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Thread: EvoHome Beta 02.00.19.31 - HW Priority Setting

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  1. #9
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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 35°C 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 15°C 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 – 0.7°C/hour if there is truly no heating – and a bath of 45°C 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 1°C 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 80°C
    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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