New System Install Advice...
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From what I've read Y-plan is not recommended for larger systems, because the maximum achievable flow with both hot water and heating demand at the same time is not as great as an S-Plan system because all the flow must go through the single central port on the mid position valve, (past the ball valve which only has a small gap on either side) whereas with two 2 port valves you have a separate input port for each valve, and the ball valve can move further away from the input.Originally Posted by kimber.kimber
Some other reasons S-Plan is probably preferred over Y-plan are that mid position valves are more complicated and not as reliable as 2 port valves and harder to diagnose if they start misbehaving, also you can't tack on additional heating zones like you can when you convert an S-Plan system to S-Plan plus, by basically adding additional heating zone valves. The latter is not so much of an issue with Evohome because you use HR92's to perform per room zoning rather than relying on multiple traditional zone valves, however if you add underfloor heating later you may want to use a zone valve there.
W plan uses a diverter valve which is a three port valve like a mid position valve, but which doesn't have a mid position - its either all hot water or all heating. Since you're never supplying heating and hot water at the same time you don't have the flow restriction issue that Y-Plan could have.
W-Plan is OK if you have a modern fast reheat cylinder and you're happy with your radiators not being heated for the entire duration that hot water is being reheated. On my old slow heat cylinder it takes about 25 minutes from cold to heat so W-Plan wouldn't work very well for me. Honeywell has this to say about W-Plan:
http://www.honeywelluk.com/products/...undial-W-Plan/
"Because the W Plan is a priority control system, it is recommended that it should not be used when there is likely to be a high hot water demand during the heating season which could lead to the space temperature dropping below comfort level. This situation is likely to occur, for example, in large family dwellings or in poorly insulated properties."
Personally I think S-Plan using two 2 port valves is the best arrangement because it is the most flexible - by default you get independent switching of heating and hot water as demanded, however you could choose to implement W-Plan style hot water priority with a simple wiring change. Or if you decide later you don't need a heating zone valve you can effectively wire it out of circuit by wiring it to the pump power feed instead of a relay. Any configuration is possible with just wiring changes once you have those two 2 port zone valves.
It doesn't have any problems with adding any extra zones later like underfloor heating, doesn't have the flow restrictions of Y-Plan, and in theory should be more reliable and easier to diagnose/repair that Y-Plan.
One difference though is that S-Plan always requires an automatic bypass valve as under some conditions both valves can close at once leaving nowhere for the flow to go, whereas in Y and W plan there is always at least one port open. (Although technically you still need an automatic bypass valve on those too if you don't have a bypass radiator - whereas on S-Plan you always need an automatic bypass even if you have a bypass radiator, as that bypass radiator can get isolated)
Without seeing the exact diagram you're referring to that's hard to answer (can you find the diagram online ?) but regarding a heating zone valve, if you have one it will TPI modulate the zone valve based on heat demand, as well as the individual HR92's adjusting their valve opening. It is debatable whether this is the best way of doing it or not and I've just been through the exercise of modifying my S-Plan wiring to avoid this, as discussed in this thread, which you might find interesting reading:I know I am going to need the hot water kit, and I believe a BDR91 to control the valve. I take it that I need a separate BDR91 per valve? Also Honeywells installation guide shows a V4073 valve offset from the T of the system. I thought this was a 3 position valve – Is this just schematically shown in the wrong position? Why does the valve move position when the HR92s are added to the radiators?
http://www.automatedhome.co.uk/vbull...ll=1#post30338
The above circuit only applies if you go for a three relay configuration using a separate boiler relay, if you just do a standard two relay configuration (where each relay powers a zone valve and its up to the zone valve switches to fire the boiler) then you have no choice but to let the heating zone valve be TPI modulated so that the boiler is in turn TPI modulated.
As above if you go S-Plan then you need an automatic bypass valve no matter what, and generally its good to have one because it regulates the differential pressure across the radiators when the number of open radiators changes. Without one even if you have a bypass radiator the pressure differential will change a lot so by the time only one or two radiators are open the pressure will be high and those remaining radiators will be noisy. An automatic bypass valve is just an adjustable pressure relief valve between pump output and return to the boiler, this sets a cap on the pressure differential when there are only one or two radiators flowing, but closes to avoid any "wasted" flow when there is a higher demand from more radiators.Also, I understand that with HR92s there needs to be some kind of bypass for the boiler. Is it better to do this as an uncontrolled radiator, or an automatic bypass valve?
Personally I think another reason to prefer an ABV over a bypass radiator is that a bypass radiator is wasteful. It will get hot no matter which other rooms are calling for heat even if the room the bypass radiator is in is already too hot. In winter you might be OK with this but in spring and autumn you may find that rooms gets uncomfortably hot when you try to use the heating somewhere else.
My original system used the hallway radiator as a bypass radiator and also located the main thermostat there, when I installed Evohome I changed the valve body on that radiator so I could fit a TRV head, added an HR92 and had an automatic bypass valve fitted - my summer gas use is now only 50% of what it used to be and apart from zoning in general I put quite a bit of that down to not having any bypass radiator cranking out heat when it is not needed. (For example when I'm just trying to warm the bathroom in summer and not any other room, now that hallway radiator won't be cranking away just because I'm drying towels in the bathroom!)
There's a bit of debate in other threads about this at the moment but I've been told by people that should know including Richard that you can't do this. Having the evotouch as a sensor with an uncontrolled radiator and able to call for heat can't be done in a multi-zone configuration. I also have the evotouch as the temperature sensor for the hallway on the wall mount but I have an HR92 on the hallway radiator as well for it to control.If the EvoHome controller is going to be positioned in the hall, could I run the hall rad without a HR92 (i.e. as the bypass) and then rely on the controller to stop the boiler when the temp in the hall is OK?
Surely a normal or corner TRV would have been much more aesthetically pleasing.....
