[OpenTRV-dev] Using TELE-temperature to add on radio-TRVs

[Thomas Hood]

Hi. First let me briefly introduce myself. I am an electrical engineer
living in the Netherlands. I am trying to reduce my energy consumption and
I think that the next best thing for me to do is to add time-programmable
TRVs to my radiators. There are systems on the market that allow
("radio-")TRVs to be programmed wirelessly and to be controlled by
wall-mounted sensors — OK — and even a few (Honeywell Evohome; Danfoss
Living Connect; RWE Smarthome are the ones I have found so far) that can
control the boiler as well as the radio-TRVs so that if a room needs heat
when the living room doesn't, the boiler will be switched on. But these
systems are all expensive, imperfect and closed, so I am interested in the
OpenTRV project which could provide a better, open, alternative.

OK, now for my first thought. On the OpenTRV website the initial list of
minimum requirements includes the following.

> The boiler end end, ideally, should be a two-wire volt-free
> 240V switch that can be paralleled with an existing room
> thermostat to control an existing non-modulating combi,
> and that will call for heat when any of our TRVs do.

I have a Honeywell Chronotherm Modulation T8851M1000. This thermostat has a
"TELE-temperature" feature such that when two terminals on the thermostat
are connected, the target temperature changes to a defined static value.
With such a thermostat the radio-TRVs could perhaps be added to the system
in the following way.

0. Assume the living room thermostat is set to 20°C.
1. Add radio-TRVs to radiators except the one in the living room.
2. On the radiator in the living room put a dumb (non-radio-controlled) TRV
set to 21°C.
3. Set up the system so that if any radio-TRV calls for heat, the
thermostat is put in TELE-temperature mode.
4. Set the TELE-temperature to 22°C.

When the radio-TRVs are not used or need no heat, the thermostat will keep
the living room at 20°C.

When a radio-TRV calls for heat the thermostat switches on the boiler to
try to get the temperature up to 22°C. The living room dumb-TRV keeps the
temperature from exceeding 21°C. Once other rooms have heated up
sufficiently, the radio-TRVs withdraw the call for heat and the living room
thermostat is taken out of TELE-temperature mode.

Advantage: radio-TRVs can be added on to an OpenTherm system without an
OpenTherm bridge.

Danger: If the dumb TRV is set too low then the living room thermostat will
keep the boiler on with all radiator valves closed.
-- 
Thomas
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