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Thread: Evohome Hot Water and boiler flow temp?

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    Default Evohome Hot Water and boiler flow temp?

    Hi,

    I've been monitoring the behaviour of my Evohome set-up now the weather has warmed up and I've switched off the central heating. I've set the heating to Off (5deg) on the Evohome so the boiler is only heating the hot water cylinder. I have an S plan and boiler relay set-up with the central heating zone valve removed so just the hot water zone valve remaining. I have the boiler flow temp set to 60deg and the evohome hot water set to 58deg. What I seem to be seeing is that the boiler relay and hot water BDR91s are almost constantly on and the boiler is doing its job and modulating the flow temperature, but holding it at around 50deg. Should I alter a setting on my boiler so that it doesn't modulate its flow temperature to allow the hot water to reach the target temp which should then allow the boiler to switch off until the tank cools to the min of 48deg?

    Thanks,

    Dan

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    Hi,

    I thought I'd post an update to see if there are any comments/thoughts on whether I have my evohome set-up correctly. I've played around with the flow temp on my boiler (Glow Worm 30sxi) and it seems to work OK if I increase the flow temp to 70deg. This allows the DHW temp to hit its setpoint of 58deg and then the evohome switches off its boiler demand. I tried 65deg on the boiler, but this seemed to modulate at about 54deg so still didn't reach the setpoint. I'd be interested to hear peoples thoughts on how the evohome DHW and boiler are meant to work together. I'm not quite sure how/why the DHW hit its setpoint whilst the central heating was on and that it didn't seem to show the same problem, or maybe I just didn't notice. I probably don't really understand how it's all meant to work, but it's triggered a few other questions about how efficiently the boiler can run when it's just heating the DHW as there's such a low temperature differential.

    Dan

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    I can't explain why your HW can't get to 58 when the boiler is set at 65 but can when it is set to 70.

    What I can tell you is that when using OpenTherm the evohome changes the flow temp target to 90 whenever there is HW demand. But because my boiler is set to a 70 max, it will never get to 90. I do find the 90 setting strange. Clearly no one will want their HW that hot. So maybe it's a 'catch all' setting to get the boiler to run at it's set maximum.

    The OpenTherm then goes to 54 (or lower) when there is only CH demand. So in my case I don't mind having the boiler set to 70 max because I know it will only get to that when there is any HW demand and having it at that means the HW temp recharge time is quicker.

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    Hi Bruce,

    Thanks for your reply and it's really helpful to hear how this works with an Opentherm set-up and this sounds similar to how my boiler's conventional controls worked. I've spent a while thinking about this and reviewing the settings on my boiler, but I have to admit that I still have no idea how the boiler settings are meant to work with the Evohome set-up. As my non-Opentherm Evohome set-up simply uses the CH function of the boiler and can't communicate a separate DHW specific demand, it seems that the DHW suffers from the modulation behaviour of my boiler and hence it settles to a lower flow temp than the setting. My guess is that this might be due to the delta between flow and return being very very small when only heating the DHW which is very different to when providing CH. I may be talking total nonsense as I admit that I have very little understanding of how this is all meant to work. With the conventional controls on my boiler the DHW and CH demands were separate with their own temp settings and this would allow the boiler to alter its modulation behaviour for the different demands in a similar way to your Opentherm setup. I would have thought that some installers would hit this problem of the DHW not hitting its setpoint when fitting Evohome, but I haven't found any information on the Honeywell site about recommended boiler settings.

    I've tweaked the boiler temperature down to 68 and this seems to work OK and hit 58, so for now I think I'll just leave it alone and see how it all behaves when the weather turns cold again.

    Dan

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    Quote Originally Posted by bruce_miranda View Post
    What I can tell you is that when using OpenTherm the evohome changes the flow temp target to 90 whenever there is HW demand. But because my boiler is set to a 70 max, it will never get to 90. I do find the 90 setting strange. Clearly no one will want their HW that hot. So maybe it's a 'catch all' setting to get the boiler to run at it's set maximum.
    The hotter the water from the boiler, the faster the water in the hot water cylinder will heat up. The actual temperature of the water in the hot water cylinder is being controlled by the Hot Water Kit sensor, and will stop calling for heat once the water has reached the programmed temperature.

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    Quote Originally Posted by DanD View Post
    Hi,

    I've been monitoring the behaviour of my Evohome set-up now the weather has warmed up and I've switched off the central heating. I've set the heating to Off (5deg) on the Evohome so the boiler is only heating the hot water cylinder. I have an S plan and boiler relay set-up with the central heating zone valve removed so just the hot water zone valve remaining. I have the boiler flow temp set to 60deg and the evohome hot water set to 58deg. What I seem to be seeing is that the boiler relay and hot water BDR91s are almost constantly on and the boiler is doing its job and modulating the flow temperature, but holding it at around 50deg. Should I alter a setting on my boiler so that it doesn't modulate its flow temperature to allow the hot water to reach the target temp which should then allow the boiler to switch off until the tank cools to the min of 48deg?

    Thanks,

    Dan
    To heat the cylinder to 60 in a reasonable time you'll need to allow the boiler to get to at least 70. Depending on what boiler you have it will automatically slow down based on the return temp.

    I have a Vaillant converted to OpenTherm and am able to datalog the boiler flow and return temp and OT control set point. As you have found if you don't let the boiler get a high enough flow temp relative to your hot water set point it will just sit there all day wasting energy.

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    Hi dan
    I've been through exactly the same experience as you, opentherm controlled evohome and a DHW cylinder that I couldn't get to heat up fast / not at all , due to open therm setting low flow temps on the boiler. I am a (very tech literate) end user not a heating engineer so I know nothing of your boiler, however this may help, as I learned a LOT about this whilst working through my issue (which took me several months).
    The solution in my case (viessman 100WB1B boiler) was that the boiler has an extra input ( viessman call it a DHW kit) that you wire to the evo hot water demand BDRs output side. 230V present on the input of this kit , it contains a small PCB that outputs a particular low DC voltage / resistance (not sure exactly what they are) which in turn tells the boiler to ignore the opentherm controlled flow temperature , and not to make much attempt to modulate down, and instead, run the flow as hot as it safely can, as fast as it can.
    without this modification I would get the same scenario as you - cylinder target of 60, boiler flow of 50-odd, cylinder never gets to temp. With this solution in place , my boiler goes from cold start up to a flow temp of 77-80 degrees as quick as possible, using full power , then modulates down the burner (but still keeps the flow at 77-80).
    the other thing we did was modify the flow plan to be a logical W - i.e. when the DHW zone valve opens it closes a master zone valve on the CH circuit, thus forcing all the very hot flow to only go to the cylinder coil. It shuts off CH for a bit, but the cylinder recovery time is so quick now, hardly notice it - with this solution in place the cylinder heats up in no time.

    might be worth you researching if there are any "turbo button" equivalent additional wiring terminals on your boiler. not all at viessman knew about it in my case, it was a difficult problem to get to the bottom off.
    cheers
    Ian

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    Hi, Thanks for your helpful reply. I'm going to take a another look at my boiler and the external controls I removed when I installed the Evohome system. My boiler is a Glow Worm 30 SXI which originally had the Climapro2 wireless controller and Smart Wiring Centre external controls. I don't think there's an easy way to attach the Evohome system to the external controls, but I'll have another think about this and see if there's a way to connect to its DHW controls without interfering too much with the CH. Although the Climpro2 is a simple single zone thermostat it also has weather compensation and its own programmer and it just seemed to much of a complication to leave this in the system, but possibly its separate DHW control may still be useful.

    Dan

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    I've spent a while searching through the documentation and wiring diagrams for my boiler and previous controls (Glow Worm 30 SXI, Smart Wiring Centre & Climapro2 wireless controller). I've come to the conclusion again that there's no way of separately connecting the DHW and CH demands from the Evohome to the boiler to trigger different target temperatures. I'll just stick with my current wiring setup and accept the higher boiler temp.

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    I have maybe another issue with the way Opentherm has been implemented on the Evohome wrt to DHW demand. My DHW tank is set to 60C on the Evohome, but most days the boiler doesn't even need to heat the tank because the Solar panels have already done that and taken the temperature up to 62C (deliberately set that way). But because the DHW schedule is triggered by the Evohome, the OT bridge seems to set the boiler target flow to 90C (i.e. max boiler flow temperature set). And it stays that way for the entire DHW ON scheduled period. Evohome / OT doesn't seem to care that it doesn't need to heat the DHW. This just means that the boiler is working unnecessarily and heating up higher than needed but is only satisfying and CH demand. Ofcourse the boiler does a pretty good job of modulating, but again it's modulating between a higher than required flow temperature. Anyone else seen or noticed that?

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