Nest@opentherm

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Nest@opentherm

Postby fritzberlin » Fri Dec 05, 2014 10:52 am

Hi,
I'm trying to get my new Nest On/Out thermostat to work together with a Honeywell Modulation thermostat. I figured out that the Nest can give a pulse to a switch that lets go the Opentherm protocol of the Honeywell to my Remeha 28W ECO. [ Honeywell in the "continu" mode, external temperature input on]. However, when Nest stops asking Opentherm by setting the 2 wires to off, Honeywell is loosing its memory and time indication after a while. I therefore need a permanent low voltage input to the Honeywell via Opentherm, without any signal on it. \

QUESTION 1 : What's the optimal voltage [without pulses] of an Opentherm connection.
QUESTION 2 : Does it harm the Honeywell Chronotherm thermostat to provide an external Alternative Current (AC) feed, switching with the real Opentherm connection [ when in operation]
QUESTION 3 : do I somewhre need a condensator during the "off" modus to prevent Opentherm pulses from the Chronotherm to the external AC input. Any idea what pF ?
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Re: Nest@opentherm

Postby hvxl » Sat Dec 06, 2014 12:49 pm

ANSWER 1 : An opentherm master device (like a thermostat) expects to be powered using a regulated current. The thermostat then controls the voltage. On an idle line the current should be between 5 and 9 mA. The current source should be able to provide at least 18V.
ANSWER 2 : I would advise against doing that. The device has not been designed to interface with a fixed voltage, let alone an AC power source.
ANSWER 3 : If you power the thermostat with a proper current source, it should not have any problems with the changing voltages the thermostat uses to send opentherm messages. Of course the thermostat will indicate a communication error if it doesn't get responses to its queries.
Schelte
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Re: Nest@opentherm

Postby fritzberlin » Sat Dec 06, 2014 2:13 pm

Thanks for your fast respons.

At the moment I use an old Chronotherm I found at Marktplaats.
So I did the next step.
A small 5V DC powerunit did lighten up the display. It also restarted the clock on the Chronotherm [ on its own] . Within a minute I got a fault : Wrong Connection ["Verkeerde bedrading" in Dutch ]. Afterwards I made the connection with the Rema again and the display shows the regular information, starting up the Remeha.
Maybe it's just a question of time when it collaps. I don't harm the Remeha inteface I guess, while there is no connection in the 5V-DC modus.
The only fear I have is the condition of the condensor in the Chronotherm, the regular back-up. I cannot see if it is getting hot, or even it's working with the 5V-DC input.

QUESTION 4 : Has anybody seen a scedule of the Chronotherm Moudulation somewhere on Internet ?
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