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Sufficient radiation for heat loss calcs

Rocky Member Posts: 121
Lets pretend you walk into an existing building and the owner wants a new boiler. IN order to size it properly you do a heat loss calc. Lets say it calls for 100,000 btu. However, there is only 100 feet of baseboard in the whole place. The owner says the building stays plenty warm. I figure 100 feet of baseboard times 580 btu/foot equals 58,000 btu of radiation. I should then size my boiler only to amount of radiation and not heat loss calcs? This happens all the time in existing buildings to me. Maybe I'm adding in areas that do not need heat..hallways, storage rooms? Reason I'm asking is bidding a hanger at airport. Each wing of hanger is 9,000 sq. ft. Figures about 380,000 btu. Yet they only have about 300 feet of baseboard installed (174,000m btu at 580 btu/ft. Do I size my boiler based on existing radiation or heat loss? Seems silly to put in a boiler capable of 380,000 btu if radiation will only deliver 174,000. What say ye?


  • Don_4
    Don_4 Member Posts: 36
    calcs ??

    Just to throw a stick in the mud are your calcs done comparing the design temperature properly compared to the actual temp the boiler (existing) is running at? Sometimes boilers are run exceptionally hot to achieve the required output from undersized radiation fixtures.
  • Gary Fereday
    Gary Fereday Member Posts: 427

    "The owner says the building stays plenty warm" your statement! now that is where the problem just may be. What did you figure for 'inside' temperture? An airplane hanger may be just 'right' to have airplanes in it at somewhat lower temp than what is called normal. Just trying to cause some extra thinking here. Bigugh
  • Rocky
    Rocky Member Posts: 121
    It seems like there is always

    something you hadn't considered, eh? Didn't think about how hot the boiler was, or if the design temp is actually what is being achieved. But regardless. Even in smaller jobs, it always seems like the amount of baseboard is too little to produce the amount of heat the heat loss says I need. Yet these homes work just fine and I have seen several of these boilers in operation and they are not running exceptionally hot. Too wierd.
    Happy heating
  • I think

    If you dare to size the boiler to the radiation, figure it with a 220° F water temperature. If you do that, and the building underheats at 180° F, you have the capacity to turn the boiler temperature up. Trust your heatloss, and re-examine the numbers, if you need to. Talk to the owner about the choices, so he knows what the compromises are.

  • sootmonkey
    sootmonkey Member Posts: 158

    could it be that building is only occupied during the daytime hours, and no one is there during the colder nights? just a thought.
  • MikeB34
    MikeB34 Member Posts: 155
    Its intersting..

    Heat loss calcs represent the extreme or worst case maximums. How often will it be and STAY at the coldest design temp? Honestly? D**N little. which is why so many systems are oversized. lots of peple round up and "add a little for safety" there already is a margin in manual J for safety, so it gets increased.

    Also what is the existing boiler BTU's? Its efficiency? GPM? running temp and Delta T. The size should probably fall somewhere between your calc and the baseboard output. If the existing boiler is sized to your heat loss, I would expect it to run on the hign limit most of the time, which means your H2O temp will most likely be 220 or so, not 180.

    My figures then go to 870 Btu per foot at 220 degree H2o and 4 GPM, this gives you a radiant of 261,000. Not right, but a lot more than your 174,000.

    Also, I remove interior spaces that do not have an exterior exposure.

    There would also be other heat gains (engines running or tested? etc..Halogen lighting can toss a lot of heat too)

    My nickle in the pot.Good luck Rocky.

  • Radiant Wizard
    Radiant Wizard Member Posts: 159
    Why it's Warm

    What alot of us seem to forget is that calculation is based on how many btu's we need on our design degree day. Here it's "0". Now how many days do you think we average "0" here in Ct? Maybe 7 all year if we are lucky. So the rest of the time we run 180 degree water through a footage of baseboard that we only need 7 days a year. Sounds stupid doesn't it?

    How about what's going on in that building? Machinery, number of people, lights so on and so forth. How about solar gain? Now is it always warm in the building because the majority of the work goes on during the day and nobody is there at night? The person is telling you is that he is comfortable not that the available footage of baseboard is equivlant to the heat loss.
  • hydronicsmike
    hydronicsmike Member Posts: 855
    Rocky Pavey!!!

    How have you been?

    I would run the heat load calc and size the boiler(s) so it can maintain the desired indoor air temp at your outdoor design. Keep in mind the total footage of baseboard (fin-tube convectors) you have. You will most likely find that you have more baseboard that you actually needed if you were to run the boiler at 180°F at the coldest day of the year. You will also most likely find that the boiler you have right now is way oversized, but thats just a guess. This is probably the case in about 95% of all the boiler replacements I work on. Now, Since you propably end up having more baseboard that you would normally need, I would assume that your boiler will be able to run at lower temperatures than normal. Thats why I would even consider running all of the baseboard off a mixing device to get full outdoor reset out of it.

    You kow about all that though. You've got my number.
    Enjoy Fairbanks at this time of year.

    Quote: "Fairbanks is a place where the people are unusual and the beer is unusually good." :-)

    You remember this saying Rocky?

This discussion has been closed.