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Thread: Evohome hot water 'off' settings

  1. #1
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    Default Evohome hot water 'off' settings

    Does Evohome use a proxy temperature to model the 'off' setting for the hot water (DHW)? I ask because my DHW has just cut in, even though it is scheduled to be off until 1700 hrs. The Evotouch is still showing "Off" but the DHW temperature was showing as 4oC. (The mains water is cold in Scotland in February!) That makes me wonder whether the DHW has a default minimum temperature of 5oC to model the 'off' setting, like the room zones use 5oC as their proxy for 'off'. Evohome has just taken it upon itself to heat my DHW until it showed 24oC and now it has turned off again. I have a large cylinder which was still supplying usable hot water throughout. (The DHW temperature is measured at the bottom, of course, and there was still hot water above the sensor. My setting is 58oC when 'On'.) How can I inhibit Evohome turning on the DHW when I don't want it to? Otherwise I will never get my teenagers out of the shower. (I ration their hot water to one full cylinder only!)

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    Interesting - nobody has reported this on the forum before as far as I'm aware, and I have not seen anything in the documentation about it but it does seem likely that this is frost protection for your hot water cylinder.

    Do you really want to risk your hot water cylinder dropping to freezing temperatures and have a cylinder containing hundreds of litres of water burst in your loft or boiler closet ? I know I wouldn't! It's not possible to disable frost protection for rooms from the controller (although the OFF setting turning the dial on an HR92 does in fact disable frost protection) so it's likely that this can't be disabled either, and I certainly haven't seen a way to turn on or off cylinder frost protection.

    As for why it heated to 24 degrees - I doubt that that temperature in particular was significant, it's more the fact that when the temperature is well below the cut-in temperature (hot water temperature minus differential) the CS92A hot water sensor only transmits updates very infrequently (about once an hour) unless a large change in temperature occurs - so 24 degrees was probably the first time a new reading was sent to the controller following the reading that was below 5 degrees.

    I highly doubt that this frost protection would allow them to run unlimited showers - although it would keep coming on to keep it above 5 degrees it wouldn't heat anywhere near your normal hot water temperature - 24 degrees does not make a hot shower.

    How far up the side of the cylinder do you have the temperature sensor ? Is it the recommended 1/3rd of the way up ?

    By the way, how come your cold water temperature is so low ? I'm also in Scotland and I've never seen the cold water temperature as measured at a cold tap go under about 10-12 degrees, even when its well into the negatives outside...
    Last edited by DBMandrake; 12th February 2017 at 01:48 PM.

  3. #3
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    Many thanks DBM for confirming my suspicions on this. It would appear to be Evohome's nanny configuration applying frost protection to the DHW whether I want it or not.

    Do you really want to risk your hot water cylinder dropping to freezing temperatures and have a cylinder containing hundreds of litres of water burst in your loft or boiler closet ? I know I wouldn't!
    My cylinder is in my boiler room on my top floor, so I would be more worried about my boiler freezing than my cylinder. And the top floor zone minimum of 5oC would protect against that. In any case, does anyone really still worry about internal pipework freezing in occupied UK houses anymore? Unless I were going away for a long holiday in the middle of winter, the chances of any of my internal plumbing actually freezing seem very slim.

    24 degrees was probably the first time a new reading was sent to the controller following the reading that was below 5 degrees.
    That makes a lot of sense - thanks.

    I highly doubt that this frost protection would allow them to run unlimited showers
    My cylinder is a Megaflo 210i. The spec for that quotes a coil heat transfer of 24.3kW and a continuous performance with a delta-t of 45K as 463 l/h. That's about 8l/min which would be a decent enough shower at 50oC (Inlet secondary 5oC + 45). So I do think my kids could stay in the shower all day with the cylinder heating in quasi-Combi-boiler mode.

    How far up the side of the cylinder do you have the temperature sensor ?
    It's in the purpose-built pocket that is provided with the Megaflo cylinder. And that is about 50mm above the cold water (secondary) inlet. I suspect that, with a secondary flow rate of 10 - 15 l/min (typical shower) the cold inlet is spraying into the cylinder pretty close to the sensor and so the sensor will reflect this.

    I've never seen the cold water temperature as measured at a cold tap go under about 10-12 degrees
    So where is the energy coming from that is heating your cold water to that temperature? I'm sure it did not leave the reservoir at that temperature.

    Thanks again DBM for your help.

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    As an update to this thread, I am having the same issue again this winter, i.e. the hot water is deciding to turn itself on whether I want it on or not. The recent cold weather has dropped the incoming mains water temperature below 5oC and so, when my CS92 cylinder thermistor registers that incoming water temperature it opens my DHW heating valve. I have my Stored Hot Water settings at 58oC with a differential of 8oC and the outlet temperature from my cylinder has hardly dipped below 50oC.

    The new firmware on my Evotouch is helpful in confirming this. My Evotouch showed the Hot Water at 4oC with the new blue condom icon, implying that the hot water should be off. But the heat demand shown on the new System Summary screen showed "Stored Hot Water" as 100%. So my assumption is that Evohome controls the hot water simply by changing the set point of the water temperature, such that "On" means 58oC (in my case) and "Off" means 5oC (in everyone's case). It also appears that the differential (8oC in my case) applies to both the 'On' set point and the 'Off' set point. The system will turn on at 5oC and stay on until the thermistor reaches 5 plus the differential (i.e. 5+8 = 13 in my case). That fits with my observation of the behaviour of my CS92 (and with the explanations of DB Mandrake and others in threads passim) in that from 4oC the Evotouch reported changes in temperature about every minute until it reached 12oC and it then stuck there, even though the demand was still at 100% and the boiler was firing hard. This was a classic CS92 overshoot but at the "Off" temperature setting. It sat at 12oC for a long time and then suddenly flipped to 23oC and the demand changed to 0% and the BDR91 turned off.

    I still find it irritating that I cannot stop Evohome from heating my incoming water but I am posting this more to share this observation of how the system appears to work.

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    Sounds like the cylinder frost protect is working as it should. I can’t see why you would want to disable it - would you prefer to have your cylinder freeze and burst in an unoccupied house ?

    Warming it from 5 degrees to 23 seems reasonable to me. On central heating frost protect systems that combine an air temperature sensor and a return flow sensor when the air temperature is below 5C the boiler will run until the return flow reaches 25-30 degrees typically.

    If it were only to heat the cylinder a few degrees above 5C then it would have to cycle back on again a lot more frequently than if it heated it to 23C so a smaller differential would cause unnecessarily frequent cycling.

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    Quote Originally Posted by DBMandrake View Post
    I can’t see why you would want to disable it - would you prefer to have your cylinder freeze and burst in an unoccupied house ?
    If I were to leave my house unoccupied for more than a few days in the middle of winter then I would reprogramme my Evohome before I left to give the frost protection I want. The chances of pipework freezing in an occupied house (i.e. one where we are not away for more than about 48 hours) seems vanishingly small to me. Has anyone on this forum actually had pipework freeze solid in an occupied house in the last twenty years? (Not an outbuilding, an occupied room?) This frost protection does seem overdone to me, mitigating a risk that is extremely unlikely to materialise. My cylinder is in my boiler room which is nice and toasty today. The outlet from the cylinder is showing about 50oC. Having the boiler firing to protect my cylinder from frost while I am sitting looking at it seems barmy.
    Last edited by Edinburgh2000; 27th December 2017 at 03:21 PM.

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    Quote Originally Posted by Edinburgh2000 View Post
    If I were to leave my house unoccupied for more than a few days in the middle of winter then I would reprogramme my Evohome before I left to give the frost protection I want. The chances of pipework freezing in an occupied house (i.e. one where we are not away for more than about 48 hours) seems vanishingly small to me. Has anyone on this forum actually had pipework freeze solid in an occupied house in the last twenty years? (Not an outbuilding, an occupied room?) This frost protection does seem overdone to me, mitigating a risk that is extremely unlikely to materialise. My cylinder is in my boiler room which is nice and toasty today. The outlet from the cylinder is showing about 50oC. Having the boiler firing to protect my cylinder from frost while I am sitting looking at it seems barmy.
    There is something not right with the configuration of your HW kit. I have an unvented cylinder and I have seen Evohome reporting hot water temperatures as low as 20C. My sensor is about a third of the way up the cylinder. I have never known frost protection to operate and Evohome has been installed now for 4 Winter periods. The HW kit is wired in series with the existing tank stats.

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    Quote Originally Posted by HenGus View Post
    There is something not right with the configuration of your HW kit. I have an unvented cylinder and I have seen Evohome reporting hot water temperatures as low as 20C. My sensor is about a third of the way up the cylinder. I have never known frost protection to operate and Evohome has been installed now for 4 Winter periods. The HW kit is wired in series with the existing tank stats.
    Yeah I have to say something doesn't seem right, the lowest i've seen ours must be 11oc which is the incoming temperature of the water. Even if your incoming mains was lower than that, you are mixing with water which has at least a bit of warmth in it (Maybe not at the bottom but you will be 1/3 up). Are you sure you probe isn't faulty, or mis correctly placed?

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    Quote Originally Posted by HenGus View Post
    There is something not right with the configuration of your HW kit.
    No, I think DB Mandrake is right in saying that everything is working as designed. My cylinder is a Megaflo 210i for which the online adverts boast "Patented cold water inlet diffuser minimises mixing of cold and hot water" . (See: https://www.plumbnation.co.uk/site/h...ater-cylinder/ on the "Description" tab.) I think the probe is measuring the temperature close to the inlet and so, while hot water is being drawn off, it will report a temperature close to the cold supply temperature. Once the flow stops, there is indeed some mixing and I do see the temperature rise a little.

    Here is a picture of the cylinder. Megaflo cylinder.jpg The temperature probe is on the end of the thin black wire and is in the purpose-built pocket in the cylinder. The mechanical thermostat is under the grey cover to the right of the probe and is wired in series with the BDR91. Its performance implies to me that the temperature reporting of the CS92 is about right, because when the CS92 overshoots and then catches up (i.e. when the mechanical thermostat has limited the temperature rise before the CS92 reports a temperature rise), the Evohome final reported temperature matches the mechanical setpoint to within a couple of degrees.

    The frost protection has come on again this morning and I attach a couple of photos showing the Evotouch showing 6oC and the blue HW "Off" icon but the System Summary screen showing "Stored Hot Water" demand at 100%. (I guess it had dipped below 5oC briefly to trigger the DHW heating.)
    20171229_100339_DHW frost protection.jpg 20171229_100353_System Summary.jpg

    I do have unusually cold mains water. In Scotland all our mains water comes from surface sources (rivers and lochs) whereas in England a significant proportion comes from underground sources (aquifers and boreholes) so I think our water is more susceptible to changes in the ambient air temperature, otherwise we would have warm mists over our lochs in the mornings. My house is a large old property and is connected to a large diameter main which, I guess, gives only a short residency time for the water between the reservoir and my house. I know that some housing developments will have a long reticulated distribution network which can mean that water takes as much as 24 hours to come out of a household tap after treatment. In those cases, with smaller diameter pipes, I guess the water will gain temperature from the ground and from the internals of the property before being used which could explain why some on this forum report winter temperatures not falling below 10oC.

    Anyway, my point in my recent posting was not to suggest anything was not working as intended. I think I am in a minority of one in wanting to disable the frost protection so I will stop advocating that. I was just keen to share the information that the "Off" setting for the Evohome hot water is actually just a change in the setpoint to 5oC + the differential. And that, if you have a cold water supply below 5oC, then the Evohome hot water will act like a combi boiler and always reheat when hot water is drawn off, whether the hot water is set to "On" or "Off".

  10. #10
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    Quote Originally Posted by Edinburgh2000 View Post
    No, I think DB Mandrake is right in saying that everything is working as designed. My cylinder is a Megaflo 210i for which the online adverts boast "Patented cold water inlet diffuser minimises mixing of cold and hot water" . (See: https://www.plumbnation.co.uk/site/h...ater-cylinder/ on the "Description" tab.) I think the probe is measuring the temperature close to the inlet and so, while hot water is being drawn off, it will report a temperature close to the cold supply temperature. Once the flow stops, there is indeed some mixing and I do see the temperature rise a little.

    Here is a picture of the cylinder. Megaflo cylinder.jpg The temperature probe is on the end of the thin black wire and is in the purpose-built pocket in the cylinder. The mechanical thermostat is under the grey cover to the right of the probe and is wired in series with the BDR91. Its performance implies to me that the temperature reporting of the CS92 is about right, because when the CS92 overshoots and then catches up (i.e. when the mechanical thermostat has limited the temperature rise before the CS92 reports a temperature rise), the Evohome final reported temperature matches the mechanical setpoint to within a couple of degrees.

    The frost protection has come on again this morning and I attach a couple of photos showing the Evotouch showing 6oC and the blue HW "Off" icon but the System Summary screen showing "Stored Hot Water" demand at 100%. (I guess it had dipped below 5oC briefly to trigger the DHW heating.)
    20171229_100339_DHW frost protection.jpg 20171229_100353_System Summary.jpg

    I do have unusually cold mains water. In Scotland all our mains water comes from surface sources (rivers and lochs) whereas in England a significant proportion comes from underground sources (aquifers and boreholes) so I think our water is more susceptible to changes in the ambient air temperature, otherwise we would have warm mists over our lochs in the mornings. My house is a large old property and is connected to a large diameter main which, I guess, gives only a short residency time for the water between the reservoir and my house. I know that some housing developments will have a long reticulated distribution network which can mean that water takes as much as 24 hours to come out of a household tap after treatment. In those cases, with smaller diameter pipes, I guess the water will gain temperature from the ground and from the internals of the property before being used which could explain why some on this forum report winter temperatures not falling below 10oC.

    Anyway, my point in my recent posting was not to suggest anything was not working as intended. I think I am in a minority of one in wanting to disable the frost protection so I will stop advocating that. I was just keen to share the information that the "Off" setting for the Evohome hot water is actually just a change in the setpoint to 5oC + the differential. And that, if you have a cold water supply below 5oC, then the Evohome hot water will act like a combi boiler and always reheat when hot water is drawn off, whether the hot water is set to "On" or "Off".
    I have a MegaFlo, although a bit older (However it has the same diffuser system). The only thing I would say is different with mine is that I have my Evohome temperature probe located in the same pocket as the mechanical probe, and from memory this is a little pocket mounted on the side wall of the cylinder not actually inside it. Where as from what I can gather from your picture the probe is in a pocket which sits inside the cylinder (I'm unfamiliar with the newer MegaFlo so the mechanical behind the cover now maybe the same). If not you could try moving the probe to the same location to see if this reduces the issue.

    Have you measured your incoming water main temperature at a cold outlet?

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