OT wiring specs

[emmeesse68]

Hi all,

In the 2003 OpenTherm spec document V2.2 I found this chapter:

3.1 Medium Definition- Characteristics of the Transmission Line

Number of Wires : 2
Wiring type : untwisted pair *
Maximum line length : 50 metres
Maximum cable resistance : 2 * 5 Ohms
Polarity of connections : Polarity-free, i.e. interchangeable.

* In electrically noisy environments it may be necessary to use twisted pair or screened cable.


Since I had no clue about it, I actually wired my boiler and thermostat using a piece of 2-wires 0,75mm2 double-insulated electric cable (the one you would have used in the old times for the mains cable of a small ungrounded appliance such as a radio set). To connect my OTGW in the middle of the line, I cut the wire and added two simple male/female connectors (I think they are called "barrrel" or "coaxial"), those used for the DC power of surveillance cameras. Now if I want to disconnect my OTGW, I just have to unplug those connectors from the OTGW and stick them together to have my thermostat pluggedd directly to my boiler.

As I checked with my multimeter, cable resistance is far lower than 5 ohms, and cable length could be 5 to 6 meters. Thus, OpenTherm specifications should be met. And I'm not having problems, except for some relibility issue of my "thermostat", a Honeywell R8810A OpenTherm bridge for my EvoHome system, that sometimes seems to be losing its wireless connection to the central controller. I can't easily understand the reason of those losses,since EvoHome logging is really poor. Honeywell specs are max. 30 meters of 2-wire 0,5 to 0,8 mm2, that's different from the OpenTherm above, but is still met by my wires.

Anyway, nothing is said about adding connectors nand the effect they could have. I'm curious about what type of cables/connectors are used to put together boilers, thermostats, OTGW and to share opinions about possible issues related to them... could my issue be some power problem induced by my wiring?

[marcelr]

could my issue be some power problem induced by my wiring?

I don't know the answer to this question, but my set-up looks like this:

toon thermostat -- toon boiler interface -- Schelte's OTGW -- female-female interface cable -- Nefit EMS-OT converter -- Nefit boiler

So, lots of connections and contact resistances. The connection between toon and the toon boiler interface is a long, standard thermostat lead from the living room to the attic.
I have connected the OTGW with power connectors (like you did), with an extra gender changing cable to be able to disconnect the OTGW (+ interface cable) without having to resolder/reconnect everything. Had no issues so far.

grtz,

marcelr

[emmeesse68]

Cool... thanks!

The connection between toon and the toon boiler interface is a long, standard thermostat lead from the living room to the attic.

By the way... What do you mean by "standard thermostat lead"? Is it something as thin as an Ethernet cable or is it more like an electric wire used for appliances?

[marcelr]

What do you mean by "standard thermostat lead"?

2 x 0.25mm^2 or thereabouts, solid core wire, red and blue, with sleeve. Nothing fancy.

grtz,

marcelr

[sincze]

I did not know about this either.

A well my problems were solved by changing the wiring.
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