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Thread: Evohome - firing boiler for no reason?

  1. #1
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    Default Evohome - firing boiler for no reason?

    Hi,

    I have a question about the behaviour of Evohome I hope someone can answer. I having logging setup to record
    - Boiler firing
    - Pump Active
    - Themostat triggered (by Evohome controller)
    - Actual temp measured by HR92
    - Configured target temp
    - Actual target temp as chosen by the Evohome controller optimization
    - Boiler out and return temperatures

    What I see is the evohome regularly requesting the boiler to fire for just over one minute even when all the rooms are at or above temperature.

    This pattern can be seen in this image of 50mins worth of logging. Most rooms are over target, with just study and dinning room at temperature. In the 2nd chart the Evohome thermostat is represented by the grey bars (labelled "front" in the legend).

    What is the EvoHome controller doing, firing for this short space of time has little to no effect on the temperatures of the radiators or rooms, it just seems to be wasting gas warming up the pipework and the boiler?


  2. #2
    Automated Home Legend paulockenden's Avatar
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    That's TPI. It's why, over those 50 minutes, the rooms hold their temperature. Without this proportional input from the boiler they'd be cooling down.

    Looking at those nice steady temperature logs I'd say it's making an excellent job of it! With old style heating controls those room temps would be oscillating within a few degrees hysteresis band.

  3. #3
    Automated Home Legend paulockenden's Avatar
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    Incidentally, Paul, what are you using for your monitoring / logging / graphing? Looks like a nice little setup with lots of inputs!

    P.

  4. #4
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    That's an amazing setup you have for capturing all that data!

    As Paul Ockenden said, the whole reason those rooms are holding so steady is because of those short bursts on the boiler that just manage to keep the radiators ever so slightly warm, and stop the rooms from cooling down. It's a beautiful thing.

  5. #5
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    Thanks for explaining - I guess that all makes sense - it just seemed that the effect of the rads was so small.

    The monitoring is done using a couple of Raspberry Pi boxes. One monitors gas usage - my gas meter has a built in reed switch which ticks every 0.31KWh. One monitors the boiler and thermostats - optical sensors on the leds on the thermostat receivers and on the boiler leds for it being active and being lit. The temp sensors on the output and return pipes. The RPi script sends a network broadcast with the status of all the sensors every second or so. A c# program on my server picks these up and when something changes writes an entry into my sql server database. The service on my server also polls the Evohome API every 5 seconds and records actual, target and configured target temps for each zone. The reports are then done using SQL Reporting Services. (Evohome API I have made public here https://github.com/paul-777/Evohome.Net )

  6. #6
    Automated Home Legend paulockenden's Avatar
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    Quote Originally Posted by paul777 View Post
    polls the Evohome API every 5 seconds
    Eeek - you're not supposed to poll at anything shorter than 10 minute intervals!!!!

    P.

  7. #7
    Automated Home Legend paulockenden's Avatar
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    Quote Originally Posted by paul777 View Post
    Thanks for explaining - I guess that all makes sense - it just seemed that the effect of the rads was so small.
    The effect on the rads is EXACTLY the right amount! Your graphs demonstrate brilliantly just how good Evohome is at holding a room at a given temp.

    P.

  8. #8
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    Sorry that should have read 5 minutes! (5seconds would make for a very large database) But who defined the 10 minute rule? It seems Honeywell are resistant to actually publishing the API and associated rules about its use. I should also mention that it seems rather lame that I have to sign up to and use an external web service to access something which sits on my network - Honeywell really should work on providing a direct local interface, I can't believe it would be a difficult change to the controller firmware and would eliminate the security risks associated with having it talk externally.

  9. #9
    Automated Home Legend paulockenden's Avatar
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    The 10 minute thing was posted by one of the Honeywell guys here.

    You don't HAVE to use the web service. Honeywell does offer a local interface. It's called HGI80 and allows you to poll the data locally. Go as fast as you want to, then

    P.

  10. #10
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    Interesting but a google of HGI80 yields very little, a few references to it but no product documentation or anywhere offering it for sale - do you have any links that would help?

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