House Heat Loss Calculator?

PastTense Posted By PastTense, Mar 24, 2013 at 5:05 PM

  1. PastTense


    Sep 18, 2010

    One specific point is entering data for a basement.
  2. GaryGary

    Feeling the Heat

    Jul 12, 2010
    SW Montana
    This is the simple and free one I offer on my site:

    It gets used several hundred times a day, so I guess that some like it :) I use it quite a bit myself.

    Basements and crawlspaces are a challenge to model well. I looked around for a simple way to handle these without a lot of luck.
    What's in there now is a slab on grade method from ASHRAE where you can change the edge insulation, but no provisions for insulation under the slab.
    Otherwise, for an unheated basement or crawl space, the main floor can be modeled with whatever level of insulation you want.

    When modeling a crawlspace or unheated basement, some methods recommend inputting a higher R value for the floor than you actually have because the crawlspace or basement is buffered from the outside and generally runs a warmer temp (so less heat loss) than if the floor were fully exposed to the outside. The HEED program and some others recommend just doubling the R value for the floor to account for this. That is, if your floor is actually insulated to R20, then use R40 for the program input.

    One way to model a heated basement would be to treat the above ground part of the basement wall as a standard wall (just like a main floor wall), and then model the rest of the basement as a slab on grade. A little trial and error with this leads me to believe it overestimates heat loss.

    Its a Javascript, and all the code for it can be looked at or changed by just using your browsers View Page Source menu. Its open to anyone to change as long as you make it clear that its a modified version.

    Bear in mind that any of these methods are approximations -- I figure its a good day if the method comes with in plus/minus 20% of the real heat loss.

    Some others you might look at are HEED, HOT2000 (Cannadian) -- there may be a HOT3000 by now -- these have pretty nice user interfaces.
    Equest is free and can model in more detail, but has a pretty steep learning curve.
    EnergyPlus is the DOE's free modeling tool and is capable of modeling very complex heat loss situations -- quite a steep learning curve, but there has been some effort to build tools to use with it to aid in input prep and viewing the output -- one of those allows you to build your house in SketchUp and then works out an EnergyPlus input file from that.
    All of these work off a weather file for your area, while mine estimates yearly heat loss from your local heating degree days.

  3. woodgeek

    Minister of Fire

    Jan 27, 2008
    SE PA
    Basements are hard to model.

    I have played with a few calculators for the rest of the house, but in the end I just compute 'UA', where U = 1/R-value and A is the square feet of an assembly. UA times the temp difference inside to outside is BTU/h going out that assembly. I just added up the UAs for the different parts of my house to estimate the total load (due to conduction), subtracted from the measured load (from utility bills) and called the difference infiltration. As Gary says, its all approximate, but adequate to prioritize different improvement projects. I initially figured that infiltration was 50% of my heat loss (!) and figured airsealing was the place to start.
  4. Laszlo


    Jan 19, 2013
    SE Pennsylvania
    You might try the Passive House Planning Package (PHPP). It's a spreadsheet-based tool to assist the design of Passivhaus standard that's been shown to be able to describe the thermal building characteristics and heating load of passive houses surprisingly accurately. It looks like it can go into a lot of detail.

    PHPP 2012 can be purchased from the Passivhaus Institut for 160 Euro, but a reduced functionality demo version of PHPP 2007 is available for free download.

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