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Thread: Boiler relay behaviour

  1. #1
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    Default Boiler relay behaviour

    We've put our heating back on full schedule due to the increasingly colder temperatures. Last season I didn't have our boiler relay set up, however this was successfully set up in Spring and has been working fine since (Apart from one blip where it had to be restarted).

    I've increasing noticed that when walking into the garage where the boiler is (Along with the heating relay for the MV) that the boiler is firing (Boiler relay - green light on) when no other BDR91s are open, if I stand and watch it can take anywhere from 30 seconds to 4 minutes perhaps before another relay opens and the heat is released. Whilst this isn't an issue as we have a bypass and a radiator in the garage which is usually open it seems a bit daft that the Evohome controller demands a heat source before opening a zone valve for instance.

    I wanted to know if 1) this is normal operation, and 2) whether anyone else has noticed this on their own system.

    System set up is:
    Boiler Relay
    Heating ZV Relay - HR92s on all rads. (Yes I know I technically don't need the ZV however if we ever sell it's easier to revert back)
    How Water ZV Relay
    Towel Rail ZV Relay
    Bypass valve fitted and garage radiator installed before all zone valves.

  2. #2
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    If the boiler relay cycles on by itself when both the hot water relay and heating zone valve relay are off or their cycles get out of time with each other then it is almost certainly intermittent comms between the controller and some of the relays.

    Even on my system where they are only 6 metres from the controller I see the boiler relay and heating zone valve relay get out of step with each other on rare occasions. Normally the controller keeps the boiler relay and heating zone valve relay TPI synchronised so that they switch on and off together.

    My suggestion would be to check things like spacing between the BDR91's - each other and nearby metal objects, any metal objects between them and the controller, any other devices that could be interfering with them etc.

  3. #3
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    After posting this last night I went to turn up the hot water temperature in the settings (As we have it hotter in the winter). Doing this made me take a look at the settings for the boiler cycles, I found that previous to having the boiler relay I had set the minimum on time to 5 minutes and 3 cycles an hour. Our boiler is 17 years old with a cast iron heat exchanger and a 4 minute pump over run. So it takes a good 2 minutes to get hot from cold. It seemed daft to bring the boiler on for 1 minute (Not that it always would however it could) and then have the pump over run for 4 minutes.

    So it made me wonder whether the Evohome controller was keeping the boiler relay on for a set period of 5 minutes, without opening any valves due to the minimum on time setting. I've changed the setting to 2 minutes to see if the same pattern occurs, this morning I was in the garage and the boiler relay was on and no other BDR91s. I ran for about 30 seconds and cut off, it's possible it was over running from the heating shutting down.

    In regards to the positioning of the BDR91s, they are probably closer than they should be. However they are mounted on a plastic surface protruding 10cm from the wall, and apart from this behaviour, I haven't had any issues with signal. As my heating MV is next to the boiler along with the relays I notice when they are 'On', on a system where you have the BDR91s apart it's possible you may not notice this behaviour.

  4. #4
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    Quote Originally Posted by mtmcgavock View Post
    After posting this last night I went to turn up the hot water temperature in the settings (As we have it hotter in the winter). Doing this made me take a look at the settings for the boiler cycles, I found that previous to having the boiler relay I had set the minimum on time to 5 minutes and 3 cycles an hour. Our boiler is 17 years old with a cast iron heat exchanger and a 4 minute pump over run. So it takes a good 2 minutes to get hot from cold. It seemed daft to bring the boiler on for 1 minute (Not that it always would however it could) and then have the pump over run for 4 minutes.
    I have a nearly 25 year old non-condensing boiler with cast iron heat exchanger too, and after a lot of experimentation with cycle time and minimum on time I found that the best settings were the default of 6 cycles per hour and 1 minute minimum on time, so I went back to those defaults.

    If you set the minimum on time too high then it increases the minimum heat demand that the HR92's have to call for before the boiler fires - any less heat demand and the boiler doesn't fire at all. (The heating zone valve does open though I think)

    This can make it difficult to maintain a steady room temperature under a light load like moderate weather or a single radiator running, leading to large room temperature cycles - this is because the room has to fall well below the set point to build up enough "heat demand" to exceed the minimum on time threshold - and when it does it will run until the temperature overshoots. A small minimum on time avoids this.

    Yes its true that the boiler won't heat up much for a single minute of on time due to thermal mass - however keep in mind that it is cumulative - so if it comes on for one minute every ten minutes it is still putting out roughly 1/10th of the boilers maximum potential output, or about 2kW and the temperature will steadily climb to an equilibrium. This short on time allows it to modulate the flow temperature under a light load.

    For cycle time - the only time you'd want to set it to 3 cycles an hour is a boiler that can't be cycled rapidly for example solid fuel. For a gas boiler 6 cycles is more optimal. 12 cycles I found was pointless because you're doubling the number of switchings per hour (more stress on the boiler) for no good reason. Also the minimum on time with 12 cycles per hour is still 1 minute but now its 1 minute out of every 5 minute cycle instead of 10, so you now need a 20% heat demand to trigger the boiler at all instead of 10% head demand with 1 minute and 6 cycles - which can lead to the same light load room temperature cycling issues as above.
    So it made me wonder whether the Evohome controller was keeping the boiler relay on for a set period of 5 minutes, without opening any valves due to the minimum on time setting. I've changed the setting to 2 minutes to see if the same pattern occurs, this morning I was in the garage and the boiler relay was on and no other BDR91s. I ran for about 30 seconds and cut off, it's possible it was over running from the heating shutting down.
    What should actually happen - as far as I know, is that the minimum on time setting only applies to the boiler relay. So if you have a three relay system (like I do) and you for example set minimum on time to 3 minutes and the current demand for heat was only equal to 2 minutes of on time then the heating zone valve relay would come on for 2 minutes but the boiler relay would stay off. Once the demand was over 3 minutes worth they would both come on and off together.

    What you describe your system doing with the boiler relay coming on but not the heating zone valve or hot water (?) relay isn't right. Something is not working properly there. Is there any possibility that the binding of the heating zone valve and boiler control relays are mixed up ?

    If heating is off and you have hot water turned on is it definitely the boiler relay and hot water zone valve relays that come on and not the heating zone valve relay ? (You do have 3x relays, right ?)
    Last edited by DBMandrake; 6th October 2017 at 08:00 PM.

  5. #5
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    I had the opposite view regarding the cycles times, when I first installed the system I found that the boiler in my opinion was cycling too much. The EvoHome was bringing the boiler on for 1 minute, 6 cycles an hour trying to maintain the temperature. In my instance this was causing a pump over run time of a minimum of 24 minutes (My garage was warmer than the house!). It was doing it's job fine in terms of temperatures, but personally I think on a older boiler less cycles is better. At the end of the day they were designed to be used on a standard clock and controls, not TPI controls. I would like to prolong my components as long as possible, along with reducing the amount of time my pump is over running.

    In terms of minimum on time, yes I agree with you. I had set it too high on 5 minutes, which is quite a long time for the boiler to be running. If I set it to 1 minute on 3 cycles an hour it's more than likely to run for longer than this anyway.

    Quote Originally Posted by DBMandrake View Post
    What should actually happen - as far as I know, is that the minimum on time setting only applies to the boiler relay. So if you have a three relay system (like I do) and you for example set minimum on time to 3 minutes and the current demand for heat was only equal to 2 minutes of on time then the heating zone valve relay would come on for 2 minutes but the boiler relay would stay off. Once the demand was over 3 minutes worth they would both come on and off together.
    I thought that in this situation the EvoHome Controller would wait until the demand was 3 minutes, then open them both at the same time. What is it achieving opening the Heating BDR91 earlier than the Boiler Relay?

    Quote Originally Posted by DBMandrake View Post
    What you describe your system doing with the boiler relay coming on but not the heating zone valve or hot water (?) relay isn't right. Something is not working properly there. Is there any possibility that the binding of the heating zone valve and boiler control relays are mixed up ?

    If heating is off and you have hot water turned on is it definitely the boiler relay and hot water zone valve relays that come on and not the heating zone valve relay ? (You do have 3x relays, right ?)
    No I have four relays. HW BDR91, CH BDR91, Towel Rail Circuit BDR91, and the Boiler Relay.

    Bindings are not mixed up, Hot water relay comes on and only the boiler relay comes on. Don't seem to have the same issue with the hot water and boiler relay. HW comes on, boiler relay comes on straight away, and then Hot water over runs for 2 minutes without boiler relay on as set in EvoHome settings.

    The issue isn't really that the CH relay is not opening, more than they don't seem to be very much in sync. It's like the EvoHome controller knows it needs a demand, turns the boiler relay on, then anywhere from 30 seconds to 3 minutes realises it need to open the CH BDR91. One theory is that it's waiting till a HR92 has opened? But that theory goes out the window when the CH BDR91 closes and boiler relay is still on for 30 seconds or so.

  6. #6
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    Quote Originally Posted by mtmcgavock View Post
    It was doing it's job fine in terms of temperatures, but personally I think on a older boiler less cycles is better. At the end of the day they were designed to be used on a standard clock and controls, not TPI controls. I would like to prolong my components as long as possible, along with reducing the amount of time my pump is over running.
    Is your old boiler a modulating boiler or does it only have an on/off flow temperature thermostat as mine does ?

    If so, keep in mind that the boiler is already cycling on and off every few minutes to maintain the flow temperature. Just because your wall thermostat is on constantly doesn't mean the boiler is. The burner will run constantly during the initial system heat up but once the radiators are up to temperature and the boiler can hit the flow temperature target it will be cycling on and off every few minutes anyway, probably with a differential of about 8 degrees if it is an old mechanical flow thermostat. (I've retrofitted a digital flow stat onto my boiler to get more accurate and repeatable flow temperature control as well as allowing me to see what it is)

    In this situation the additional TPI switching of the BDR91 doesn't result in that much more switching per hour (in general) it just ends up modifying the proportion of on time, since both BDR91 and flow stat have to cycle on at the same time for the burner to fire (logically AND'ed) so if their cycle times are about the same, total number of switches per hour will be only a little bit higher statistically. (I've measured it) If TPI was so damaging for boilers I don't think all the manufacturers would have moved to it.
    I thought that in this situation the EvoHome Controller would wait until the demand was 3 minutes, then open them both at the same time. What is it achieving opening the Heating BDR91 earlier than the Boiler Relay?
    You'd have to ask Honeywell that - I don't have an answer. Also when you only have a two relay system (no boiler control) cycles are fixed at 6 per hour and there is no minimum on time, which means that it can cycle on for such a short time that zone valves don't have time to open to fire the boiler...
    The issue isn't really that the CH relay is not opening, more than they don't seem to be very much in sync. It's like the EvoHome controller knows it needs a demand, turns the boiler relay on, then anywhere from 30 seconds to 3 minutes realises it need to open the CH BDR91. One theory is that it's waiting till a HR92 has opened? But that theory goes out the window when the CH BDR91 closes and boiler relay is still on for 30 seconds or so.
    I'm pretty sure we are just back to unreliable comms unfortunately.

    According to parts of the wireless protocol that have been reverse engineered, there is a synchronisation command that the controller sends periodically to the BDR91's to reset the "time zero" of the TPI switching cycles. If this wasn't done then their nominal 10 minute cycles (when using 6 cycles per hour) would drift out of time fairly quickly and it's obviously pointless to have boiler relay and heating zone valve coming on for 3 minutes each, but not the same 3 minutes!

    This is like keeping your clock from drifting too far out by using ntp or a DCF radio clock.

    I believe this synchronisation happens every 20 minutes, also a new heat demand transmission is made both when the heat demand changes, or periodically every 20 minutes if the heat demand stays exactly the same. (Which it would at 0 or 100% but unlikely to occur at in between values)

    Another wrinkle when comparing the TPI cycles of boiler relay and heating zone valve relay is that "time zero" seems to count from when the relay switches off, not when it switches on. So if a change from say 30% to 70% duty cycle is sent to both relays but one doesn't receive it for some reason then the on times will seem out of step between the two but the off times will still be synchronised. When this happens it may take at least 20 minutes for them to get properly back in sync again.

    I've seen the exact same symptoms you describe - boiler relay switching on without heating zone valve relay, or both switching on but with very little overlap, or even the reverse - heating zone valve switching on by itself and no boiler relay.

    However it happens rarely enough that I have only observed it a few times, and it usually rights itself within half an hour or so. And as far as I'm concerned it's caused by a lost wireless transmission - probably a collision, but I'd need to analyse it with an HGI80 to be sure.

    Where is your controller positioned relative to the BDR91's ? Is it possible to try re-situating it a bit closer (say a couple of metres away) on its dock for a few days to see if the synchronisation problems go away ? That would be one way to prove whether it is comms issues...
    Last edited by DBMandrake; 7th October 2017 at 08:18 PM.

  7. #7
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    Quote Originally Posted by DBMandrake View Post
    Is your old boiler a modulating boiler or does it only have an on/off flow temperature thermostat as mine does ?

    If so, keep in mind that the boiler is already cycling on and off every few minutes to maintain the flow temperature. Just because your wall thermostat is on constantly doesn't mean the boiler is. The burner will run constantly during the initial system heat up but once the radiators are up to temperature and the boiler can hit the flow temperature target it will be cycling on and off every few minutes anyway, probably with a differential of about 8 degrees if it is an old mechanical flow thermostat. (I've retrofitted a digital flow stat onto my boiler to get more accurate and repeatable flow temperature control as well as allowing me to see what it is)

    In this situation the additional TPI switching of the BDR91 doesn't result in that much more switching per hour (in general) it just ends up modifying the proportion of on time, since both BDR91 and flow stat have to cycle on at the same time for the burner to fire (logically AND'ed) so if their cycle times are about the same, total number of switches per hour will be only a little bit higher statistically. (I've measured it) If TPI was so damaging for boilers I don't think all the manufacturers would have moved to it.
    No it's not modulating (I've range rated it down to 12kw output), and yes in terms of the burner cycling you are right. It won't add much more cycles onto the burner switching on and off. However my boiler seems to eat PCBs for fun (We are on the third in it's lifetime), so the constant switching on and off with the relays i'd like to keep to a minimum, along with the 4 minute pump overrun I find 3 cycles an hour more suitable. Now if I had a new condensing boiler (Such as a Vaillant) i'd be having it on 6 cycles per hour, and with this I could turn the pump over run time down to 1 minute.

    Quote Originally Posted by DBMandrake View Post
    I'm pretty sure we are just back to unreliable comms unfortunately.

    I've seen the exact same symptoms you describe - boiler relay switching on without heating zone valve relay, or both switching on but with very little overlap, or even the reverse - heating zone valve switching on by itself and no boiler relay.
    You are most probably correct. I've monitored it tonight. I've had the heating off as we've been in and out working in the garage. Switched it from Heating off at 19:30 and within 1 minute both relays came on at the same time. Will continue to monitor over the following days.

    Quote Originally Posted by DBMandrake View Post
    Where is your controller positioned relative to the BDR91's ? Is it possible to try re-situating it a bit closer (say a couple of metres away) on its dock for a few days to see if the synchronisation problems go away ? That would be one way to prove whether it is comms issues...
    The controller is about 5 metres away in the hall, the garage is on the otherside and the boiler and BDR91s are at the back of the garage. It isn't far and we don't seem to have any comms issues with the HW and Boiler relay, it seems more it's the boiler and CH relay getting out of sync as you say.

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