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Thread: EvoHome Advice

  1. #11
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    Quote Originally Posted by yngndrw View Post
    I found the same thing with Nest and OpenTherm, I called their technical support line to ask some OpenTherm questions and nobody had an answer even when they asked around. I think it's one of those things that I'll just have to take the plunge and give it a go. I might start off with just a basic system (No HR92s) and then add to it as I learn how it works.


    One question that I have around the installation of the hot water kit is that earlier you mentioned that you have the EvoHome HW kit in series with the existing cylinder stat. My unvented cylinder is a ThermaQ Evocyl 210 which has a stat pocket going into the cylinder. There's an ESi cylinder stat mounted directly in front of the pocket with presumably its sensors going directly inside.

    The ESi documentation suggests that there's two thermostats inside, an adjustable one and a separate 80C limit thermostat with a manual reset inside - Does the EvoTherm HW kit not fully replace both of these or do I need to find a replacement for the limit thermostat to go with it ?

    The HW Kit does not replace either of the two built in cylinder thermostats. These must not be by-passed. The HW Kit is wired in series with the existing stats. My HW Kit controls hot water in the range 55 to 60C. The cylinder stat is set at 65C. It follows that if the HW Kit fails or there is a temperature overrun (an often reported occurrence) then the cylinder stat will close off my cylinder heating at 65C. As I am sure you know, work on an unvented cylinder should normally be undertaken by a G3 qualified GSR engineer.

  2. #12
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    That clarifies things a lot, I was under the impression that the HW kit replaced one of the thermostats. (The user-adjustable one) In which case, I might just fit a HR92 to replace the hot water timer and use that with the existing thermostat and valve setup initially. That saves any work on the cylinder and I'm not certain that the HW kit itself adds a huge amount of benefit for my usage, as long as enough of the radiators are calling for heat throughout the day.

    Do you know how boilers typically work with OpenTherm and the standard call for heat signal ? I believe that normally a boiler with OpenTherm would just have the permanent live and the OpenTherm signal does all of the calling, but what if I wanted both (For example, if I wanted the radiators to call via OpenTherm and the HW via the HW zone valve's contacts) - Would this typically work okay ? (Under the proviso that all boilers will implement it slightly differently)

  3. #13
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    How do you plan to use an HR92 to replace your hot water timer ? Does not compute...

  4. #14
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    (Based on my understanding of S-Plan+, I need to do some tracing to find out exactly how it's been wired) So the HW "zone" is essentially five things in a chain:
    • The HW zone in the 7 day timer
    • The HW thermostat (User adjustable)
    • The HW limit thermostat (Fixed at 80C)
    • The HW zone valve
    • The boiler (Triggered via the switch in the HZ zone valve)

    The timer and both thermostats are in series, requiring that all three are calling for the zone valve to operate. The boiler is then triggered if any of the zone valves (HW or CH1 / CH2) are open.

    I'm proposing to replace just the HW timer with a HR92 so that the EvoHome can decide when hot water should be available, but it would not be able to decide the temperature of it.

  5. #15
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    An HR92 is a radiator controller - it doesn't have any switch contacts to connect to your zone valve. And when it calls for heat it will trigger the heating relay not the hot water relay. So again, I am confused as to why you are trying to dream up some non standard frankenstein configuration.

    I think you're misunderstanding how the different components work together.

    Normally with Hot water control on Evohome you have the hot water temperature sensor which sends the measured cylinder temperature back to the Evotouch controller (either insertion or strap on sensor as appropriate - insertion in your case) and the Evohome then switches on or off the BDR91 hot water relay which then opens or closes the hot water zone valve you already have.

    You can either choose to fire the boiler from the switch contact on the hot water zone valve as you probably do now, or it is possible to add a separate boiler relay directly connected to the boiler, which is the configuration I have. (3x BDR91 in total)

    If the hot water temperature is below the configured temperature and the hot water is scheduled to be on, the hot water BDR91 relay will turn on, at all other times it will turn off. Simple.

    With an unvented cylinder you should always leave the limit thermostat in circuit, but you don't need to go as far as leaving the user adjustable thermostat in place, as Evohome is responsible both for setting the temperature and scheduling it. You could leave it in circuit as well and turn it up above your desired temperature as HenGus suggests but it isn't really necessary and you may not have any other usable pocket to insert the Evohome sensor if you do that.

    So the normal method of installation would be to remove both the hot water timer and user adjustable thermostat in the cylinder, fit the insertion sensor in the adjustable thermostats old pocket, and connect the hot water BDR91 in place of both of those, leaving the high limit protection thermostat in the cylinder in series as well.

    So you'll have BDR91 relay contacts, limit thermostat and zone valve motor all in series.
    Last edited by DBMandrake; 14th March 2018 at 04:44 PM.

  6. #16
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    Sorry yes I meant a BDR91, I copied the wrong code - I should probably stick to the names rather than the codes! My proposal was just to replace the timer with the BDR91 relay and use the existing thermostat.

    The high limit thermostat is built into the user-adjustable thermostat that I have, so I'd either have to use both together or find another limit thermostat if I were to use the EvoHome HW kit to control the temperature. I think it's easier just to leave it in circuit if I decide to go with the EcoHome controlling the HW temperature as well as the time.

  7. #17
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    Taking it out will probably invalidate your unvented cylinder warranty. It is also there if Evohome fails and you want to use the electric immersion heater.

  8. #18
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    Good point, I think they do sell them as a partially-made kit so it was likely fitted from the factory so it likely would affect the warranty - A little annoying as that's the only thermostat port on the cylinder but I probably don't need the EvoHome to control the water temperature, just the time for hot water.

  9. #19
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    Quote Originally Posted by yngndrw View Post
    Good point, I think they do sell them as a partially-made kit so it was likely fitted from the factory so it likely would affect the warranty - A little annoying as that's the only thermostat port on the cylinder but I probably don't need the EvoHome to control the water temperature, just the time for hot water.
    If you go that route you still need the temperature sensor otherwise it won't operate the hot water relay, so you'd need to leave it connected but lying loose somewhere. And the Evohome would always report the wrong temperature and always (when scheduled on) call for heat to heat up the cylinder.

    The only thing that would prevent the boiler from firing is if the boiler was fired through the orange wire for the hot water zone valve, because the cylinder thermostat would prevent the zone valve from opening. Not an ideal solution really.

  10. #20
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    Thanks that's good to know, I thought the EvoHome could just use the relay without knowing the temperature so that's a shame. I guess I could strap the sensor to the hot water outlet of the cylinder so it's at least a ballpark figure, but still just use it for scheduling. Not ideal as you say, but it will let me try the system and I can then get someone in to fit the sensor properly and switch to a more suitable limit thermostat if needed.

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