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Thread: EvoHome Beta 02.00.19.31 - HW Priority Setting

  1. #21
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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 05:03 PM.

  2. #22
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    Quote Originally Posted by Andy the Minion View Post
    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.
    All valid points and can see where you are coming from, don't think we're taking this out on you Really appreciate your answers and support through the forum. But surely is it that hard to add a simple 'ON/OFF' to the system for this feature?

    I feel like i'm going to get a few phone calls from customers with existing systems that when their radiators aren't on because their HW is reheating - and they don't realise this. It'll be a difficult situation to explain - especially when you're changing how an existing system worked. Think it'll confuse a lot of people. You're taking the view that many people don't have the Central Heating Zone valve and just HR92s, when I suspect in reality many do as they've just converted their existing systems and kept existing zone valves for easiness or to revert back if required.

    On an additional note - are there features built in where if the water isn't heating, or it's been heating for 30mins with no rise the system will override the feature? (like Vaillants systems do)

  3. #23
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    HW priority is a brilliant feature. Just please make it optional.

  4. #24
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    This is an interesting issue and discussion from my perspective. I've been an Evohome user since it was originally launched in b/w form to industry rather than consumers. Like DBMandrake I have an old - well very old - slow-to-heat-up hot water tank so right from the start I have only used Evohome for controlling CH and have never implemented hot water control relying on the old timer.

    One of my CH issues from the very beginning is that heating up the hot water tank hogs all the heat from CH system leaving radiators on the cold side. I avoid this by turning hot water on for half an hour before the CH gets turned on by Evohome at 06:00. When I had two extra family members in the house, I topped it up at 12:00 when having the radiators cool down for 20 minutes was an acceptable solution.

    A few years ago, I was thinking about using DBMandrake's solution of fitting an ABV to share the hot water between CH and heating the hot water tank but felt it was not worth the effort at the time.

    So, even though I have decided this time around not to participate in the Beta programme which is unusual for me, I understand the call for an on/off switch can only support the call for such a feature to be added.

    I also very understand and support Resideo's need to update Evohome for more Avant-garde heating systems but not at the expense of the vast number of Evohome users like myself that are stuck with an old-fashioned gas boiler based heating system with a large, slow-to-heat hot water tank in the loft. Maintaining compatibility and managing the transition between old and new is always a challenge but to ignore what knowledgable long-standing customers are saying will never end in well.

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    Because I have a Solar Thermal HW system my storage tank is very large. In the winter this can take a long time to heat using gas. HW priority would result in relatively long periods without CH. This will probably mean that I will have to disable HW control via EvoHome. This is very disappointing and I strongly support the request that HW priority be made an option. How hard can it be?

  6. #26
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    What is the main reason why people still have a motorized valve on the CH loop, even when all the radiators have HRxx controllers on them? Is it just to stop the hot water rushing to radiators when the HW cycle kicks in?

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    Quote Originally Posted by bruce_miranda View Post
    What is the main reason why people still have a motorized valve on the CH loop, even when all the radiators have HRxx controllers on them? Is it just to stop the hot water rushing to radiators when the HW cycle kicks in?
    I'm not clear on that myself. The most logical is that there are numerous or large rooms without HR92 but I know a lot are fitted with both. Often this is because the system had one originally and getting into somebody else's Sundial wiring is rarely a pleasant job.

    This does however limit the ability to control the boiler efficiently as we dont want to constantly cycle a Sundial valve, they just weren't designed for continuous control and cannot modulate. We run the valve in On/off mode, so the CH valve opens 100% when any zones exceeds 10%. A wired boiler therefore gets a 100% switch live from the orange wire for the full duration of any of the zones heat demand and the boiler will cycle constantly at high temperature on its stat. This is very far from ideal for system efficiency.

    Some people have a boiler BDR91 and a CH valve which is far better, the valve was redundant but this is how we are achieving the DHW priority function, to get this function the system needs both boiler and CH BDR91

    @CTI can you pm me your MAC address, I would like to have a look at the DHW performance?
    Regards AtM

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    Quote Originally Posted by bruce_miranda View Post
    What is the main reason why people still have a motorized valve on the CH loop, even when all the radiators have HRxx controllers on them?
    I suspect that mostly it's historical. They had existing S-Plan systems when they had Evohome fitted.

    And let's not forget that the original default Evohome wiring instructions, back in the day, was to fire the boiler off the orange wires on the two 2-port valves. So many people will have systems installed like that.

  9. #29
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    Quote Originally Posted by bruce_miranda View Post
    What is the main reason why people still have a motorized valve on the CH loop, even when all the radiators have HRxx controllers on them? Is it just to stop the hot water rushing to radiators when the HW cycle kicks in?
    I think the most common reasons are probably;

    1) It's an existing system with either a two 2 port valves or a 3 port valves already in situ. You have to remember EvoHome isn't cheap to buy, never mind having to start doing pipework alterations to convert systems from 3 ports or removing the old 2 port valve.
    2) Again coming down to cost - most of my customers don't want to splash out on HR92s throughout - so some rads are left without HRXX control. If there was no Motorised Valve then these would come on during a HW reheat.
    3) People don't stay in houses forever; if you've got this system you're probably going to take it with you as it costs a fair bit. Having the motorised valves in there allows you to convert it back very easily without any alterations.
    4) Self Installers - most won't want to be doing wet work, you can install the system yourself and leave existing valves in situ.

    I think the issue is here the main thing is costs get overlooked, buying the system can cost you nearly 1000+, never mind having it installed as well! Having an installer come and have to maybe do substantial pipework alterations to remove a valve that in reality can stay there seems daft to me, especially when you possibly come to move and then need it adding back in!

  10. #30
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    Quote Originally Posted by Andy the Minion View Post
    I'm not clear on that myself. The most logical is that there are numerous or large rooms without HR92 but I know a lot are fitted with both. Often this is because the system had one originally and getting into somebody else's Sundial wiring is rarely a pleasant job.

    This does however limit the ability to control the boiler efficiently as we dont want to constantly cycle a Sundial valve, they just weren't designed for continuous control and cannot modulate. We run the valve in On/off mode, so the CH valve opens 100% when any zones exceeds 10%. A wired boiler therefore gets a 100% switch live from the orange wire for the full duration of any of the zones heat demand and the boiler will cycle constantly at high temperature on its stat. This is very far from ideal for system efficiency.

    Some people have a boiler BDR91 and a CH valve which is far better, the valve was redundant but this is how we are achieving the DHW priority function, to get this function the system needs both boiler and CH BDR91

    @CTI can you pm me your MAC address, I would like to have a look at the DHW performance?
    Regards AtM
    The main reason I have a motorised valve for the CH, is that EvoHome replaced an existing system and the installer used what was there. To change now would mean paying him to come back and modify the system for what looks like no reason except to avoid the HW priority.

    Andy, I am happy to provide my MAC address, but do not think it will help you much as I operate my EvoHome in a semi manual manner to get the best out of my other Solar powered options and because my household does not operate to a regular pattern. The HW control is entirely manual at present and used to give things a boost before or after showers or to ensure temperature is raised to 60 when necessary to provide sterilisation. Unfortunately it is not possible to integrate/automate everything.

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