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Thread: Alternarive solution to Evohome hot water sensor lost comms error

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    Default Alternarive solution to Evohome hot water sensor lost comms error

    I think we have now proven categorically that the HW sensor lost comms is actually a radio message that the controller doesn't pick up from the Hot water sensor, for some reason.
    Removing the batteries, tightening the battery terminals etc on the Hot water sensor appears to fix the problem because the Hot Water sensor transmits a message on startup that the Controller picks up.
    I've even hard wired my CS92A, so that it doesn't need batteries but even that doesn't solve the problem.

    Yesterday I discovered another way to force the CS92A to send a message to the controller. Just pressing the HW BDR91 button on the relay seems to force the CS92A to transmit a message that the controller then picks up.

    So the next time you see a lost comms for the HW sensor, do this first before playing around with the sensor itself.

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    Quote Originally Posted by bruce_miranda View Post
    Yesterday I discovered another way to force the CS92A to send a message to the controller. Just pressing the HW BDR91 button on the relay seems to force the CS92A to transmit a message that the controller then picks up.
    I don't understand how that could possibly work. The CS92A is a send-only device. There's no way for it to know that you've pressed the BDR91 button.

    Unless.... pressing the button causes the tank to heat up a bit, which then wakes the CS92A so that it can transmit a new reading.

    p.

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    I have cured my hot water sensor problem with 100% success. Complete success! I removed it! New combi, no water tank. I guarantee doing this cures the problem. Not a cheap route though if it is all you want to do.

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    Fixed ours by swapping the batteries to Lithium, we’ve had no overshoots since.

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    I tried that some time back. There was a slight improvement for a short time in that the log did not report errors as much but it did eventually revert to the same frequency error as standard AAs.

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    Our behaviour was different, no faults logged just appeared to miss the message to turn the boiler off.

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    Quote Originally Posted by paulockenden View Post
    I don't understand how that could possibly work. The CS92A is a send-only device. There's no way for it to know that you've pressed the BDR91 button.

    Unless.... pressing the button causes the tank to heat up a bit, which then wakes the CS92A so that it can transmit a new reading.

    p.
    Spot on Paul, you need to have the CH going round the system so that when you press the HW BDR91 button the CS92A suddenly detects a temperature change which causes it to make a transmission, which otherwise wouldn't happen. The issue with the CS92A has nothing to do with batteries, terminal tension etc. It is just a shortcoming of using a radio protocol with no acknowledgement. So the CS92A has no idea that the Controller missed its rarely sent message.
    Some older Honeywell kit were programmed to send each message 2/3 times each, in the hope that one would succeed. Its quite bonkers actually that the larger Relay boxes have no radio TX capability. So they can only listen to instruction and don't say a word. The controller just has to "hope" the relay heard the instructions sent. The same is true with the HM80 mixer valve too. I can finally see why Honeywell quickly moved to make them obsolete. The newer BDR91 allow 2 way comms. They even changed the Radio test messages to accommodate the devices that can do 2 way messages vs those that can only receive messages.

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    Quote Originally Posted by daniel@paynefamily.co.uk View Post
    Fixed ours by swapping the batteries to Lithium, we’ve had no overshoots since.
    Mine has Lithium batteries - unfortunately it didn't help. If anything I'm seeing it more often now.

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    Quote Originally Posted by bruce_miranda View Post
    Spot on Paul, you need to have the CH going round the system so that when you press the HW BDR91 button the CS92A suddenly detects a temperature change which causes it to make a transmission, which otherwise wouldn't happen. The issue with the CS92A has nothing to do with batteries, terminal tension etc. It is just a shortcoming of using a radio protocol with no acknowledgement. So the CS92A has no idea that the Controller missed its rarely sent message.
    Some older Honeywell kit were programmed to send each message 2/3 times each, in the hope that one would succeed.
    Yes, the CS92A sends its updates too rarely (1 hour between updates if no changes have occurred) and does not send every message twice like the DTS92 does. It doesn't take many lost messages before the controller has heard nothing for hours and assumes a problem.

    Quite bizarrely, when the CS92A sends a battery level report or periodically queries the hot water set point and offset values, even though the controller responds to the set point/differential query it does not clear an outstanding comms "fault" !! Only a temperature measurement packet will clear the comms fault.

    When I see a comms fault on my hot water sensor now I just dismiss it and ignore it because I know that if I actually run the hot tap and cause the temperature to drop it will send an update and the fault will clear itself.

    These bogus comms faults could easily have been worked around in the controller firmware - increase the amount of time before assuming a problem and also accept a set point/differential query from the CS92A as a "ping" that says it is still alive!
    Last edited by DBMandrake; 16th February 2021 at 02:58 PM.

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    I am so very late to the DHW party - but lockdown job #2345 was to install it
    I have had nothing but errors/failures in the 2 days - which frankly has really put me off.

    The BDR91 associated with my CS92A - is as I type - flashing Red and has the green light on.

    Is this related?

    I am thinking possibly not, as the Evohome controller is not seeing the BDR91 when I RF test. BUT do I even bother if all I am going to get is hassles from this CS92A issue?
    The Heating BDR91 is in the same location and works fine, so perhaps I have a faulty BDR91?

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