Zone Heating with 3-Port Valves very complicated

why are you making such a simply job so complicated, 2x zone valves forget the mid pos valve option
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Wouldn't say it was complicated.

All you need is a Drayton RTS 5 or Honeywell T6360 to replace what would normally be the cylinder stat.

L & N

1- Com, 3 in the prog.

2- Sat, grey and 1 in the prog.

3- call for heat-orange.

The other room stat will be as normal and power the white.
as doitalls post above in its entirety
But as others have said it will be a lot easier and service friendly to go with s-plan, but if you want to go the 3port route then just follow DIA's post

Doing this sort of thing with 3-port valves gets very complicated."

This is exactly what i want to do. Why is it very complicated?

You could do it, quite easily IMHO, if you figured out the wiring, but why would you want to?

With 3-port valves, water will flow through the by-pass port (HWS, usually) when the valve isn't powered. This could cause a major short-circuit and a loss of flow to other zones; you can get around this by fitting a regulating valve on the by-pass port and adjusting it so that the flow rate is the same whether the valve is powered or unpowered. You then have a constant flow system; air handling unit heater batteries were usually done like this.

You'd then have another problem with the hot flow water entering the return pipework, raising the return temperature and reducing boiler efficiency on a condensing boiler. It's a pipework problem, not just a wiring problem.

You'd be better off with pumped secondary circuits if you're determined to follow this path and have the cash. If not, stick with 2 ports, like everyone else is recommending.
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Not sure what you're saying Onetaps. There's only two zones, A & B, so certainly shouldn't affect anything else.

My initial thought was, the Y plan was existing and the boiler was changed for a combi, and it would work as my post above, provided you use a stat with the changeover.
To add, the stat with the changeover controlling the B port will also be a priority circuit
Not sure what you're saying Onetaps. There's only two zones, A & B, so certainly shouldn't affect anything else.

Yes, sorry, you're right. Ignore that. I'd just skimmed through it and hadn't taken that in.

I wouldn't use a mid position valve for it, which is what the OP seems to be proposing. The flow rates will vary enormously as zones go on or off.
Without a doubt the S plan would be much better, although I suppose it depends on the size of the system to an extent.

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