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Thread: Evohome Opentherm Boiler decision and set up

  1. #91
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    Actually, apologies, the Honeywell suggestion is a valve on the hot water side only for an Opentherm build. But I have a normally closed valve on the heating too so when the water heats up in summer, it doesn't feed the rads too

    Apparently this will be in the revised installation guide coming out soon according to their head of tech

  2. #92
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    Quote Originally Posted by 1animal1 View Post
    Actually, apologies, the Honeywell suggestion is a valve on the hot water side only for an Opentherm build. But I have a normally closed valve on the heating too so when the water heats up in summer, it doesn't feed the rads too

    Apparently this will be in the revised installation guide coming out soon according to their head of tech
    So basically you've got a standard S plan. So if you want to stop overshoots when your HW is reheating it's easy. Just wire the BDR91 for the CH motorised valve as follows (Assuming you've got 3 BDR91s, a CH, HW and Boiler Relay). From the C Port on the HW BDR91 take a wire to the A port on the CH BDR91. This will then give you HW priority. So when the HW is off or Heated up the BDR91 for the CH valve get's power to the A port. If there's a demand on the CH the BDR91 will close opening the MV.

  3. #93
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    What you describe I should already have... I'm using Opentherm so you don't use a BDR91 to fire the boiler. I have one BDR91 which is wired for the HW. The hot water valve is normally closed so when the heating is on, the is no power to the valves, so the flow will only go through the heating. When HW is requested and the BDR91 powers up, this opens the normally closed HW valve, and closes the normally open valve on the heating.... This is how Honeywell had my heating engineer wire it

  4. #94
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    Quote Originally Posted by 1animal1 View Post
    What you describe I should already have... I'm using Opentherm so you don't use a BDR91 to fire the boiler. I have one BDR91 which is wired for the HW. The hot water valve is normally closed so when the heating is on, the is no power to the valves, so the flow will only go through the heating. When HW is requested and the BDR91 powers up, this opens the normally closed HW valve, and closes the normally open valve on the heating.... This is how Honeywell had my heating engineer wire it
    Yes that makes sense, you'd said earlier on the Heating Valve was a NC valve not NO.

    Quote Originally Posted by 1animal1 View Post
    Actually, apologies, the Honeywell suggestion is a valve on the hot water side only for an Opentherm build. But I have a normally closed valve on the heating too so when the water heats up in summer, it doesn't feed the rads too
    And yes you are right i'd forgot you were using Opentherm, a Boiler Relay or Opentherm reciever does the same job in respect to that's what tells the boiler to fire instead of the Micro Switches from the valves.

    Well if that's the case then, I'm not sure why you're experiencing overshoots when reheating the hot water? Are you sure the Heating Valve is closing when HW demand is being requested.

  5. #95
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    I'll have to have a listen, see if there's flow....

    Difficult one isn't it, it should just be as straight cut, so that's the only thing it could be in theory. Although the heat cycle at the time of the overshoot was when downstairs was powering up for the morning. But all rooms were consistently over by 1-2C. Unless that's not a large enough shoot to worry about?

  6. #96
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    Quote Originally Posted by 1animal1 View Post
    I'll have to have a listen, see if there's flow....

    Difficult one isn't it, it should just be as straight cut, so that's the only thing it could be in theory. Although the heat cycle at the time of the overshoot was when downstairs was powering up for the morning. But all rooms were consistently over by 1-2C. Unless that's not a large enough shoot to worry about?
    I'd say that's a large enough Overshoot, especially if you are experiencing it every day. I tend to find all mine quite reliable (A mix using the HR92 sensors and remote sensors) some occasionally overshoot by 0.5 or 1 degree but not often.

    You should be able to tell if the Heating Motorised valve is closed or open on the little manual override lever on the valve when the HW is on.

  7. #97
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    Definitely worth digging deeper in that case.

    I would check the lever, but the normally open valve on the Heating doesn't have one. I did read the instructions when I sourced the parts and this is normal apparently.... I've tried to listen and feel the pipes, but this isn't telling

  8. #98
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    Quote Originally Posted by 1animal1 View Post
    I'll have to have a listen, see if there's flow....

    Difficult one isn't it, it should just be as straight cut, so that's the only thing it could be in theory. Although the heat cycle at the time of the overshoot was when downstairs was powering up for the morning. But all rooms were consistently over by 1-2C. Unless that's not a large enough shoot to worry about?
    If some zones are partially flowing maintaining a set point with a low heat output and other zones come on full to heat up to a set point some overshoot of the first zones can happen. This is because those first HR92's have adjusted their water flow rate for a low flow temperature to get room temperature equilibrium, then suddenly the flow temperature is going to maximum due to the demand from the other zones. Until the first HR92's sense a room temperature rise and react by closing down the room will overshoot a bit.

    0.5C overshoot is typical in these conditions although I have occasionally seen 1C. Any more than this and I would suspect one of the following:

    1) Maximum flow temperature set too high for the radiators/weather conditions.
    2) Individual radiators in the problematic zones (if there are specific problem zones) over-speced compared to other radiators in the house.
    3) Sticky or not fully compatible TRV valve bodies that are not allowing optimal control by the HR92.

    I had issues with significant overshoot/oscillation in some of my rooms that turned out to be due to due to some cheap TRV valve bodies that had become "sticky". I replaced all the valve bodies with Valencia's and also ended up having to set them to stroke mode 1 (long story, covered by another thread) and after that the overshoots in a zone caused by another zone coming on full is less than 0.5C and most oscillation are gone or are very minimal compared to before.

    I would try reducing the flow temperature a bit if you think it's too high. If that doesn't seem to help you could try setting the HR92's to stroke 1 mode - some valve bodies will work best with Stroke set to 0, some work best with stroke set to 1, it depends on the mechanical design of the valve and sealing washer inside.

    If the stroke mode is set wrong for your valves the calibration between when the HR92 starts to call for heat from the controller and when it actually starts to allow water flow can be a long way out - this can lead the zone being prone to overshoot, among other things. Although I'm not sure if it's necessary, when I change stroke mode I always remove the HR92, turn the black wheel anti-clockwise and refit it to force it to do a re-calibration.
    Last edited by DBMandrake; 25th December 2018 at 09:19 AM.

  9. #99
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    Morning chaps! Happy New Year....

    After investigating this for the last few days, it seems me adjusting the ON time for the CH to coincide beyond the main CH morning warm up, has solved the issue. Temps throughout the day appear exactly where they should be with the odd 0.5 degree overshoot - so pretty happy. This must be related to the max temp water remaining in the system from the HW request, filtering into the HW, I can't think of anything else.

    My only remaining question is, with my modified system having a 'normally open valve' on the CH and 'normally closed valve' on the HW - what happens if both the CH and HW request max heat within the same time slot - which does Evohome give priority to? Presume CH given my recent experience.
    Last edited by 1animal1; 1st January 2019 at 10:46 AM.

  10. #100
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    Quote Originally Posted by 1animal1 View Post
    Morning chaps! Happy New Year....

    After investigating this for the last few days, it seems me adjusting the ON time for the CH to coincide beyond the main CH morning warm up, has solved the issue. Temps throughout the day appear exactly where they should be with the odd 0.5 degree overshoot - so pretty happy. This must be related to the max temp water remaining in the system from the HW request, filtering into the HW, I can't think of anything else.

    My only remaining question is, with my modified system having a 'normally open valve' on the CH and 'normally closed valve' on the HW - what happens if both the CH and HW request max heat within the same time slot - which does Evohome give priority to? Presume CH given my recent experience.
    The system that I had in the house that I sold in July had an ATag is24 with OT control with hot water priority. The normally open CH valve closed whenever there was a HW heating demand. A HW demand would always result in a TMax Set flow temperature. CH would only demand TMaxSet when a zone/or zones were outwith their set temperature (c2C). Given your situation, I wouldn’t expect to see Evohome making an informed decision; ie, if there is a HW demand, I would have thought that TMaxSet will be demanded irrespective of any CH demand until HW re-heating is no longer required.

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