alternatasha a site to catalog random projects


home heating index

How efficient, in terms of heat, is your home? The Home Heating Index (HHI) is a simple calculation that gives you a number with which to categorize your home heating efficiency. A simple guide here takes you step by step through the calculation.  Basically, you find the ‘Heating Degree Days’ (HDD) for your area here. Then look at your gas bill. There you can find how many therms or ccf’s of gas your house consumed in one month. Over the entire season, take the total therms (or ccf) of the year and subtract 12 times the usage in the hottest summer month. This would remove the therms used solely for hot water for a more realistic number for heating gas. In my case, since 1 therm natural gas is about 100 ccfs of 1000 BTU’s each, if I use one whole round and a half of my gas meter (1500 therms), then the home heating index (HHI) is calculated as such:
HHI = Total Winter Fuel Energy / (House Area * HHD)
HHI = (1500 x 10^5 BTU) / [(3000 sf)(4804 DD)
HHI = 10.4 BTU/sf-DD

Then find your number in the following chart :

Home Heating Index (HHI)

HHI Comment
0-2 Excellent! House consumes very little energy for its size, climate and user habits
2-4 Very Good! Some improvements could likely be found. House is well insulated and sealed against infiltration. Efficient heating system.
4-8 Moderate to Average. Improvements could be made
8-12 Low Average. You could save money by improving the house
12-18 Poor! There are many opportunities to improve the house
> 18 Paying a fortune to heat a drafty house with an inefficient heating system!

For me?  Low Average.  I am not surprised — I need to seal up and fix drafty windows and doors, install insulating blinds, and, most importantly, figure out what’s going on with the radiators so that I can balance my steam heating system.

8 Responses to home heating index

  1. Eric says:

    Over the last 12 months I’ve used about 650 heating therms for about 1400 square feet and about 7255 HDD; that’s 65,000,000 therms / (1400 * 7255) = 6.4.

    Over the last 5 years (better average) it’s about 6.7. I think; the numbers are a little weasely since I can’t easily separate out heating therms. Another arduino project, if I ever get a pulsing gas submeter 😉

    The above is for a 1931 house which has had a reasonable amount of weatherization work, but has only a bog-standard 10-year-old boiler with a standing pilot.

    • admin says:

      Nice! 650 therms is nice and low. I’m using double or triple that! After this season I should have a better idea of what I’m really consuming here; and then I’ll be able to tell how much the upgrades really help. And I do have a long list of future upgrades…

      • Eric says:

        Have you had a home energy audit done? A blower door test & IR camera scan can really tell you a lot about where to concentrate your efforts & rank your projects…

        • admin says:

          No, I haven’t done that. I’m still working on basic things like drafts. The thing is, this is a century-old townhouse with original double-hung windows which I refuse to replace. Unfortunately they happen to be only 1/4″ single-pane, and to fix one requires weeks… I did four of them in the past, but haven’t gotten around to the worst ones yet. It’s a shame I started this blog only a month ago, but if I embark on another round of window-fixing I’ll definitely take pictures and post the process. Other things like roof insulation are expensive and can wait, possibly.

          • David Fay says:


            You’re right to tackle air sealing first — cheap and effective.

            I’d love to see the pictures when you do more window repair. I plan to restore the original windows on my 1850’s summer house in the next few years. I could use some tips.


            • admin says:

              Hi David,
              Check out the book, “Working Windows.” After reading it I understood more about windows than I ever thought needed to be understood. If you start your project before I do, let me know and I can write you a list of the products I used.

          • Eric says:

            Yep I suppose if there are blazingly obvious leaks, that’s the place to start. Blower + IR can help identify smaller leaks you didn’t know about.

            The other thing I’ve heard (and I’m no real expert, just a hack) is to really concentrate on the leaks at the top & bottom of the structure due to the stack effect – so attic hatches & roof electrical boxes matter a lot, as do leaks down below like the rim joist in the basement.

    • Eric says:

      Hm I’ve got too many spreadsheets etc. 650 is too low, my best estimate for average is 707, and with a 60F base it’s about 6200 HDD, so more like HHI of 8.

Leave a Reply

Your email address will not be published. Required fields are marked *