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Thread: Evohome + Opentherm v Evohome + Boiler Outside Weather Compensation

  1. #31
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    Thats a shame. Because that would have even given weather compensation too. Am still wondering if I should go down the Opentherm route or not. Given that I will be taking a risk of installing a VR33 in my Vaillant boiler which I am "not supposed" to in the UK.

  2. #32
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    If you look at ErP directive - evohome + OpenTherm + 3 or more heating zones ranks higher than Weather Comp. I am not really sure why people get so hung up on WC - it is just a modulating control system based on outside air temperature. OpenTherm is a modulating control system based on internal temperature and actual demand. I would hedge my bets that modulation based on internal reference (actually what is required) would save more than influence based on external temperature (which can fluctuate massively).

  3. #33
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    Quote Originally Posted by The EVOHOME Shop View Post
    If you look at ErP directive - evohome + OpenTherm + 3 or more heating zones ranks higher than Weather Comp. I am not really sure why people get so hung up on WC - it is just a modulating control system based on outside air temperature. OpenTherm is a modulating control system based on internal temperature and actual demand. I would hedge my bets that modulation based on internal reference (actually what is required) would save more than influence based on external temperature (which can fluctuate massively).
    I agree with this. I've never understood the weather compensation argument. You need whatever you need indoors, not out. OpenTherm seems way more useful in making the system run optimally.

  4. #34
    Automated Home Legend paulockenden's Avatar
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    The one advantage weather compensation could give is to not heat south facing zones on a spring morning if solar gain is likely to heat them anyway. A kind of optimisation++

    But it would need to be based on a weather FORECAST rather than an outside temperature sensor.

    I already do something /almost/ like this using IFTTT.

  5. #35
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    Quote Originally Posted by rotor View Post
    I agree with this. I've never understood the weather compensation argument. You need whatever you need indoors, not out. OpenTherm seems way more useful in making the system run optimally.
    On a more conventional system there is a good argument for weather compensation.

    The warmer it is outside the less heat loss through the walls and the less heat the radiators need to put into the rooms. Manual wax-pellet TRV's are not the most accurate things around (and suffer from problems such as Offset error among others) and in this case reducing the flow temperature when the there is less heat loss helps to avoid large temperature oscillations. On a condensing boiler it will also improve efficiency not having the flow temperature any higher than you need for the amount of heat lost from the rooms.

    I also find it more comfortable if rooms are maintained at their set temperature with as low a flow temperature as possible - again with manual TRV's a high flow temperature when the heat loss through the walls is low tends to result in large temperature oscillations as the TRV overshoots, completely closing and then opening again in ~30 minute cycles, and I find the radiant heat from a high panel surface temperature during the "on phase" in warmer weather can be quite uncomfortable.

    I think most of us probably turn our flow temperatures down manually a bit in warmer weather for the reasons above.

    With an evohome system with OpenTherm control most (but not all) of the problems above are negated. Once rooms are up to temperature it will drop the flow temperature as low as it can while still satisfying the needs of the most demanding room, this means you get the comfort of a low panel temperature and minimal temperature oscillation/overshoot, and condensing boiler savings even in the winter. (Which you wouldn't normally get as you would be forced to turn your static flow temperature up high to get a satisfactory warm up time, and weather compensation would also keep the flow temperature high due to the cold outdoor temperature)

    What evohome+opentherm doesn't address as far as I can see, is that during the "100% demand" warm up phase it will still ramp the flow temperature up to the boilers user-set maximum even in warm weather, reducing condensing boiler efficiency and possibly causing a little bit of initial overshoot and discomfort. Once the rooms are warmed up it will then throttle back the flow temperature however.

    In this case, manually turning the "maximum" flow temperature down a bit (or having weather compensation) should still make the warm up phase a bit more efficient in warmer weather and less likely to have rooms initially overshooting until the flow temperature is ramped back. (It only takes a single zone with a high demand to trigger maximum flow temperature, which can easily happen when optimal start staggers the start times of different zones, or just from having a complex schedule)
    Last edited by DBMandrake; 31st March 2016 at 02:03 PM.

  6. #36
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    Quote Originally Posted by paulockenden View Post
    The one advantage weather compensation could give is to not heat south facing zones on a spring morning if solar gain is likely to heat them anyway. A kind of optimisation++

    But it would need to be based on a weather FORECAST rather than an outside temperature sensor.

    I already do something /almost/ like this using IFTTT.
    Weather Compensation cannot account for solar gain and the rooms are heated according to outdoor air temperature. The WC outdoor sensor is normally located on the north face of the building away from solar energy to prevent inaccuracies.

    IFTTT is more adventitious in this scenario using 'OFF' or 'AWAY' in 'Zone X' if 'SUNNY' (using local weather information).

  7. #37
    Automated Home Legend paulockenden's Avatar
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    Exactly, that's why I said it needs to be a forecast rather than a sensor. It would still, technically, be "weather compensation".

  8. #38
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    Personally I think the Evohome zoning will automatically take care of solar gain on the sunny side of the house, as long as the zone temperature measurement points in the rooms are well chosen. (No direct sunlight on the sensors and the measurement location being representative of the room etc)

    Now that we're starting to get some sunny weather I'm already seeing it deal well with afternoon sun in the living room versus the colder side of the house at the same time of day, with no changes to schedules or set points.
    Last edited by DBMandrake; 31st March 2016 at 05:36 PM.

  9. #39
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    Ok I am taking a plunge into the unknown now. Richard seems to have convinced me to jump in at OpenTherm. So have ordered it from the Evohome shop. I've also ordered the VR33 from Netherlands. I'll let people know how I get on.

  10. #40
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    My current setup uses a normal timer for the hot water. Will i need to move to the hot water kit if I plan to use OpenTherm? Otherwise how will Opentherm know there is demand for Hot Water too?

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