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Thread: Why is Evohome calling for heat when all zones are below setpoint ?

  1. #1
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    Default Why is Evohome calling for heat when all zones are below setpoint ?

    I have evohome with eTRVs and a BRD91 controlling the boiler, plus another BRD91 opening a zone valve on some UFH (which is bound to the main unit).

    All "optimisations" are turned OFF.

    It mostly works, but it does some decidedly odd things at times. For example - this morning: all zones are well below their setpoints, AND economy mode is switched on (thus: -3deg C, or so I'm told).

    And yet: evohome is CFH roughly every 10 minutes, burning for about 1 minute, until sometime around 10:30 when just as mysteriously it stopped.

    Is there any way of figuring out WTF evohome believes it needs to CFH ?!

  2. #2
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    Others here are way more expert than me but I don't understand your question in that I would expect Evo to call for heat if zone(s) are below setpoint. Apologies if I've misunderstood your question.

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    Sorry, yes, brainfart - I meant _above_ the setpoint. That said, it's also doing the inverse behaviour.

    I.E: I have times where >1 zone has a current temperature below the setpoint temperature, and yet evohome is occasionally (every 10 mins) switching the boiler _off_ for 2 minutes.
    I have times where *all zones* current temperatures are above their setpoint temperature, and yet evohome is occasionally CFH (firing the boiler, every 10 mins) for about a minute.

    The latter, in particular, seems incredibly wasteful - opening the zone valve, the startup cost of wasted gas in firing up - to run for 60 seconds before turning off again. That can't possibly be enough time to pump heat to the furthest radiators.

    And this morning, even though 2 zones temp were 1C below setpoint (and one 0.5c) - the boiler was being turned off for ~30% (3-4 minutes out of 10). This seems utterly daft, given my monitoring shows the flow temperature hadn't even reached the boiler setpoint, let alone it having a chance to modulate down to some sort of constant burn.

    I was expecting to see something like "if any zone is cold, run the boiler until it isn't", and "if all zones are now at temperature, turn off the boiler". It seems to be either confused, or attempting to be too clever by half (like trying to vary on/off to somehow control speed of increment, which is frankly a losing game as it'll have zero idea what's happening in the other, non-evohome zones or even what the boiler flow temperature has been set to today).

    How is it supposed to be operating? Is this normal?

  4. #4
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    Quote Originally Posted by magnayn View Post
    Sorry, yes, brainfart - I meant _above_ the setpoint. That said, it's also doing the inverse behaviour.

    I.E: I have times where >1 zone has a current temperature below the setpoint temperature, and yet evohome is occasionally (every 10 mins) switching the boiler _off_ for 2 minutes.
    I have times where *all zones* current temperatures are above their setpoint temperature, and yet evohome is occasionally CFH (firing the boiler, every 10 mins) for about a minute.

    The latter, in particular, seems incredibly wasteful - opening the zone valve, the startup cost of wasted gas in firing up - to run for 60 seconds before turning off again. That can't possibly be enough time to pump heat to the furthest radiators.

    And this morning, even though 2 zones temp were 1C below setpoint (and one 0.5c) - the boiler was being turned off for ~30% (3-4 minutes out of 10). This seems utterly daft, given my monitoring shows the flow temperature hadn't even reached the boiler setpoint, let alone it having a chance to modulate down to some sort of constant burn.
    I recommend reading up on TPI or "Time Proportional Integral" controls - your expectation of how it should be working is a little naive and/or outdated. The behaviour you seem to be expecting is that of an antiqated mechanical thermostat, where if the room is below the set temperature the boiler runs continously until it is above the set termpature, and then turns off completely until the room falls below the set point again.

    This traditional control method is not comfortable or accurate because the room temperature ends up cycling up and down about 2 degrees every hour or so, and it is not efficient because you end up setting the average temperature higher so that the minimum temperature during the dips is not perceived to be too cold.

    TPI uses proportional control to maintain a more steady room temperature by varying the average boiler duty cycle over a period like 10 minutes. This proportional control in the case of Evohome applies across about a +/- 1.5 degree range around the set point.

    So if you pretend for a moment only one zone is active, if the room temperature is more than 1.5 degrees below the set point the boiler relay will be on continously, if its more than 1.5 degrees above the set point it will be completely off, and if it is within that +/- 1.5 degree band the boiler will be on intermittently with a minimum on time of 1 minute out of 10 and maximum of 10 minutes out of 10 to maintain the room temperature.

    If multiple rooms are active at once the highest heat demand from the different zones is passed to the boiler relay, and its then up to the other HR92's that need less heat to reduce the heat output of their own radiator by restricting flow instead.

    So yes, it's perfectly normal for the boiler not to fire continously if the room is only say 1 degree below the set point, and to fire intermittently if it is slightly above the set point.
    I was expecting to see something like "if any zone is cold, run the boiler until it isn't", and "if all zones are now at temperature, turn off the boiler". It seems to be either confused, or attempting to be too clever by half (like trying to vary on/off to somehow control speed of increment, which is frankly a losing game as it'll have zero idea what's happening in the other, non-evohome zones or even what the boiler flow temperature has been set to today).
    Why do you think it's a losing game ?

    It doesn't need to know what the flow temperature is, it will find its own equilibrium through a process of negative feedback. If zones are too cold the duty cycle of the boiler is progressively increased until the set point is reached, if they are too hot it is reduced until the set point is reached. There is an automated, simple learning process going on all the time. (It's actually just integration over time)

    In effect it is controlling the average flow temperature going through the radiators by preventing the boiler from being able to reach the set flow temperature when a partial heat demand is required, only when a high heat demand is required for example a room first warming up.

    If you fiddle with the flow temperature a lot then it will confuse the learning process a bit as it will have to keep readapting so you're best to leave it alone unless you really need to change it.

    As for not knowing what "non-evohome zones" are doing - how could it ? It's not psychic. If you have non-evohome zones then you are going to have some issues - the main one being that if all your evohome zones are satisified and don't need to call for heat you wont get any heat to your non-evohome zones.

    The same can be said of a traditional system with a single hallway thermostat which has been satisfied resulting in other rooms in the house no longer getting heat.

    Evohome doesn't suffer from this shortcoming because all zones are able to call for heat from the boiler, however if you have non evohome zones then it's not Evohome' fault that they can't call for heat and it doesn't know what they need.

    I ran with some zones being traditional TRV's for a while as I gradually built out my system and I don't recommend it. Put HR92's on all your radiators if you can.
    Last edited by DBMandrake; 24th October 2019 at 12:54 AM.

  5. #5
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    WhatDBmandrake says is spot on from a function point of view. Once my house is up to temperature (21 degrees) and things have stabilized then the flow temperature often drops to just 40 degrees depending on the outside temperature.

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    Thanks for this - the TPI bit makes sense. I'd eventually sort of guessed what it was trying to do by observation

    My 2nd (non-evohome) zone has it's own zone valve (and thermostat) as it's heating a completely separate annex (and separate heating 'circuit'). This does, however, mean that aggegate demand can suddenly vary quite significantly.

    It's not apparent to me how a TPI approach is meant to figure if your boiler has weather compensation, which is potentially adjusting the flow temperatures, of which evohome will be unaware.

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    Quote Originally Posted by magnayn View Post
    It's not apparent to me how a TPI approach is meant to figure if your boiler has weather compensation, which is potentially adjusting the flow temperatures, of which evohome will be unaware.
    Weather compensation will set a cap on the maximum possible flow temperature when the Evohome system calls for 100% heat, such as when any zone is more than 1.5C below the set point.

    However with a lesser heat demand due to all zones being within their proportional bands (or off) the TPI cycling of the relay will cause the average flow temperature to be less than that dictated by weather compensation. So evohome can effectively request an average flow temperature lower than Weather compensation provides, but not higher.

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