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Thread: S-Plan vs Y-Plan for Evohome ?

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  1. #1
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    Default S-Plan vs Y-Plan for Evohome ?

    In another thread I was talking about adding the hotwater kit to my system which is currently gravity circulation for the hot water cylinder, and decided that I would do a conversion to S-Plan next year:

    http://www.automatedhome.co.uk/vbull...ll=1#post28246

    However I'm now wondering, what are the pros and cons if I were to make it Y position with a mid position valve instead of S-Plan ? Since I'm converting it anyway I could go either way.

    My main point of interest, is how does the Evohome's control of the two, 2 port zone valves in S plan or the mid position valve in Y plan differ in terms of valve positions during both TPI modulation and after the system goes off ?

    In an S plan configuration, if heating is active and hot water is not, when the heating is turned off is it correct that the heating zone valve would close immediately, thus forcing the entire flow from my boiler during the pump overrun period to flow through the automatic bypass valve ? This is not ideal from the point of view of dissipating excess heat in the heat exchanger to avoid kettling. I already have a fairly long pump overrun period to avoid the boiler generating small steam bubbles when the pump flow stops.

    My question is, what does the Evohome do on a Y plan configuration when hot water is disabled and heating is running, (mid position valve fully in the A position) and the heating is then also turned off - does it keep the mid position valve energised in hold mode in position A, or does it completely remove power from it to let it fall back to the HW position. (Port B open)

    What I would ideally like is that when hot water and central heating are both turned off that the mid position valve drop back to Ports A and B or B only so that the latent heat from the boiler heat exchanger can circulate via the hot water cylinder during the pump overrun period, rather than circulate via the automatic bypass valve like it would on an S plan configuration. Of course this would raise the hot water temperature very slightly, but the benefit is the boiler heat exchanger is quickly and effectively cooled to prevent kettling as an automatic bypass loop won't remove much heat compared to the cylinder loop.

    Does anyone here use Y plan and know what the valve does under these conditions ?

    Another difference - in S plan mode does the CH zone valve modulate on and off in each TPI period ? I'm assuming it must since most people would fire their boiler using the limit stop switch in the zone valve.

    However what does a mid position valve in a Y plan configuration do during TPI modulation of the boiler during the period of time when the boiler is supposed to be off - does the mid position valve stay in the B (heating only) position, or does it drop back to the mid position, or even the hot water only position ? I'm assuming it must stay in the heating only position otherwise it would cause undesired heating of the hot water cylinder during each TPI cycle in heating mode ?

    Any other pros and cons of S plan vs Y plan in relation to Evohome or in general that I should consider before deciding which way to go ?
    Last edited by DBMandrake; 14th October 2016 at 03:34 PM.

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