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Condensing boiler & home heat loss sizing for baseboard heat

Is conventional home heat loss calculation coming in to question when you have baseboard heat combined with a high efficient boilers.  Due to outdoor reset and low boiler temperatures.  True/false we need to measure the baseboard heating element when we do heat loss calculation, to make up for the low temperatures.  Ending up with a bigger boiler.


  • CMadatMe
    CMadatMe Member Posts: 3,086
    edited February 2011

    A Heat Loss is a heat loss. The boiler should be sized to overcome the heat loss of the home. Once the heat loss is done we then calculate the existing output capability of the baseboard based on that zones flow rate to find the water temp that is required to overcome the heatloss for the rooms.

    The attached chart will help you in de-rating the board to figure your heating curve. Do a heat loss and pick a boiler that will overcome it.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • icesailor
    icesailor Member Posts: 7,265
    False, or put another way:

    Put another way, the boiler is always sized to the heat loss of the building and domestic hot water demand. The radiation is sized to overcome the heat loss. The temperature of water you use determines the size/amount of radiation you will need. The cooler the water, the more radiation you will need. The hotter the system temperature water you use, the less the amount of radiation you will need. The boiler stays the same though.
  • KevinNY
    KevinNY Member Posts: 4
    Related question

    We're getting quotes to replace our hot water boiler in our home in western New York.  The house was built in 1937, has cast iron baseboard and a handful of radiators.  I've had 5 quotes and I'm getting different answers on how to size a boiler properly.  One guy measures all the baseboard/radiators, another just wants to know the sq ft of the house, etc. 

    You guys say the size is determined only by heat loss. Any way I can estimate that myself? 

    Also, say you determine I need 70,000 BTU.  Do I look at the IBR BTU capability of the boiler, or (for lack of the appropriate term) the "gross" BTU capability?

    Lastly, if I have more radiation than I need - the upstairs especially, where the radiators are, gets much hotter than the main floor - are you saying I should have the water temperature lowered?  Put another way, should these guys look at how much baseboard/radiators I have and use that to determine the proper water temperature?
  • Bill H.
    Bill H. Member Posts: 30
    SlantFin type emitters

    I've always wondered about this. HVAC contractors seem always wanting to replace boilers with high efficiency condensing units, without regard to existing baseboard radiation. Unless one can add more sections, and knowing that condensing boilers need lower water temperature to be efficient, wouldn't a non-condensing boiler be better assuming fin-type emitter efficiency is temperature dependent?
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    My former contractor was like that too.

    My former contractor paced two sides of my house. That is how they did a heat loss.  I did not know better at the time, but I fortunately found John Siegenthaler's book and this web site. So I did my own heat loss calculations. By then I knew I wanted a mod|con boiler, and that is what my contractor recommended anyway. But I calculated I needed one size smaller boiler than he recommended (the smallest from that manufacturer).

    The other problem was that he did not even look at the emitters upstairs. Downstairs was radiant in slab. Upstairs had an inadequate amount of fin-tube baseboard, even with the old boiler. I wanted it to condense when heating the upstairs, too, so I calculated how much baseboard I would need to heat up there with 135F supply water temperature when it was 0F outside (design temperature is actually 14F). So I have 14 feet of baseboard in each of those rooms now.

    This former contractor sells mod|cons, but obviously does not know how they work. Perhaps my former contractor is typical. I am an environmental nut, so I do not even care if my mod|con pays for itself or not. But I suspect, from my gas bills, that it will. I was not enough of a nut to go solar or geothermal, because I had even less confidence in those contractors around here. They are all 'johnny come lately' types that remind me of aluminum siding contractors of yesteryear. This does not mean they are incompetent, but just that I have no idea how to evaluate their abilities to design the system, do the work, and remain in business as long as I need them.
  • Ifollowinstructions
    Ifollowinstructions Member Posts: 27
    heat loss

    If you have had 5 prices for a boiler job you should have a better idea from the contractors what your problems are and what needs to be done.  If the guy drives a plain white van with a pvc pipe on the roof don't let him into your house.  If the 2nd floor of your home is hotter than the 1st floor... someone added insulation in the attic/ walls. 1st floor has no insulation in the walls.  Cast iron radiators and baseboard on the same zone will never work! Unless you have 2 boilers. heat loss is home square footage, insulation and windows.  Average home needs 100.000 btus input, they don't make enough different boiler sizes to make it complicated.

    Here is a old Gas Company formulae to get the experts motivated!  2 story home:  Square footage times 86,4 times .63 this is a home with insulation & storm windows. Heat loss. This is for a zero degree day and older homes.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Heat Loss Estimate

    After doing thousands heat losses I use this rule of thumb formula for budget quoting non radiant jobs.  Get the job then a proper heat loss...An estimate is that an estimate. Exacts cost money...

    I'm in a 0 degree design temp climate

    Minimal Insulation bad windows sqft x 40

    R11 Walls, decent windows x 32

    R19 Walls, nice windows x  25

    Current new construction x 20
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Ironman
    Ironman Member Posts: 7,180
    I Have To Kindly Disagree...

    About your heat loss formula. It calculates out to 54btu's per sq. ft. That house would be blowing the curtains straight out with a 5mph wind outside.

    As someone who has done countless heat losses, both manually and by computer for over three decades, I totally agree with Chris's estimating formula listed above. It's spot on.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    edited March 2011
    Heat Emmitters

    I always measure length of heat emitter in each room and compare them to the heat loss. Without the heat loss and the knowledge of how to figure the emitters capable output or outputs at different temps how the heck could you give the consumer the proper information to make the best decision of how they want to spend their money? What I've found and others please chime in. Most homes are 35% over-radiated. Especially the older homes where the board was never changed but changes have been made to windows and insulation.

    The experienced pro can see that the minute he walks into the home. The heat loss is the tool that allows us to provide the homeowner the answers to all the questions and to questions that they never ask.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • bob eck
    bob eck Member Posts: 930
    baseboard & high eff gas boiler

    do a heat loss on the house. then see how much element is in the house. if you have enough element you can run the boiler on the outdoor reset control. if you need to add more baseboard with copper element so the boiler can run at lower that water temps that come from a cast iron regular gas boiler with water temp going out to the system at 180*F. the lower the water temp going out into the system the better. if you need to run a high efficiency gas boiler at 180*F I would look at putting in a cast iron boiler. If you can not add baseboard to the system look at taking out old baseboard and installing high out put copper baseboard. also check out http://www.smithsenvironmental.com/html/he.html this baseboard will work great with mod-con boilers and low water temps. Take a look at Triangle Tube Prestige Solo & Prestige Excellence 95% AFUE boilers.
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