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Thread: Hot Water issue - Random Overshoots

  1. #1
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    Default Hot Water issue - Random Overshoots

    I've got a problem with the hotwater on a install and was wondering if anyone else has had the same issue? I haven't had it occur on mine or a few other systems just this one.

    The hot water randomly seems to overshoot by 10 to 15oc, but only once every week. The set point is 50oc with a 10oc drop and 0 min overrun time. It overshoots to around 63oc which I think at that point the actual Cylinder/High Limit stat is cutting off the motorised valve. I've tested the signal strength which is fine, and the Hot Water relay isn't opening with any other zones/calls for heat so that eliminates that. There is no boiler relay just BDR91s set up as heating & hot water valves.

    Rung Honeywell support which were less than helpful but have said to replace the Hot Water kit and to see if it sorts the issue.

    So has anyone else come across the issue? And also if you delete the hot water kit from the EvoHome can you just repair it without having to reset up your zones?

  2. #2
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    Could it be an Anti-Legionella function in EvoHome?

  3. #3
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    That was my initial thought, however there is no option for it in the settings (It would be a great idea though) plus I don't have the same issue with any other systems with the Hot Water control. It appears just to be this system. Unless this is a new feature i'm not aware of?

  4. #4
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    You can delete hot water without affecting the heating zones.

  5. #5
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    It could be the loose battery issue that several of us have had. Open the transmitter up and gently bend the battery contacts so they make a better contact with the battery and see if that helps.

  6. #6
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    Quote Originally Posted by mtmcgavock View Post
    I've got a problem with the hotwater on a install and was wondering if anyone else has had the same issue? I haven't had it occur on mine or a few other systems just this one.

    The hot water randomly seems to overshoot by 10 to 15oc, but only once every week. The set point is 50oc with a 10oc drop and 0 min overrun time. It overshoots to around 63oc which I think at that point the actual Cylinder/High Limit stat is cutting off the motorised valve. I've tested the signal strength which is fine, and the Hot Water relay isn't opening with any other zones/calls for heat so that eliminates that. There is no boiler relay just BDR91s set up as heating & hot water valves.
    Yes, I had this problem a lot when I initially installed my hot water kit. I had my hot water set to 50 at the time (I now use 54) and it was overshooting to 62 degrees on a fairly regular basis - about 5 to 10 times a week. Always to 62 degrees.

    I've discussed this problem at length and its (mostly) solution in these two threads and possibly in other threads, so you might want to skim read them both:

    http://www.automatedhome.co.uk/vbull...vershoot+water

    http://www.automatedhome.co.uk/vbull...vershoot+water

    I'll give you a quick summary before you read those for more detail. In my case there were two issues.

    1) The CS92A seems to be very sensitive to low battery voltage, and starts reporting in very infrequently or not at all if there is any problem with the battery voltage or battery contacts. This leads to hot water overshoots as the sensor doesn't report that the set temperature has been passed in a timely fashion.

    I've heard from a source that should know that it's a relatively common problem that the battery contact tension behind the PCB is inadequate from the factory on some units - the solution is to carefully remove the PCB board from the casing, (just push back a couple of plastic tabs at the top then carefully tilt it out) and gently bend up both the battery interconnects that press up on the underneath of the board about 2-3mm to increase their tension, then re-install the board. In my case this solved about 90% of the overshoots right away.

    I haven't tried it yet but I suspect that running the CS92A on Lithium batteries instead of the supplied Alkaline batteries may also help, because lithium batteries have both a higher initial terminal voltage than Alkaline (about 0.1 - 0.2 volts higher) and more importantly the voltage stays nearly the same until the cell is nearly exhausted before dropping, compared to Alkaline where there is a steady decline in voltage as the cell is depleted.

    Just in the last few weeks (after over 6 months of no trouble) I've started to have a few overshoots again and I took the batteries out (factory original Alkaline) and they measured exactly 1.5v - which should be fine for Alkaline, however I swapped them for some new Alkaline's of a different brand which measured 1.6v and the problem went away immediately and hasn't returned, which tends to confirm my voltage sensitivity theory. I would have tried some Lithium's but I didn't have any on hand at the time, but I think I will get some to give them a try.

    2) Changing the hot water set point, especially increasing it can lead to overshoots because the set point is "missed". Hard to explain why without going into a lot of technical detail - check those two threads for the explanation. The workaround is a few minutes after changing the hot water set point or differential, reboot both the controller and the CS92A (by unplugging the controller and removing its batteries temporarily, and removing the batteries of the CS92A temporarily) and this will fix this problem.

    3) Minor comms problems from the CS92A to the controller can cause some temperature update messages to get lost. The symptoms are less frequent temperature reading updates when the cylinder is heating and occasional overshoots. I spent a lot of time on this problem and it turned out to be the location of the CS92A. The CS92A and my 3x BDR91's are in the boiler closet and were previously mounted on the same wall in the same orientation stacked vertically with about 300mm between each one, which should be fine according to Honeywell's specs. The signal test from the CS92A was also always 5/5.

    Unfortunately I've found the signal test fairly useless at troubleshooting comms issues, as a good signal doesn't guarantee trouble free comms, nor does a lesser signal (like 3/5 or 4/5) indicate problems! What I ended up doing was moving the CS92A to a wall of its own (at 90 degrees to the original wall) in the boiler closet a bit further away from the BDR91's and at a 90 degree orientation to what it was before. Signal test now reports 3/5 or 4/5 and line of sight to the controller is not as good as before (it's now going through two walls instead of one to exit the closet) but despite this the overshoots went away. So it seems that the CS92A doesn't like to be too close to BDR91's, and the 90 degree orientation between them probably helped. (Depending on how the antennas are orientated inside the casing of the devices, a 90 degree orientation difference would cut any interference between the two down dramatically)

    BTW 62 degrees seems to be a safety trigger temperature for the CS92A sensor - it uses a battery saving algorithm to only send temperature updates when the temperature is either changing rapidly, or changing within the differential range or crossing the set point, however it also seems to always send updates when it reaches 62 degrees regardless of what the set point is, hence why in my case with my 50 degree set point it was always overshooting to exactly 62 degrees on the occasions that it did overshoot.

    Rung Honeywell support which were less than helpful but have said to replace the Hot Water kit and to see if it sorts the issue.

    So has anyone else come across the issue? And also if you delete the hot water kit from the EvoHome can you just repair it without having to reset up your zones?
    To delete the hot water kit you would unbind the BDR91's (15+ second press on the relays until they flash fast) and set stored hot water to none. Then you can bind the new ones. You'll lose your hot water schedule and set point but your heating zones will be unaffected. So take a note of the hot water schedule and hot water settings first.
    Last edited by DBMandrake; 29th June 2017 at 09:52 AM.

  7. #7
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    Quote Originally Posted by dty View Post
    It could be the loose battery issue that several of us have had. Open the transmitter up and gently bend the battery contacts so they make a better contact with the battery and see if that helps.
    In my case it was the tension between the contacts and the rear of the PCB that needed improving, I had previously tried increasing the contact tension to the batteries themselves and it made no difference.

  8. #8
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    Quote Originally Posted by blowlamp View Post
    Could it be an Anti-Legionella function in EvoHome?
    Best "it's not a bug its a feature" comment ever.

  9. #9
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    Out of interest, can anyone say how long their CS92A batteries (alkaline) are lasting?

    All Evohome HR92 valves I installed October 2015 have been replaced over the last few months (barring three upstairs rooms, little used) whereas the CS92A is still running on original batteries.

    And although I have seen sporadic comms failures on the sensor, strangely since I turned OFF the central heating, the CS92A happily behaves whereas I would have expected comms + battery issues by now.

  10. #10
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    Quote Originally Posted by killa47 View Post
    Out of interest, can anyone say how long their CS92A batteries (alkaline) are lasting?

    All Evohome HR92 valves I installed October 2015 have been replaced over the last few months (barring three upstairs rooms, little used) whereas the CS92A is still running on original batteries.

    And although I have seen sporadic comms failures on the sensor, strangely since I turned OFF the central heating, the CS92A happily behaves whereas I would have expected comms + battery issues by now.
    CS92A batteries last more than 2 years. I changed them after 2 1/2 years to see if it resolved the occasional comms sensor fault but it didn't. I also find that my HR92 batteries also last well over 2 years. I replace them when there is a warning on the Evotouch but I am still using up the old HR92 batteries in torches etc and they work just fine. Given that the cost of good AA batteries is c.25p for a bulk buy, I can see why some people just elect to change all the batteries just prior to the start of each heating system - particularly, if the property is not always occupied due to work commitments etc.

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