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Thread: Evohome/Opentherm - Hot Water Cylinder Heating

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    Default Evohome/Opentherm - Hot Water Cylinder Heating

    In the 5 months since a new boiler with OT was installed, I have noticed that irrespective of how much hot water is drawn from the 250L unvented cylinder, re-heating takes exactly 46 minutes. I can hear a BDR click on at 5.57am (Evohome 3 minute clock error) and off again at 6.43am. The boiler has a TMax temperature of 70C and the cylinder re-heats to 60C. Initially, I thought that this was down to our boring pattern of hot water usage; however, the hot water took a hammering last night (visitors) but, once again, it was 46 minutes to re-heat.

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    Automated Home Ninja Dan_Robinson's Avatar
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    Laws of physics day you are misinterpreting the things you are observing.

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    I am not sure that it is that simple. Take a kettle for example with say 1 Litre of water at varying start temperatures. Physics would dictate that the lower the water temperature, the longer it will take to boil. The two givens here are water volume and the size of the kettle element in kW.

    A boiler isn't like a kettle as the heating output varies. What I believe that I am seeing is Evohome/OT/boiler varying the boiler thermal output. The amount of time needed to re-heat the cylinder may be in the order of 46 minutes irrespective of starting temperature but the amount of gas used each day for HW re-heat most certainly varies depending whether a shower, bath or shower and bath were taken on the same day. Whether my reasoning is true or not, it is what I am observing.

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    Automated Home Ninja Dan_Robinson's Avatar
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    The heat output of the boiler varies depending on the dt and energy transferred through the coil. But that doesn't mean that it'll take 46 minutes everytime.

    Depending on boiler rating, coil size, flow temperature, cylinder temperature target, the chances of it being the same time every day are slim to non existent.

    I

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    Quote Originally Posted by Dan_Robinson View Post
    The heat output of the boiler varies depending on the dt and energy transferred through the coil. But that doesn't mean that it'll take 46 minutes everytime.

    Depending on boiler rating, coil size, flow temperature, cylinder temperature target, the chances of it being the same time every day are slim to non existent.


    I
    Theoretically, I agree, but it most certainly isnít what I am experiencing. Heating time over the past week has been 46 minutes but the gas usage has varied: (kWhs)
    5.6
    6.7
    5.6
    6.7
    5.6
    9.0
    7.9
    5.6
    12

    HW heating is ON from 6 to 9.30am.

    I will be interested to see what happens when I return from a week away in early September.

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    Automated Home Ninja Dan_Robinson's Avatar
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    It's more to do with heat transfer rates due to low dt between the boiler flow and the cylinder.

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    Quote Originally Posted by Dan_Robinson View Post
    It's more to do with heat transfer rates due to low dt between the boiler flow and the cylinder.
    I bow to your superior knowledge.

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    Automated Home Ninja Dan_Robinson's Avatar
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    I would be looking at the actual cylinder temperature compared to what you have it set to.

    Does it get close to target (don't rely on Evohome to tell you this), then sit there ticking over for a while not quite getting to switch off point?

    Don't concentrate on the gas consumption. That is a direct result of the quantity used.

    Also worth looking at the position of the cylinder sensor.

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    You may be overthinking this. My cylinder reaches 60C every time (no overshoots) and this has been the case for the last three years. The CS92A was replaced last year as it was throwing up a lot of loss of sensor comms faults. I am not for a moment suggesting that there is anything wrong with my system, boiler or Evohome. All I am saying is that it appears to take the same amount of time each morning to re-heat the cylinder up to 60C. This simple fact - based on repeated observation - has just piqued my interest.

    If the BDR was clicking off at random times between 6 and 7 am then I probably would not have noticed anything Ďunusualí. Just because something shouldnít happen doesnít mean it isnít happening. I know: I spent 20 years of my life watching modern flight avionics attempt doing things that the system designers said just wasnít possible. Unlike Evohome, we usually had recorded data on all unusual occurrences.

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    Quote Originally Posted by HenGus View Post
    In the 5 months since a new boiler with OT was installed, I have noticed that irrespective of how much hot water is drawn from the 250L unvented cylinder, re-heating takes exactly 46 minutes. I can hear a BDR click on at 5.57am (Evohome 3 minute clock error) and off again at 6.43am. The boiler has a TMax temperature of 70C and the cylinder re-heats to 60C. Initially, I thought that this was down to our boring pattern of hot water usage; however, the hot water took a hammering last night (visitors) but, once again, it was 46 minutes to re-heat.
    Reheating when ? Immediately after hot water use during the day enough to trigger a re-heat, or only after it has sat overnight with the hot water scheduled to be off ?

    If you're only talking about the scheduled re-heat in the morning after it has been off overnight then yes, it will take about the same time every day regardless of how much hot water you used the previous day provided that you don't use a lot of hot water late at night after it is scheduled to be off.

    Like you, our hot water takes pretty much the exact same time to re-heat each morning. I don't recall what it is exactly, but with our current flow temperature of 65 degrees it's about 30 minutes. (It's only a 100 litre cylinder)

    As long as you don't use much hot water after it is scheduled off at night then you have roughly the same starting point every night in terms of heat stored in the cylinder. It will then lose heat at a rate that is determined by the mass of water in the cylinder and how well insulated it is, and the ambient temperature around the cylinder, which won't change a lot. In the morning it will take about the same time to reheat it back to your target.

    Nothing strange about that ?

    If you ran a full bath after it was scheduled off at night so that it wasn't able to re-heat then and there, I'd expect the re-heat time in the morning to be a bit longer, but not a lot longer as the lower temperature in the cylinder means less additional heat loss overnight, and heat loss is going to be asymptotic to the ambient temperature of course.

    Re-heat time after hot water use will vary wildly depending on how much hot water you draw - due to stratification only a small amount of water needs to be reheated. For example if you only draw 1/3rd of the hot water out of the cylinder and that triggers a reheat, only the 1/3rd of the cylinder with cold water at the bottom needs to be reheated - in fact the convection currents won't even flow around the top of the cylinder, it will only convect around the element and as high as the top of the cold layer, which will act as a boundary.

    On our cylinder if I run the hot water only long enough to trigger a reheat (probably about 1/4 of the cylinder) it only takes about 8 minutes to reheat at the same flow temperature.
    Last edited by DBMandrake; 27th August 2017 at 10:55 AM.

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