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How to make a mod con boiler happy?

Uni R_3
Uni R_3 Member Posts: 299
If a permit is involved they may insist that the IBR exceed the heatloss.

That said, you probably wouldn't even run the 6-24 very hard. I'll bet your heatloss is way overstated!


  • PointBlueBullet
    PointBlueBullet Member Posts: 8
    Boiler sizing / heat loss calcs...

    I've run every piece of software, paperware, etc. on sizing a mod con boiler for a new construction 2 story 2200sf house and they suggest boiler sizes ranging between 75 MBH and 88 MBH (some suggesting as high as 107MBH depending on what size fans I put in the range hood(!) and bathrooms(!!!))..

    ..and am considering a Vitodens 6-24 as the spec'd boiler, but know my "Pro" installer (we have a choice of 1 in my area) will recommend a 8-32 - out of "oversize" habit.

    Will the smaller 6-24 be happiest running flat out trying to keep up with heat load (if calcs are correct), or will my "Pro" recommended 8-32 be better?

    Also considering the Buderus GB142-24 which has a max capacity of 75 MBH.. but leaning toward the SS heat exchanger of the Vitodens as I need to run a high percentage of glycol in the system as the house will not be heated regularly in the winter which can stay well below zero for days/weeks on end. (AL ok glycol perform as well as non-AL ok glycol?)

    So I guess my question is how to make a boiler happiest (and have the most reliable, efficient system possible) - run flat out, or have some reserve capacity which will never be used...???

    [building highlights: 3250sf wall area (3" spray foam (R-19) insulation), 18% glazed (U-.33), ceiling 1020sf (6" spray foam), 13 loops pex (2 zones) max length 300', 9000 degree days, -10 degree design temp for area]
  • Perry_3
    Perry_3 Member Posts: 498
    Vitodens 6-24

    Much of this was said in the other thread - but I have added information:

    I would personally install a 6-24 in that situation.

    I live south of you in Wisconsin. The heat loss program said, without looking it up, about 60,000 Btu/Hr for -15 Design day (and somewhere over 50,000 Btu/Hr at 0 F). This is on a 1700 Square Foot 1950's house with not the best insulation and only half of the windows replaced.

    I clocked the meter recently at 0 F after several days of sub 0 weather at about 35,000 Btu/Hr (way less than the heat loss calc). My Vitodens 6-24 also cycles on and off on days above 20 F. Thus the minimum firing rate of 20,000+ Btu/Hr is to much to keep up on a normal 20 F day.

    I believe that the heat loss calculations have a fair amount of conservatism in them. So if you did indeed get 85,000 Btu/Hr maximum from a heat loss calculation I believe that the Vitdens 6-24 will do just fine as your actual heat needs are going to be below 85,000 Btu/Hr. If I am wrong - then on the few days of the year when you reach your design temperature the house will get a few degrees cool.

    I believe you will benifit from having a boiler that will modulate starting in the 20,000+ Btu per hour instead of in the 30's (or whereever the 8-32 does); which will allow it to better operate more efficiently during the early spring and late fall (instead of cycling on and off).

    If there are permits for the area - use the heat loss that says 75,000 Btu/Hr to make the Vitodens 6-24 acceptable.

    Edited to add: I also agree with your desire to avoid the Aluminum Block HX's. I did a bunch of research on them and did not find any good indictions of long life in Europe (and lots of indications of short life). They have very strict water chemistry requirments that would require frequent testing and treating of your system. Somthing almost never needed on other HX materials. Also, the warranties are generally invalid if water chemistry is not maintained.

    Perry (note I am a homeowner).
  • ALH_4
    ALH_4 Member Posts: 1,790

    Using the smallest boiler possible will be most efficient.

    In my experience, 2-story, 1100sf/floor with spray foam should be quite safe with a 6-24. I think the 8-32 would be quite oversized.
  • Plumb Bob
    Plumb Bob Member Posts: 97

    > I've run every piece of software, paperware, etc.

    > on sizing a mod con boiler for a new construction

    > 2 story 2200sf house and they suggest boiler

    > sizes ranging between 75 MBH and 88 MBH (some

    > suggesting as high as 107MBH depending on what

    > size fans I put in the range hood(!) and

    > bathrooms(!!!))..

    For new construction these numbers are way excessive. What's this house made of, ticky-tacky?


    > Will the smaller 6-24 be happiest

    > running flat out trying to keep up with heat load

    > (if calcs are correct),

    The calculations are padded. The 6-24 will not be running flat out. It will have reserve capacity.
  • PointBlueBullet
    PointBlueBullet Member Posts: 8
    Heat Calcs and Happy Boilers...

    Here behind the Cheddar Curtain, all that's required in the township I'm building is the "Wisconsin UDC Energy Worksheet" to size boilers from a "code" standpoint. In doing the calcs, precision in calculating heat losses through walls, windows, ceilings, etc. is pointless as there is an extreme weighting to infiltration losses from bathroom and range hood fans.

    The envelope calc alone says a heat loss of 67 MBH, the difference between 75 - 85 MBH (and even up to 107 MBH) is dependent on what I say I'm using for ventilation fans!

    I agree in a practical sense I'd just not turn on every exhaust fan simultaneously when it's -20 outside.

    My only other "concern" or "consideration" is the fact that I'll need to run glycol mix, which isn't as efficient in carrying heat as water, thus will be reducing boiler capacity as a result....

    On a related question, would it be worth the extra couple of $ to use bronze pumps and thus eliminate any ferrous metals in the system entirely? Where I'm building, reliability is paramount as service personnel are a long way out.

  • ALH_4
    ALH_4 Member Posts: 1,790

    Glycol doesn't reduce the boiler output, it decreases the circulator output. Generally this is not a major consideration in home heating systems. Head loss should be computed at the propylene glycol concentration that will be used and circulators sized accordingly.

    Iron pumps should be no problem as long as you have tube with an oxygen barrier. There is no getting away from iron components completely with a 24/32 Vitodens since a few internal parts are iron.

    I think your heat loss calcs are quite conservative for new construction. A 6-24 should have no problem at all carrying your design heat load.

  • Bernie Riddle_2
    Bernie Riddle_2 Member Posts: 178
    In the ball park

    There were post before that stated the avg newly construtced home home is in the 25-35btu X per sg ft, range
    you seem to fall there [very eff built is under 25]

    My "New" home came in at 32K-39K btuh... currently running an Ultra80> In the low 20°'s, I run constant circ at 1300 RPM or 16K btu input>> DD of 0° had me around 28-32K btuh
    Also Running AL-glycol, so no huge loss there
  • ALH_4
    ALH_4 Member Posts: 1,790
    heat loads

    It obviously depends greatly on the construction methods, but I would say most new homes, particularly with spray foam insulation, SIP, or ICF and a reasonable window area, are in the 15-20btu/sf range. Basement area is usually less than that. From my experience, my guess is that the heat load for this 2200sf home could be a bit more than half what Bluebullet is planning for. The heat load calculations don't "lie", but they may be quite conservative. Before mod/con boilers, it was less detrimental to size the boiler one size larger than you need. Now, with these low-mass modulating boilers capable of highly efficient burns at low modulation, it is more important than ever not to oversize the boiler.

    Just my take on the whole thing.
  • Mitch_4
    Mitch_4 Member Posts: 955
    I did the same

    in my forced air retrofit, and all said 50-52k, so I put in a 60k 93% eff high eff gas unit.

    remember I did it with 3 software and the old fashioned build a wall with the heat transfer mulipliers just to be sure. My design temp was -10C.

    So far I gather I am about 30-50% oversized as we just experience -30C weather for 3 days, well beyond my design parameters, and I was worried, but it ran 15 on, 10 off or at 60% capacity.

    There is a LOT of padding in the heat loss calculations
  • jp_2
    jp_2 Member Posts: 1,935
    bath fans

    got to think about how often you really run those fans?

    you might find, not very often.
This discussion has been closed.