Help with Evohome / Opentherm / Intergas

[Dan_Robinson]

I wish someone had said that about a million posts ago! ;-)

Perhaps having the original question would have helped

[HenGus]

Wires crossed I think. I think that answer refers to the fact that you cannot pair evohome to eco rf internal wireless receiver. And you still can't. You need to use Opentherm bridge

TB - you might think that, but I know the question that I posed which specifically mentioned concerns about the multi-zoning function that Evohome offers when connected to the Intergas RF Eco boiler via Opentherm. I also had a follow on call from Intergas' Technical Department to discuss the matter at some length. The e-mail that I copied from was a request for an update.

[DBMandrake]

The evohome system is using fuzzy logic and Honeywell control algorithms. I was told on several occasions this is not TPI control when using Honeywell to Honeywell technology (such as the Honeywell chip inside the ECO RF) or a bus connection like OpenTherm.

Correct.

PID controllers and TPI shouldn't be confused. A PID controller is a feedback system that produces a proportional (instead of simple on/off) output in response to its inputs. TPI is just one method of taking that proportional output from the PID controller and then using it to control something via duty cycle that would normally only accept an on/off input, like a switched live boiler control.

If you are using OpenTherm the exact same proportional output from the PID controller is used to directly request a flow temperature rather than try to approximate it by duty cycle modulation - which is clearly a superior control method. (Not to mention faster responding to changes, since the heat output of a TPI controlled system has to be averaged over at least one full cycle - typically 10 minutes, whereas a request to change the flow temperature can potentially be actioned by the boiler quite quickly, in maybe a minute or two depending on the load and the requested change)

From what I have read and understood, a fuzzy logic control system can be implemented without PI algorithms, just as long as the process is not 'dumb' (i.e. just an ON/OFF switch controlling a fixed output heat source). The control bandwidth is then defined by the software and the fuzzy logic is told to keep it within that parameter by changing the control environment (closing TRV's and reducing boiler setpoint - the main features I have witnessed in multi zone evohome and OpenTherm).

"Fuzzy logic" can mean anything that someone wants it to mean really - its a very vaguely defined term. I'll tell you what I think the fuzzy logic is in Evohome, feel free to disagree. I think it can be broken into two parts:

One is the PID controllers - which in an HR92 controlled radiator zone is performed by the logic in the individual HR92's. A PID controller needs its parameters tuned to match the characteristics of the system it is controlling to be useful, which in a heating system is everything about the system - flow temperature, radiator size, room heat loss, thermal mass in the room etc etc. Evohome doesn't provide any direct end user control of the parameters, (although I would argue the stroke setting does affect the "proportional gain" parameter somewhat) so it stands to reason that for it to "adapt" to individual rooms that it must be "self tuning", and that this is part of the "fuzzy logic".

So whilst the basic feedback control is a PID controller (no doubt optimised to the peculiarities of a heating system, like the fact that it is asymmetrical in nature - it heats quickly and under control, but has no active cooling so can only cool passively through heat loss - problematic in warm weather) in the background there will be a learning algorithm that watches the response to changes and makes gradual changes to the PID tuning to adapt to each room and optimise response time.

For example if the learning algorithm (lets call it the fuzzy learning algorithm) detects persistent overshoot each time a step increase to the set point is made, it will make an appropriate adjustment to the PID tuning parameters to avoid that overshoot next time. I've seen classic signs of self tuning PID behaviour in my system where the system will adapt to the reduced heat output of a radiator with a towel over it - initially it will "undershoot" for a day or so during warm up but after a while will adapt, when I remove the towel the following day it will overshoot during the same set point change and then in a day or two will adapt again and no longer overshoot.

The second part of the fuzzy logic will be the optimal start and optimal stop algorithms - which reside on the controller, not the individual HR92's. This is learning the heat up rate of each room, so it knows how long the room takes to warm up, combined with how many degrees short it is, it can calculate when to start pre-heating. As the seasons change and the warm up rate changes it learns this and adjusts its calculation to the new rate.

There is no point second guessing about this 'control bandwidth' tolerance either - it is plus or minus 1 degrees C on setpoint which was confirmed by Honeywell at my meeting on Monday.

If I had the time, HGI80, and inclination to measure it I would, but I would be surprised if its +/- 1 degree for Evohome when its +/- 1.5 degrees for a DTS92 on its own directly controlling a BDR91:

http://www.honeywelluk.com/documents/All/pdf/TPI%20Functionality%20explained.pdf

I believe you can use a DTS92 directly with an OpenTherm bridge instead of a BDR91 as well ? Would it be the same proportional band then ?

Also there have been a number of posts like this recent one from top brake which certainly don't dispute the 1.5 figure:

http://www.wordpress-1219309-4387497.cloudwaysapps.com/vbulletin/showthread.php?5238-Honeywell-evohome-and-OpenTherm-integration&p=32860&viewfull=1#post32860

Until I see hard evidence to the contrary or get time to measure it, I'm going to stick with +/- 1.5.

And as Dan said earlier, I don't think +/- 1 is sufficient anyway for efficient control as if you make the window too narrow you'll hardly ever be in the proportional control range at the boiler itself in a multi-zone system. If I was designing the system I would have made it +/- 2 degrees or even made it system installer adjustable...

[top brake]

TB - you might think that, but I know the question that I posed which specifically mentioned concerns about the multi-zoning function that Evohome offers when connected to the Intergas RF Eco boiler via Opentherm. I also had a follow on call from Intergas' Technical Department to discuss the matter at some length. The e-mail that I copied from was a request for an update.

There has been some confusion that has is resolved. Hope that the situation is now clear. You bought an ATAG didn't you?

[DBMandrake]

I'll write a fresh thread sometime to clear up all these misconceptions, or maybe we can do a webinar.
I'm surprised that there is so much fascination with how evohome works rather than just being happy that it does work

Then you don't know the target audience on this forum.

There are some of us (and I include myself) that like to know every tiny detail of how something works and analyse it in detail. And some of us have backgrounds in programming, electronics or engineering that put some aspects of Evohome right in our wheelhouse to analyse... So no, some of us are not just happy that something works, we want to know how in fine detail, especially when it is installed in our own house.

[HenGus]

And as Dan said earlier, I don't think +/- 1 is sufficient anyway for efficient control as if you make the window too narrow you'll hardly ever be in the proportional control range at the boiler itself in a multi-zone system. If I was designing the system I would have made it +/- 2 degrees or even made it system installer adjustable...

Even with my limited technical knowledge (as demonstrated many times on this forum), I can understand where @DBM is coming from. From a condensing boiler's perspective - in terms of efficiency - it would seem to make more sense not to use Evohome with Opentherm (I note that Richard disagrees). I suspect that a boiler would struggle to get in to full condensing mode in a normal home with people in and out of rooms etc. Surely, in the same way that an installer can adjust boiler parameters, it would make sense to allow for some adjustment within the Evohome settings?

[top brake]

Then you don't know the target audience on this forum.

There are some of us (and I include myself) that like to know every tiny detail of how something works and analyse it in detail. And some of us have backgrounds in programming, electronics or engineering that put some aspects of Evohome right in our wheelhouse to analyse... So no, some of us are not just happy that something works, we want to know how in fine detail, especially when it is installed in our own houses.

I understand that you and others do and where possible we will try to provide answers; however please understand that there are not public domain answers to some of the questions and the response needs to be either obtained or researched before responding. And sometimes it will not be possible to provide an answer or response as it may become a conflict with intellectual property.

Hope this explains.

[top brake]

Even with my limited technical knowledge (as demonstrated many times on this forum), I can understand where @DBM is coming from. From a condensing boiler's perspective - in terms of efficiency - it would seem to make more sense not to use Evohome with Opentherm (I note that Richard disagrees). I suspect that a boiler would struggle to get in to full condensing mode in a normal home with people in and out of rooms etc. Surely, in the same way that an installer can adjust boiler parameters, it would make sense to allow for some adjustment within the Evohome settings?

Sounds like there is some disconnect in your understanding of what OpenTherm control is and how boiler flow temperature is requested and acted upon. No manual setting adjustment is needed as evohome fuzzy logic does an auto tune to create an optimal control system in harmony with the zone controls

[DBMandrake]

Even with my limited technical knowledge (as demonstrated many times on this forum), I can understand where @DBM is coming from. From a condensing boiler's perspective - in terms of efficiency - it would seem to make more sense not to use Evohome with Opentherm (I note that Richard disagrees). I suspect that a boiler would struggle to get in to full condensing mode in a normal home with people in and out of rooms etc. Surely, in the same way that an installer can adjust boiler parameters, it would make sense to allow for some adjustment within the Evohome settings?

I think Richard has already explained that it should be possible for a properly set up system to condense even with the boiler running at a "full" installer configured flow temperature of say 70 degrees, provided that you have a 20 degree return temperature drop.

But regardless of possible inefficiencies, having the boiler switch to going full tilt just because one zone has been scheduled to come on outside its current proportional band can be problematic when it causes other zones that are in their proportional band and maintaining a nice steady temperature to suddenly overshoot a degree or more before the HR92's can adapt to the change in flow temperature.

An example of this would be a bedroom that is maintaining 18 degrees over night (as the only radiator running) requiring only a low boiler flow temperature - when the downstairs rooms come on in the morning and demand maximum flow temperature to heat up quickly the bedroom can temporarily overshoot by 1-2 degrees due to the sudden increase in flow temperature that happens before the HR92 can respond to the change.

This is an issue that I see to some degree depending on the time of the year, and the overshoot is often enough to wake me up or at least leave me feeling a bit dry and thirsty when I wake up.

The only way to minimise this effect is to limit the maximum flow temperature to be only as high as needed under the prevailing weather conditions, hence I manually turn my flow temperature down in the warmer months and only turn it up in winter. If I left the flow temperature on maximum year round then I would have significant schedule induced room temperature overshoots in warmer weather, where the schedule change of one room causes significant overshoots in other rooms.

This is also the same reason why people who are unable to limit the maximum flow temperature under OpenTherm control were having issues with overshooting, especially at this time of year.

[HenGus]

There has been some confusion that has is resolved. Hope that the situation is now clear. You bought an ATAG didn't you?

I did buy an Atag on the basis of the problems reported by Richard, Dan and others (and confirmed by Intergas). As I said in other posts, I was surprised by the time that it took my Atag boiler's TSet flow temperature to fall in the 3 weeks when my CH was on. I now understand why: it was just responding to one or more of the 19 HR92s in my home demanding heat. My schedule has been one of zones coming on in a morning to meet the flow of the family through the home. I will now have to re-think this strategy and possibly my choice of Evohome. It would seem to me that the greater the number of zones (and HR92s - I have 12 zones and 19HR92s), the greater the possibility that the boiler will remain at max flow temperature under Opentherm control. I confess that I am no longer as sure as I was that Evohome is the best heating control system for my home (and my wife is no great fan of the level of complexity attached to it). Single zone with Opentherm control might be a better all-round solution.

[Dan_Robinson]

This is why you need a boiler that can limit its heating Tmax.

Can we all acknowledge that OpenTherm has no influence on the gas valve. That is the the boiler to manage as 8t sees fit.

[Dan_Robinson]

An alternative to OpenTherm is a hybrid tpi and weather compensation. It is essential to design and adjust the system correctly to make this work.

[HenGus]

This is why you need a boiler that can limit its heating Tmax.
.

Atag IS boilers have a Parameter P0 which has 4 settings - CH TMax of 80C down to 50C.

[top brake]

Indeed Dan. The room control system is one element of a heating system. The boiler sizing and minimum modulation, the sizing and balancing of the radiators or underfloor circuits; all vital to get these working in harmony to get an optimal system. This is why it is best to get a heating engineer to install your evohome system

[blowlamp]

This is why you need a boiler that can limit its heating Tmax.

Can we all acknowledge that OpenTherm has no influence on the gas valve. That is the the boiler to manage as 8t sees fit.

Agreed. If it was my previous post that you had in mind, then what I meant to convey was that the flame would modulate by virtue of OpenTherm requesting a specific flow temperature from the boiler.

I still don't see any reason for Evohome to not be controlling CH maximum flow temperature though - other controllers can & do.