New Install - Banging Head Against The Wall - Hot Water Sensor!
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Hi Dan,
Sorry to hear you've having these problems. You're not alone. It looks like you have researched the problem well. I'm sure you've read some of the threads I've posted about this issue in too.
Unfortunately after having the CS92A hot water kit for 3 something years and having spent countless dozens of hours troubleshooting this issue and talking to other people with the same problem I believe it is a systemic problem which cannot be solved. In other words there is one or more design flaws in the system.
I think there are actually two interwoven issues that interact with each other.
One is unreliable comms from CS92A to Evotouch - in short, sometimes a temperature update sent by the CS92A is not received and processed properly by the Evotouch as if it didn't "hear" the message. I've actually gone to the lengths of building a DIY receiver for the Evohome's protocol (NanoCUL with CC1101 receiver and software called evohome_rf) to independenty monitor and log all transmissions in the house for later analysis.
This is situated about a metre from the Evotouch (and operates receive only, so no chance of interference) so should be able to receive anything the Evotouch can and vica versa.
And after comparing these logs with the times when I see hot water issues the conclusion is unequivical - the Evotouch is missing temperature transmissions from the CS92A which my independent receiver is correctly receiving, confirming that the "missed" messages are in fact being sent. This was quite a surprise to me because before building the monitoring system I had always assumed the fault lay with the CS92A not sending a temperature update.
Whether there is an issue with the receiver built into the Evotouch or there is something a bit "funny" about the timing or quality of the transmission from the CS92A is hard to determine for sure, but at the end of the day the independent monitor receives the "missing" transmissions just fine.
In theory a collision with the Evotouch and CS92A both sending at once could cause the Evotouch not to hear the CS92A - however if that was the case neither would my independent receiver, which is much closer to the Evotouch than the CS92A, so I don't think that's the reason.
The second issue is the transmission schedule of the CS92A is "sub-optimal" shall we say. As you've noted, if the temperature isn't changing it will send an update once an hour. If the temperature is changing within the differential band, it sends frequent updates, however below or above the differential band it only sends infrequent updates with changing temperature.
This is very unfortunate. What seems to happen is that as the hot water reheats through the differential band it sends frequent updates and everything seems fine, however once it goes above the differential band (above 55c in your case) it seems the CS92A only sends one further update to say that the hot water temperature has been satisfied, then goes silent for at least 20 minutes despite the temperature still rising. That means if the Evotouch doesn't receive that one message (due to the reception problem described earlier) the reading will sit at 54c or whatever the previous reading was for something like 20 minutes. If you have a fast hot water cylinder the result is a major oveshoot.
This is a fundamental flaw in the power saving algorithm in the CS92A - when the temperature is above the differential band and still increasing it should not go into low power mode sending infrequent updates.
While the firmware in the CS92A is not field upgradable and hence can't be fixed without hardware replacement, it's the Evotouch which tells the CS92A what the differential range is, so in theory the Evotouch firmware could be modified to "lie" about the differential range to make the CS92A send fast updates over a wider temperature range. (for example it could lie and say your differential range was 45-65C instead of 45-55C) Whether that would cause compatibility issues with other devices that might monitor the hot water parameters I don't know.
Honeywell don't officially acknowledge any of these problems or that there might be a design flaw and I've basically given up trying to solve this problem now. It doesn't happen very often for me (maybe 2 or 3 times a month) but when it does it's certainly annoying. I've considered ripping out the hot water part of my Evohome system and replacing it with a traditional programmable timer.
Sorry to sound like a wet blanket but I don't think this problem can be solved, at least not by us end users.
Originally Posted by mtmcgavock
