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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.

  2. #2
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    After a bit more research I've decided to stick with my original intention to convert to S plan, and not consider using Y plan.

  3. #3
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    Can someone running Evohome on an S plan system using three BDR91's instead of two answer some questions for me:

    The 3rd "boiler relay" makes possible the "hot water overrun" setting, which I understand will keep the hot water zone valve open for the specified number of minutes after the boiler relay turns off after there was hot water demand to allow latent heat from the boiler to be removed during pump overrun, rather than closing the valve immediately forcing the latent heat to be (slowly and wastefully) removed by the automatic bypass loop.

    1) Is there any equivalent "heating overrun" setting that would cause the heating zone valve to remain open for a specified delay after the boiler relay turns off following central heating demand ? Or would I need to add an external timer to achieve this ?

    2) If the answer to (1) is no, is there any way to trigger the hotwater overrun (hot water zone valve open, boiler relay off) after central heating demand ends, or will it only apply after hot water demand ends ?

    3) When using a three relay configuration, when the boiler relay modulates using TPI under partial load conditions for central heating, does the central heating zone valve also modulate on and off in time with the boiler relay, or does it remain open so long as there is some demand ?

    In a two relay S plan configuration the central heating zone valve must be TPI modulated since the same control must be used to fire the boiler (either via a standard relay or the limit switch in the zone valve) but in a three relay configuration its at least theoretically possible for the boiler relay to be TPI modulated while the heating zone valve just stays open so long as there is some heating demand.

    The reason why this would be a good idea is if the valve closes at the same time the boiler turns off in each TPI cycle, with short cycles like 1 minute the latent heat built up in the boiler heat exchanger will be forced to dissipate through the automatic bypass loop rather than being circulated through the radiators because the valve has closed. This seems very wasteful, and is another reason to add an external 2-3 minute timer on the central heating zone valve if the Evohome doesn't support a "heating overrun" setting.

    Anyone know for certain ?
    Last edited by DBMandrake; 15th October 2016 at 08:16 PM.

  4. #4
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    Out of interest I'm using the Y-Plan, so in answer to:

    '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.'

    The Honeywell (and AFAIK Drayton) 3-port valves at rest go back to the DHW valve position and latent heat does indeed flow through the DHW heat exchanger during the pump overrun period. There was no obvious over-heating of DHW that I have noticed, but it does not waste that latent heat. Both valve types are sprung loaded to DHW on no power or no demand. I have not tried extending the pump overrun duration in my system because I have noted when the tank is at temperature there is still heat left in the boiler water and exchange of heat takes a while if the differential is small, every little helps

    Since I have fitted the DHW control with Evohome and can see water temperature (unknown accuracy) I have noticed that the DHW does heat up by about 1-deg some days, it's very difficult to tell especially as the resolution is 1-deg and it may have just about to have moved up a degree.

    There is an appreciable delay inherent in the valve operating speed that takes ~10-secs to transition from say DHW to HTG and about 5-secs from heating to both (mid-position) HTG and DHW.

    And:
    '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)'

    My system goes back to DHW under control of the spring in the actuator.
    Last edited by g6ejd; 18th October 2016 at 10:06 AM.

  5. #5
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    Thanks for the reply - I decided to go with S Plan, for a few reasons:

    1) I already had one two port zone valve that I was going to use in a C Plan before I decided to go the whole hog and convert to fully pumped, if I went to Y plan I'd have a two port zone valve left over that never even got used. With S plan I just ordered another identical 2 port valve.

    2) Reading I've done suggests that two port zone valves are a lot more reliable than mid position valves, and after looking at the complex design of a mid position valve I can see why.

    3) Simpler more straight forward design wiring, easier to test, and easier to override in case of problems.

    As for heating zone valve overrun - nobody has replied on that point so meanwhile I've been thinking up ways of providing that with an external timer and came up with a great circuit using an MRT16-REM/VF. This is a similar timer to the one I already use for a pump overrun timer, but has full changeover volt free relay contacts which allow some clever configurations.

    I came up with a circuit for the timer taking a feed from the heating zone valve relay and also a feed from the hot water zone valve normally closed contact which does the following:

    If the hot water relay is off, you get a "zone valve overrun timer" which will keep the heating zone valve open for X number of minutes (adjustable on the timer) after the heating relay turns off to allow time for latent heat to be distributed to the radiators - just like it does on a system without a heating zone valve.

    However when the hot water relay is on the overrun is disabled so that the heating zone valve will only remain open as long as the heating relay is on, and immediately close when the heating relay goes off.

    Even an already counting overrun will be cancelled immediately when the hot water relay turns on - so say heating was on and hot water was off, then the heating relay went off, the zone valve would remain open for a few minutes however if during that overrun period the hot water relay came on the heating zone valve would then immediately cancel the overrun and close.

    The reason to do this is you don't want your heating valve staying open beyond the TPI duty cycle the Evohome is asking for if the hot water relay is on, because hot water will be keeping the boiler on constantly and raising the boiler flow temperature as well - if the heating valve stayed on for several extra minutes in these conditions the radiators would go way over temperature due to loss of TPI control, causing spikes in room temperatures when hot water demand occurred. (HR92's would compensate after a while, but not before there was a big spike in heat output in the rooms)

    I have this circuit drawn up as plan B if I find I need to implement this. I should point out that the timer idea above is only applicable with a 3x BDR91 configuration that also includes a boiler relay, which is what I will be using, because the overrun timer would obviously just keep the boiler on longer if it was being powered via the limit stop switch in the zone valve instead of a separate boiler relay.
    Last edited by DBMandrake; 18th October 2016 at 04:38 PM.

  6. #6
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    I recall a few years ago there was a gadget being sold that prevented boiler cycling and all it
    Was just a monostable like relay that once triggered or retriggered it would keep the pump running for minutes beyond the end of demand, I don't know what happened to those products, but I can see what your doing and well worth the effort IMO, can't think why it is not s default design fit.

    I am in total agreement about 3-way valve reliability, just last week we had our third fitted and the BGas variants not Honeywell or Drayton I think as unbranded last just a couple of years it seems, that what the 'engineer' told me. The first lasted about 10-years from the property bring new, to the last that was fitted in Jan '15 to now another about 2-years later at 130 each or 60 for the actuator it gets expensive. Although the failure is nearly always the synchronous motors which are now freely available as a spare for 15 few maintainers seem to replace them opting for the repair by replacement of the whole unit. Our last failure was a fracturing of the valve drive shaft it looks like fatigue but who knows. So yes S-plan is a good choice, plus when the valve fails as you say you can override things.

    We would have been without heat for 5-days the last but one time the valve failed because it kept returning to the default DHW position, in the end I moved the override lever and locked it in the centre.

    I'd be interested to know how your going to bind three BDR91's to inform the controller which does which, I think the binding process lacks clarity for the 3-port option anyway, I ignored the supplied documentation and followed the on-line variant and that works.

    My best by so-far has been the Honeywell wiring centre which enabled me to produce a really neat wiring solution and I've wired in-parallel the old style heating controls so that when the maintenance guys come in they don't glaze over and freak out, like the last guy said to me, I'll have to call one of our experts I don't understand how all this works... Perhaps the time has come to stop paying 271.20 per year and do it all myself.

    Good luck with your system upgrade.

  7. #7
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    Also think about what happens when the Boiler runs on its own for cycles like Frost protection. You will need to make sure you have a water loop before any zone valves via a ABV or similar.

  8. #8
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    When relying on the Frost Protection of the devices, you need to also think about pipe runs that drop below the room temp. e.g. ones in the loft or under the floors. The rad stats and boiler will only protect themselves and not the entire system.

  9. #9
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    Quote Originally Posted by bruce_miranda View Post
    When relying on the Frost Protection of the devices, you need to also think about pipe runs that drop below the room temp. e.g. ones in the loft or under the floors. The rad stats and boiler will only protect themselves and not the entire system.
    Indeed, and this is one limitation of an Evohome system - you can frost protect the boiler using a boilers built in frost stat (or 3rd party frost stat as in my case) and Evohome automatically frost protects the rooms themselves as HR92's will open and call for heat if the room gets below 5C. (Unless you set them to the OFF position on the HR92 - so don't do that...)

    But you can't frost protect the pipes because unless the rooms are below 5C the HR92's won't open therefore no flow through the pipes. In contrast a conventional system with manual TRV's will generally have the TRV's set to normal room temperatures when the system is scheduled "off", so the TRV's are open and ready to flow should a frost stat somewhere in the system fire the boiler.

    I haven't worked out an elegant solution to this problem. Possibly the only way would be to have a frost stat under the floor and/or in the loft which then sends some sort of wireless signal to schedule the evohome to run it's normal schedule once every few hours, but not great, especially if it relied on the unreliable internet API! (The only other way would be Domoticz and an HGI80, but would you trust frost protection to a Raspberry Pi ?)

    A specific crawl/loft space wireless frost protection sensor for the evohome system that could put the system into an active frost protection mode where it periodically gives a few minutes of warm water circulation through all zones that aren't already on is certainly a feature that Honeywell could add, but demand is probably low as most people don't realise the risk until it bites them.
    Last edited by DBMandrake; 3rd December 2018 at 09:58 AM.

  10. #10
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    What may also help this situation would be a custom menu which you select when going away and it is configured to bring the heating on at a designated temperature for the whole house once a day for a short time

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