Originally posted by DBMandrake
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Evohome Opentherm Boiler decision and set up
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Originally posted by Edinburgh2000 View PostIs there a differential term? I thought Evohome uses TPI (i.e. Proportional and Integral only). Or is it just the Evohome controller that applies TPI control and the HR92s do something different?
The HR92 is a PID controller which is a feedback system where measured temperature and set point are the inputs and valve position is the output, which is then also modified by a transfer function to produce the heat demand percentage sent to the controller. The aim of it is to adjust the valve position and heat demand to reach the target set point as quickly and as accurately as possible without overshoot or oscillation. (Quite a challenging task, as it happens)
TPI (time proportional integral) is something completely different. It's really just a fancy name for duty cycle modulation or pulse width modulation where there is a constant cycle time (default 10 minutes on the BDR91) and the on time is proportional to the desired heat demand - hence the "proportional" bit. Integral is referring to the total heat output of the boiler averaged over time over multiple cycles. (Since integration calculates the area under a graph)
So, not related at all. The PID controller is what measures the temperature and compares it to the set point to vary the heat output from the radiator, TPI is just a method of modulating the power output of a boiler that doesn't normally have any means to do that. (Fixed user set flow temperature) If you are using OpenTherm you are not using TPI but the HR92 is still behaving as a PID controller.Last edited by DBMandrake; 29 January 2018, 09:32 PM.
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Originally posted by paulockenden View PostAny savings with Evohome are more likely to come from not heating unused rooms.
For example we don't heat our bedrooms during the day, and at night the bedrooms are on at a low temperature but the downstairs is all off. In the early evening most of the downstairs is on at 20'ish (except for a spare room which is 15) but in the later evening all the downstairs except the living room drops a couple of degrees. (I've compared gas usage from not doing this - the two degree drop from 8pm to 11pm in 4 rooms makes a big difference!)
TPI is, to my mind, more about comfort than money.
If you compare it to the normal thermostats that came before (first mechanical, then electronic) they typically had a 2 degree hysteresis. Set it to 20 and the boiler would run at full set flow temperature until the room reached 21 degrees then the boiler went completely off. With thermal inertia the room might gain another degree and end up at 22 degrees before it started to drop back down again.
The boiler would remain off, potentially for an hour or two until it reached 19 degrees then turn on again. Rinse and repeat. You end up with a very long period of time where no other rooms can get heat because the boiler is off for so long each cycle and the room temperature is fluctuating 2-3 degrees every hour or two as the boiler cycles on and off. Not very comfortable.
TPI means that in the proportional band the boiler is cycling on and off at a regular rate - for example every 10 minutes, and by adjusting the on period the average heat into the room can be very precisely controlled (theoretically infinite resolution) and thus the exact balance of heat into the room can be established and the room temperature held within a fraction of a degree.
No more repetitive 2-3 degrees swings, no more hour or more period where the boiler isn't on where rooms with manual TRV's can't get any heat - as long as there is at least some heat demand the boiler will fire for at least some time in every 10 minute cycle which means rooms with manual TRV's get a chance to get some heat in every 10 minute period.
OpenTherm takes it one step further of course and directly controls the flow temperature which leads to even greater precision in heat output control from the boiler, but perhaps more importantly is far quicker and more responsive. As TPI is based on average heat output over a (by default) 10 minute cycle when the heat demand changes it is on average 10 minutes before that change has any tangible effect on radiator flow temperatures, whereas a change from say 45 to 55 degrees via OpenTherm will be actioned by the boiler immediately and it may reach the target in under a minute. This faster response time can help give better overall control as the more delays you add to the system response the more prone it will be to overshoot and oscillate.Last edited by DBMandrake; 29 January 2018, 09:52 PM.
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Originally posted by Edinburgh2000Is there a differential term?
Originally posted by DBMandrake View PostDon't confuse two completely different things. The HR92 is a PID controller ....TPI (time proportional integral) is something completely different.
(And thanks for the best explanation of "TPI" that I have seen on this forum. That penny has now dropped with me.)Last edited by Edinburgh2000; 29 January 2018, 11:23 PM.
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Originally posted by blowlamp View PostNo contest in my opinion.
Intergas are built like no other -
Does Veissmann still have rubber hoses to and from the heat exchanger?
The intergas would be simple to descale if ever necessary (are you in a hard water zone?) and don't forget that you'll be paying for the ten year warranty of the Veissmann in the purchase price.
Martin.
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Originally posted by Edinburgh2000 View PostIs that a "Yes"? (The control method of determining the % heat demand applied to zone valves (rather than HR92s) by the controller including a differential term.) Does the controller react to the rate of change of zone temperature?
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Originally posted by paulockenden View PostDespite what we've always been told, I think it's +/- 1.0 degree.
I then subtracted the (raw) measured temperature from the set point and compared it to reported heat demand percentage. The results were:
Code:Temperature difference - Heat Demand Living Room 1.32C - 67% 1.82C - 100% Bathroom 0.51C - 7% 0.78C - 21% 1.28C - 100% Kitchen 0.71C - 7% 1.21C - 35% 1.71C - 100% Study 0.78C - 14% 1.24C - 65% 1.54C - 100%
In summary, the proportional band (at least in the positive, set point higher than measured direction) must be at least 1.32C and is probably less than 1.54C. (More samples needed though to confirm the latter) I suspect it will be the same in the negative direction but to test that I'd need to redo the test taking the valve position reading directly from each HR92 as the valve position readings below 30% are truncated and appear as a 0% heat demand all the way from 30 down to 0.
A bit more sampling (with some luck with the precise temperature readings) would narrow it down a bit more but its definitely more than 1C, and 1.5C is a nice round number, just saying.Last edited by DBMandrake; 30 January 2018, 12:22 AM.
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@DBMandrake
Simon - your detailed knowledge and willingness to pass on the information in a clear and reading style is much appreciated. Noting the above, what conclusions have you come to read the boiler demand setting?
IMG_0944.jpg
What does the boiler control of 37% actually mean in practical terms.(NB: the percentage is still higher than the highest zone demand; that said, the particular zone comprises three small single panel radiators)
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Originally posted by G4RHL View PostI am wary of any warranty. Many long term warranties, if you read the fine print, are only covering the bits that are unlikely to fail. Then you have the likes of one well installer offering wonderful warranties, and expensive installations, but you have to use them for servicing every year and they are not cheap. There is probably enough said in these pages to persuade you to go Intergas!
Can anyone answer if a replacement pump for the IG can be bought off the shelf or do i need a particular fitting?
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Originally posted by 1animal1 View PostWow this thread has moved on! I can't thank you chaps enough, even not fully understanding every element of the conversation, it has helped me massively - i now feel confident in making an informed decision. I must admit i have wavered and considered a standard TPI approach instead of OT, given the conversations i've had with various plumbers. Given that I have the Honeywell I think, why the hell shouldn't i have a fully optimised system... You make a good point G4 & blowlamp regards the IG. I completely get the warranty thing too - it seems which ever way i go i am taking a risk, and i gather the parts for the IG are much cheaper in comparison.
Can anyone answer if a replacement pump for the IG can be bought off the shelf or do i need a particular fitting?
I think Intergas spares can be had from Plumb Centre and the like, but any decent Intergas installer should carry a good range of parts.
I'd say that rather than worry about the pump failing, rejoice in having a boiler with no diverter valve, rubber hoses, or any of the other stuff that commonly fails on modern equipment.
I'd get another Intergas, because, engineering wise, I can see what is inherently right about them - a case of form follows function, as far as I'm concerned. Failing an Intergas I think I would plump for an ATAG.
Martin.
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I think after the mention of Atag at the start of this post and the reviews, that is my first option. I am just not prepared to change heating engineers to get one.... The Intergas will be his first sight. This is of course depending on what quote I get of course. Such a shame Atag won't supply without accreditation, with the warranty.... Thinking of asking my chap to contact them to price up and see what warranty they will giveLast edited by 1animal1; 30 January 2018, 11:29 PM.
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Originally posted by 1animal1 View PostI think after the mention of Atag at the start of this post and the reviews, that is my first option. I am just not prepared to change heating engineers to get one.... The Intergas will be his first sight. This is of course depending on what quote I get of course. Such a shame Atag won't supply without accreditation, with the warranty.... Thinking of asking my chap to contact them to price up and see what warranty they will give
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