Combining S Plan and Y plan on same heating system

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Is it possible to combine a Y plan 3 port valve and an S plan two port valve onto the same boiler system?

This enquiry is made to see if it is possible to achieve 1 x HW zone and 2 x Heating zones within the same system, but without the ineherent bypass problems of achieving pump overrun in an S plan system with three No. two port valves? I'd also like to avoid installing a fixed bypass to say, a manually valved radiator / towel rail.
(Although many boilers now have integral bypass valves fitted, some manufacturers are advising against relying on the bypass valve to overcome the closed head problem because of 1. noise when the valve operates and 2. insufficient heat dissipation when relying on the integral bypass valve).

There is an assumption in my query that there would be at least one radiator per circuit without TRV's fitted.

 
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Is it possible to combine a Y plan 3 port valve and an S plan two port valve onto the same boiler system?
Simple answer: no.

The reason: there is nowhere you can make the connection for the second heating zone. There are three possibilities and all have problems:

Before the 3 port: HW can work without the 2nd CH zone; but the second CH zone cannot work without the HW;

Teed into the feed to HW cylinder: The CH zone will only work if the HW port of mid-pos is open;

Teed into CH feed from mid-pos: The second CH zone will only work if the CH port of the mid-pos is open.


without the ineherent bypass problems of achieving pump overrun in an S plan system with three No. two port valves? I'd also like to avoid installing a fixed bypass to say, a manually valved radiator / towel rail.
What do you see as the inherent problems of a bypass? Incidentally using a fixed bypass through a rad is no longer permitted.

some manufacturers are advising against relying on the bypass valve to overcome the closed head problem because of 1. noise when the valve operates and 2. insufficient heat dissipation when relying on the integral bypass valve.
Which manufacturers are saying that?

There is an assumption in my query that there would be at least one radiator per circuit without TRV's fitted.
That's taken as read; you need it for the room thermostat!
 
If I was doing it, i'd have a 2 port on each heating circuit which loop off the mid position.

then you can still room stat the two heating circuits seperately with the 3 port getting the last say in it all.

so bit like a Y plan with the heating side going to the 2 ports before it hits the boiler.
 
Thanks D_Hailsham,

Speaking with Viessmann technical, they did not recommend relying on the internal bypass as a sole means for arranging pump overrun.

I'd be interested to hear feedback from installers as to whether the integral bypass valve has proved in practice to be an acceptable arrangement for pump overrun and without any associated problems.

Incidentally, the system configuration I was exploring was a tee upstream of the three port valve, to serve the second heating zone (via its own two port valve).
 
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It would be better to use a second 3 port for the two CH zones.
 
If I was doing it, i'd have a 2 port on each heating circuit which loop off the mid position.

then you can still room stat the two heating circuits separately with the 3 port getting the last say in it all.

so bit like a Y plan with the heating side going to the 2 ports before it hits the boiler.
I hadn't thought of that option. You would wire both heating valves so either of them would put 240V on the mid-position white wire when they called for heat.
 
Check out OSO unvented cylinder web sites. you should find some usefull information there
 
Incidentally, the system configuration I was exploring was a tee upstream of the three port valve, to serve the second heating zone (via its own two port valve).

As already explained by D_Hailsham you cant do it that way as then you would have control issues with the hot water
either do it as suggested with an extra 3port (the control wiring will not be simple and you will need one of the room stats to have an off/cooling terminal)
or extra 2x2ports
or fit an auto bypass and convert to s-plan plus (this will make for simpler wiring)

Matt
 
Make your 22mm connection before the mid position valve but after the bypass. You will only need one two port valve. It will work.. you don't have to have a bypass when you have got a mid position valve. But you have to have one if its an s-plan as when the 2 port valves close completely when not energised which would happen if they became faulty.
 
Make your 22mm connection before the mid position valve but after the bypass. You will only need one two port valve. It will work..

no it wont
if the extra added zone is calling for heat but the hotwater and the other zone are both satisfied control wise then how exactly are you going to shut off a valve that is always open to one port ?
 

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