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

  1. #11
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    '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.'

    In the S-Plan schematic the pump and boiler are wired in parallel, so when the pump runs the boiler runs. So do you have a separate boiler relay to switch off the boiler when overrun is actuated? Without one when the boiler runs during overrun it will most likely fire thereby not allowing latent heat to be dissipated. This is why I've disabled the overrun (made=0) on my Y-Plan system, I think overrun is irrelevant when there is no direct boiler control via the Opentherm method.

    '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.'

    Also in the S or Y plan systems the demand is either DHW or HTG or both, but in either case when the valve are shut the boiler demand stops and power removed so this results in instant off control. Because those valves has the in-built switch that only opens and closes after valve actuation, and that switch provides power to the pump and boiler, the only way to achieve energy recovery/latent heat extraction is to separately control the burner while keeping the pump running, both S and Y plans don't allow that to happen.

    Currently I can't see a way of implementing Overrun and I wish Honeywell would explain its system function, such that latent heat can be extracted, however I believe an Opentherm controlled boiler would allow this to happen.

    If Overrun is set, it could be that when the demand temperature has been achieved the system continues to heat the load for the overrun time, but what would be the point as the loads would likely cool the water and re-ignite the burner so wasting heat generation and driving the room/DHW over-temperature.

    '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.'

    I'm not sure how the relay can be actuated and not provide power to the valves and after their actuation power to the pump/boiler, this suggest the BDR91 is switching on the LED but not the relay. Or vice-versa if a valve has actuated which is supplied with power via a BDR91, then the relay in the BDR must be on without the LED being switched on to indicate operation. Could be a BDR91 issue under certain circumstances.

    A lot of my queries stem from inadequate documentation of the system operation or the system use of parameters that can be set by the user/installer, maybe if I had attended a Honeywell training course I'd know. That said, often the odd Honeywell data sheet pops up that explains e.g. the protocols and communication methods, so there might be one out there.

  2. #12
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    Quote Originally Posted by g6ejd View Post
    In the S-Plan schematic the pump and boiler are wired in parallel, so when the pump runs the boiler runs. So do you have a separate boiler relay to switch off the boiler when overrun is actuated? Without one when the boiler runs during overrun it will most likely fire thereby not allowing latent heat to be dissipated. This is why I've disabled the overrun (made=0) on my Y-Plan system, I think overrun is irrelevant when there is no direct boiler control via the Opentherm method.
    Sorry I should have been more clear about my setup.

    I opted for a 3x BDR91 configuration instead of 2x BDR91 for the S-Plan conversion, so there is a hot water relay, heating relay, and a separate boiler relay.

    The boiler BDR91 fires the boiler via an external digital flow temperature controller which I retrofitted on the boiler about a year ago. (It's a 20 year old boiler with an old mechanical stat) The boiler BDR91 also goes to the trigger input on an external timer which powers the pump. So the pump is triggered by the boiler relay but has an overrun timer that is independent of the Honeywell system.

    "Hot water overrun" in the Evotouch can only be used if you use 3x BDR91's, or 2x BDR91 with OpenTherm. If you only have 2x BDR91 and no OpenTherm there is no way that it could keep the zone valve open for longer than it fires the boiler...

    Also in the S or Y plan systems the demand is either DHW or HTG or both, but in either case when the valve are shut the boiler demand stops and power removed so this results in instant off control. Because those valves has the in-built switch that only opens and closes after valve actuation, and that switch provides power to the pump and boiler, the only way to achieve energy recovery/latent heat extraction is to separately control the burner while keeping the pump running, both S and Y plans don't allow that to happen.

    Currently I can't see a way of implementing Overrun and I wish Honeywell would explain its system function, such that latent heat can be extracted, however I believe an Opentherm controlled boiler would allow this to happen.
    What you are describing is a two BDR91 configuration - in this case yes, hot water overrun can't be done because the boiler/pump are powered typically by the limit stop switches in the zone valves, so if the hot water valve remains open the boiler remains fired.

    However if you use 3x BDR91 (or 2x and OpenTherm) the boiler is not powered by the limit stop switches but by a separate BDR91 - now when you enable hot water overrun the Evohome turns the boiler relay off first, waits for the hot water overrun period and then turns of the hot water relay. It works just fine.

    Of course for it to be effective, your pump must have its own overrun which is at least as long as the hot water overrun period otherwise no water would be flowing.

    I don't use the limit switches in the zone valves to fire the boiler or pump - I do use the limit switch in the hot water zone valve as a flow temperature "boost" though - it is connected in parallel with the relay contacts of the external digital flow temperature controller I use - so when there is hot water demand the switch closes and bypasses the external flow temperature controller so I get the temperature set on the boilers own conventional stat - which I have set to about 78 degrees for a nice fast hot water reheat time.

    When the hot water zone valve is closed the limit switch opens, so the external flow temperature controller is able to function again and it is set at a lower temperature (55-75 depending on time of year) for central heating flow temperature.

    The limit switch in the heating zone valve I don't use at all.
    If Overrun is set, it could be that when the demand temperature has been achieved the system continues to heat the load for the overrun time, but what would be the point as the loads would likely cool the water and re-ignite the burner so wasting heat generation and driving the room/DHW over-temperature.
    Nope, that's not the way it behaves. From observation it works like this:

    a) If there is only hot water demand, boiler and hot water relays are on, when the target hot water temperature is reached the boiler relay is turned off immediately but the hot water relay stays on for the configured number of overrun minutes then closes. It relies on your pump (or pump control in the boiler) to have its own overrun facility as the water must continue to be circulated for this period of time for the overrun to actually achieve anything useful.

    b) If there is both heating and hot water demand (all three relays on) then when the hot water temperature is reached the boiler relay stays on (because there is still a heating demand) and the hot water relay turns off without any overrun delay.
    I'm not sure how the relay can be actuated and not provide power to the valves and after their actuation power to the pump/boiler, this suggest the BDR91 is switching on the LED but not the relay. Or vice-versa if a valve has actuated which is supplied with power via a BDR91, then the relay in the BDR must be on without the LED being switched on to indicate operation. Could be a BDR91 issue under certain circumstances.
    I think you're just confused here because you haven't realised that I'm using a 3x BDR91 configuration, or even that one is possible.
    A lot of my queries stem from inadequate documentation of the system operation or the system use of parameters that can be set by the user/installer, maybe if I had attended a Honeywell training course I'd know. That said, often the odd Honeywell data sheet pops up that explains e.g. the protocols and communication methods, so there might be one out there.
    I've done the free on-line training course for the Evohome (a while ago now) and while it's good and worth doing it's really just aimed at your typical heating engineer who just wants to get the install done and doesn't go into the intricacies of how it really works at a low level control and algorithm level... for that level of detail we just have to observe and reverse engineer the system and its protocols...I think Honeywell's attitude on this low level stuff is just "trust us, we know what we're doing", but I always like a bit more detail than that, even if I have to reverse engineer things myself.
    Last edited by DBMandrake; 23rd October 2016 at 12:36 PM.

  3. #13
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    OK, got it now, using three BDR91 will solve all of my observations and make for maximum efficiency of operation. With the 3rd BDR I can see how some of the energy saving functions can be implemented and I see the advantages of using the BDR relay for control than indirectly via the valves, I'm going to re-think my system design based on yours now. But I presume you maintain your system yourself, when the BGas engineer was here 2-weeks ago, he freaked out when he saw an Evohome, saying he could not fix the system and would need to call in an expert, until I pointed out it was only the 3-port valve that had failed, so he reluctantly agreed to replace it and I drove the heating when he asked for the boiler to be turned on/off. So to counter that, I'm now producing comprehensive documentation and drawings to help any maintainer.

    I'm going to wait a while before I install a 3rd BDR as I'm still getting communication errors that look like this:

    RESTORE 22-10-2016, 17:29, COMMS FAULT BOILER RELAY
    FAULT 22-10-2106, 20:39, COMMS FAULT BOILER RELAY
    RESTORE 21-10-2016, 17:29, COMMS FAULT BOILER RELAY
    FAULT 21-10-2106, 20:39, COMMS FAULT BOILER RELAY

    The repetitive fault(?) always gets triggered at 17:29 and always gets restored 3-Hr and 10-mins later, but that does not sound like a random communications fault to me and there are no schedule transitions during these times. I'm going to try a full reset.

  4. #14
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    Quote Originally Posted by g6ejd View Post
    OK, got it now, using three BDR91 will solve all of my observations and make for maximum efficiency of operation. With the 3rd BDR I can see how some of the energy saving functions can be implemented and I see the advantages of using the BDR relay for control than indirectly via the valves, I'm going to re-think my system design based on yours now. But I presume you maintain your system yourself, when the BGas engineer was here 2-weeks ago, he freaked out when he saw an Evohome, saying he could not fix the system and would need to call in an expert, until I pointed out it was only the 3-port valve that had failed, so he reluctantly agreed to replace it and I drove the heating when he asked for the boiler to be turned on/off. So to counter that, I'm now producing comprehensive documentation and drawings to help any maintainer.
    Yes apart from some work that we had done when we first moved in to get a barely functioning system back into usable state (leaking valves, a lot of single end feed microbore, no TRV's anywhere in the house etc) I've done all the maintenance and modifications myself including the external flow temp controller, pump overrun timer, installing the Evohome system last year and now the S-Plan conversion. I like to do as much as I can myself, and managed to do all the plumbing of the conversion as well as the wiring.

    Going from a 2x BDR91 to 3x configuration is actually very easy - instead of powering the boiler/pump from the two limit switches you simply power them from the 3rd BDR91, then in the Evohome installer menu you bind a boiler control relay - job done.

    It remains to be seen how beneficial a 3rd relay is in practice - it allows for hot water overrun but I have ended up setting that to zero anyway, as I don't like the temperature overshooting. It gives a bit more flexibility in the wiring plan and it does solve he problem of 15 seconds worth of boiler run time per TPI cycle being lost.

    Minimum on-time for TPI is one minute but if you power the boiler via the zone valve limit switches you loose about 15 seconds worth of boiler run time every cycle as the boiler won't even turn on until the zone valve fully opens, and then takes time to heat. With the three relay config the boiler fires at the same time that the zone valve starts to open, so is already warming up by the time the zone valve finishes opening.

    Other than that, I'm not sure what other benefits the independent boiler relay provides.

    I'm going to wait a while before I install a 3rd BDR as I'm still getting communication errors that look like this:

    RESTORE 22-10-2016, 17:29, COMMS FAULT BOILER RELAY
    FAULT 22-10-2106, 20:39, COMMS FAULT BOILER RELAY
    RESTORE 21-10-2016, 17:29, COMMS FAULT BOILER RELAY
    FAULT 21-10-2106, 20:39, COMMS FAULT BOILER RELAY

    The repetitive fault(?) always gets triggered at 17:29 and always gets restored 3-Hr and 10-mins later, but that does not sound like a random communications fault to me and there are no schedule transitions during these times. I'm going to try a full reset.
    I think someone mentioned in another thread, there is a periodic "keep alive" signal sent between some devices every few hours - I've found that if there is a fault with communication with a BDR91 that it usually takes about 3 hours for it to be flagged as a fault - so I'm guessing that there is a 3 hour keep alive signal that is sent even if there are no changes to heat demand to be sent.

    It took roughly three hours for boiler relay faults to appear when I had the system powered down in the last couple of days doing the conversion work.

  5. #15
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    How close together are your two BDR91s Dave? Only almost every time someone has posted about them giving comms errors, the problem has been fixed by moving them further apart. And away from HW tanks as far as possible.

    Given the quality of the system as a whole, I find the fact that the RF comms is so sensitive to position a bit shocking. Over the years I've tested plenty of wireless devices, including some horrible cheap Chinese tat, and none has had this problem. The BDR91s are particularly susceptible, but other bits of the system can be sensitive too - there's advice, for example, that when binding HR92s you move the controller a few feet away.

    I'm surprised that a company which has been making wireless devices for as long as Honeywell has can't do better than that.

    (Sorry Ramses and Top Brake!)

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    Paul, they are 22cm apart and it would have been so easy to make it 30cm or more to remove this possibility, that will be my next step. But for now, so I've recorded the time when the rest took place and now ill wait to see if it will run without a comms. fault, which currently I think it won't.

    The Honeywell data sheet says the system is built-around/based on ZigBee protocols/modules according to their documentation which are well known and reliable, usually used in systems that need over-the-air updates; so that figures, but the repetitive nature of the fault then restore log entries appears to me to be a bit more than a comms fault unless they are keep-alive signals that don't get acknowledged, I don't know.

    Yes, most other products on the market just don't care about siting or other signals or metal or anything, my Davis Weather station has run continuously on 868MHz for the last 6-years 24/7 and never had a single comms fault in the log, so is does rather point to a protocol issue, being it can't cope with errors of any type. All these modules with their 'open' receiver front-ends are susceptible to blocking from other RF sources and it appears to me these Honeywell units are no different, but why does it not recover the link and handshake after what would be a short transient RF interference period, or whatever is causing the issue, I just don't understand it.

    One thing I found interesting is at 12:00 today I rest the controller and took the batteries out, then was on my way to the BDR91's when I heard a radiator whirring and then the heating came on, I was intrigued to see the heating BDR91 had turned on and the boiler of course was running, so the system was working autonomously from the controller that was definitely non-operational, maybe it's some form of providing heating during a controller fault, which is good if so.

    Dave

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    Where did you see that the system is based around ZigBee?

    I thought it predated ZigBee. In fact I'm pretty sure Hometronic (which first used this comms system) was around in 2003-4, whereas ZigBee wasn't announced until 2005, and products didn't hit the market until sometime after that.

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    https://www.google.co.uk/url?sa=t&rc...5QGJzSSqDQVcaw

    Although the syntax makes it a little vague, but my reading is they used Zigbee and/or Z-Wave protocols. 'It is a well controlled relatively ‘quiet’ band used by manufacturers broadcasting ZigBee & Z-wave protocols and it is the protocol adopted by Honeywell whose bespoke protocol Ramses II works in this area.

    Could have been written:

    It is a well controlled relatively ‘quiet’ band used by manufacturers broadcasting ZigBee & Z-wave protocols and the bespoke protocol adopted by Honeywell Ramses II also works in this area.

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    Quote Originally Posted by DBMandrake View Post
    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.
    Think I might have discovered the underlying cause for this, and it was operator error!

    Today while poking at the system testing things I noticed the boiler relay cycled on for about 3 minutes but neither hot water or heating zone valve relays came on, thus the boiler uselessly heated the automatic bypass loop. I knew it wouldn't be hot water demand so it was the heating zone valve that should have turned on.

    Then I suddenly realised what had happened - I had just a few minutes previously used the manual override button on both zone valve relays to open and then close the zone valves manually.

    The button on the relays whether used to turn on or off the relay acts as a kind of "temporary override" which eventually resumes normal operation automatically. At the time the relay should have turned on it must have still been in the override state. After about another 10 minutes they started working again.

    DOH! The question is (and perhaps top brake or Ramses can answer this) how long does the manual override from pressing the button stay in effect ?

    Is it a fixed time period like 10 minutes, or does it only last until the controller sends a change of heat demand ? I think it's the latter, but I'm not certain.

    If I only had a one or two relay configuration I would have never noticed a missed relay activation, it's only because the three relay configuration normally has both a zone valve and boiler relay come on and go off at the same time that one of the relays not responding was noticed.

  10. #20
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    Quote Originally Posted by g6ejd View Post
    https://www.google.co.uk/url?sa=t&rc...5QGJzSSqDQVcaw

    Although the syntax makes it a little vague, but my reading is they used Zigbee and/or Z-Wave protocols. 'It is a well controlled relatively ‘quiet’ band used by manufacturers broadcasting ZigBee & Z-wave protocols and it is the protocol adopted by Honeywell whose bespoke protocol Ramses II works in this area.

    Could have been written:

    It is a well controlled relatively ‘quiet’ band used by manufacturers broadcasting ZigBee & Z-wave protocols and the bespoke protocol adopted by Honeywell Ramses II also works in this area.
    Yes it is poorly worded but it looks clear that what the mean to say is that they use the same 868Mhz frequency band as ZigBee and Z-wave, but using their own proprietary protocol.

    How well Honeywell's protocol stands up to interference from ZigBee and Z-Wave is an interesting question if it pre-dates both of those other protocols development, and we know that it is a largely acknowledgement free protocol...

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