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Thread: Single vs Multi - what is actually happening?

  1. #1
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    Question Single vs Multi - what is actually happening?

    I'm trying to understand the differences between single and multi room modes.

    Fundamentally, I understand that for a zone in single room mode the only sensor that is used is the first one that is bound to the controller, and with multi room each radiator uses it's own sensor. so in multi mode each radiator can act autonomously once is has been told the temperature to aim for.

    What I don't understand is in single room mode, how should each TRV behave? Should they all call for heat at the same level? Do the other sensors listen to the 'master' sensor? Can anyone explain at a protocol level what is actually happening?

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    Quote Originally Posted by lloyd View Post
    I'm trying to understand the differences between single and multi room modes.

    Fundamentally, I understand that for a zone in single room mode the only sensor that is used is the first one that is bound to the controller, and with multi room each radiator uses it's own sensor. so in multi mode each radiator can act autonomously once is has been told the temperature to aim for.

    What I don't understand is in single room mode, how should each TRV behave? Should they all call for heat at the same level? Do the other sensors listen to the 'master' sensor? Can anyone explain at a protocol level what is actually happening?
    In single room zones one device is the sensor - it broadcasts the temperature measurement to the controller periodically, (how often depends on the kind of sensors - HR92's send more often than DTS92 for example) and this is displayed on the controller.

    The controller then sends those zone temperatures together with the set points to the HR92's in each zone every 3 1/2 minutes. Interesting point - if you have a single room zone with only one HR92 in it which is also the sensor the HR92 sends the temperature measurement to the controller which then sends it back again a few minutes later. The measurement has to go to the controller and back to the HR92 before it is acted on, it isn't used directly. (But is in multi-room zone mode)

    As to whether the heat demands should be the same for all HR92's in a single room zone - yes, but they won't track exactly.

    They all have the same firmware version and have the same inputs (set points and measured temperatures) so in theory they should track exactly (deterministic algorithm using the same inputs) however in my two HR92 zone they can be as much as about 10% valve position apart.

    I suspect this is caused by factors such as rebooting one HR92 but not the other when changing batteries, missed transmissions (one HR92 misses one of the temperature measurements) etc. Ideally they should track together.

  3. #3
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    Thanks, that makes sense. It's not obvious from a packet dump using evohome_listener, but trying evohome_rf it does show it.

    So I suppose the other question is how does a HR91/2 know that it is in multi-room or single room mode?

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    Quote Originally Posted by lloyd View Post
    Thanks, that makes sense. It's not obvious from a packet dump using evohome_listener, but trying evohome_rf it does show it.

    So I suppose the other question is how does a HR91/2 know that it is in multi-room or single room mode?
    It's one of the zone configuration values/flags that is sent to the HR92's in the zone periodically. Another example of zone configuration values that get sent at the same time are the min/max temperature settings (which restrict minimum and maximum override temperatures) and the "override disabled" flag which stops you making overrides directly on the HR92 essentially locking the controls on the HR92. (Soft tamper proofing)

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