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

  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.

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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.

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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.

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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.

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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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    Well that's the plumbing part of the S Plan conversion completed today.

    About 45 minutes from completely cold (15 degrees) to 50 degrees as reported by the strap on sensor, which I find too hot to keep my hand under. That's with the flow temperature manually set to 70 and all radiators in the house turned on at the same time with both zone valves open.

    It's an old cylinder so is not "quick heat" but its still a big improvement on what we had before with the useless immersion heater and the useless slow and unreliable gravity loop! (And a bigger, modern quick heat cylinder could be easily fitted in the future now that it is in an S Plan configuration)

    I just have the wiring modifications to do tomorrow - the temperature sensor is installed and reporting but the two additional BDR91's are still sitting on the test bench connected only to power, so the heating zone valve is just wired to the pump supply and the hot water zone valve is in the manual fill position until I can get the wiring done, so we have heat and hot water in the morning!

    I don't think people realise that the manual fill position on 2 port valves only half opens the valve...so with it just on the manual fill position the flow to the radiators was noticeably restricted, hence wiring it to the pump temporarily to let it fully open.

    As Paul suggested in the temperature sensor thread it does indeed seem to be stratification that was the problem with the sensor reading being so low when using the immersion heater. It now easily gets up to 50 degrees and beyond on the reading, however it does feel hotter than 50 to me though, so I'm not sure how accurate it is, but I don't have a contact thermometer handy at the moment.

    Also as Paul reports, running the hot water for a few minutes results in the reading dropping enough to click the relay on, and it then takes about 7 minutes for it to heat back to 50 for the relay to go off again. About 5 minutes of running the hot tap drops the reading from 50 to 37 even though the water coming out of the tap feels the same temperature - which is to be expected due to stratification.

    Now to find good mounting locations for two additional BDR91's in a cramped boiler closet dominated by a hot water cylinder!
    Last edited by DBMandrake; 21st October 2016 at 01:14 AM.

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    Out of interest, the water from our tank set at 55 is unbearably hot at the taps, I think it approaches boiler water temp, so there is a temp gradient in the tank, I may turn it down a bit to get a usable average hot water temp whilst preventing nasties from growing in the tank. Having done the legionella training at work I now understand a lot more about it. Apparently most people catch a lower strain of it from their showers and it exhibits itself as cold or flu like symptoms. Hence 55-deg is the recommended minimum to prevent the growth, but that's too hot!

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    Our shower is an instant heat electric shower so no problem there, the hot water cylinder is only used for dishes in the kitchen sink, bath and bathroom hand basin.

    I suspect the temperature reading on the casing of the cylinder is lower than the actual water temperature inside, as it feels more like 60 to me when it says 50, but there doesn't seem to be a sensor offset/calibration feature in Evohome for calibrating this error out, so it seems to be just a matter of finding a comfortable temperature by trial and error and adjusting the target down a bit!

    By the way the solution to Legionella is to run the cylinder at 60 degrees then use an automatic tempering valve on each hot water output to regulate the temperature at the tap to a safe 46 degrees or so. I seriously considered doing this but good automatic tempering valves are expensive, especially a compact one that can fit in a pedestal hand basin, and getting access to the pipes for the bath is quite difficult so I eventually decided against it, although I suppose I could still install them in the future.
    Last edited by DBMandrake; 21st October 2016 at 01:28 AM.

  10. #10
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    Quote Originally Posted by g6ejd View Post
    Out of interest, the water from our tank set at 55 is unbearably hot at the taps, I think it approaches boiler water temp, so there is a temp gradient in the tank, I may turn it down a bit to get a usable average hot water temp whilst preventing nasties from growing in the tank. Having done the legionella training at work I now understand a lot more about it. Apparently most people catch a lower strain of it from their showers and it exhibits itself as cold or flu like symptoms. Hence 55-deg is the recommended minimum to prevent the growth, but that's too hot!
    I completed the wiring and controls side of the S-Plan conversion yesterday so everything is up and running.

    One thing I noticed was that the hot water at the tap measured about 2 degrees hotter than claimed by the evohome. Even though the copper looked clean and shiny after I'd cleaned it with a kitchen scourer I grabbed my multimeter and checked for electrical continuity across the cleaned area and discovered I had none unless I pushed the probes hard. Whoops!

    Obviously a thin layer of invisible lacquer or glue still remained between the sensor and the copper, so this time I got to work with some sandpaper until I had bare sanded copper. Now the temperature reading is almost spot on. After a full reheat the tap temperature is +/- 0.5 degrees from the Evohome reading. (Good considering evohome rounds to the displayed figure to the nearest degree anyway!)

    I noticed that the Hot water overrun only seems to apply if the heating zone valve is currently closed, this makes sense - if the heating zone valve is open then there is no need for hot water overrun as there are still radiators to absorb the latent heat. The hot water zone valve closes immediately when the hot water temperature is reached if the heating zone valve is open.

    After some testing I ended up deciding not to use hot water overrun though, mainly because it does result in significant and unpredictable temperature overshoots. Rather than doing the clever thing, which would be to predict how much earlier it can turn the boiler off and still get the right temperature when keeping the zone valve open X minutes, it simply waits until the temperature is correct then turns off the boiler, but keeps the zone valve open for X number of minutes - so overshoot is inevitable, especially if you have a large mass heat exchanger.

    As I expected, there is no heating zone valve overrun of any kind - every time TPI cycles the boiler on and off for heating demand the zone valve also opens and closes. So as soon as the boiler goes off the zone valve immediately closes. This applies whether or not there is hot water demand, the only difference is if there is hot water demand the boiler stays on the whole time and only the heating zone valve is TPI modulated.

    After observing the system in action last night and today I've come to the conclusion that an overrun for the heating zone valve that I proposed in an earlier post may not actually be necessary for optimal performance. Although it's a perfectly valid approach, (and simulates a system with no zone valves for TPI control of heating) it might be trying to solve a problem that doesn't really exist.

    The idea behind it was to allow all the latent heat present during a TPI off phase to continue to circulate to the radiators instead of circulating around the bypass loop and being dissipated there. By doing this the average flow temperature drops.

    But what I've found in practice is that the flow temperature doesn't really drop significantly in each 10 minute cycle during the period where the zone valve closes (since the bypass loop can't radiate much heat and the boiler has a lot of thermal mass) so little energy is actually "wasted", it is just stored in the heat exchanger and bypass loop for the next heating zone valve TPI on phase when it continues to flow to the radiators.

    Furthermore, from initial monitoring it looks like proportional control on light loads (minimal heat loss through walls due to mild conditions) is actually better with the zone valve opening and closing on each TPI cycle rather than just cycling the boiler relay on and off and allowing pump overrun to continue to circulate to the radiators.

    This is born out by the absolutely flat temperature graph in the living room for over 3 hours tonight where it stayed between 20.86 and 21.08 the whole time with no sign of any cyclical oscillations. (As measured by a remote wall mounted DTS92) This despite the number of people in the room varying between 4 people and 2 people, and the door sometimes being open, both of which normally do make a noticeable change to the temperature.

    If I check a few days back before the conversion was done and the zone valves were added the same room was fluctuating between 20.64 and 21.75 with quite noticeable oscillations.

    I've been giving some thought to why this might be and I think I know - if the pump is allowed to circulate continuously (I have a very long 12 minute overrun on purpose for several reasons) and only the boiler is TPI modulated the actual average flow temperature that you achieve by doing this is somewhat unpredictable - it depends on the load drawn by the radiators, and how many previous cycles the boiler has been through at that duty cycle. (It is an inherently unstable balance, where a constant duty cycle will tend to cause either a continuously increasing or continuously decreasing flow temperature)

    However if you open and close the zone valve at the same time the boiler is fired and stopped then the situation changes. Now the average flow temperature coming out of the boiler will quickly increase and stabilise at the set flow temperature (for example 70) since only during a small proportion of the time will the radiators be drawing heat.

    Now instead of flowing constantly but with a varying flow temperature you have a higher more stable flow temperature but the flow is intermittent in duty cycle. This also controls the total amount of heat delivered to the radiators but in a more proportional way than an unpredictable change in average flow temperature - if you assume the flow temperature is fairly stable the heat delivered must be proportional to the zone valve TPI, so you get more precise control. It seems to work anyway, so I'm going to leave it as is for now!

    It's quite interesting sitting in front of the living room radiators - which are a string of three 800x600 panels in a bay window piped together and controlled by one HR92 - previously when the system was trying to maintain a low average temperature through TPI all three radiators would be at about the same surface temperature (since the flow was continuous) but that temperature itself would cycle up and down every few minutes in a somewhat unpredictable way from nearly cold to quite warm.

    Now the radiator on the supply side is getting moderately warm/hot but the one on the return side is almost cold. Because the flow period is short only the radiator on the supply side is getting a chance to warm up before the flow is stopped again - the far one doesn't heat up at all. The one that does heat up seems to stay at a very even temperature. Sounds strange but it does seem to give a better proportional control!

    When there is full demand and the valve is staying open all three radiators heat up as normal. I'll keep an eye on the behaviour of the system over the next few weeks to see how it behaves.

    I did have a few weird potentially wireless issues since installing the 2nd and 3rd BDR91's - they are all spaced out (Heh) but I have seen a couple of occasions where:

    1) The boiler relay and heating zone valve relay were both cycling on and off even though the controller had been in "Off" mode for over 10 minutes and no HR92 in the house was demanding heat. I ran around and checked every one individually to make sure they were all set at 5 degrees but the relays persisted in cycling on and off for at least 20 minutes before I gave up waiting. After a reboot of the controller the problem went away.

    2) I have seen occasions when the boiler relay comes on without either of the zone valves, for minutes at a time, and also occasions where the heating zone valve opens for minutes at a time without the boiler relay coming on. I can only assume this is a communication problem ? No problems have been reported, and the boiler relay is located in the exact same position it always has been.

    3) I have noticed times when the boiler relay and heating zone valve relay have got "out of sync" with each other's TPI cycles, where the boiler relay will cycle on for a minute then off, then the heating relay will cycle on afterwards, or there will be a partial overlap, making it look like there is an overrun in action - but really there isn't.

    To be fair this has mainly occurred at times not long after the power has been switched off to the heating (and all BDR91's) so it's possible that it might take them time to "sync up" their TPI cycles with each other. Most of the time the boiler relay and heating zone relay do stay in sync within a second or two.

    All relays give Excellent/5 signal readings. I'll keep an eye on this to see it happens again now that I have stopped fiddling with the system.
    Last edited by DBMandrake; 23rd October 2016 at 12:29 AM.

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