pump pressure to open it, needs to be inserted into the HW circuit.]

[Keefiedee]

I thought the difference between S plan and Y plan was that S plan used 2 port motorised valves, and Y plan used 3 port valves?

Keith

[Roger Mills]

That is true, but the thing which they have in common is that they both use only *one* pump.