Phil! You know how a two way switch functions. with 1 'live' wire going into the switch and two 'switched live' wires coming out but not both at the same time. So you have either 'com' to 'L1' or 'com to 'L2', but never 'L1' to 'L2'
Right ! well a cylinder stat is the same 'live' comes from programmer into stat. and 'switched live' goes out to boiler/pump, then when the demand is satisfied it breaks this connection and makes the other and this will feed the 3 port valve.
If you dont need the 3rd connection then you don't use it. Just like the two way switch you can use it in a one way mode.
Best thing to do is carry out a bench test before you fit it by using a continuity meter across each set of two terminals with the stat turned high so there a demand for heat.
Say terminals are A,B & C
AB .............BC............AC only one pair will give reading and the omitted terminal must be the one for the 3 port valve (if needed)
The pair giving the reading must be live in and switched live to boiler.
But which is which? Well turn stat down and repeat test, again only one pair will give a reading and it won't be the same pair.
This time the omitted terminal will be the switched live to the boiler
So if you know 2 you must know the third. example 1st test AC gives reading B is to 3 port 2nd test BC gives reading A is to boiler so C must be the 'live' problem solved!
