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Thread: EvoHome Beta 02.00.19.31 - HW Priority Setting

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  1. #1
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    Default EvoHome Beta 02.00.19.31 - HW Priority Setting

    In the welcome beta email it briefly mentions about HW Priority dependent on your system set up but doesn't really expand on this

    'And finally. Stored hot water priority and a hot water heating state if you use a Sundial central heating valve.'

    Could someone expand on this? As I believe i've got the correct set up (Zone valves, Boiler Relay etc) for this to work however it doesn't appear to be an option on my panel running the beta firmware.

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    Wasn't that a coming later feature, rather than a current beta one.

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    Quote Originally Posted by bruce_miranda View Post
    Wasn't that a coming later feature, rather than a current beta one.
    No it's included in the email as a new feature....I suspect you need certain hardware for it to show, but I figured I had the right hardware to do it.

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    So after some observation -

    It appears HW priority is just enabled - with no way to turn this on or off as you require. Can someone from Resideo confirm this?

    However think i've found a bug - i've got HW heating (And shown as heating on the new icon). All zones are at set point. When I override a zone to request for heat the HW switches off on the BDR91 (HW not to temp and new icon still showing as heating) and switches the heating BDR91 on - HW BDR91 goes off. So hot water not to temp and zone calling for heat seems to be overriding HW priority.

    Addition - Canceling zone override reverts back to HW.

    Must add too whilst HW wasn't fully to Temp (49oc, set point 55oc) it was within it's 7oc differential. Don't know whether this makes a difference...?

    Further info - changed HW temp to 65oc with a 5oc differential. When HW is at 54oc it doesn't call for heat when a zone is heating even though HW not at set point.
    Last edited by mtmcgavock; 2nd April 2020 at 03:11 PM. Reason: More info.

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    For anyone wondering going forward;

    HW Priority is enabled by default if you have a heating zone valve - if you've got this it can't be disabled. There is no On/Off in the settings.

    Current bug where if you manually override a zone the HW priority doesn't function correct but development team are aware.

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    Could you describe how HW priority works in Evohome now.

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    Quote Originally Posted by DBMandrake View Post
    Whoa a second. So this new hot water priority cannot be turned off ???! Please tell me this isn't true.

    What is the reason for that ? It's definitely not appropriate to my system with a slow heating cylinder - I do not want hot water priority, and I actually even have an ABV in series with the indirect loop for the cylinder (to prevent it dropping the radiator differential pressure to zero) precisely so radiators and hot water reheat can work in parallel, and this works well for me.

    If this new feature can't be disabled this is a big step backwards for me and anyone else will a slow reheat cylinder.
    DB, I am concerned by this. What the effect been on your system been please? Could you pm me and let me know your MAC address and I will put the system on trace to look at the hot water performance before and afterwards

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    Quote Originally Posted by Andy the Minion View Post
    DB, I am concerned by this. What the effect been on your system been please? Could you pm me and let me know your MAC address and I will put the system on trace to look at the hot water performance before and afterwards
    Hi Andy,

    I registered for the firmware update on Thursday night but haven't actually received the update yet. So I don't actually have any measured effects to report. (I'll PM my MAC address anyway - because I have authorised the monitoring even though I don't have the update yet)

    However I know exactly what hot water priority would do without even testing it - my system has a "slow" hot water cylinder that takes about 30 minutes to reheat from fully used and as a result I've designed the system to allow simultaneous hot water reheat and heating to radiators, so I'm naturally quite concerned about a software update that takes that ability of simultaneous heating and hot water reheat away from me and does not even give me an option to turn off priority to restore original behaviour.

    An example of where this would be a problem would be if we started to run a bath and then a few minutes later turned up the bathroom radiator - a fairly common situation.

    The measured cylinder temperature would drop to the reheat trigger point after running the bath for just a few minutes, (long before the bath was finished running) and as soon as that happened the heating zone valve would close and no heat would be available for radiators, so that turned up radiator which didn't get turned up until after the zone valve closed would remain completely cold for over 30 minutes that it took for the bath to finish running and the reheat to continue to completion.

    As well as that all the radiators in the house would largely cool down in that 30+ minutes causing the rooms to undershoot significantly in the winter, probably followed by an overshoot as the HR92's tried to over compensate while they were denied hot water.

    I'd not be happy with that and I suspect I'll get complaints from my other half about the radiator in the bathroom not heating up with a bath is run.

    Hot water priority is fine for a system with a cylinder reheat time <10 minutes, and if the owner/installer decides that priority is right for them and their system, but it seems very short sighted to force it on everyone, including systems where it's not appropriate without any way to turn it off.

    It also violates the "principle of least surprise", where a (later general release) software update that an end user didn't ask for, didn't know was coming, and can't refuse makes a fairly major change to the behaviour of their heating system, which when they discover it, can't be changed back as it's impossible to roll back the firmware or turn off the feature.

    In my opinion the hot water priority feature needs to have an on/off configuration setting in the installer menu and needs to default to off during the firmware update process (and perhaps during a factory reset) to not violate the principle of least surprise... It can't be that hard to add an on off preference setting surely ?

    Anyway hopefully the engineers working on this beta release can take this as constructive critisism. I'm still looking forward to testing the beta release, in particular the load scaling feature.
    Last edited by DBMandrake; 3rd April 2020 at 10:35 PM. Reason: fixed some mistakes

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    Automated Home Sr Member Andy the Minion's Avatar
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    Quote Originally Posted by DBMandrake View Post
    Hi Andy,

    I registered for the firmware update on Thursday night but haven't actually received the update yet. So I don't actually have any measured effects to report. (I'll PM my MAC address anyway - because I have authorised the monitoring even though I don't have the update yet)

    However I know exactly what hot water priority would do without even testing it - my system has a "slow" hot water cylinder that takes about 30 minutes to reheat from fully used and as a result I've designed the system to allow simultaneous hot water reheat and heating to radiators, so I'm naturally quite concerned about a software update that takes that ability of simultaneous heating and hot water reheat away from me and does not even give me an option to turn off priority to restore original behaviour.

    An example of where this would be a problem would be if we started to run a bath and then a few minutes later turned up the bathroom radiator - a fairly common situation.

    The measured cylinder temperature would drop to the reheat trigger point after running the bath for just a few minutes, (long before the bath was finished running) and as soon as that happened the heating zone valve would close and no heat would be available for radiators, so that turned up radiator which didn't get turned up until after the zone valve closed would remain completely cold for over 30 minutes that it took for the bath to finish running and the reheat to continue to completion.

    As well as that all the radiators in the house would largely cool down in that 30+ minutes causing the rooms to undershoot significantly in the winter, probably followed by an overshoot as the HR92's tried to over compensate while they were denied hot water.

    I'd not be happy with that and I suspect I'll get complaints from my other half about the radiator in the bathroom not heating up with a bath is run.

    Hot water priority is fine for a system with a cylinder reheat time <10 minutes, and if the owner/installer decides that priority is right for them and their system, but it seems very short sighted to force it on everyone, including systems where it's not appropriate without any way to turn it off.

    It also violates the "principle of least surprise", where a (later general release) software update that an end user didn't ask for, didn't know was coming, and can't refuse makes a fairly major change to the behaviour of their heating system, which when they discover it, can't be changed back as it's impossible to roll back the firmware or turn off the feature.

    In my opinion the hot water priority feature needs to have an on/off configuration setting in the installer menu and needs to default to off during the firmware update process (and perhaps during a factory reset) to not violate the principle of least surprise... It can't be that hard to add an on off preference setting surely ?

    Anyway hopefully the engineers working on this beta release can take this as constructive critisism. I'm still looking forward to testing the beta release, in particular the load scaling feature.
    DB
    I really welcome your comments and especially the effort you spent making them. They are constructive, based on your experience in the profession, and the reason we released this trial on the forum. It is also why I’m am here.
    So firstly, thank you!

    In return I will try to explain some of the how or why behind the functions and what we can or can’t do. Resideo is a big company and as my user name suggests I am just a small part of the development team. The Evo system is a big part of my working life, I have had all three version one in my home, sometimes two. They will have to take it from my cold dead hands if they ever want it back again
    So….. I will certainly accept the criticism on behalf of the team. I will try to explain where I can and not come over as defensive, but some things are outside my power to change.
    If the forum can accept that position, I would greatly appreciate you allowing me to participate.
    I will probably repeat this screed in the release feed as not everybody is following both.

    To start, the first part of what follows won’t address the main points of your comments but I would like to explain the reasons for DHW priority.

    Firstly we received a considerable amount of pressure from OpenTherm users, the argument they made was that because the boiler water temperature calculation is a sum of all the demands. When a CH system is happy ticking along at 35C and an 80C DHW demand appears, the heating gets an unwanted jolt of hot water.
    This is not normally a huge problem (an annoyance, yes) because standard tanks recover in 10-15 minutes (usually its a top up rather than heating from the cold inlet temperature) and they can sometimes recover almost as fast as they fall.
    However, for the slow cylinder being described, this is a considerable problem. The CH gets overheated for as long as the tank takes to recover.
    This is the exact opposite of the argument against DHW priority. It was the opentherm guys that noticed this, but it is also completely true for all TPI systems as well.
    An excessively long 100% demand caused by the cylinder will cause overshoots in the central heating. The time constants in room temperature control makes it impossible for even a HR92 to stop the overshoot because the hot water is already in the radiator by the time it notices it. We now have a lot of evidence to back this up because of remote diagnostics.

    The second reason is that we are heading into a world where 3 or 6kW heatpumps and 35C flow temperatures will become the norm. The Dutch are already there and in 2025 we won’t have the option of a big ass combi either. This is just about okay for a correctly insulated underfloor system in new build, but I know from helpdesk calls and site visits that heatpumps are being installed in conversions with large radiators.
    Even in a good new build the DHW becomes really critical, 55C flow is a real struggle, CoP values plummet and they will be relying on a direct electric element as boost/backup. In this case the expensive, high CO, high temperature MUST only be put into the tank. If the CH valve were open the tank will (again) be very slow and the heating will be partially supplied by a direct element.
    It’s a personal view but if mis-selling of heatpumps becomes a big thing, zoning systems will be a potential solution to some very unhappy OTC based customers with very high bills.

    The final reasons is cooling, European heatpump installers are selling them with a ‘free cooling’ sales pitch, and a non heat/cooling control will just not be acceptable. The current UK regulations exclude this from any rebates but we needed to cover this immediately. Heatpumps are two pipe systems, they can either heat or cool so again a DHW demand must close the CH valve or there will be unhappy times. The system can’t allow pump overruns into the CH system either, back in the 1990’s those new-fangled Combis had enough trouble with this, and a cooling customer will simply go up the wall

    I know this didn’t answer the ‘why no option’ point, but this was a rational decision made with the application experts in UK sales as @mtmcgavock mentioned, and it was based on technical knowledge, a balance of probabilities and experienced feedback.

    1. We made DHW priority conditional on having a CH valve, so most systems will not be DHW priority. We were actually expecting this to be picked up as a problem as it needs additional components, wiring etc in the system (short pause as the OT guys rush to the keyboard)
    The CH valve is needed because not all systems are fully HR92 and the constant opening and closing of all the HR92 will bring its own problems.

    2. The argument about underheating of the central heating with a slow cylinder. This is a 50/50 call, it is either going to be central heating overheating or underheating depending if there is priority or non-priority DHW.
    Normal cylinders just won’t notice and will work equally well with either system.

    3. We also made a call based on knowledge of the response times in central heating systems.
    a. Water is fast compared to air, a stratification layer in a tank means seconds between hot and cold water, plus the Evo has an adjustable differential so a cylinder doesn’t have to fall 15C before a demand starts like old bimetal cylinder stats.
    b. Temperature fall in a room is generally in the order of 0.2 – 0.7C/hour if there is truly no heating – and a bath of 45C water is definitely not ‘no heating’, the room will overshoot. The water in the radiator will also need to cool before room cooling starts.
    The human body cannot detect temperature changes much below 1C so the system has over an hour to respond.
    c. Finally, most cylinders do actually recover well, and because of the high flow temperature in DHW only mode this will get getter still. Importantly the slower ones will also improve because the boiler doesn’t have to drag the whole system volume up to 80C

    So having said all this, I am not closing the topic down. If we can get evidence of a significant population of Evo system that have been noticeably adversely affected, we will look to see how it can be addressed.
    Resideo employee. Comments are personal, and likely to get a hard stare from Rameses

  10. #10
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    Quote Originally Posted by Andy the Minion View Post
    However, for the slow cylinder being described, this is a considerable problem. The CH gets overheated for as long as the tank takes to recover.
    This is the exact opposite of the argument against DHW priority. It was the opentherm guys that noticed this, but it is also completely true for all TPI systems as well.
    An excessively long 100% demand caused by the cylinder will cause overshoots in the central heating. The time constants in room temperature control makes it impossible for even a HR92 to stop the overshoot because the hot water is already in the radiator by the time it notices it. We now have a lot of evidence to back this up because of remote diagnostics.
    What you're saying makes the assumption that all systems implement explicit flow temperature boost on hot water reheat. Mine doesn't. Many others don't.

    Yes, this is probably the default case with OpenTherm since OpenTherm can explicitly command the flow temperature. But for any non-OpenTherm system like mine there will only be a flow temperature boost if the system is explicitly wired this way by using a boost input on the boiler. I know a couple of members here have done this.

    I don't see any rooms going over temperature during hot water reheat - probably for a couple of reasons. One is I have a heating zone valve, so even though the boiler might be running at its set flow temperature (100% duty cycle on the boiler relay) during hot water reheat the flow through the radiators is still TPI controlled, eg intermitent for a low demand. The second reason is when the cylinder indirect loop is in circuit it drops the differential pressure applied to the radiator circuit, further reducing flow through the radiators. In fact I have an ABV in the indirect loop so I can adjust the differential pressure during a reheat.

    This is a good illustration of how different systems can behave very differently - the problem you're trying to solve with hot water priority (overheating radiators during hot water reheat) doesn't exist for my system in the first place, and the solution causes me other problems.

    The second reason is that we are heading into a world where 3 or 6kW heatpumps and 35C flow temperatures will become the norm.

    [...]

    The final reasons is cooling, European heatpump installers are selling them with a ‘free cooling’ sales pitch, and a non heat/cooling control will just not be acceptable. The current UK regulations exclude this from any rebates but we needed to cover this immediately. Heatpumps are two pipe systems, they can either heat or cool so again a DHW demand must close the CH valve or there will be unhappy times. The system can’t allow pump overruns into the CH system either, back in the 1990’s those new-fangled Combis had enough trouble with this, and a cooling customer will simply go up the wall
    All very interesting, but not applicable to the gas boiler systems we're currently using. So we shouldn't be suffering from a design decision based partly on systems we aren't even using. Another reason for there to be a setting to enable or disable it.

    I know this didn’t answer the ‘why no option’ point, but this was a rational decision made with the application experts in UK sales as @mtmcgavock mentioned, and it was based on technical knowledge, a balance of probabilities and experienced feedback.
    There are good reasons to use hot water prioirity on some systems, I don't disagree with that. However there are equally as good reasons to NOT use it on some systems. And there are also good reasons to not abitrarily make a major change to the behaviour the system has had for 5+ years without some way to disable it for systems where it will cause a problem.

    On old firmware it was possible to implement hot water priority with a relatively trivial wiring change of the BDR91's. With hot water priority implemented in software with no preference setting to disable it I can't think of any external wiring change to defeat it and restore original behaviour. So you would be forcing your entire customer base to accept hot water priority whether they want it or not.

    I don't understand the reasoning behind this and why there can't simply be an on/off setting in the installer menu like there is for the other new features like load scaling and cold weather boost. It's a trivial software change.

    2. The argument about underheating of the central heating with a slow cylinder. This is a 50/50 call, it is either going to be central heating overheating or underheating depending if there is priority or non-priority DHW.
    Normal cylinders just won’t notice and will work equally well with either system.
    I just can't agree with this unfortunately.
    3. We also made a call based on knowledge of the response times in central heating systems.
    a. Water is fast compared to air, a stratification layer in a tank means seconds between hot and cold water, plus the Evo has an adjustable differential so a cylinder doesn’t have to fall 15C before a demand starts like old bimetal cylinder stats.
    Yes, which means with a typical cylinder stat location of 1/3rd up the cylinder you've only used about 1/3rd of the hot water before a reheat cycle starts, at which point radiators stop being supplied.
    b. Temperature fall in a room is generally in the order of 0.2 – 0.7C/hour if there is truly no heating – and a bath of 45C water is definitely not ‘no heating’, the room will overshoot. The water in the radiator will also need to cool before room cooling starts.
    The human body cannot detect temperature changes much below 1C so the system has over an hour to respond.
    It was cooling in other rooms I was more referring to while the bath was running.

    0.2-0.7C change in air temperature maybe, but that's not all there is to comfort - part of what we feel is a result of the direct IR radiation from the radiator, just like outdoors where part of our sense of temperature is air temperature and part is direct IR from the sun.

    When the radiator goes cold occupants will start to feel cold even before the air temperature has dropped noticably or any change has registered on a remote thermostat. Also I'm not sure where you get your 1C sensitivity figure from. Humans are actually more sensitive to changes in temperature than they are to absolute temperature, as they can acclimitise to different temperatures but will feel an increase as "hot" and decrease as "cold". The old one hand in cold water and one in hot water then both into warm water trick is an example of this.

    This is actually a big advantage of Evohome and TPI systems is that they usually maintian the temperature very constant without the cyclic fluctuations of an old fashioned thermostat system where you would feel hot on the upswing and cold on the downswing even though the average temperature might be deemed to be comfortable.

    Best comfort is obtained by avoiding any oscillations in temperature at all.
    c. Finally, most cylinders do actually recover well, and because of the high flow temperature in DHW only mode this will get getter still. Importantly the slower ones will also improve because the boiler doesn’t have to drag the whole system volume up to 80C
    Again, you're making an assumption of an explicit flow temperature boost for hot water reheat, which many systems don't apply. On a system where the boiler has sufficient output and the flow temperature doesn't change there is no speed advantage to reheating a "slow" cylinder in hot water priority mode - because the reason it's slow is a lack of thermal conductivity between the indirect loop and water in the cylinder.

    For example even at an elevated flow temperature my cylinder can only draw approx 6kW from the indirect loop in the cylinder. I actually did a test when I first converted my system to S-Plan and the reheat time with hot water priority temporarily wired in was almost identical to no hot water priority with already hot radiators.
    So having said all this, I am not closing the topic down. If we can get evidence of a significant population of Evo system that have been noticeably adversely affected, we will look to see how it can be addressed.
    It's pretty simple really - just make it possible to enable or disable hot water priority in the installer menu at the installer/owners discretion. Then everyone is happy and catered for. I'm still struggling to see why this can't be a preference setting when the other new features being introduced can be turned on and off.

    While I don't mind testing beta firmware with this setting enabled (its effect on my system would be annoying, but not the end of the world) if forced hot water priority does make it through to the next general public release I'll be seriously considering pulling out the hot water part of the Evohome kit and installing a regular timer and thermostat for hot water control.

    I'm still having intermittent lost comms issues with the CS92A despite several years of troubleshooting and don't find remote control of the hot water schedule useful so hot water priority which I don't want and can't disable may be the final straw for me to switch Evohome back to heating control only, which it does very well.

    Edit: I don't want my post to come across as overly negative, it isn't intended to be. Hopefully I've made my case and I don't think I really have any more to add on the matter of hot water priority.
    Last edited by DBMandrake; 4th April 2020 at 04:03 PM.

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