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Thread: Boiler with separate HW and CH max and OpenTherm

  1. #41
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    Quote Originally Posted by paulockenden View Post
    Isn't that likely to just be the time delays inherent in the system?
    After I posted, I went and made a cup of tea and returned about 20 minutes later. There was no change in the heat/zone % demands.
    That said, the max zone demand at the moment is 24% which matches the boiler demand. I have noticed this event too often to believe that what I am seeing is just system lag.

  2. #42
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    Quote Originally Posted by HenGus View Post
    After I posted, I went and made a cup of tea and returned about 20 minutes later. There was no change in the heat/zone % demands.
    That said, the max zone demand at the moment is 24% which matches the boiler demand. I have noticed this event too often to believe that what I am seeing is just system lag.
    Interesting. I am yet to see a discrepancy between boiler demand and maximum zone demand, and I've kept tabs on it quite a bit.

  3. #43
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    Quote Originally Posted by blowlamp View Post
    Could you expand a little on what you mean by this please?
    If you were to combine OpenTherm control from Evohome with Weather compensation from a boiler module, the ideal way for it to work would be for the weather compensation to derive a maximum flow temperature based on outside temperature (and your slope/offset adjustment when you set up weather compensation to match the characteristics of your house, radiators etc) and then "scale" the requested flow temperature from the OpenTherm bridge down based on this.

    As far as I know no boiler does this. All boilers seem to set flow temperature maximums by capping the requested flow temperature - in other words say Weather compensation calculated that in 10 degree weather you only needed a flow temperature of 60 degrees, it would honour the requested flow temperature from the Evohome via OpenTherm up until 60 degrees but beyond that it would cap it to 60 degrees if the Evohome requested 70, 80, 90 degrees... this introduces a lot of deadband (yes this is a thing) where the Evohome varying the requested flow temperature doesn't actually do anything.

    Worse still the amount of deadband would vary with the outside temperature as the flow temperature cap changes. This is far from optimal.

    But it's even worse than this - from reports we've seen on here almost no boilers allow you to combine their weather compensation module with OpenTherm control - as soon as you use OpenTherm the weather compensation module basically gets disabled and the OpenTherm request overrides the flow temperature limit that the weather compensation would normally apply. I think we have one confirmed report of a boilers Weather compensation module and OpenTherm working together peacefully (although still with the undesirable deadband) and all the other reports are saying that they can't both be enabled at once as one overrides the other.

    Doing it all in software in the Evotouch would get around all these problems - the heat demand can be scaled properly when you set a flow temperature limit in the installer menu instead of being capped which means no deadband, and if they went as far as to incorporate weather compensation (I don't think they ever will) then you wouldn't even need a weather compensation module for your boiler - it could just use the weather report temperature that is now displayed in the top left corner as the input to calculate the maximum flow temperature and then scale the heat demand to not exceed that temperature, but avoiding introducing any deadband. Win win.
    Last edited by DBMandrake; 25th January 2018 at 02:43 PM.

  4. #44
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    In a perfect world it would all be done as a software update to the existing controller, but if required a new evotouch, I would certainly buy it.

  5. #45
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    Quote Originally Posted by fergie View Post
    In a perfect world it would all be done as a software update to the existing controller, but if required a new evotouch, I would certainly buy it.
    Personally I think a manually configured maximum flow temp setting in either the installer menu or the normal settings menu for OpenTherm installs is a no brainer.

    It's both easy to implement purely in a software update (and as Paul points out they already expose cycle rate and minimum on time for BDR91 installs, so there is precedence for having this kind of setting available) and more importantly I would argue it is actually pretty necessary for all those boilers that disable their built in maximum flow temperature setting when being controlled with OpenTherm - which is a lot of them.

    Any system using OpenTherm where the boiler maximum flow temperature setting is disabled will be running very non optimally as by running at the boilers maximum temperature every time a zone is below the proportional band it will probably be experiencing room temperature overshoots and oscillation, interaction between rooms where one room coming online causes other rooms to overshoot, may not be condensing very well (if at all) and will be just unnecessarily stressing the boiler and pump by running them at maximum temperatures for a significant period of time.

    One thing is for sure, if I was considering an OpenTherm compatible boiler I would not buy one that does not allow me to set a maximum flow temperature when used with OpenTherm, and if I already had such a boiler I would not bother buying the Honeywell OpenTherm bridge, I'd just stick with the BDR91.

    However if they implemented this feature I would have no qualms about using Evohome OpenTherm with any (?) OpenTherm boiler.

  6. #46
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    Thanks for your reply, DBMandrake.

    I’m giving details of my setup that you may already be aware of in the hope they might be of use to you & others

    My system is an Intergas boiler with Remeha iSense controller-thermostat. The sytem uses OpenTherm and weather compensation control together, the weather compensation sensor being wired directly to the boiler.

    If no additional OpenTherm boiler controller-thermostat is connected, then the user adjustable weather compensation curve programmed into the boiler is utilised, provided it is sensed that an outdoor sensor is in-circuit.

    However, when an additional OpenTherm boiler controller-thermostat is used, the weather compensation curve programmed into the boiler is ignored and instead, the one within the connected controller-thermostat is used, if available and it is sensed that an outdoor sensor is present.

    The boiler passively sends the iSense the outside temperature value and from it, the iSense calculates a flow temperature to suit the current indoor temperature & weather conditions, which it returns back to the boiler. The boiler now only has to maintain that temperature according to its inbuilt logic and until the next request from iSense.

    The iSense has a definable maximum flow temperature. There is also a Room Temperature Effect (RT Effect) setting, which deviates flow temperature from the programmed weather compensation curve, depending on how far away the measured room temperature is from set point. This effect is user adjustable and governs how high the boiler must fire to achieve set point.

    The weather compensation curve of iSense is usually based on a set point of about 20C and is raised or lowered in relation to the actual requested set point. The default curve might be programmed to give a room temperature of 20C when it’s 6C outside with a flow temperature of 50C, whereas a room temperature of 22C whilst at 6C outside might dictate a 60C flow temperature. The RT Effect setting will also combine to shift the curve higher or lower along its temperature scale.

    So iSense is quite unlike Evohome in many ways, but a major difference to me is that there is no ongoing learning required. In effect iSense is taught about the property it inhabits when the weather compensation curve is adjusted and other parameters are set. Also, because of the weather compensation curve, higher than necessary flow temperatures are very, very much reduced.


    Martin.

  7. #47
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    You guys are looking for full load/weather/optimisation to be implemented in a product that is in fact a glorified S-plan +

    Why try and jury rig Evo? If that is what you want(need?) then why not start with a control family that is designed for purpose?

    Most of the better northern European boiler manufacturers already offer 'domestic' optimiser/compensator controls, one of them has even added TRV motors to their range.
    The trouble is weather/load compensation (in the real world) is not really suited to the rapid demand change of many small loads that is representative of a domestic installation, a situation that is agravated by systems such as Evo. The two are differing control philosophies from the outset - feed forward vs feed back - continuous flow variable temp vs variable flow continuous temperature.
    To my mind Evo already does an exceptional job of keeping stable under challenging unknown circumstances.

    Simon

  8. #48
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    Quote Originally Posted by Simon Willett View Post
    The trouble is weather/load compensation (in the real world) is not really suited to the rapid demand change of many small loads that is representative of a domestic installation, a situation that is agravated by systems such as Evo. The two are differing control philosophies from the outset - feed forward vs feed back - continuous flow variable temp vs variable flow continuous temperature.
    Like so many people you seem to be laboring under the illusion that OpenTherm control and Weather Compensation are two competing systems that try to do the same thing and can't work together, purely because they both attempt to control the flow temperature. This is simply not true.

    You are right that Weather compensation is feed forward and OpenTherm control is feedback - but feedforward and feedback systems can and frequently are combined into composite control systems.

    They are both measuring different things and their resultant can be used to control the flow temperature in a way that provides better control than either system on its own, but it has to be done right and it is not being done right (or at all) in most cases right now.

    Evohome using OpenTherm varies the flow temperature based on the instantaneous heat demand from individual rooms. The idea is that the flow temperature should be no higher than that required by the most demanding room. Rooms that are less demanding modulate their radiator flow down further to achieve the heat output they need.

    This works well on it's own but it still has some drawbacks. The main one is that if any zones are below their proportional band then it simply demands maximum flow temperature, which may or may not be limited by a boiler control!

    This will happen both during initial warmup when you turn the system on, and also when a zone is newly scheduled to come on. This can lead to large overshoots in other rooms that were already at a stable temperature. This excessive flow temperature may cause some zones to dramatically overshoot and never stabilize (they keep oscillating) and it's certainly not very efficient to run the boiler at full tilt so often. It's also not comfortable in warmer weather to have excessive panel surface temperatures.

    A partial solution is to manually turn down the maximum flow temperature to something that is appropriate for the radiators, house (heat loss properties etc) and outside weather. You want flow temperature set so that it's just a few degrees higher than what is needed to reach your target set points in a reasonable amount of time. Doing so will minimise or perhaps even avoid overshoots and oscillations if the speccing of the radiators between different rooms was well chosen and balanced. (No weak radiators and/or massively oversized radiators in relation to other rooms)

    There are two problems with this. The first is that some boilers simply don't let you set the maximum flow temperature when controlled by Evohome OpenTherm, and the Evohome itself also currently has no way to set a maximum flow temperature. This has been discussed to death on this forum and it is a MAJOR problem, to the point that it makes the system practically unusable as achieving stable room temperatures and good comfort can be nearly impossible, especially if the boiler is trying to run at 90 degrees. I'm sure you've read those threads.

    But even on boilers where you can manually set the maximum flow temperature - what do you set it to ? There is no one temperature you can set it to that will be optimal year round, or even across the span of a week if weather is shifting dramatically.

    In the winter you need high flow temperatures or your house will take far too long to warm up and some rooms will never achieve their set points. In warmer weather you need a lower flow temperature otherwise the rooms will all overshoot and keep oscillating, panel temperatures will be uncomfortably high when radiators come on (too much direct IR) and there will be a large amount of interaction between rooms where a single room coming on can cause several other previously stable rooms to overshoot a degree or two. (Due to the sudden massive increase in flow temperature) It's probably also less efficient and won't promote good condensing performance.

    So in warmer weather you need to turn the maximum flow temperature down and turn it up again in winter. This should be the job of weather compensation, not you as the house owner to run around changing this all the time when the weather shifts from +6 to -6 in the space of a few days. (Or just put up with the poor performance of having the flow temperature too high most of the year - which is what I suspect most households do, especially if there is no-one particularly technical at the helm to turn it down when it would be a good idea)

    Far from confusing Evohome it should reduce the amount of adaption it has to do as the outside conditions change - even during a single day. (Early morning vs early evening for example, where the outside temperature can be radically different)

    Evohome has no knowledge of the outside weather, nor the heat loss through the walls that result from that. It is purely a negative feedback system that only measures the temperature in individual zones, and adjusts the heat demand and flow through the radiators using PID controllers in an attempt to meet that target. (Contrary to what you say above, Evohome controls both flow temperature and flow rates, it is not variable flow fixed flow temperature as you state)

    The heat demand required to reach the same target temperature varies depending on the heat loss. If we consider a fixed maximum flow temperature for a moment say 80 degrees, to reach 20 degrees in winter it might require a "heat demand" of 60% but in the warmer months it might only require a heat demand of 20%. The way this is achieved is through the I (integral) term of the PID controller - over a period of time it will gradually adapt to a fixed offset and this will allow it to find the new heat demand that will give the target temperature.

    The other adaption that has to take place with a change in weather is the tuning of the D (differential) term. This is the one that controls and tries to avoid temperature overshoots and allows the temperature to climb quickly to the target and settle with minimal overshoot. The hotter a radiator is (due to higher flow temperature) the more thermal mass and inertia it has and the more tendency there will be to the room overshooting the target as the amount of heat it puts into the room after the valve is closed off increases.

    An HR92 will observe any overshoot and make adjustments to the D term to attempt to avoid this in the future, but this adaption takes time and many warm up cycles (a few days) to optimise. Essentially what the D term does is predictively starts to throttle the valve back well before the target set point is reached based on an assumption of how quickly the temperature is rising and how long it will keep rising as the valve closes. This is really hard to get right - close it too late and the temperature overshoots, close it too soon and the temperature levels out below the target and then will eventually start to rise again thanks to the I term.

    The problem comes when outdoor temperatures shift suddenly like a cold snap. If it suddenly gets colder outside than yesterday the prediction for how soon to close the valve before the target is thrown out of kilter - it will close too soon and the target will be undershot. Conversely if you get an unexpected warm shift rooms that were hitting their targets perfectly will start to overshoot and it can take a few days for the system to adapt. It will get there eventually but there will be a messy adaption period. If the flow temperature is a lot too high - like set for the middle of winter and its late spring, it will not be able to adapt far enough and it will overshoot regardless.

    Again this is where weather compensation could come in. If it is adjusted properly to model the heat loss of the house correctly you can arrange things so that the individual HR92's don't have to adapt very much to changing weather - they call for about the same "heat demand" whether it is cold or warm outside, but that heat demand is scaled down by the weather compensation based on outside temperature and implied changes to building heat loss... which means there is little if any adaption required so the system will respond quickly to shifts in the weather, even on a daily basis without going through re-learning cycles.

    For example in our 80 degree example above requiring a 60% heat demand in winter, in the cold weather the weather compensation may allow that high flow temperature, then in the warm weather where previously the HR92's heat demand would have had to have fallen to 20%, the weather compensation may have scaled back a 100% heat demand to say 55 degrees so that when the HR92 calls for 60% heat demand it is getting about the same flow temperature as it would have got from a 20% call for heat had it not been for the weather compensation.

    Now the HR92's only need to adapt to your radiator and room characteristics, but are relieved from having to constantly adapt to unknown (to them) changes in weather and therefore heat loss. Year round you should see optimal performance - fast warm up time and set points being reached in the winter, but avoiding overshooting, oscillating and unnecessarily high flow temperatures in warm weather, all without the owner lifting a finger to adjust the maximum flow temperature, or changing their set points. And it can adjust automatically on both a seasonal and daily/weekly basis.

    Perhaps overkill, but that is my ideal system. The fact that I find I need to turn my flow temperature up in the winter and down in the summer for optimal results tells me that not all is well with the concept of a single fixed maximum flow temperature.
    Last edited by DBMandrake; 26th January 2018 at 11:34 AM.

  9. #49
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    That has to be one of the longest posts for quite a while 😎

    Rather than messing around with sensors, Id be happy for evohome to use forecast temps (with option to vary it if your garden is normally hotter or colder than the station).

    For me (in a modern well insolated property), being able to set independent max temps for heating and water is the biggest priority.

  10. #50
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    OpenTherm is a 2-way communication protocol/language, rather than a feedback system, as far as I am aware. OpenTherm's utility depends on how many words from its 'dictionary' are understood by the boiler & controller-thermostat and also how they are applied under any given set of circumstances. Once a message is delivered and acknowledged, it is down to the internal logic of the receiving device to act in a suitable fashion. Obviously, the smaller the vocabulary of any device, the less utility it will have and can be seen by Evohome not implementing things like a maximum flow temperature setting or using the outside temperature feature.



    Martin.
    Last edited by blowlamp; 26th January 2018 at 02:31 PM. Reason: typo

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