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replacement boiler size?

Somethings sounds amis. I'd double-check everything.

That's 45 btu/sq.ft. heat loss. Very high unless the climate is extremely cold and/or the shell is in awful shape--leaky with essentially no insulation.

Amount of radiation (214 sq.ft. EDR) sounds low for an old hot water system. Are you positive you're using the correct table for the rads in question? Using a tube-type table for column rads would result in far less EDR than is actually there. Can you post a photo of a rad?

At 170F average water temp the resulting 20 btu/hr per square foot of floor area sounds very low for an old system. Have radiators been removed?

Perhaps the rads were sized for significantly hotter temps than 170 average. Is this a steam system converted to hot water? Perhaps an old gravity system with something like a Honeywell "heat generator" that allowed significantly hotter water temps and higher pressure? Is there something that looks rather like a butter churn on the floor next to the boiler?


  • Paul_64
    Paul_64 Member Posts: 4
    replacement boiler size?

    I did a heat loss calculation for a 1600 sqft home and came up with a total btuhr need of 72000. So at 83% eff. I need a 90,000btu boiler. I measured the radiators and converted them to sqft edr. 1sqft edr = 150btuhr with an average water temp of 170. I have 214sqft edr x 150= 32,100btuhr. My question is Why put a boiler in that more than doubles what the connected radiation can give off? What am I missing?
  • Brad White_172
    Brad White_172 Member Posts: 53

    Where is the house and do you have windows and doors? :)

    A 1,600 SF house with insulated 2x4 walls, average areas of single pane windows and storm windows should have a heat loss (assuming a 0 degree outside design temperature and 68F indoors) in my experience would fall between 25 and 36 BTUH per SF depending on how "tight" the house is to infiltration.

    That would put you at about 40,000 to 56,000 BTUH. You would be in the range of about 45 BTUH per SF not bad but room for improvement. Let's stipulate that it is "in range"- it seems more so than the radiation part.

    The median level of 48 BTUH PSF if taken at zero outside- to get to your target heat loss of 72,000 your house would have to be in a -34F design area. Is that about (or "aboot") correct?


    Now, your radiation: I think it is time for you to check it again. Unless you had steam (and even if you did), your radiation would only emit about 54,300 BTUH, still shy of your calculated target. Double-check your radiation.

    If there is room to spend money on improving your envelope, that is to me the best investment.

    But do check your radiation take-offs; those numbers are more out of whack than your heat loss could be.
  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
    The radiation is your btu load.

    If the house heated ok before, then it would be safe to say your radiation is your btu heat load. Putting a boiler in that is substantially larger than this is not efficient and will have a tendency to cycle more often - shortening its useful life, and wasting fuel. Thats doesn't mean it was the right amound of radiation to begin with, it could be more than you need to heat the house.

    I use this chart to make quick verifications of manual heat loss calculations. I got this from WeilMclain over 25 years ago. It has served me very well.

    Note, the chart is based on a 70 differential design temp. If your home is similar to code 9, a 1600 sq ft home is approx 37,000 btu. Load. Notice how close this is to your radiation load of 32,100.

    I agree with the others, you've got to recheck your numbers. I agree with the Brad, something is out of whack.
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