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Baseboards and Mod/Cons...What's the word?

MSmithHH Member Posts: 25
Hey folks,

As mentioned in my other post, I am replacing my natural gas boiler in my 30 year old house in Massachusetts. I have 3500 sq ft of heated space and it uses baseboards. My slant fin heat loss calc shows 65k loss at 20 degrees outside and 89k at 1 degree outside with 180 design temp. I have gotten a few quotes and all highly recommend I go with a mod/con. My leading quote if for a Weil Mclain WM97+110CT. In my research I have found many posts say that mod/cons don't have the advertised savings when dealing with baseboards, because baseboards are high temp, so it will rarely be condensing. There may be some savings due to the modulation, but when you factor in maintenance, modulation alone probably won't be realized.

So, what's the truth? Do Mod/Cons still make sense if you have baseboards in a large house?



  • Zman
    Zman Member Posts: 7,588
    If you take the room by room breakdown you have for the heat loss and compare the amount of baseboard you have to the heatloss of the space on design day and determine what temp you can run your baseboards on that day.
    You can then do the same thing for your average heating day.and determine what water temp you can run on that day.
    This will tell you how often the boiler will save you energy when condensing.
    The Mod/con will always save you a few percent as it modulates to match the load perfectly rather than cycling on and off.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Tinman
    Tinman Member Posts: 2,808
    The answer to your last question is maybe.

    The more baseboard, the higher the EDR.

    The higher the EDR, the lower the return water temperature.

    The lower the return water temperature, the more efficient the modcon will be.
    Steve Minnich
  • MSmithHH
    MSmithHH Member Posts: 25
    edited August 2019
    Thanks @Zman and @Steve!

    "The answer to your last question is maybe."

    >> Perhaps a better question, if a mod/con is used in a situation where it doesn't get a chance to condense, could it be the wrong solution to the point that I'm shortening it's life because it's working harder than it was designed for? In other words, if efficiency-wise it's no worse (and perhaps could be better), and if price-wise, due to Mass-rebates, it's no more expensive, and reliability/longevity-wise it's equivalent, than I'm happy to go with a mod/con. But if longevity/reliability decrease because it's being used in an application it's not designed for, then perhaps it's the wrong solution. Is that the case here, or not at all?

    Thank you again!
  • Ironman
    Ironman Member Posts: 7,385
    How many lineal feet of BB ELEMENT do you have?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Zman
    Zman Member Posts: 7,588
    Even if it runs at high temp all the time, the Mod/con will run beautifully and should not have any issues with premature failure assuming it is properly sized and installed. If you set your outdoor reset properly, you will condense some of the time.

    All boiler should have annual maintenance performed. This is especially true of mod/cons.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Keith M
    Keith M Member Posts: 78
    People often state baseboard is for high temperature systems and it is in some cases while it is not in other cases.
    If your heat loss is 65,000 Btuh with a 20F outdoor design, your heat loss at a 10F outdoor design is 78,000 Btuh. These are based on an indoor design temperature of 70F.
    Very rarely do we ever reach a heating outdoor design temperature. Remember it is NOT to coldest it gets...it is the coldest it remains at over an extended period of time. If my memory is correct, this extended period of time is 48 hours and yes I know this is a simple definition. But we do not take into account other heat gains such as people, cooking, appliances, lighting, solar...etc. These heat gains are just "icing on the cake" for heating systems.
    Because of above I recommend designers use 140F as design water temperatures if condensing boilers are being used. At 140F water temperature and 1 GPM flow, Slant/Fin Fine/Line 30 output is 320 Btuh per foot. With a design heat loss of 78,000 Btuh we would install 244 feet of Fine/Line 30 with element. Naturally, since this is "actual fin length" the amount of cover will be slightly higher.
    If I was using a low mass condensing boiler I would also use a buffer tank. I would certainly use the outdoor reset function with the condensing boiler as well.
    While it is not as efficient a system as a "wet" radiant system, it is still an efficient and comfortable system.
    Keith Muhlmeister
    Slant/Fin Corporation