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Thread: What would you like to see in evohome? (have your say)

  1. #281
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    All of which would require firmware updates for the HR92s, so never going to happen (at least, not retrospectively for existing customers).

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    Automated Home Legend paulockenden's Avatar
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    I think it would probably require a CPU and RAM update too! ;-)

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    Quote Originally Posted by dty View Post
    Zone names longer than 12 characters. I mean, come on...

    Master Bedroom
    Guest Bedroom
    etc.

    I use "Master Bedrm", but that totally confuses Alexa because whatever I try and say, it hears "master bedroom" (of course) and then can't match it to any zone.
    You can edit the names in the iPhone app so they have more than 12 characters. They will still display on the controller with just 12 characters, but Alexa should pick up the full name (not got Alexa so I can't be sure) as the API returns more than 12 characters.

  4. #284
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    Quote Originally Posted by RichardP View Post
    You can edit the names in the iPhone app so they have more than 12 characters. They will still display on the controller with just 12 characters, but Alexa should pick up the full name (not got Alexa so I can't be sure) as the API returns more than 12 characters.
    That's nuts! But it works. Thank you!

  5. #285
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    Quote Originally Posted by paulockenden View Post
    Not really. I was thinking how weather comp would work best with a system like Evohome.

    As you say, traditionally it dials back the flow temperature of the boiler on warmer days, but that's useless for Evohome because the HR92s (or whatever) have 'learned' how to heat a room based on the flow temp they see from the boiler. Start to mess with that and things will get really messy. A bit like how if you adjust your boiler stat you'll start to see overshoots and more ocscillations than normal for a few days on your temp graphs.

    No, with Evohome I think if weather compensation were to be done it would need to be at zone level.
    I couldn't disagree more, and I still don't think you're appreciating why controlling the flow temperature is the right way to do weather compensation and why trying to apply weather compensation on a zone by zone basis just won't work.

    The only way it could be applied on a per zone basis would be to inform the HR92's of the outside temperature, and then what would they do with this information ? All they could do is vary how much they open the valve, but they already do this now based on feedback from the room temperature. It does absolutely nothing to help with overshoots, and here's why.

    The fundamental issue is that controlling a radiators average surface temperature by varying the flow through it with a valve is a very imprecise control method which only has a certain useful range of adjustment (modulation ratio, if you will) even if you have a geared down motor adjusting the pin.

    If you need more heat than you can achieve by fully opening the valve (because the flow temperature is too low) you won't be able to reach your target temperature or it will take a very long time to get there. Conversely if you only need a small percentage of the heat available with the valve fully open (the flow temperature is too high) it becomes uncontrollable and imprecise. The valve only needs to be fractionally open to achieve the necessary heat output and the tiniest error in the valve position will cause a large error in the heat output from the radiator.

    This leads to a very unstable system where the tiniest change in the valve position triggers an overshoot in the room temperature. (Increasing the flow temperature increases the "loop gain" of the feedback loop formed by the entire system, which at some point will lead to instability as the response time of the radiator is so slow)

    Couple to that the large thermal mass of radiators (they stay hot for a long time after the flow stops) and the fact that an HR92 only samples the temperature and makes and adjustment around every 4 minutes, doesn't actually know what flow temperature is passing through it, and also doesn't know exactly what pin position the valve starts to flow, and overshoots, oscillation and poor temperature control are inevitable and unavoidable if the flow temperature is too high for the conditions, such as large radiators in warm weather.

    No amount of informing individual HR92's of the warm temperatures outdoors will help with this situation when their only real avenue for control is to vary the valve position. The only solution is to reduce the flow temperature when the outdoor conditions are warm and heat loss of the building is low. This reduces the overall loop gain of the system, reduces the amount of heat stored in the radiator and helps minimise or prevent overshoots and make the temperature more controllable.

    If the HR92 does make a "mistake" with its adjustment at the lower flow temperature the consequences are less severe - the overshoot may be very small and not lead to an ongoing oscillation as it would with a high flow temperature where it keeps over compensating.

    As for the HR92 being confused by a changing flow temperature - I don't believe that's the case at all, provided that the flow temperature does vary smoothly in proportion to the outside conditions. Overshoots following a manual increase in flow temperature are a result of a "step change" in the flow temperature that doesn't track the heat loss change in the house - as the weather has got progressively colder you've left the flow temperature the same and forced the HR92 to do the compensation instead, then a few weeks later you make a sudden change of 5-10 degrees, now the HR92's are overcompensating so you experience an overshoot until they learn again.

    However if that flow temperature varied gradually as the weather gradually changed then there is no sudden step change, the weather compensation has done most of the compensation for you, leaving to the HR92 needing to do less compensating for outdoor conditions. (The change in required valve position to achieve the target room temperature under different outdoor conditions will be much less with properly adjusted weather compensation than no weather compensation at all)

    This is achieved by reducing the heat input into the room (for the same radiator valve position) by the same amount that the heat loss through the walls has reduced.
    That way it could take account of how sunny weather affects south facing zones more (thermal gain), whereas cold weather is more likely to affect rooms on the other side of the house.
    But weather compensation isn't trying to (and can't) account for differences in the temperature on different sides of the house due to direct sunlight, but it doesn't need to. All it needs to do is adjust the flow temperature smoothly in a way that is proportional to the average heating needs of the house, and the HR92's will deal with differences in the sunny and shaded side of the house - just as they do now without any weather compensation.

    The point is to bring the required heat output of most of the radiators in the house into the portion of the HR92's control range that is accurate rather than trying to have them operate with their valves nearly completely closed or fully open due to too high or too low a flow temperature.
    Last edited by DBMandrake; 30th June 2017 at 07:39 PM.

  6. #286
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    Quote Originally Posted by DBMandrake View Post
    The only way it could be applied on a per zone basis would be to inform the HR92's of the outside temperature, and then what would they do with this information ?
    At the moment they build a very simple relationship of how much boiler output is needed to raise a room by a given amount. I'm not even sure it builds an internal graph - it's probably just a single parameter - very simple. That's why pissing with the flow temp is going to break things big time. Have you ever tried adjusting your boiler flow rate then monitoring the zone temp graphs for the next week or so? It isn't pretty.

    I'm suggesting that the HR93 (or whatever) could be given the ability to auto-generate proper compensation curves. So in the way that Evohome moves overall heating control from system to zone level, the same principle could be applied to weather compensation.

    P.

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    Quote Originally Posted by paulockenden View Post
    At the moment they build a very simple relationship of how much boiler output is needed to raise a room by a given amount. I'm not even sure it builds an internal graph - it's probably just a single parameter - very simple.
    They're a lot smarter than that...

    As far as I can tell (I can only observe and analyse the behaviour, not look at the code...) each HR92 is a full PID (proportional/integral/differential) controller, with automatic self tuning of at the very least the differential term. (which controls overshoot tendency)

    By adjusting the differential term it can compensate for overshoot induced by thermal lag of the radiator etc - when it learns not to overshoot it is increasing the differential term so that it starts closing the valve sooner, before the target is reached when the temperature is rising quickly, allow it to "coast" to the target instead of overshooting it.

    The important thing to keep in mind is that this compensation has a limited range of effectiveness and there will be limits on how much it will tune this parameter - if the temperature rises too quickly and continues to rise too far after the radiator is closed (as it will when the flow temperature is too high) then it will be unable to compensate so there will be an overshoot.

    The integral term controls how quickly it adjusts for a change in offset error - offset error being that a different valve position is required to get the same measured room temperature when the heat loss of the room changes. I can't tell whether the integral term is self tuned or not - it may well be fixed.

    The proportional term mainly controls how far the valve will turn immediately in response to a change in set point. This may also be fixed and not self tuned - although it seems to differ between stroke 0 and stroke 1.

    I'm not sure how familiar you are with PID controllers but if you've not looked into them before you'll probably find it interesting reading and it will help to explain how the HR92's behave.

    (Pretty heavy on the maths, but this is quite good: https://www.allaboutcircuits.com/tec...using-matlabs/ )
    That's why pissing with the flow temp is going to break things big time. Have you ever tried adjusting your boiler flow rate then monitoring the zone temp graphs for the next week or so? It isn't pretty.
    I think I explained that adequately in my previous post - you're introducing a large step change which will of course cause a temporary instability until the system settles down. (Look up step response of a PID controller)

    If the flow temperature adjustments are small, smooth and inversely proportional to the actual heat loss of the building it won't cause a temporary instability, it just becomes a system wide "feed forward" loop outside of the individual feedback loops of the individual zones.
    I'm suggesting that the HR93 (or whatever) could be given the ability to auto-generate proper compensation curves. So in the way that Evohome moves overall heating control from system to zone level, the same principle could be applied to weather compensation.
    And my argument is that it's not that the HR92's can't learn properly - as self tuning PID controllers they are perfectly well equipped to learn the changes that result from outside temperature changes, the issues is on the control side not the input side - the valve that they operate and the thermal mass of a radiator conspire to make it impossible for the HR92 to smoothly and accurately control the heat into the room with excessive flow temperature regardless of what information they have available at the input side or clever algorithm they might employ. The system becomes inherently unstable, and that's why you see large overshoots and/or ongoing oscillation.
    Last edited by DBMandrake; 30th June 2017 at 08:21 PM.

  8. #288
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    But for weather comp to work properly it wouldn't be small changes would it? We live in the UK. It can be low twenties one day, ten degrees the next!

    If you're only going to make tweaks so small that the TPI control in the HR92 will cope then I wonder whether it's worth bothering at all? After all, at that micro level TPI will itself adjust for external conditions, as the Honeywell dudes have told us so many times.

    I'm envisioning something more radical. And like I said, you'll bu&&er things up big time if you make radical changes to the boiler output / flow temp.

    I think we're trying to achieve different things!

  9. #289
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    Quote Originally Posted by paulockenden View Post
    But for weather comp to work properly it wouldn't be small changes would it? We live in the UK. It can be low twenties one day, ten degrees the next!
    You don't just change the flow temperature one degree for every degree of outside temperature change though.

    That's where the slope adjustment comes in. See page 4 here for an example:

    http://viessmanndirect.co.uk/files/8...ather_comp.pdf

    The slope required basically depends on how well insulated your house is, in a well insulated house the slope will be pretty flat so it will take several degrees of outside temperature change to change the flow temperature by 1 degree. Or if you have a badly insulated house you'd need a higher slope that affects the flow temperature more.

    So in the well insulated example outside temperature going from 20 degrees to 5 degrees might only affect the flow temperature by 5 degrees for example. By adjusting the slope and offset you can emulate the change that you would have manually made previously.
    If you're only going to make tweaks so small that the TPI control in the HR92 will cope then I wonder whether it's worth bothering at all? After all, at that micro level TPI will itself adjust for external conditions, as the Honeywell dudes have told us so many times.
    Small changes is probably the wrong way for me to describe it. What I'm trying to say is that the changes are smooth, continuously variable, and directly proportional to the conditions rather than being a step change made manually and semi-arbitrarily by a person who has decided after a few weeks that it's finally too cold for their summer flow temperature to do the job or vica versa.

    The step response of most feedback systems is not nearly as stable as the response to a smoothly varying input signal, so you're unlikely to see the temporary overshoots after a manual change of flow temperature that you're talking about.

    Anyway I've probably said all I can on the matter - I'm not holding my breath for Honeywell to implement weather compensation as it seems to go against their mantra with the Evohome system ("it doesn't need it" - but if it doesn't need it why do we still need to adjust our flow temperature between summer and winter for optimal performance...) but if I ever get a boiler with OpenTherm support I'll either be choosing one that does support weather compensation and opentherm working properly together (could be tricky based on the discussion in other threads!!) or implementing my own man-in-the-middle OpenTherm decoder and encoder where I can apply my own maximum flow temperature and weather compensation in some custom coding of my own.

  10. #290
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    Quote Originally Posted by DBMandrake View Post
    You don't just change the flow temperature one degree for every degree of outside temperature change though.

    That's where the slope adjustment comes in. See page 4 here for an example:

    http://viessmanndirect.co.uk/files/8...ather_comp.pdf

    The slope required basically depends on how well insulated your house is, in a well insulated house the slope will be pretty flat so it will take several degrees of outside temperature change to change the flow temperature by 1 degree. Or if you have a badly insulated house you'd need a higher slope that affects the flow temperature more.

    So in the well insulated example outside temperature going from 20 degrees to 5 degrees might only affect the flow temperature by 5 degrees for example. By adjusting the slope and offset you can emulate the change that you would have manually made previously.

    Small changes is probably the wrong way for me to describe it. What I'm trying to say is that the changes are smooth, continuously variable, and directly proportional to the conditions rather than being a step change made manually and semi-arbitrarily by a person who has decided after a few weeks that it's finally too cold for their summer flow temperature to do the job or vica versa.

    The step response of most feedback systems is not nearly as stable as the response to a smoothly varying input signal, so you're unlikely to see the temporary overshoots after a manual change of flow temperature that you're talking about.

    Anyway I've probably said all I can on the matter - I'm not holding my breath for Honeywell to implement weather compensation as it seems to go against their mantra with the Evohome system ("it doesn't need it" - but if it doesn't need it why do we still need to adjust our flow temperature between summer and winter for optimal performance...) but if I ever get a boiler with OpenTherm support I'll either be choosing one that does support weather compensation and opentherm working properly together (could be tricky based on the discussion in other threads!!) or implementing my own man-in-the-middle OpenTherm decoder and encoder where I can apply my own maximum flow temperature and weather compensation in some custom coding of my own.


    It's not tricky at all, our Intergas boiler uses OpenTherm and weather compensation together just fine - all controlled via a competitor's (Remeha) thermostat.

    Pegler I-Temp TRV's regulate the radiators to our satisfaction despite having no feedback to the boiler, so obviously this means that individual zones can't activate the boiler either. However, room temperatures are still well controlled through the varying seasons, which makes me think that weather compensation would work if incorporated into Evohome and might prove a benefit in reducing some of the excessively high flow temperatures users report under OpenTherm, although I do still believe Honeywell's Evohome implementation of OpenTherm is below par.


    Martin.

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