[OpenTRV-interest] OpenTRV board as 868MHz transceiver

[Marko Cosic]

On 10 January 2016 at 13:30, Alasdair Macdonald <
alasdairgmacdonald at gmail.com> wrote:

> I can read the
> open-ness (but only from the locally-connected "Hub" device) and "M"
> sends a command that relates to open-ness. But does it send a command
> to a paired TRV to open (precisely) to that percentage? I thought that
> the logic that relates to achieving a target temperature was more
> important.


FHT8V valves are fairly useless creatures. In ideal conditions they can
reliably move a TRV from "off" to "on" every 2 minutes. That is all. In
typical conditions they can reliably move a TRV from "vaguely off" to "on"
every 2 minutes. They are not in any shape or form capable of precisely
controlling valve opening at the flow rates needed for a radiator.

This is similar to how mechanical TRVs actually operate in the field: few
are proportional controllers because the combination of high gain (to
minimise temperature changes in the room - they only have the P-term not I
or D terms) and static friction (stiction/hysteresis) mean they do 'nothing
nothing nothing shut' and 'nothing nothing nothing open.'

Flowrate during reheat conditions is controlled entirely by the lockshield
valve or presetting within the TRV body. For continuous heating mechanical
TRVs give good performance without these. For intermittent heating
mechanical TRVs that aren't set properly on the lockshield valve or
presetters have awful balance: extended reheat periods and high return
temperatures result.
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