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Thread: Evohome Self Install

  1. #11
    Automated Home Legend paulockenden's Avatar
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    Quote Originally Posted by DBMandrake View Post
    Risk of scalding ?
    Yes, you're probably right. Unless there's a blend valve on the tank output.

  2. #12
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    There's a number of reasons, although it's part of the Unvented regulations that all heat sources have a cut out device in addition to the standard temperature control. And yes I have seen them cut out on with a boiler when something somewhere has failed, usually when someone else has fitted something incorrectly! But you are quite correct, it does take a number of devices to fail before it should ever really come into action, however there's no harm in having it there.

    It's also prevents it overheating if the CS92 fails, which they can be a bit problematic from time to time, although touch wood mine has been OK for a while.

  3. #13
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    Quote Originally Posted by paulockenden View Post
    Yes, you're probably right. Unless there's a blend valve on the tank output.
    For what it's worth I have a vented cylinder (which was originally just gravity circulation before I converted it to S-Plan) and I don't have an additional cutout, (technically it isn't required to have one) however I tend to have the flow temperature set to 65 in summer and 70 in winter, and never any higher than 75. When hot water heating is occurring it's not boosting the flow to 90 like many modern boilers do, it just stays at what it's set to for heating.

    I have an external digital flow thermostat I added connected in series with the original mechanical flow stat in the boiler, with the digital stat controlling the temperature and the mechanical stat turned up right up to 82 as a safety cutoff for the boiler in the event of the external stat failing. For a while I had the switch in the hot water zone valve configured to bypass the external stat thus boost the flow temperature to 82 during hot water heating, however I found it wasn't really necessary, and put a lot of unnecessary stress on the boiler which has a tendency to kettle going that high.

    So I ripped that boost wiring out and also set a high limit on my digital stat to stop anyone setting it higher than 75.

    So I do rely on the flow temperature limit preventing hot water getting dangerously hot - typically it can't get more than about 5C below the peak flow temperature so 65C when flow is set to 70C, and it does take it a long time to get those last few degrees.

    The problem with adding an additional safety stat in my circumstances is that they typically have an 8C hysteresis. So in order for it not to interfere with my normal 54C hot water set point I'd have to set it to at least 62C, or in practice say 65C - by which time its not really giving me any additional protection over relying on the flow temperature limit. On the other hand if my boiler was boosting to 80/90C for hot water heating as many do I would definitely fit an additional stat for the times when the CS92 malfunctions.

    I really wish Honeywell had not designed a hot water control system that relies on a wireless communication loop from a battery powered device (CS92) to a sometimes battery powered device (controller) back to the relay. Such as design is just never going to be 100% reliable.

    Instead of a CS92 and BDR91 there should have been a single hot water control relay box that had both the relay and temperature sensor input in the same box, mains powered and hardwired. The controller then just tells the box what the hot water temperature and differential should be, then the box can sense the temperature and control the relay directly without any intervention from the controller, just providing notification back to the controller so the controller is informed of what's going on. (And can control the boiler control relay if one is fitted)

    It's something Honeywell could theoretically do with a firmware update in the controller to support a new combined hot water control device, but I'm not holding my breath!
    Last edited by DBMandrake; 14th July 2018 at 08:09 AM.

  4. #14
    Automated Home Legend paulockenden's Avatar
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    I agree that the CS92 is quite a weak point in the Evohome system.

    Trouble with combining the sensor and relay in one box is that they might need to be in different places.

  5. #15
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    Realistically how far apart would they need to be ? In my case they are in the same boiler closet. Even if it was a cylinder in a loft space with a boiler on the ground floor you might need say 4 metres of figure 8 flex to extend the sensor.

    The relatively high resistance of the sensor used means that a few ohms of cable resistance would have negligible effect on the temperature reading, so I don't see it as a major issue in most installations, given the increased reliability and robustness it would offer.

  6. #16
    Automated Home Legend paulockenden's Avatar
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    Hmmm... the whole POINT of Evohome is no long cable runs.

  7. #17
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    Quote Originally Posted by paulockenden View Post
    Hmmm... the whole POINT of Evohome is no long cable runs.
    What about the 8 metre run of cable under the floor from the evotouch power transformer located in my wiring centre box in the boiler closet to the wall mount base in the hallway where the evotouch sits ?

    Not everything can or has to be wireless. It's enough that the radiator valves are wireless IMHO, I don't care if there is a bit of extra wire from my hot water cylinder to my hot water zone valve even if they weren't already both right next to each other.

    Besides, most traditional control systems that Evohome would be replacing would already have a hard wired connection from the hot water cylinder sensor back to the zone valve.
    Last edited by DBMandrake; 15th July 2018 at 08:34 PM.

  8. #18
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    Quote Originally Posted by paulockenden View Post
    Hmmm... the whole POINT of Evohome is no long cable runs.
    Is it? That was not what attracted me to Evohome at all. I liked the idea of a multi-zone control system that gave me flexible scheduling and remote access via my phone and (eventually) via my PC. I saw the wireless components as downsides, not as an attraction. I would have been happier to hard wire all my wall thermostats.

    It is the wireless element of Evohome that I see as its biggest weakness and the most frequent cause of problems with its operation. The hot water (DHW) control via a wireless battery-powered CS92 is the most extreme example of an unreliable component. I like DBM's suggestion of a mains powered combined BDR91 / CS 92 which would be a huge improvement.
    Last edited by Edinburgh2000; 17th July 2018 at 08:36 AM.

  9. #19
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    If you're a bit handy yourself, this might be an idea: https://www.youtube.com/watch?v=Sa18...szHaYC&index=1

    and here's another one: https://www.youtube.com/watch?v=H3kMedqDSdg
    Last edited by gordonb3; 18th July 2018 at 02:13 PM.

  10. #20
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    Quote Originally Posted by gordonb3 View Post
    If you're a bit handy yourself, this might be an idea: https://www.youtube.com/watch?v=Sa18...szHaYC&index=1

    and here's another one: https://www.youtube.com/watch?v=H3kMedqDSdg
    The issue is not so much that the CS92 is battery powered - it's a combination of unreliable wireless comms (for Evohome in general) and the battery saving algorithm in the CS92 that is very stingy about when and how often it sends temperature updates, so if a transmission is lost at an inopportune time (like the temperature update that would indicate to the Evotouch that the hot water is now up to temperature) it can be 20 minutes or more before another update is sent by which time a large overshoot can occur.

    Neither of these issues is solved by adding a mains operated battery substitute as the battery saving algorithm will still be in effect, and the device will still be using wireless comms.

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