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Most new Mod/Cons seem too small for my application?

Mike E_2
Mike E_2 Member Posts: 81
Again, you need a heat loss, and I bet you will need alot less than 196K BTU. My house had a boiler with 210K input / 180K output. It is being replaced with a 80K input mod/con (most likely a Knight).

If you need something larger, Lochinvar Knight goes up to 500K, Munchkin goes up to 399K, Weil McLain Ultra goes up to 310K, Laars Mascot goes up to 241K, Viessmann Vitodens up to 230K.

If in fact your house does need this large of a boiler, I would suggest going with 2 smaller ones. This way, if there is ever a problem with one, you will still have another one to provide heat. Also, you will be able to have a much wider overall modulation range with 2 boilers.



  • Al Roethlisberger
    Al Roethlisberger Member Posts: 194
    Most new Mod/Cons seem too small for my application?

    So my current 15 year old Carrier(rebadged Dunkirk) atmospheric boiler is about an 80% efficient unit, with an input BTU/hr of 245k and output of 196k. This output figure seems about right based on my EDR calcs and existing 80yr old how water system.

    We have made some improvements with insulation, and will probably do more over time that will help with efficiency. But I'd also like to add DHW to the boiler, which my current boiler does not provide.

    But as I shop for a possible replacement boiler, I've noticed that most seem to go up to only about to about 190kish input, with outputs in the 170k range. This seems too small.

    Is there another consideration that may make these new smaller rated boilers still an appropriate fit?


    Just a DIY'er trying to learn, and improve and maintain his converted ca 1929 overhead gravity hot water system since there is no one local that can.
  • Uni R_2
    Uni R_2 Member Posts: 589
    heat loss

    Al, you really should do heatloss calculation before shopping. There is a link for heat loss calcs at the top of the page. That net 196K may be grossly oversized. Another way of looking at it is how long did your boiler run on the coldest day of last winter?
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    You may be pleasantly surprised, Al...

    Forget your EDR for the moment, that is just the surface area available to you. It tells you as much about your actual heat loss as a baggy suit in the closet can tell you your weight.

    Your older boiler was probably quite oversized as well. No worse way to size a boiler than to match the old one. Well, there are worse ways, actually but let's not go there.

    Do you have a current ("as it is today") calculated heat loss?

    Until you have that information, you will be at a stand-still as far as selecting the boiler of your dreams.

    Now, on the off-chance that your heat loss might be too large for a single ModCon, you can go to multiple boilers for even greater efficiency. See the thread on the Buderus install by Ray Landry to get an idea of larger than boiler heat losses and how they can be dealt with.

    EDIT: The Viessmann Vitodens goes up to 230 MBH input which may be of greater comfort to you. Historically also, unless your DHW needs are unusual, no oversizing is required nor desirable. Most of us have the smallest available ModCons that are still too big and so never know the margin in all fairness.
  • ALH_4
    ALH_4 Member Posts: 1,790
    heat loss

    It is likely your existing boiler is oversized. Size your new boiler based on a new heat loss calculation. The Vitodens 15-60 has an output of 206MBH with natural gas and 192MBH with LP. However, it is very likely you can get by with an 11/44 or maybe even an 8-32 depending on what insulation improvements you have made and whether the existing boiler is oversized.

    edit: Also, the Triangle Tube Prestige 250 has 230MBH output. There are several in this range.
  • Dave_4
    Dave_4 Member Posts: 1,405

    I concur that you should have a new heatloss calc made and ignore the size of your current boiler. A vitodens 8-32 is fully capable of heating a 2500sqft 4 bedroom house here in Toronto (winter design temp of -4F). I am running a DHW indirect on a vitocell 300. The old boiler the 8-32 replaced had an input of 200K but due to its efficency I was likely only getting 120K into heat.

    The good news is that if you really have a high EDR is that you will be able to run the Vitodens at a very low temp and maximize your condensing efficiency. The design temp for my house is 140F and I have yet to reach it.
  • Al Roethlisberger
    Al Roethlisberger Member Posts: 194
    Thanks, and will do on the heat-loss calc

    As I mentioned in a couple other threads, I don't seem to have any informed/willing local pros here. And the joke has been made by the few that came by to even look that, "hey, you must have the last one or two boilers running left in Sanford!"

    They basically look at it and note that nothing seems to be leaking, and ask "does it heat?"... and leave it at that.

    Great... *laugh*

    Well, anyway, this leaves me with what I think is a great opportunity to learn and do this as a DIY install. Which I honestly don't mind. It's a fantastic new challenge, and I'll know all the ins/outs of the system when I'm done.

    Since this is primarily just a boiler swap-out, I think my system would be pretty basic and possible to upgrade as a DIY based on my research, advice received so far(many thanks again), and reading. There is a lot to consider of course, but I've already learned a tremendous amount here.

    Right now the system is bone stock, other than being converted to a closed circulated system, no TRVs, etc. ... just standing iron radiators as emitters.

    But we have insulated quite well in the attics and crawlspaces.

    So I'll check out the heat loss calc for sure.

    Right now we are dealing with a 4000sqft 2story home with about 65 windows... sooooo... who knows. But then again, when it is above 20, the house stays an honest 20 degrees warmer inside all night without any heat. Below 20, that margin starts to slide though. Fortunately we really don't have that many super cold days through the year.

    Thanks for all the help. This has proven to be very interesting.

    Just a DIY'er trying to learn, and improve and maintain his converted ca 1929 overhead gravity hot water system since there is no one local that can.
  • GW
    GW Member Posts: 4,676

    heat loss schmeet loss, whose gonna check the infiltration that tells the heat loss program how much air change there is?

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  • Brad White
    Brad White Member Posts: 2,398
    You have to start somewhere, Gary

    and this brings up the possible need for a blower door test to nail that one down. Yes, it is otherwise a guess, but an educated one.

    Personally, I bracket by the crack method, empirical ACH factors (summed room by room) and "sum of building transmission" plus a global ACH rate added in rather than summing room infiltration (which will not happen all at once).

    But one has to start somewhere.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Boilerpro_5
    Boilerpro_5 Member Posts: 407
    Or, go to the big Veismann modcons (and others)

    They go up to 3.3 million btu/hr input. MY own home had a 270,000 btu/hr input cast iron sectional that was well matched to the radiation. AFter making improvements to the home and recalculating my heat loss, my current 100,000 input boiler short cycles due to too light of a load even with a buffer tank. Heat loss is now under 50,000 btu/hr at -4F design.
    If your current boiler is in good shape, and has a stack damper to cut losses up the chimney, I'd baseload size a new modcon to about 65%of peak load and use it as the first stage boiler, with the second stage being the current unit. I would expect very little efficinecy to be gained by using 2 modcons since the second stage unit would be alsmost never on (unless you use a control that fires both boilers at low fire on typical days).


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