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Vitodens 95.2% AFUE

S Ebels
S Ebels Member Posts: 2,322
Actually, not the testing but the conditions under which the boilers are tested. The test conditions are simple but virtually impossible to replicate in the field. A lot like the CAFE mpg rating for cars. This past summer I was at Riello for commercial burner training and had a chance to watch and talk to the two techs doing an AFUE test on a boiler. The parameters are 140* out and 120* in for the water temps. (how many boilers have you seen in the field running those temps?) They played with the burner to get it dialed in and then set the flow through the boiler to attain that 20* temp drop across the boiler. (They have some SERIOUS heat dumping capacity in that place)

The resulting efficiency number became the AFUE. How many boilers have you ever seen in the field that are operating under continuous fire conditions and steady 20* temp drop across the HX? I've never seen one and I doubt very much if anyone here has. Everything has to be perfect to attain those conditions and it doesn't happen out in the field. Therefore the AFUE numbers don't reflect real life at all.

This is exactly why the modulating/condensing boilers, which are rated only 5-10% higher AFUE than their cast iron brothers, manage to produce savings of over 40% out in the field. The cast iron models NEVER duplicate the AFUE numbers or even come close to them because the testing criteria is so far from real life it's pathetic. More than meets the eye in those misleading AFUE numbers.

A variable fire/variable flow boiler such as the Vitodens, which has an excellent control package tailored to the boiler, can actually come close or even duplicate the AFUE numbers while a cast iron or even a lot of other M/C boilers don't stand a chance. The key is matching the firing rate AND the flow to the existing conditions.


  • Brad White_9
    Brad White_9 Member Posts: 2,440
    This came to my attention

    something many of us had long suspected, the Viessmann Vitodens has tested to an AFUE of 95.2% (for what AFUE is worth but that is, yes, yet another thread).

    Still- this is excellent news and the Federal $150 Tax Credit (again, FWIW) applies retroactively with some limits I am sure.

    I was not sure if this had been posted before, I took a brief look.

    Just passing it on.

  • S Ebels
    S Ebels Member Posts: 2,322
    Makes you wonder

    Makes you wonder what AFUE is worth and what it really means. I'm pretty certain that the good Germans didn't redesign the Vito to achieve 1% more efficiency here in the US. So, where did the 1% come from?

    I've noticed a goodly number of the F/A crowd have done the same thing with existing equipment also. An existing model suddenly appears with a new model number and a new AFUE rating. How does that work?
  • jp_2
    jp_2 Member Posts: 1,935

    why is afue such a mistery? anually efficience uses short cycling inefficiencies as well doesn't it?

    plus, don't ya throw in some statistics for good measure?
  • S Ebels
    S Ebels Member Posts: 2,322
    The mystery

    The mystery is...how does an appliance be that furnace or boiler jump in efficiency without any modification. How does it all of a sudden "test" better.
  • jp_2
    jp_2 Member Posts: 1,935
    mystery, mistery

    I knew my spelling looked wrong, thanks for the correction.

    thing is, I've never heard a discussion here on the method for testing afue, so not sure why everyone thinks its so incorrect?

    so i can see they were able to fidget with the statistics.....pulling another 1% here or there.

  • jp_2
    jp_2 Member Posts: 1,935
    big question is?

    my question steve, How many boilers out there in fact use constant cir?

    the boilers around here I know of do not, but I do not work on them so my numbers are few.

    I might guess around this area, lower incomes, older houses, old bang,bang boilers that probably see DT at 20F.

    i've yet to see P/S set up.
  • subcooler
    subcooler Member Posts: 140
    No Mystery Here

    Just Slick Advertising and a little --- ---.
  • S Ebels
    S Ebels Member Posts: 2,322

    Very few old systems use constant circ but I have converted about all I've been given the opportunity to do so with. It's all about matching the boiler output to the load as much as possible.
  • S Ebels
    S Ebels Member Posts: 2,322

    You're refering to what, The AFUE numbers or M/C boilers?
  • Uni R_2
    Uni R_2 Member Posts: 589
    2nd tests

    I've always scored better taking a test the 2nd time as well! ;-)
  • S Ebels
    S Ebels Member Posts: 2,322
    Good one

    Some guys are like that. I remember taking my mechanical test and talking with the rest of the guys there before the session. The dude who was seated next to me was on his 5th attempt, I was there for the first time. Scared the pud right out of me.
  • Constantin
    Constantin Member Posts: 3,796
    Never mind...

    ... oil burners/boilers being set up to run at 13% CO2 for AFUE testing. The soot, combustion byproducts, or the questionable burner/boiler longevity under these conditions are not objectionable and these operating conditions are representative of what's out in the field, right?

  • OOHH OOHHHhh (Arm flailing wildly in the back of the class)

    I saw a 20 degree DT once. It was on a brandy new system, hot water base bored, cast iron boiler with the house at design condition (-10F) and the inside of the house right at the freezing (35 degrees F) level. 20 degrees F with all zones running their hearts out and the boiler running full tilt boogy.

    That's the ONLY time I've witnessed a 20 degree F delta T for ANY period of time.

    And that's within the 1/3 of a century span of my meager hydronic career.

    Now THAT'S a standard I can hang my hat on...

    More gubernmint interaction...

    Congrats to Viessmann. I suspect more will follow.


This discussion has been closed.