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Thread: Drayton Wiser Experience on a large system.

  1. #11
    Automated Home Jr Member
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    Apr 2013
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    14

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    This system is doing my nut in.

    Chose it, because as far as I can see, its one of the only system's on sale which has 'by design', the ability to have each device as a 'main mover', i.e. bring the boiler on when 'duty' is required. Yay, I thought!

    Seem's their control loop algorithm is a farce.

    In all my years of automation (industrial / commercial) using PLC's / Scada control, you specific a setpoint, and as long as you were <= sp - deadband, heating would 'call', duty on... achieve setpoint, duty off.

    With this system, they operate a fuzzy on / off, around the actual setpoint, with no figure for duty on / off.... but what's worse, when the difference between sv and pv is small, say 0.5 deg, the system calls for duty, but the TRV which initiates that call, is only open by a small margin (say 10%), and the result is a luke warm radiator, incapable of raising its own temp (and thus satifisying).... so the boiler just runs, and runs, and runs... Meanwhile, the bypass (and other rads with old school TRV's), continue to 'load' the system, burning gas.

    The 'only' way I've found, to get the system to work and maintain comfortable room temps, is manually control the heating with setpoint swings, whereby you change the setpoint on the app (one way or the other), to either bring it on, or shut it off.

    Just ridiculous...

    I've been in touch with drayton, but the 1st line support have never heard the words 'deadband', 'duty', or 'satisfied' and in the period of a week, have failed in their promise to escalate this to the software team responsible for such ludicrous cloud control.

    Very dissapointed, completely 'mickey mouse'
    Last edited by skiv71; 19th December 2019 at 03:44 AM.

  2. #12
    Automated Home Legend
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    Sep 2014
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    Quote Originally Posted by skiv71 View Post
    This system is doing my nut in.

    Chose it, because as far as I can see, its one of the only system's on sale which has 'by design', the ability to have each device as a 'main mover', i.e. bring the boiler on when 'duty' is required. Yay, I thought!
    For the record, both Evohome and Tado V3 (with TRV valves) allow any and all zones to call for heat from the boiler. It's one of the things I like about my Evohome system vs a traditional system where only one zone is actually able to call for heat from the boiler.
    Seem's their control loop algorithm is a farce.

    In all my years of automation (industrial / commercial) using PLC's / Scada control, you specific a setpoint, and as long as you were <= sp - deadband, heating would 'call', duty on... achieve setpoint, duty off.

    With this system, they operate a fuzzy on / off, around the actual setpoint, with no figure for duty on / off.... but what's worse, when the difference between sv and pv is small, say 0.5 deg, the system calls for duty, but the TRV which initiates that call, is only open by a small margin (say 10%), and the result is a luke warm radiator, incapable of raising its own temp (and thus satifisying).... so the boiler just runs, and runs, and runs... Meanwhile, the bypass (and other rads with old school TRV's), continue to 'load' the system, burning gas.

    The 'only' way I've found, to get the system to work and maintain comfortable room temps, is manually control the heating with setpoint swings, whereby you change the setpoint on the app (one way or the other), to either bring it on, or shut it off.
    It's a little bit hard to follow your explanation when you say "no figure for duty on / off" etc.... ? Also, you seem to be expecting a deadband around the set point however none of these systems use a dead band as far as I know. (Evohome definitely doesn't have a deadband) Any deadband is only going to cause the temperature to fluctuate unnecessarily. In some control systems where there is a penalty for making an operating point change (such as consuming fuel in a guidence system) a deadband can be appropriate, however if there is no "penalty" then there is no reason to have it.

    The challenge of balancing radiator valve opening for water flow and heat demand (duty cycle) for the boiler is not as easy as you make it sound, and there might be a slight disconnect between your expectations and what is possible to achieve using traditional TRV valve bodies which lack any sort of feedback mechanism to let the control system know whether water is flowing or not. (TRV valve body pin displacement to water flow rate is extremely non-linear)

    I'm not familiar with the precise workings of the Wiser system however I know the control system of Evohome inside out, so I'll describe that and see how it lines up with your expectations and how the Wiser system seems to be working for you.

    On Evohome there is a +/- 1.5 degree C "proportional band" around the set point, so if a zone is <1.5 degrees below the set point the zone will always wirelessly call for 100% heat (duty cycle) from the boiler, while if it is >1.5 degrees above the set point it will always call for 0% heat.

    Within the proportional band there will be an intermediate heat demand calculated by the self tuning PID controller in the HR92. Because it has an integral component it can achieve the set point precisely regardless of what actual heat demand it requires. So it could be calling for 20% heat at the exact set point or 80% etc...

    It also seems to use self learning heuristics to tune the differential parameter, by measuring overshoot after a step change in set point. For example it might overshoot the target by 1C when going from a 10C set point to 20C but the next day it will overshoot less and after a few days of the same set point cycles it adapts to the point where it can hit the target bang on without over or undershoot. (Only if the room has reasonable lag characteristics though)

    When it's maintaining the temperature around the set point there is no deadband that I'm aware of, it will make small, subtle adjustments to keep the temperature as steady as possible.

    Every zone calls for heat via the central controller and the controller then takes the highest head demand from any zone and sends that on to the boiler relay which does TPI with a default cycle time of 10 minutes, although this can be changed. There is a minimum on time of 1 minute which means there needs to be at least a 10% heat demand before the boiler relay goes on at all.

    If you use Opentherm instead of the relay the 0-100% heat demand is instead translated to a call for heat of (I think) 10-90C instead of a duty cycle, and of course the boiler itself will clip the lower and upper limits as appropriate, so not all of that control range is useable.

    One of the major challenges I've seen which Evohome gets mostly right but still stumbles with slightly is trying to balance the opening of a valve with the call for heat demand - it's not easy due to the extreme non linearity and interaction with differential pressure in the system which can vary with the activity of other zones.

    The way Evohome handles it is to perform a "calibration" of the TRV valve body by compressing the pin down until a certain amount of resistance is met, this becomes the 0% position, it then counts out X number of rotations of the collar that controls the pin based on whether it's set to short or long stroke mode. Typically long stroke mode will hit the limit stop on the collar. This end becomes 100%.

    With most TRV valve bodies this works out so that the bottom 30% of pin travel is essentially "lost" due to the compression of the washer in the valve - over that bottom 30% of pin movement no water will flow. From about 30% to 70% you have a somewhat "linear" region where the valve position can modulate the water flow. Above about 70% there is little or no increase in water flow right up to about 100%.

    An HR92 in Evohome takes that 0-100% valve position and passes it through a lookup table which results in:

    0-30% - heat demand 0%
    30-70% - heat demand increases with a slow linear ramp.
    70% - 100% - heat demand increases with a steep linear ramp.

    So below 30% no water flows and there is no call for heat.
    Between 30-70% there is little change in call for heat but a lot of variation in water flow - the radiator temperature is primarily controlled by water flow.
    Above 70% there is little change in water flow but a large change in heat demand called for from the boiler - the radiator temperature is primarily controlled by the flow temperature coming from the boiler.

    This works fairly well but inevitably there is a bit of "luck" in that 30% pin position corresponding with water just starting to flow through the radiator. If water didn't start flowing until say 40-50% you can be in a situation where there is a call for heat from the boiler but water can't flow through the radiator. Likewise if it starts flowing below 30% you can be in a situation where heat called from a different zone can cause this radiator to start heating up at an opening where it doesn't actually want any heat. Neither is desirable.

    Unfortunately there is no direct way for the TRV to know whether the valve body is actually open enough for water to flow and if so how much. There is a lot of assumption about the valve body characteristics built into the algorithms. The HR92 does have a stroke setting which changes those assumptions between two different sets which does increase valve compatibility however it's possible for a valve body to simply not be properly compatible if for example the compression range of the washers is too long or too short.

    I suspect the problem you're seeing with the Wiser system is that the TRV unit is not calibrating correctly for you valve bodies, or the valve body properties are not fully compatible in terms of pin travel, pin length, washer compression and position vs flow characteristics. Or the wiser valves just aren't very good...

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