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Vitodens Problems

Lest you think that I'm only going to tell the good, here are my negative criticisms so far:

1) Sizing. Even the smallest model (the 6-24) still has an utter minimum maximum output of 81 mbh. True maximum output is probably more like 88 mbh. Granted my local climate isn't particularly cold (8° design), but mine is a big, old house and I venture to say that the smallest model would be greatly oversized for the vast majority of homes in this area--including new McMansions.

That almost freakish ability to match output to load on a minute-by-minute basis has <I>serious</I> ramifications for heat loss in familiar terms.

Perhaps the reasoning is that the high cost precludes installation in modest homes--I don't know.

Perhaps the reasoning is that you need some extra boiler capacity for DHW production. It's been my understanding that you should never oversize a boiler just to meet DHW loading, instead relying on the reserve ability and proper sizing of the indirect. Maybe the Vitodens breaks this rule--I'm not sure.

2) Low-end Modulation. It's pretty good, but still quite a way from perfection. Again, when your output is so closely linked with your load, things change considerably. 22 mbh or so minimum output might not sound like much, but it goes a LONG way in moderate weather. Get it down to 11 mbh or so and WOW!!!!!

3) Poor Pre-Sales Information for Contractors. Yes, we know that the burner is unique and awesome. Yes, we know that the design is "uncompromising". But we DON'T know what to expect in operation! Your competitors at least give general information on control operation--surely you can give something more without fear of giving away trade secrets!

Since the Vitodens is a closed system where it's the Viessmann way or no way, you REALLY need to give some details on how everything actually performs as a system and why such is necessary for best operation! Your competitors generally offer MUCH more freedom in system design and component choice.

Use your website for its best use, but give EVERYONE access to the information. Those homeowners (or their children) doing research will greatly appreciate some good information on just what to expect and what they should expect to see!

4) Essentially Zero Mention of the "German" Way. Considering that the boiler comes from Germany where TRVs are required by law, don't TRVs deserve at least a paragraph or two in the Technical Data Manual? You have almost everyone believing that the low-loss header is a requirement--even if your system has proportional flow control via TRVs!

Not everyone on this side of "the pond" believes that hydronic systems must be filled with digital flow control devices, pumps and gillions of digital wall thermostats! Give us some good literature and web references and you might find that Americans aren't quite as stuck in their "backward" ways as you seem to believe!

Problems 3 & 4 are highly linked and (I suspect) are MUCH of the reason that MANY wonderful heating contractors shy away from the Vitodens! In that regard, I believe that EVERY heating contractor is from the Show-Me State! If you're going to produce a system that requires a different way of thinking (and designing) then you better damned well give the reasons why with the operational details for verification!


  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 15,425
    Why not

    submit all of this (good and the not so good) for the On the Job section? That way it will stick around for others to consider for longer than three days. Thanks.
    Retired and loving it.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
    Will Do Dan

    Can it be an ongoing review? As my data collection time increases and the weather gets colder, it's going to be interesting. Well at least interesting to a "Show-Me" citizen like me!
  • Nron_9
    Nron_9 Member Posts: 237

    If btu loads are very small instead of a low loss header look at a 20 gal buffer tank that is well insolated to help with temp swings
  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 15,425

    just send them along though the On The Job section and I'll post them as I receive them in the one place. Thanks.
    Retired and loving it.
  • Darren_6
    Darren_6 Member Posts: 13
    ahhh the German mind at work...

    I am an engineer in the food industry and we have quite a few machines here from Germany..and its not just the boiler guys that hold back on info. The german mind is an interesting thing. i too have trouble getting any info on how their units work etc prior to purchase. While most of my purchases have been happy ones, I find that getting things adapted to US or Canadian conditions is difficult, but not impoosible. The first response I get from German engineers on any of my modification ideas is ":It's impossible" You simply ignore that and push on..gently. In time, they'll get tired of hearing form you and set an engineer down to thiNk about it. A few weeks later, they are designing something for you and a few weeks after that, its installed and working. Then they incorporate it into future designs and take credit for the invention (no sweat off my nose) .

    If its any comfort, the German food machinery guys are the same as the German boilers guys...and probably the rest of them too..

    They make nice stuff...they resist adapting to our market place...but they will....

  • Dale
    Dale Member Posts: 1,317
    Everywhere but the US

    Not just the Germans, only in the US can you call and get good information from corporations or govt. agencies. I had friends in Spain who said a reasonable reason for a Spanish person to miss work was to "get their papers fixed". We don't have a clue about how poor information exchange is outside the US.
  • Constantin
    Constantin Member Posts: 3,782
    Ummmm... that depends.

    I had a friend point out to the INS that they had mistyped his date of birth on the green card. The INS officer insisted that it was my friends fault, even though the (typed!) application showed the correct date. So even though my friend had proof that the INS had made the mistake, the INS officer simply took the green card away and told my friend to re-apply. Several hundred dollars and several days down the drain. Way to go, team!

    A different friend had to submit a "sex change" form at the MA DMV because the DMV had transcribed her drivers license incorrectly when she took up residence in MA. A bouncer at the door of a nightclub was the first to notice, hilarity ensued.

    I guess all I'm saying is don't think that the US doesn't have its tangle of bureaucratic snafus. However, I will agree that in my experience the tangle in the US is somewhat less ridiculous than in other countries like France or Germany. At least the US has clearly stated requirements (for the most part) when it comes to getting papers in order, whereas the French love to make up stuff on the fly.
  • Glen
    Glen Member Posts: 855
    amazing -

    I was just chatting with a Viessmann service tech the other day about these very issues. It's not they are not aware of these "problems" - it's more on the line that they have built such a sophisicated machine - that the engineers still dont' fully comprehend what they have built. Tis true! They are experiencing field conditions that were never considered - but the boiler and its integral controls are responding - sometimes negatively. Keep us posted Mike - Warwick may have a job for you!
  • S Ebels
    S Ebels Member Posts: 2,322
    Patience Mike

    From my conversations with Viessmann it's apparent that they don't understand all they know about it. I don't mean that in an unkind way. The control is a slow "learner" but it does learn. I have had a couple that acted differently as far as firing rates and modulation when fired up. After a couple weeks though both of these settled down and behaved "normally". It seemed to figure out what was going on after time. What does seem to keep it on a constant "hunt" is a zone valve or other similar system where the demand varies widely in an on/off mode. It doesn't like those at all. The control was made (as far as I can tell) to see constant flow with a varying btu requirement. Under those circumstances it operates as one would expect. If your flow is getting squeezed down to nearly nothing, you have a clear need for a LLH. From my experience, it won't "learn" a system with flow rates that go from very low to full bore. Also bear in mind that the integral circ speed is controlled by outdoor temp. In your case with the TRV's all nearly squeezed off and rather warm O/D temps, causing slow circ speed, the unit would "hunt" because your system BTU requirement is below the minimum firing rate. I have yet to see a burner that would meet that kind of demand. Something with a 50:1 turndown ratio would be needed. My advice is to give it some time and see what happens when you get into a little more wintery weather.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
    Is What I Call the \"Pulsing\" Mode Normal?

    Part of the learning process?

    With outside temp approximately 52° or higher the boiler delivers heat in little batches with firing times of only a few seconds. I was honestly a bit surprised by this operation.

    BUT, when I thought about it, with the boiler driving a fully proportional system with continual circulation and a very low heat loss, it seems quite an effective way (comfort-wise) to deliver heat. Judging by fuel consumption, it's a very efficient way as well. The old W/M acted similarly, but it lost the vast majority of it's input up the flue--both as the HX heated and as it cooled.

    As a "virgin" the cycles seemed extremely close with up to 15 per hour! As the boiler has learned, it seems to have determined that 5 firings per hour are ideal. (At least I get 5 peaks and 5 dips per hour on my sensors.)

    I'm almost positive that the boiler has learned to use the heat contained in those big mains as a buffer, and as I suspected, return temperature is generally a "lead anchor" even though the burner is fully capable of changing the return temperature rapidly during periods of moderate loss!

    Only in what I call "digital mode" when loss is approaching low-end modulation ability does the return temperature vary significantly and it's a nearly perfect (but slightly delayed) image of the supply temp change!

    Since there appears to be no ability to set a fixed warm-weather shutdown point, it's interesting to see how it deals with quite warm weather. Previously I just shut down the system at 55°. While the house might fall a bit in temperature, it was never drastic. To my knowledge the built-in circulator has never stopped, but only one day found an outside temp exceeding the room temp setting and I didn't go down in the basement and forgot to open a TRV.

    There's NO WAY I'm going to kill power to the Vitodens similarly, so I guess it will be spring before I really know what happens. In future years, I'll probably still use forced air electric resistance in the shoulder seasons as our winter rates are exceptionally low and I'm used to the forced air from the A/C anyway at that time...

    The other curiousity is the various different temperatures displayed by the boiler. The standard-display "boiler temperature", the "target temperature" and "actual temperature" all seem to relate VERY curiously to my measurements, but NOT in the way the names would imply!
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    There's NO TRVd system that won't go down to nearly zero flow in warm weather--that's why the differential pressure bypass is required. I DO hear bypass in extremely mild weather. Neither calculations based on delta-t nor observations lead me to believe that low flow is causing any kind of problem as both the internal and outboard differential pressure bypasses are doing their job.

    There is also definite evidence that the boiler is learning to adapt to this system and my reset curve settings. By no means did I expect this to happen immediately. I've made only one small change to the curve and that was just a few days after turning it on.

    There is utterly nothing digital in this system besides the boiler brain! Even with all of the TRVs satisfied, there is still flow through the mains via the small radiant bath loops.

    "The control was made (as far as I can tell) to see constant flow with a varying btu requirement."

    Can't say I agree with that entirely. The variable-speed, weather responsive pump is obviously designed to vary flow along with those varying btu requirements... Continual flow yes, but not at a constant rate.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
    Forgot To Say

    I'm not at all disappointed--just curious...

    Lack of patience is NOT one of my faults!
  • marc
    marc Member Posts: 203

    usually mean something is wrong, not complaints about the design, try adjusting the curve / shift to lower the supply temps. on warmer days, when you buy a new vehicle the dont tell you how to adjust the idle, viessmann gives the homeowner all of the adjustments neccessary to tweak the control. if the burner needs adjustment call a pro. it does sound like you really like the vitodens. we have installed alot and they continue to keep the customers very happy. marc
  • larry
    larry Member Posts: 91

    "What does seem to keep it on a constant "hunt" is a zone valve or other similar system where the demand varies widely in an on/off mode."

    Just curious... Would you consider a (relatively) low supply temperature (120F to 140F) hydronic air handler as falling into this category of an on/off mode device? I'm looking at doing a Vitodens with a combination of AHers plus some radiant floor and panel radiators and don't want to go down the Vitodens road if it's not a good fit.
This discussion has been closed.