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  • Understanding Boiler BTU Rating

    My hot water boiler is on it's last leg. I've had to pump it with boiler sealer again. The rating on the boiler tag says 150,000 BTU input/hr and 118,000 BTU Heating Capacity per hr. Could someone explain why there are two BTU ratings and am I looking for a 150,000 or 118,000 BTU furnace. I've seen nice used boilers for sale and it will just simply say 300,000 BTU's or whatever the furnace is. What is important here?
    Thanks again Jake

  • #2
    Re: Understanding Boiler BTU Rating

    Input is the amount of BTU's of the fuel you put in...

    Output is the actual amount of heat that the heater puts out for the Input.

    The difference between the input and the actual heating output allows you to figure the efficiency of your boiler / heater.

    The higher the efficiency the closer the input and heat output numbers will be to each other
    ( 150,000 BTU Input with 150,000 BTU Output= 100% efficient heating)

    By your numbers your boiler is 78% efficient.


    The higher the efficiency the higher the cost of the equipment.

    When I buy a furnace I order it by input and efficiency ( I'm not a boiler guy)

    Hope this helps...



    Bill
    Last edited by OkieBill; 10-22-2009, 09:34 PM.

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    • #3
      Re: Understanding Boiler BTU Rating

      The only way to know what size it should be is to have a professional do a heat load calculation on your home. Especially since your boiler is older (used to last 30 yrs) and you may have had windows replaced and insulation changed or added. Is there an addition that wasn't originally there that maybe wasn't piped right? Also, going off existing doesn't factor in your efficiency increase. Have it checked out by getting a few estimates.......for free. Notice I didn't say you had to buy from them, simply let the smarties help you size it.

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      • #4
        Re: Understanding Boiler BTU Rating

        if you get a "free" estimate you might end up paying more in the long run. I bet most places would just look at your current boiler and size it the same way. Make sure they do a heat loss analysys or it will cost you in the long run

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        • #5
          Re: Understanding Boiler BTU Rating

          You have been given some Great advise in the previous posts, it is very true that free sizing has no commitment to accuracy, they don't care if it is a little too big and wastes your money. Ask a contractor if they personally do the heat loss (heating) or Heat Load (cooling) calculations, then pay them to do it with the understanding that the cost is deducted from the job if you proceed. Then no one has anything to loose, contractor or yourself.
          As for the BTU ratings of your equipment it works like this:
          Highest Btu rate is the energy input (150,000)
          Next highest rate is the output depending on efficiency when your boiler is installed in a heated space (78% eff. = 118,000) But if your boiler is installed in an unheated space then look for the lowest BTU rate as this is the absolute minimum output your equipment is capable of. Heat loss happens everywhere and you do need to account for the space where your boiler is installed, not to heat it but for the energy it consumes from your boiler whilst running to heat the rest of your home, this includes the uninsulated piping you have in that same area. Ask your contractor if he has downloaded the free Taco Flopro designer software, it can size a heat loss for a home fairly accurately in about 20 min. of untrained use. Just make sure your wall thickness, insulation properties, room and window sizes are accurate. Good luck.

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          • #6
            Re: Understanding Boiler BTU Rating

            Originally posted by Solartech View Post
            it is very true that free sizing has no commitment to accuracy, they don't care if it is a little too big and wastes your money. .
            Think about this......if a contractor sizes it wrong, they own the issues. My company sent a rookie salesman to sell a job and ended up have to remove a furnace and reinstall the right size one.
            If you get a few quotes from reputable companys THIS WILL BE FINE!
            Who ever heard of paying for an estimate. Rediculous.

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