I've ventured in building an OpenTherm (OT) master controller, aka Thermostat.

Online I found plenty of diagrams (one and two) that roughly are all the same. The foundation of them is fine.
Someone invented this like in 2012 or even before and everybody has been replicating it,
sometimes introducing bad ideas, sometimes good ideas but never nailing it.

Bottom line, find below an improved Opentherm design.
EDIT: see post #5 below why the idle state issue is not an issue

It solves the following issues these bad designs have:
- The specification states in 3.2.2.1 that there is an IDLE state, which is a wire voltage below 7V.
However, the bad designs have an idle voltage of 17V, a HIGH
This actually means that if your microcontroller freezes or has no power, your boiler will heat forever!
- The transition sensitivity is terrible if R5 is too low. You'd like a steep transition.

With the addition of one NPN (Q2) and one resistor (R6) and making D4 a 3V9 instead of 4V3 the idle issue is solved.
With the change of R5 to 4k7ohm we have a transition range of just 1.5V

The LTspice output shows the diagram where I have created two inputs
- a wire current source that ramps from zero to 23mA in 23 seconds, to show the transition for input
- a voltage pulse every second that shows the wire voltage based on a high or low (or dead) master input.

When applying this, do not forget to invert in hardware or software to send the right bits ;-)

My question:
- Review if this makes sense.
- Post wherever you have accounts to refer to this design and prevent overheated houses in the odd chance of bad luck.

 

For the record, changing D4 is not needed... Then idle voltage is 6.5V which is also below 7V maximum

 

OK, certainly an interesting collection of comments and ideas. BUT, for starters, what is "opentherm"?? And what does it do for us, and more importantly, even is why should we be interested in it at all???
I sort of gather that it is some sort of temperature control for something.
AND, what is "wire voltage"?? I understand wires and voltage very well, but this tends to leave me a bit lost.
Certainly an inventor is totally involved with the invention, as is proper. But those of us not soinvolved, and very poor at mind-reading, tend to get left behind. That is the case here.

 

Hi Bill,

I recommend that you follow the links in my post, which give you the background you are asking for.
In short OpenTherm is a protocol to control heating systems (and HVAC) and is supported by all relevant brands in the industry. Being able to interface with this is relevant to many people trying to improve their house heating.

 

Reading the specifications one more time, I realise that my analyses is incorrect for the idle concepts versus the heater starting. My Vaillant boiler must be interpreting things wrong, since it reacts to failing communication with Vhigh as switch on the boiler.
The spec explains in 3.5 that this should only happen in Vlow state.
Also chapter 6 is confusing. A PWM duty cycle of 100% means Vlow all the time, against expectation.
So, the specification is defining inactive opposite to idle... but it does so consistently.

The only valid point that is left from my post is that R5 should not be 330 ohm...

 

Interesting indeed. Much more jargon than I am used to seeing in technical stuff.
As for a system failure causing constant boiler operation, THAT IS A FATAL DESIGN ERROR. Boiler explosions are quite nasty events.

I went thru the specification and my take on it is that it is a solution desperately in search of a problem.

Not only would it create a very profitable new market for incredibly complex hardware and software systems, it certainly creates new fields of required expertise in data communication.
In addition, the whole concept of connection via a single wire pair, not twisted , and apparently not grouped as a pair, does not seem aimed to make it reliable.

Certainly as a business plan it would succeed in creating a whole new industry producing lots of profit. The new required skill sets will allow a new group of practitioners to become wealthy.

And even more, it would assure that the number of folks able to maintain their own systems would be vastly diminished.
If it seems that I am not in favor of a vastly computerized home HVAC world, that is exactly correct.