2- way valve wiring

[Handyman]

I have a customer who wants me to replace a 2-way valve on his Stanley-powered central heating system, the object being to have a switch on the wall which enables him to manually select "hot water only" or "hot water and central heating". No other options are required, and no timer is required. Just a simple 2 position switch. (It used to work, but doesn't now).

Please could somebody suggest a website where I can find wiring diagrams for 2-way (and 3-way) valves? I've tried various search options in Google, but found nothing relevant.
Thanks

 

[Paul Barker]

http://content.honeywell.com/uk/homes/Sundial.htm

S plan uses two zone valves

Y plan uses mid position valve

 

[Handyman]

Excellent. Thank you, Paul B. Exactly what I wanted.

 

[Handyman]

Paul, just to be absolutely clear, am I correct in saying that (in simple terms) the 'S' Plan has two 'on/off' valves: One for the hot water, and one for the CH, each operated by its own thermostat. Whereas the 'Y' Plan has one 'mid-position' valve which diverts heated water to either 'hot water' or 'CH'?

In the latter case, does "mid-position" mean that there is actually a mid position where heated water can flow to both hot water and CH, and as such, there is not an "off" position?

I ask these questions because in my case, there must not be a situation where no water flows through the boiler heat exchanger - as could be the case with 'S' Plan, where both valves could be closed if both thermostats call for no heat, whereas in the 'Y' Plan, this cannot happen, and water will always flow through the boiler heat exchanger.

 

[Paul Barker]

Yes you are correct in your thinking.

However in the case of unvented cylinders a zone valve ius required because in the event of a power cut they rest in the off position. The reason this is important is because overheating the unvented cylinder could leed to a massive explosion sufficient to blow the whole block to smitheeines. True there is an expansion valve and a tpr valve, two further safety mechanisms plus within the expansion valve is a further fail safe in that if it fails eventually the cap has a built in weakness which would also give way and cause a flood, but better a flood than a major explosion.

Just worth mensioning in case that is your situation. However it is true to say that where S plan is not used it's a simple matter to install a zone valve additionally on the dhw side. You would need to bipass that ideally with an autobipass.