[OpenTRV-dev] Feedback Mechanisms.

[Thomas Hood]

The commercial systems can be divided into those that only control
radiator valves and those that also control boilers and other heating
system components. The first kind of system is relatively easy to make
and is low-risk: the system only does what the user can already do
with manual or thermostatic radiator valves; and the worst it can do
is make part of someone's house a bit chilly.

The second kind of system is very difficult to make and involves much
higher risks since the system might either run the boiler with all
radiator valves closed (which could damage the boiler if it hasn't
been installed properly) or might fail to switch on the boiler during
extreme cold (allowing plumbing to freeze). To my knowledge only
Honeywell, Danfoss and eQ-3/RWE offer such systems. So far only
Honeywell dares to control an OpenTherm boiler and doesn't do so
"intelligently". I'm not surprised. A "smart" modulating living room
thermostat just has to learn how the temperature in the living room
equipped with a static radiator valve responds to boiler modulation. A
smart modulating controller for a system like Honeywell Evohome would
have to learn how the temperatures in multiple rooms equipped with
mutually independent thermostatic radiator valves respond to boiler
modulation. Multiple boilers, electric floor heating, etc., add to the
difficulty. According to the account I read (of unknown reliability),
the Evohome OpenTherm gateway just calls for boiler power proportional
to how open the most open valve is.
-- 
Thomas