Do Viessmann boilers do weather compensation[1] while still being able to work with simple demand-live call4heat external programmers?
[1] I'm taking about the weather compensation that adjusts flow temperature up if its super cold outside. Not a set-forward on the time it starts, as the heating controller I am looking at learns that itself.
This is purely so the boiler can run cooler and more efficiently when it's not super cold outside.
WB don't - well, they do, but you have to have their whole package and that does not play nice with external heating controllers.
Very easy to DIY I have the WB 24 RI gas boiler with a basic flow temperature control and it is relatively easy to switch in a resistor in parallel with the thermistor to make the control system think that water is a bit cooler than it really is.
I consider this and simply making the add on parts via piggy back spade connectors, does mean the evidence of tampering is easily removed before making any warranty claim or calling in of a service bod. I plan to use a relay to do the switching of the parallel resistor so that there is full isolation between the boiler control supply and my additional weather compensator. The same method can be used to boost the flow temperature during water heating cycles too - another 'feature' omitted in the more basic WB boilers.
Interesting. On my WB2A - quite a few years old now - weather compensation involved a new programmer (on the boiler). You can use a second programmer (Viessmann) which is what I've done to allow switching the heating or hot water on/off from a more convenient location than the actual boiler. It's actually a zone controller and has a sensor associated with it, so can be used to control the temperature in that zone individually. But the whole lot is digital, so use of a basic call for heat thermostat (etc) would be difficult.
The basic non weather compensated boiler allows either a simple thermostat or sensor.
I suspect questions like these may need an answer from the boiler maker, since its a somewhat non standard configuration.
I know my boiler has both ebus and normal controls and you can use either - but I don't know what happens when you have a weather compensated controller on the ebus and then activate the traditional "call for heat" control in parallel. I am sure it would fire - but I don't know at what temperature.
For anyone thinking of trying this, please note my deliberate error :-( Most boilers will employ negative temperature coefficient (NTC) thermistors which means the resistance of the sensor DROPS with increasing temperature. Therefore to fool the control system into thinking the water is cooler than it is, the resistance has to be increased by adding series resistors.
An alternative is to turn the boiler setting up to the highest temperature that you will want for either weather compensation or hot water boost and then switch in parallel resistors to drop the operating temperatures to your desired optimum economy/flue condensation point.
It is vital that you do not allow a fault condition in your extra circuitry that allows the sensor to be open circuit. This would tell the boiler that the water was very cold and it would roar away and possibly over heat although I'd like to think that manufacturers would have built something in to limit this, there are no certainties!
For a specific boiler you need to know the characteristics of the sensor. Most NTC thermistors are defined by the resistance at 25C and a constant Beta. I simply bought a spare one designed for my boiler and measured its resistance at a few points (including 25C) by strapping it to a heating pipe together with a known thermometer probe. You can then fit the points to a curve using excel and work out the value of beta. The thermistor elements are almost certain to be standard ones ( for economy) so you can check against common (= cheap) thermistor data sheets.
It seems the 100 range has built in weather compensation with the addition of a simple external sensor. Fixed curve (which can be shifted side to side, but not changed in slope) - eg according to page 4 of the PDF:
Outside T (C) Flow T (C)
15 45
10 53
5 60
0 65
-5 70
-10 77
which all looks very sensible. So that seems to tick the box. Will ring Viessmann this week to confirm that it really is happy to do with with a dumb call-for-heat signal (looks that way in the wiring part of teh installer's manual).
The 200 series is less clear. Very fancy weather compensation with adjustable slopes on the compensation curve, but what's not clear is if this plays nice with external control.
I did something similar with my Keston, but rather than tampering with the thermistors, I changed the water temperature setting. On the Keston, this is a 100k linear pot between 5V and 0V, with the wiper providing
0V through to 5V back to the control board, representing 30C through to
82C demand. In principle, I could simply feed 0-5V into the control board, but with it all being under waranty at the time, I was anal about isolating all my control circuitry.
There is a ribbon cable connecting the front panel board (the potentiometer above, 8 indicator lamps, and on-off switch). The pot runs on 5V supply, and all the rest of it on a 24VDC supply. I made up a replacement front panel board with the pot and LEDs all in the same places. However, my board provided opto-isolated outputs for all the LEDs, and the pot was replaced by a dual gang motorised pot - one gang connected to the boiler just as the original was, and the other gang was positioning feedback for the motor control circuit. It was driven from an old PC with a parallel printer port doing the i/o, and the games port reading the potentiometer positioing. I used it for a short time, but didn't leave it connected permanently. By the time the boiler was out of warranty, the 120MHz Pentium system was a bit long in the tooth, and new PC's had dropped games ports, so I never got around to connecting it back up.
If I was doing it again now, I would install a raspberry pi in a die-cast case in the boiler, using direct connections, and rely on the ethernet connection providing external isolation.
Someone else here with the same boiler also made up a circuit which simply switched the 0-5V signal to 5V (full output) whilst reheating the hot water cylinder, leaving the pot controlling only the radiator temperatures, and not the hot water.
Like you I want to maintain complete isolation. I've yet to measure how the pot is connected in my boiler. At the moment I'm considering an optical interface with an led shining onto a light dependent resistor in parallel with the sensor.
Generally, north facing. There may be specific situations where you might vary this, and it can be a problem if the house has no outside walls which never get any sun to position it on.
I know the real world heating load with some accuracy at a couple of points (1-2C outside all day, and 5-7C outside) thanks to electric heating and some recording of power consumption. I have a spreadsheet with candidate radiators based on what will physically fit.
I suppose it's mostly now going to depend on "how much more efficiently does the boiler run at dT=30 vs dT=40 vs dT=50"...
I can at least answer the question "how much time will the boiler spend in dT=30 mode vs other flow temps".
OTOH the Viessmann 100 W has weathercomp more or less for free (well the price of a sensor on a wire, probably a thermistor or basic I2C Dallas type doobrey).
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