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Thread: Evohome overshoot and boiler control

  1. #1
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    Default Evohome overshoot and boiler control

    Hi all,
    I have recently installed evohome in my home. My setup is as follows-
    Oil fired condensing boiler.
    3 zone valves; upstairs radiators, downstairs radiators, hot water.
    21 radiators with TRVs.
    8x HR92s and 4x HR81s. All other rads using closed TRVs because we donít use the room (yet).
    2x BDR91 relays. One operates the hot water zone valve using the wireless cylinder sensor. The other operates both heating valves.
    NO boiler control. The relays operate the valves, then the valves signal the boiler when they are fully open.

    90% of the time it works perfectly, maintaining temps within 0.5 degrees. However it tends to overshoot in the mornings when most zones and HW all call for heat.
    My theory is that it does most of itís ďlearningĒ while the boiler is just ticking over to maintain room temperatures, and the boiler never gets properly hot. Then in the morning, when there is a larger heat demand the boiler gets a chance to get hotter than normal, confusing the system and causing the overshoot? Guesswork on my part.

    Would installing boiler control help here?
    I was wondering if this can be done using the two relays I already have. One to operate the HW, and the other as boiler control, if I manually override the heating valves to the open position. Obviously Iíll have to disconnect the boiler control signal from the valves.
    Or would I be best forking out for a third relay and do it properly?

    Lastly, I have two zones using a HR91 and a HR92 in the same room. I noticed in the bedroom this morning that the rad with the HR91 was hot while the rad with the HR92 was barely lukewarm. Is it bad practice to mix them in a room? Is it normal for the two rads to operate differently or should they heat up together and operate as one?

    Thanks for any advice.
    Last edited by johnk; 2nd November 2020 at 10:07 AM.

  2. #2
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    I forgot to mention, a new boiler, HW zone valve, TRVs and automatic bypass valve were all installed about a month ago before I installed the evohome system. All rads were balanced by the plumber at the time.

  3. #3
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    How long has the Evohome system been installed? As said in another thread it takes up to 3/4 weeks to fully learn your system.

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    It has been running for about 6 weeks now

  5. #5
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    Quote Originally Posted by johnk View Post
    Hi all,
    90% of the time it works perfectly, maintaining temps within 0.5 degrees. However it tends to overshoot in the mornings when most zones and HW all call for heat.
    My theory is that it does most of it’s “learning” while the boiler is just ticking over to maintain room temperatures, and the boiler never gets properly hot. Then in the morning, when there is a larger heat demand the boiler gets a chance to get hotter than normal, confusing the system and causing the overshoot? Guesswork on my part.

    Would installing boiler control help here?
    Probably not, although there are other advantages to a 3x relay system (like mine) such as hot water overrun.
    I was wondering if this can be done using the two relays I already have. One to operate the HW, and the other as boiler control, if I manually override the heating valves to the open position. Obviously I’ll have to disconnect the boiler control signal from the valves.
    Or would I be best forking out for a third relay and do it properly?
    If you do what you describe by eliminating the heating zone valve you'd probably see room temperature overshoots during hot water reheating unless you wired in hot water priority. Exactly what you don't want!

    This is because the HR92's will adapt their valve position and heat demand request to achieve a nice equilibrium - as soon as a hot water reheat cycle started the boiler will go to 100% duty cycle to heat the hot water cylinder thus the flow temperature would jump up suddenly and the rooms would overshoot. This effect is largely mitigated when you have a heating zone valve as the flow to the radiators will be intermittent and under control of the heat demands from the HR92's even when the boiler is going at 100%.

    Good news though on the overshoot front - the latest beta firmware (19.33) discussed and tested elsewhere in this form has a new feature called "load scaling" which can be enabled on relay controlled systems (not Opentherm) which goes a long way towards solving overshoot on "maximum demand" scenarios like initial warm up in the mornings, or when a zone is scheduled to come on when other zones are already at equilibrium. (This includes zones that are staggered automatically by optimal start)

    It adaptively scales down the load of individual zones based on prior heat demands from the zone and pretty much avoids the problem where one new zone is scheduled on, requests 100% heat and causes every other room in the house which was previously at a nice equilibrium to overshoot.

    It works so well that I don't even turn my flow temperature down in warmer months like I used to have to to avoid overshoots. I'm not sure when this firmware version will be released to everyone as an official update though, but it must surely be soon as we've been beta testing two different versions since about April...

    So when you do eventually get the 19.33 firmware update, look through the installer menu for a feature called load scaling and set it to "partial" mode, let it adapt over a week or so, and I think you'll see a big improvement in overshoot reduction.

    Lastly, I have two zones using a HR91 and a HR92 in the same room. I noticed in the bedroom this morning that the rad with the HR91 was hot while the rad with the HR92 was barely lukewarm. Is it bad practice to mix them in a room? Is it normal for the two rads to operate differently or should they heat up together and operate as one?

    Thanks for any advice.
    If the zone is configured as a single room zone (the default) both radiators should act pretty much in lock step as there is one temperature measurement taken for the whole zone and sent to both radiator controllers which usually results in nearly the same pin position, within a few percent. However the characteristics of the HR91 and HR92 in terms of the motor that pushes down the pin is quite different, so it's very possible that they won't "track" very well with each other when used in the same zone, and thus the radiator turn on thresholds may not be well matched.

    So I would be inclined to swap one of them out so you have the same model radiator controllers in a given zone in a multiple radiator zone. It's also possible that the valve bodies themselves are not a good match - are they both the same make and type of valve body of approximately the same age ?

    As a temporary workaround you could try changing the zone to a "multi-room zone" in the zone configuration - this will tell the two radiator controllers to measure their own temperature and act independently whilst still sharing the same scheduled set point. There are some drawbacks to using multi-room zones that I won't go into here but it's worth a try if you are waiting for another radiator controller.
    Last edited by DBMandrake; 3rd November 2020 at 02:33 PM.

  6. #6
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    First off, Thanks for the detailed response. Some great info.

    I think I'll leave the relays as they are but if I can spare the pennies I will install a boiler control relay eventually. Occasionally I can here the relay clicking on, and by the time the valve closes and then the boiler goes through its purge cycle, the boiler is only actually burning for a few seconds before the relay clicks off again. Probably not great for the boiler in the long run.

    Quote Originally Posted by DBMandrake View Post
    So I would be inclined to swap one of them out so you have the same model radiator controllers in a given zone in a multiple radiator zone. It's also possible that the valve bodies themselves are not a good match - are they both the same make and type of valve body of approximately the same age ?
    All of the TRVs are brand new and the same brand. They are South EMTRVB. I have no idea if they are decent quality or not; they were supplied by the plumber. But I will rearrange the HR91 & 92s so that each room only has one type.

    Thanks again for the info, and I'll keep an eye out for the firmware upgrade.

  7. #7
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    Quote Originally Posted by johnk View Post
    First off, Thanks for the detailed response. Some great info.

    I think I'll leave the relays as they are but if I can spare the pennies I will install a boiler control relay eventually. Occasionally I can here the relay clicking on, and by the time the valve closes and then the boiler goes through its purge cycle, the boiler is only actually burning for a few seconds before the relay clicks off again. Probably not great for the boiler in the long run.
    That's one of the disadvantages of the two relay configuration - there is no minimum on-time configuration option.

    It takes typically 15 seconds for a zone valve to fully open and close the switch contact before the boiler starts, and with no minimum on-time in low load conditions the heating relay can actually go back off again before the boiler even fires, or go back off just seconds after the boiler had started.

    With a dedicated boiler control relay the 15 second delay while the zone valve opens is eliminated (the boiler starts instantly since a relay directly controls it) and a minimum on-time is enforced. The minimum on-time minimum setting is one minute and can be increased in one minute intervals. I use the default one minute on my system.
    All of the TRVs are brand new and the same brand. They are South EMTRVB. I have no idea if they are decent quality or not; they were supplied by the plumber. But I will rearrange the HR91 & 92s so that each room only has one type.
    Let us know how pairing radiator controllers of the same type works out - I haven't seen any reports yet from anyone else mixing HR92 and HR91 in the same zone either positive or negative but my hunch is that sticking to the same radiator controller model within a given zone will solve your problem.

    Just be careful when you're swapping them between zones to properly clear the bindings at both the radiator controller end and the controller end.

    The radiator controllers have a button sequence to press to clear the binding (long press on BIND on the HR92 - not sure what it is on the HR91!) and also at the controller end. The best way to remove all bound devices from a heating zone on the controller side without losing the schedule is to temporarily change it to some other zone time (like zone relay) then cancel then change it back to a radiator controller zone type.

    This will leave it with no devices bound to the zone but the schedule will be intact. Then just bind your devices for that zone again.

    Failure to clear the bindings properly before swapping devices between zones will have "interesting" results to say the least.
    Last edited by DBMandrake; 6th November 2020 at 09:01 PM.

  8. #8
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    Thanks again.

    However, the plot thickens...
    I have been having an issue with the new boiler where occasionally it will lockout and have to be reset with the red button on the burner. maybe two or three times per week. sometimes never in a week. It's still under warranty, and the plumber said he will try a few things next time he is here.

    The problem is that the evohome has no idea that the boiler has locked out, and continues to call for heat, possibly for hours with no increase in room temp. It's only when I happen to notice that a room is cold that I reset the boiler. Surely this must be having an affect on how it learns how the zones respond to heat?

    On top of this, when it happened again yesterday, I turned off the heating using the quick actions on the controller, but the BDR for the heating valves stayed on. I'm not sure exactly how long it stayed on for but it was long enough to make me think that something was wrong.

    So, worrying about a binding issue, I removed all bindings from everything and factory reset the controller just to be sure. While I was at it, I swapped the valve controllers so that there is only one type in a room with two radiators. So complete system reset.

    I'll keep an eye on it for the next few weeks to see if it improves after it has had a chance to learn the rooms again.

    The problem is, if the overshoot issue is fixed, I won't know for sure if it was the system reset or swapping around the HR91 & 92s that did the job...

  9. #9
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    I recently fixed an overshoot issue on my setup by adjusting the maximum boiler temperature. The Evohome controller likes to request super hot water in the morning when the house needs to warm up from the night - on a NanoCUL + evofw3, I see an OpenTherm message for "Boiler setpoint: 81.0".

    As efficiency of an condensing boiler dramatically increases with <55C return water temperature, I went into the installer's menu and found a setpoint limiting feature and put it at 60C - it won't heat up water to more than that. While I'm there, I also put the boiler at 30% power max, or ~8 kW. Now the house heats up far more evenly, no more massive overshoots in the morning.

    On an on/off relay controlled boiler, you should be able to find this setting too. Modern houses are isolated well enough to run on tiny partial loads, 1m3 gas ~= 9.7 kWh, and I only burn 2-3 per day, with the new limits my boiler runs for about 2-3 hours per day, instead of burning it all in less than 1 hour with superheated water running through the system. Maybe I'll need to turn it up when it is really freezing but it works fine for now.

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