Open Circuit Voltage

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Open Circuit Voltage

Postby mmaz » Wed Jan 06, 2016 11:25 am

I am new to the forum and I am approaching for the first time OpenTherm interface.
I have an ATAG Q15S boiler which at the moment doesn't have a dedicated controller (on/off room thermostat are controlling only floor valves).
I'd like to add an OTGW to control the boiler, but before buying I want to be sure it is compliant (because it is not listed inside the compatibility list).
The ATAG Wize controller (which is instead present in the list) claims to be compatible with my boiler.
I did this preliminary checks with multimeter and few resistors I have at home:
Open circuit voltage: 10.69V
Resistor load 10.90k: 8.87v
Resistor load 3k25: 8.77v
Resistor load 1k717: 8.59v

So it seems the boiler is providing 5mA only with low load.
I would have expected an higher voltage with open circuit (and with 3k25 something like 16V).
I am right?
Which is the experience of other users?

Thanks in advance,
m.
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Re: Open Circuit Voltage

Postby hvxl » Thu Jan 07, 2016 10:49 am

Those measurements do not meet Opentherm specifications, so the boiler is probably not currently configured to use Opentherm, which ATAG calls Z-bus. According to the installation manual of the boiler, the factory default is for the boiler to auto-detect the communication protocol. I'm not sure if that works with non-ATAG thermostats. So it's probably best to manually set it to Opentherm by changing parameter 60 to 2. Then measure the line again.
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Re: Open Circuit Voltage

Postby mmaz » Fri Jan 08, 2016 7:10 pm

Thank you very much for your reply.
I checked my user manual and the parameter 60 is not present, my model could be too old (2007).
I will ask to ATAG dealer if it is only a metter of FW upgrade instead of a newer control board (with different HW).

m.
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Re: Open Circuit Voltage

Postby hvxl » Sat Jan 09, 2016 11:00 am

An ATAG boiler is supposed to have the Z-ready logo if it can support opentherm:
Image
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Re: Open Circuit Voltage

Postby mmaz » Fri Jul 01, 2016 4:56 pm

I have just changed the control board with a newer one.
After changing the parameter 60 to 2 now I see 18.54V on the terminals, now I can go with my project.
Thank you very much.
m.
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