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Thread: Hot Water Differential setting

  1. #1
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    Default Hot Water Differential setting

    Just thought I should give readers an explanation on why the Domestic Hot Water [DHW] BDR91 relay might appear not to be working if you have several timed calls for hot water.

    My system was originally set so that Hot Water was timed to come on at 7.30 a.m. and again at 5.30 p.m. for an hour or so.
    About two weeks ago, I then added a third time slot for DHW around 12.30 p.m. but I soon noticed that the lunchtime call for DHW didn't always seem to show an increase in water temperature.

    I thought the CS92 sensor was not responding as the controller did not appear to be turning on the Hot Water BDR91. I checked the wireless signal quality to the CS92, which was fine showing five red LED flashes. I kept turning on the request for hot water via the Evohome app or controller but the BDR91 green LED would not turn on.
    After hours playing around with the controller/app/BDR91/CS92, I have found the problem.

    There is a parameter setting on the controller's installer settings for Hot Water labelled "Hot Water Differential". This is defaulted to 10⁰C.
    What this does is prevent the controller turning on the DHW BDR91 if the temperature reported by the CS92 sensor is within the differential of 10⁰C of the maximum setting, which is by default 60⁰C. So if water temperature is say 55⁰ in the cylinder, this is only 5⁰C from maximum required of 60⁰C, so no switch on command is sent.

    The solution is to change this Hot Water Differential to say 5⁰C if you want more than one morning and evening timed call for hot water. The temperature drop will naturally occur overnight and between morning and evening but not usually between morning and lunchtime. If you always want water turned on, you could even drop the differential to 0⁰C

  2. #2
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    Quote Originally Posted by killa47 View Post
    Just thought I should give readers an explanation on why the Domestic Hot Water [DHW] BDR91 relay might appear not to be working if you have several timed calls for hot water.

    My system was originally set so that Hot Water was timed to come on at 7.30 a.m. and again at 5.30 p.m. for an hour or so.
    About two weeks ago, I then added a third time slot for DHW around 12.30 p.m. but I soon noticed that the lunchtime call for DHW didn't always seem to show an increase in water temperature.

    I thought the CS92 sensor was not responding as the controller did not appear to be turning on the Hot Water BDR91. I checked the wireless signal quality to the CS92, which was fine showing five red LED flashes. I kept turning on the request for hot water via the Evohome app or controller but the BDR91 green LED would not turn on.
    After hours playing around with the controller/app/BDR91/CS92, I have found the problem.

    There is a parameter setting on the controller's installer settings for Hot Water labelled "Hot Water Differential". This is defaulted to 10⁰C.
    What this does is prevent the controller turning on the DHW BDR91 if the temperature reported by the CS92 sensor is within the differential of 10⁰C of the maximum setting, which is by default 60⁰C. So if water temperature is say 55⁰ in the cylinder, this is only 5⁰C from maximum required of 60⁰C, so no switch on command is sent.

    The solution is to change this Hot Water Differential to say 5⁰C if you want more than one morning and evening timed call for hot water. The temperature drop will naturally occur overnight and between morning and evening but not usually between morning and lunchtime. If you always want water turned on, you could even drop the differential to 0⁰C
    Hi. Forgive me but I think that the differential is working as it was designed to do. It is maintaining the temperature of the HW in a defined band whenever the zone is ON. I have mine set in the range 55 to 62C as my shower has a minimum HW temperature of 55C. Clearly, the greater the differential you set, the less the boiler cycles resulting in less energy usage.

  3. #3
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    Hi Hengus.
    Don't disagree with your comment. If you leave water on throughout the day, it will be maintained within 10C of your max.

    But for anyone choosing to have several time period settings for calling for hot water, the problem I experienced would manifest itself - I think.

  4. #4
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    What you describe is not a "problem", it's the system working as it should. If you have a differential of 10 degrees it means that if the temperature is within 10 degrees of your set maximum then there is no need for it to turn on again because it is already within the desired temperature range.

    Because you added an additional on period the hot water cylinder doesn't have as much time to cool as it did before, so it is still within the acceptable temperature range when the next hot water demand period is due. This is not surprising because a hot water cylinder can stay hot for a long time if hot water is not used - ours will easily stay hot all day (8+ hours) if no hot water is used.

    If a 10 degree differential is too much for you then reduce it a bit, but as already pointed out this means the cylinder will cycle on and off more frequently than is necessary. And don't set the differential to 0 degrees, unless you want your element constantly going on and off.

    No mechanical thermostat has a differential of zero degrees. Traditional mechanical hot water thermostats have a hysteresis (differential) of about 6-10 degrees. No traditional thermostat maintains the temperature to an exact temperature - they all switch off at the set temperature and do not switch on again until the temperature drops below the set temperature minus the differential.
    Last edited by DBMandrake; 31st January 2016 at 10:48 PM.

  5. #5
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    Appreciate further explanation given by DBMandrake and understand that and HenGus. It did "appear" as a problem for me exactly because the temperature was within an undocumented differential range until I discovered the default 10C setting. I was only adding the post if this helped any others who might set a mid-day time call for water and find the BDR91 green LED (or indeed the temperature) not behaving as might be expected - because of the reasons given by DBMandrake and HenGus.

    I thought it might just save other frustrated users not knowing about this differential. Apologies if I have muddied the water - so to speak.

  6. #6
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    First thing most people do, I suspect, when they have Evohome installed, is turn down the DHW differential. I think the default of ten degrees is just too wide.

    I also have mine set to five. That seems FAR more sensible. Ten is actually worse than most bimetallic strip (old-school) thermostats can achieve.

    P.

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    Reduced mine to 5c a long time back but what Evohome has enabled me to do is reduce the time DHW needs to be on for. Having seen how quickly it gets it to the right temperature I ended up knocking an hour per day from the on times.

  8. #8
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    I think that one of the lessons that Evohome users have to learn is that zoning is more than the old timer plus hall thermostat. For example, during last week's cold snap, I woke up in the early hours to hear the pump running. I had to remind myself that the system was just responding to the minimum room temperatures that I had set. Similarly, there is an argument to say that if HW is required throughout the day, then just set it to heat up 30 mins before it is needed and leave it on until 30 mins after the last shower/bath in the evening.

    With cold fill washing machines etc, our demand for HW during the day is low so I do still select 2 HW periods. Two hours on from when the HW went to OFF, my tank has only dropped by 1 degree so leaving the HW zone ON would have made absolutely no difference in terms of boiler use or cost.

  9. #9
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    I agree with G4RHL - I think 10C is too big a range. I took a shower this morning just when the DHW was about to go off. The temp dropped by around 20C - We have an unvented Megaflow cylinder but not sure on capacity (will check). If I had let the DHW run continuously, yes it would have gone back to 55C and used some energy maintaining that level. I prefer selecting the timeframe when I need DHW and keeping within a tighter differential.

    Still agree everybody's perspective - horses for courses I guess.
    Reflecting on all your comments - I will stick with 55C with a 5C differential, set three times a day.
    Thanks for enlightenment to all.

  10. #10
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    Quote Originally Posted by killa47 View Post

    Still agree everybody's perspective - horses for courses I guess.
    Reflecting on all your comments - I will stick with 55C with a 5C differential, set three times a day.
    Thanks for enlightenment to all.
    You may be better setting DHW to 60c to ensure there is no risk of legionnaire's disease. I believe 55c is too low and it can survive that temperature, yet it is the default setting I think for many boilers. It starts to die at 50c to 55c but can take linger - I think!

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