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Thread: Heat load calc help

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    DIY Member Handymaner's Avatar
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    Sep 2008

    Default Heat load calc help--8/19 Starting Install


    I'm about to pull the trigger on a new boiler/indirect water heater. I've narrowed it down to the Lockinvar Knight, just need to decide between the 80000 and 110000. It's a 2500 sq ft house with half in a daylight basement, in Anchorage, AK. It's very air tight (full envelope spray foamed), but the r value is weak, with 3" sprayed on the underside of the roof sheathing of the vaulted ceiling hot roof (~r18), and maybe only a couple inches on the walls and I'm guessing nothing underneath the floor downstairs. A lot of glass on the north side (where the view is) with old 1978 dbl pane glass. There is also a large 4 car garage which is drafty but will be tightened up. There is 70' of standard (I believe 600btu/ft) tube and fin baseboard on each floor with another 25' in a hot tub room. The hot tub is also heated with the boiler, but most of the loss would be in the heated space so I don't think it adds much for load. The garage will be heated with a 61000 btu radiator/fan unit, kept at approx 45 degrees most of the time. Design day outdoor temp for Anchorage is -18f, I believe.

    There is a govt program up here that reimburses you for energy improvements. They use AK warm software for their heat load calc, which uses measurements of windows, floor space, solar gain, etc. as well as a blower door test and it came up with some numbers, as follows:

    Gross Loss: 338.2 MMBtu (not sure what MMBtu is?)
    Gross internal: 31.4 MMBtu
    Useable internal: 31.4MMBtu
    Internal utilization 1.00

    Gross solar: 29.9 MMBtu
    Useable solar: 27.9MMBtu
    Solar utilization: 1.932

    Net heat Load: 278.9MMBtu

    December UA Values and Design Heat Loss

    Living Space UA: 1148.3 Btu/hr/deg-F
    Garage UA: 0.0 Btu/hr/deg-F
    Design Heat Loss of Conditioned Space (70 deg F indoor and garage): 101054 Btu/hour

    "The above value is the required amount of heat delivered to the conditioned spaces. If you need to determine the required input rating of the heating system, you must increase this figure to account for the inefficiency of the heating distribution system, the inefficiency of the furnace or boiler, and an appropriate sizing safety margin."

    I don't fully understand all of the above. I got the idea that they add a fudge factor or two. And this is prior to doing any of the improvements they recommend, most of which I will do. At the end of their list of improvements, the design heat loss, Btu/hr is listed at 79563. I don't know how accurately the program can predict all this. I know when we closed the garage off, the door fan really bogged down, and he said the living space was REALLY tight. No infiltration could be felt on electrical boxes, etc. unlike my last house.

    I want to double check with my actual gas usage, but am unsure of the math there either. Last year (from the gas bill) my highest usage month was Jan at nearly 400CCF, with avg temp of 12f. In the summer with no heating (except the stupid boiler keeps itself hot all summer) it runs about 110 ccf (for the clothes dryer, cook stove, hot tub and 75 gal gas water heater). Leaving I guess approx 300CCF for heating in Jan. The current boiler is a giant Utica with 299999 input. The output spot on the tag is blank, but it also has spaces for DOE heating cap: 225000 and water: 195700Btu, so I'm not sure how to use these numbers to figure my gas usage and heat load? Or how inefficient that grossly over sized thing is? There is also a gas fireplace insert downstairs that could help out on those coldest days, and another fireplace upstairs that we plan to install a wood insert in.

    I would really like to properly size my system. I am leaning toward the 80000 Btu unit, but everyone recommends the 11000 (to be safe!) With my current baseboard at 600Btu/ft there is about 99000Btu capacity, plus the garage heater at 60000BTU, so there is enough radiant capacity for either boiler. In the future I want to add radiant flooring on the upper level (with warmboard or something similiar). And we are replacing windows as we can afford it. Also considering some more foam in the hot roof. Any help or links is greatly appreciated!


    Last edited by Handymaner; 08-20-2012 at 02:23 AM.

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