OpenTherm EU standard

Teemok

I was going over the control instruction of the Intergas made IBC SFB-120 and found an interesting TT connection compatibility.

OpenTherm is a Eurpoean communication protocol started in 1996 for controlling modulating appliances (boilers) over two wires. This looks like send and receive coding for t-stat control and conditioned space feedback changing the SWT target dynamically. Looks like lots of boiler data is visible, maybe changeable, from the thermostat. Not much on here about it. Anyone have any experience with it?

While our latest smart phone t-stat integration benefits are cool, the "smart" thermostat is really still just a dumb switch with motion sensing and a mic. This looks smarter. Well, but then again, so is the metric-system and that's a no go apparently

Jamie Hall

Sounds like it may be some sort of serial bus communication — probably relatively low rate, so no real need for fancy wires (like twisted pairs or shielding). Be interesting to pursue this further. Put a scope on one and see what it's sending?

Such arrangements are quite common (some folks would say all too common, but that's a different argument!) in automotive applications today — mostly a high speed serial bus (sometimes an additional low speed bus — sometimes multiple busses), but not so high as to need fancy wires. All sorts of signals go back and forth. Some digital conversions of an analogue signal (throttle position, mass air flow, several temperatures, targe fuel pressures, vehicle accelerations, that sort of thing) and some purely digital (spark timing, injector timing, cam timing… down to the mundane: power window up/down, horn, brake lights… you name it). There may be 10 or more computers talking on a single bus…

(And of course, when something goes sideways, even a slightly corroded connection, the money light comes on and the mechanic is very happy planning his or her next vacation!)

Teemok

It's all open source code so no scope needed. I hear the complication concern. Always valid but it's the way of the world and turning away from it has it's costs too. I remember when the question was who's gonna fix your stupid complex CD player?

Teemok

It seems like it would be great for single zone radiant or low temp radiators. The funny thing is it would make my repeated Ted talk to customers about a thermostat not being a gas pedal completely untrue. I'm going to hook one up and explore how it works. Maybe document with and without performance.

PRR

https://www.opentherm.eu/

http://files.domoticaforum.eu/uploads/Manuals/Opentherm/Opentherm%20Protocol%20v2-2.pdf

https://en.wikipedia.org/wiki/OpenTherm

talk to customers about a thermostat not being a gas pedal completely untrue

So it appears. OTOH most burners have very limited output settings. Like my old lawnmower: half or full. Just because the software byte can be set to 68% doesn't mean any burner will be too literal about it.

Teemok

13-200k btu is a good turn down range. The smooth ramping is quite good not sure why 68% demand wouldn't produce a 68% firing rate. How direct the t-stat command is to the boiler is questionable. I'm sure it's limited buy the guard rails of the installer programing settings and some algorithms. 68% is just a symbol who cares if it's dead accurate. My take is, if you ramp the t-stat set point up real high the supply temp curve will shift up and the boiler will fire to achieve that. Actual firing % rate is likely also a factor of the delta T. A cool start high mass system would fire at 100% for a while. It's not an actual gas pedal but it's much closer to it than the old switch demand.

clammy

Open therm has been used in europe for at least 20 to 25 year years it s old bag and in my own research over 20 years ago i came to the conclusion that due to our wanting to mirco zone heating system that a indoor reset in conjunction w a outdoor sensor would yield nothing but issues in a American mirco zoned heating system aside from having to add low temp protection . Now if it where a single zone either high or low mass emitters then your looking for success , But remember that there the slippie slide of how low can you go without having some boiler protection for prolonged low temps. And lets no forget the fact that this system would be a constant circ and unlike europe there things and ways they do things that most here would never figure out mainly because we like to be in the drivers seat when it comes to controlling things.The other issue is that all of these system are using either panel radiators w trv or in floor high mass radiant a rarity for most european homes but possibly now due to the pushing of even lower temps and to be some what compatible w awhp supply temps . I believe the big v is pushing this w there air to water heat pump ,i would gather that a simple on off t stat just aint gonna cut it nor provide any logic that would be able to limit cycling and control any capacity limiting for the compressor and fluid temps . Always trying not to sound negative but i feel that on both the heating and cooling side that these newer awhp system shall be quite a handful and it will take a little more then the information provided by manufactures to properly design install and most of all meet the home or builders expectation of savings ,performance and comfort without losing one shirt in the details . As stated open therm is a excellent concept which does work based on indoor input but one must look at the parasitic energy use of constant circ and meeting a minor building lose .I feel that on a large building that there plenty to save in a roi but for a smaller home i believe the pay back for shaving a few degrees of supply water temp will never recover the initial investment over even a long period of time possibly in a complete radiant system but never discount minor electrical use and short cycling being in all cases the cycling of equipment would be again controlled by set points temp drops and delays omg . As for using w a air to water heat pump i think u may not have a choice being we all know ( i would hope by now ) that w a tight out door reset curves it some time hard to keep up w heat lose when the temps drop fast and or high winds indoor reset deal w that especially when it not the control limiting the supply temps but its the limit of temp it can produce at low outdoor temps . Open therm to me is just old bag european stuff that they will sell to us as new junk .As stated and probably wrong its most likely over 25 year old technology hustled out of a stock room and sent our way . On a side note any open therm stuff i ve come across clearly is a step above a normal t stat and in some cases incorporate the ability to raise and lower domestic water temp aside from a temp boost . please remember where this stuff comes from which is some where there energy cost have always been at least double or triple out cost bearing that in mind . peace and good luck clammy

Jamie Hall

The reason for the 'scope, @Teemok ,is not so much so you — or someone like you — can figure out how it is supposed to work — it's to figure out, when it doesn't work (which it won't, from time to time), what's gone wrong. Do you have bad data from module X? Or is module Y not understanding the data? Or do you have a lousy ground on module Z? Or is the power missing on pin 48 on module Q? Maybe the termination resistor in the triple encabulator sensor is shorted? Or a mouse ate one of the wires? Do pins 23, 24, and 25 of connector T128 have green crusties and are shorted?

Now I'll grant you the usual response to a wonky network is to throw parts at it until something works. Trouble with that is, it's not usually a bad part, but something simple (and cheap).

GGross

Viessmann used the open therm connections on their boilers for a long long time, it used to really mess up new installers as the wiring diagram would show "OT" on the two terminals you were meant to wire TT to, they would think it was for the outdoor temp sensor and all sorts of fun would ensue!

I tried finding thermostats that utilized open therm in the US for a little bit and failed (this was 8 years ago-ish). My understanding is that the general usage of open therm for residential boilers was as a room feedback or indoor reset type device, the thermostat essentially becomes the "gas pedal" that many folks assume the thermostat already is

Teemok

Should be used in a design ideal for it's use. Agree. A wench when you need a hammer is never great. I hate micro zones but realizes they are what the market wants. It's hard to not to exploit the hydronic zoning advantage. It makes installers/manufactures money and gives the customer the control they are convinced they need, regardless of it being the poor design practice. I agree it's a euro design application.

Since I stared in hydronics in the 80's I've always seen the future of the trade coming from Europe. Pex, Mod-cons, solar, HP's etc. For a long time we've been getting things second hand from Europe. The boiler and T-stat are need, the standard doesn't really add much complexity or cost. Why not have a better self tuned system? it's a very low investment for better change performance and whatever positive efficiency gains. It makes it possible for an algorithm to control the boiler with net data and site history forecasting coming changes. Unnecessarily complex? Yes, for most but if it works well and reliably why not. I see this working well with San Francisco fog changes or vacation home warm ups. Or faulty design or installation mitigation. I really I see this type of thing working best for smaller, comfortable, efficient, individual systems housing. Like it or not that looks to be what we desperately need here.

Higher fuel costs would make it more attractive. That is to say, we aren't hungry enough yet.

Complexity: I have to admit, I like the open source concept. Linux reliably runs much of the internet for a damn good reason. Standards for com. channels are functioning all around us. I don't see this as some problematic rats nest. I change control boards and zone controls form time to time, its the nature of the beast. Getting parts might be harder for sure, until it or something like it is widely adopted. The tech cost for it is next to nothing to add. It could just be an option available use or not. For other markets with better design ethics isn't really a negative, except that it's not what we do. The Swedish build much more efficient structures with little ec=extra cost or special materials. It's design and social ethics is at the root of that.

Old and foreign: True but that doesn't mean it's not any good. Lots of the protocol used today is very old. The C language was started in the 70's. PLC stuff is quite old.

Confusing us American techs isn't all that hard to do. I mean look at the education and training investment made. Could the labeling and explanations be better? Yes! Do we need smarter techs for the the modern world? Yes! I remember when Tekmar's UI design was more than many wanted to learn. it's still beyond many contractors, never mind a typical customer. VCR time clock flashing 12:00 into the dark of the room. There's always reasons the better techs do better. People with lots of money want the "best" systems and they pay for people who provide them and fix them. I can see that this tech is not necessarily in that game. It's more of a common design ethic that seeks the highest efficiency as simply as possible. I know, it reeks of things we individualist profit devotes despise.

Jamie Hall

There is an absolutely fascinating interaction between sophistication, efficiency, reliability, maintainability, and cost. Turns out you just can't get all of them all at once.

As to European tech. Yes, I agree, a lot of it is more sophisticated and so on and on, and under regulatory forces we are being obliged to adopt much of it — I suspect sophisticated computer controlled heating systems (most likely required to be electric) are coming. But… I like to do cars, and I can assure you that I can keep a 1970 Chevy running a lot longer and a lot cheaper than I can a 2024 BMW (which is a bear to work on, by the way — you'd better have at least ten grand in electronic test equipment to properly diagnose anything beyond a flat tire). You nice electric car? Wonderful machine. Something goes wrong? Better to push it off a handy bridge when no one is looking…

Teemok

I know that engineering is a compromise game. Don't trust electronics? Here comes the future anyway. I've always been more mechanical than electrical but I've tried to study electrical some so I'm not completely ignorant. It gets very strange when you get to the history of the first transatlantic cable failure. The idea that really, the energy travels outside of the wire is still attractively mysterious to me. You might enjoy: https://www.youtube.com/watch?v=bHIhgxav9LY

This is a clarification due to some controversy: https://www.youtube.com/watch?v=oI_X2cMHNe0

The need for 10K in test equipment is less and less now. Cheap diagnostic scanners are quite powerful these days. The mind set and skill set to repair modern systems isn't all that different. Critical thinking being a primary requirement and necessity is a good motivator. I cut my mechanics adult teeth at 15 with a 76 Vega. One of the worst cars made. Willies jeeps and muscle cars evolved to VW's with Bosch fuel injection, then turbos and ECU tuning. The whole way the old cats all cried and cried about the new stuff. I have one of those then new stuff trucks that they complained about and It's known to be very good for 500k+ miles. I admit it, I complain changes too.

I had a Leaf EV for 5 years not a single thing ever broke. It was used as designed. Many short in town trips. I had good luck, it was new to 40k. There are many young people hacking and repairing what you might push off a bridge. They are doing it with tools the global market offers. Oh that market, it cuts both ways. Good diagnostics and swap parts, that's the way it is. That said, I like older cars. The problems are known, managed or fixed and yes they are cheaper. I'm not afraid of a an open protocol maintained by a large community. Funny, if a loved brand promoted it hard, I mean like, repeated it over and over in our favorite spaces, many of the unsure would embrace it like its a great thing. Preference prejudice and paradigms, we are well hacked.