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Thread: Evohome - boiler on VERY frequently

  1. #11
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    There is a potential for continual short cycling as follows:-

    1) Only one or two rooms need heat therefore the loop of water being heated is small
    2) With a boiler that only has no modulation (either on or off), then it is heating that water as fast as it possibly can. It will get up to temperature quickly causing it to stop and go into pump-overrun. For most boilers that is around 2 minutes. Then it fires up again to top up the heat which will be a short run..followed by over-run
    3) When the HR92 detects that room temperature has been reached, it goes into a "sustain" mode. It does this by flicking the BDR91 relay for short periods.
    4) When you overlay 2) and 3) together, you get a lot of on/off cycling

    I never thought much of this to start with because I assume modern boilers are designed for continual on/off and it has no real relevance. However, EVOHOME when configured in a different way has the ability to increase the minimum run time of 3) and the number of times it will do so in an hour. Even with this capability, the very fact you have a boiler designed for a large loop, heating up a much smaller one now, means that boiler will be cutting in and out over and over regardless.

    So the question is? Do we save ourselves a load of money with reduced gas usage.....and get enthusiastic nods from green enthusiasts....only to be clobbered with a 2000 pound replacement boiler bill 5 years earlier than expected?

  2. #12
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    Quote Originally Posted by jonstatt View Post
    So the question is? Do we save ourselves a load of money with reduced gas usage.....and get enthusiastic nods from green enthusiasts....only to be clobbered with a 2000 pound replacement boiler bill 5 years earlier than expected?
    absolutely !

    there is a setting (in advanced settings) called "min on time" mine is set to 5 minutes. I assume this is a minimum boiler on time ? I also assume this is to prevent short cycling with point 3) ?

    either way it's ignored in my setup as I get lots of 1 minute type boiler demand (you can tell it's boiler demand as the green light in the BDR91 lights up). Support haven't responded to my requests for clarification on this so I guess we'll never know.

  3. #13

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    The BDR91 makes a call for heat. The boiler is in control of how it fires. Whilst the 'green light' is on - this does not mean that the boiler needs to operate as the BDR tells it. The thermostat calls for heat, the BDR91 interprets that call, the boiler then chooses what to do with that call. The fact that the minimum 'on' is 5 mins - has nothing to do with the boiler firing. This is controlled by the boiler logic and circuits. An old cast iron boiler has no PCB so it will be very 'dumb' - a newer system will have a lot more intelligence.

    (Note: we see some older boilers take 45 secs to just heat up and fire - when min settings are set to 1 min - this means that the system only has 15 secs of heat to apply - just a note as this can make the system appear like it is more on & off than usual - again a reference to the boiler / manufacturer controls is useful here)

    Set parameters within the boiler itself ( modern high efficiency combi boilers , you can set a limit on the short cycling for example) can be tweaked (if you know what you are doing)

    To get this very very very granular level of control then a 'figure 4' setup is optimal (see appedix of the manual). (removes the heating valve from the Splan level and controls the system using the boiler relay)
    Last edited by Rameses; 13th March 2015 at 10:42 AM.
    getconnected.honeywell.com | I work for Honeywell. Any posts I make are purely to help if I can. Any personal views expressed are my own

  4. #14
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    Some very interesting information here. Am also reading through thw following post with interest, as it seems to be a similar problem:

    http://www.automatedhome.co.uk/vbull...-Evohome/page5

    Might seem like a daft idea, but last night I set up a video camera to record the status display of the boiler all night. I will review the footage in more detail this weekend but from a quick glance this morning, it seems the boiler is coming on for approx 1 minute every 10 minutes, all night. All HR92's were set to 5C overnight except our bedroom, which was set to 18C (we have a 4 week old baby so room needs to stay warmish).

    Also, had a quick look in the advanced settings. Am I right in thinking that as I don't have an actual boiler relay (boiler controlled by zone valves) then I can't set variables such as the cycle rate and minimum on time?

  5. #15
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    Rameses: isn't orange saying he often sees the BDR91 light coming on for 1 minute even though minimum on time is set to 5? Regardless of what boiler is connected and regardless of what logic/intelligence is built in the boiler, that bdr91 behavior is just wrong.

    I personally haven't seen this behavior by the way. When I have set minimum on time to 2 or 3, it usually respects that setting.

  6. #16

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    Quote Originally Posted by garmcqui View Post
    Some very interesting information here. Am also reading through thw following post with interest, as it seems to be a similar problem:

    http://www.automatedhome.co.uk/vbull...-Evohome/page5

    Might seem like a daft idea, but last night I set up a video camera to record the status display of the boiler all night. I will review the footage in more detail this weekend but from a quick glance this morning, it seems the boiler is coming on for approx 1 minute every 10 minutes, all night. All HR92's were set to 5C overnight except our bedroom, which was set to 18C (we have a 4 week old baby so room needs to stay warmish).

    Also, had a quick look in the advanced settings. Am I right in thinking that as I don't have an actual boiler relay (boiler controlled by zone valves) then I can't set variables such as the cycle rate and minimum on time?

    You can change the 'cycle' time in the installer menu>systems parameters > cycle rate. But the default is 6 cycles per hour. You could change down to 3 - but the system will dump more heat, less frequently. You cannot have Zero cycles per hour when there is a requirement for heat. 12 is used for electric heating usually - hence 6 is the recommended default.
    getconnected.honeywell.com | I work for Honeywell. Any posts I make are purely to help if I can. Any personal views expressed are my own

  7. #17
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    Quote Originally Posted by garmcqui View Post
    Some very interesting information here. Am also reading through thw following post with interest, as it seems to be a similar problem:

    http://www.automatedhome.co.uk/vbull...-Evohome/page5

    Might seem like a daft idea, but last night I set up a video camera to record the status display of the boiler all night. I will review the footage in more detail this weekend but from a quick glance this morning, it seems the boiler is coming on for approx 1 minute every 10 minutes, all night. All HR92's were set to 5C overnight except our bedroom, which was set to 18C (we have a 4 week old baby so room needs to stay warmish).

    Also, had a quick look in the advanced settings. Am I right in thinking that as I don't have an actual boiler relay (boiler controlled by zone valves) then I can't set variables such as the cycle rate and minimum on time?
    You can't set it with S or Y-plan models but Honeywell have not given any explanation as to why those options "couldn't" be enabled for this scenario, only that they haven't "yet" done so. We are also still waiting to hear what the defaults for cycles and run-time are in those modes.

  8. #18
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    Quote Originally Posted by Rameses View Post
    You can change the 'cycle' time in the installer menu>systems parameters > cycle rate. But the default is 6 cycles per hour. You could change down to 3 - but the system will dump more heat, less frequently. You cannot have Zero cycles per hour when there is a requirement for heat. 12 is used for electric heating usually - hence 6 is the recommended default.
    Thanks Rameses,

    This setting is available even with my S-plan setup?

  9. #19
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    Quote Originally Posted by erik View Post
    Rameses: isn't orange saying he often sees the BDR91 light coming on for 1 minute even though minimum on time is set to 5? Regardless of what boiler is connected and regardless of what logic/intelligence is built in the boiler, that bdr91 behavior is just wrong.

    I personally haven't seen this behavior by the way. When I have set minimum on time to 2 or 3, it usually respects that setting.
    Yes - that's exactly what I'm saying. I know the difference between evohome requesting heat and the boiler logic firing up the boiler.

    It makes no sense and seems inefficient to me ? Plus what is that min time setting ?

  10. #20

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    Quote Originally Posted by garmcqui View Post
    Thanks Rameses,

    This setting is available even with my S-plan setup?
    Yes - and the action of opening of the valve will fire the boiler (have the same at home)
    getconnected.honeywell.com | I work for Honeywell. Any posts I make are purely to help if I can. Any personal views expressed are my own

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