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Community and Q&A

Sanity check on very preliminary rough estimates of heating energy usage

Roger Smith | Posted in GBA Pro Help on

Hi to the Forum.

A few months back we bought a home in zone 5 (Northern Nevada at 5000′) that was solely heated with electricity. We intend to install hydronic heating throughout and want to do it right. I was going to do a BEopt calculation but the computer I downloaded that software to was damaged and is currently out for repairs and bringing my heating upgrade project to a standstill.

So, I just today received my electric bill for December. Looking back it dawned on me that during November we did little heating but during December almost every day has been a high heating load. This is also confirmed by looking at degree-day historical charts for this area. My average electrical energy usage for November was 32.1 Kwh/Day and for December it was 63.3 Kwh/D. The Delta is therefore 31.2 Kwh/D the majority of which I would attribute to the additional heating load. Converting Kwh/D directly to BTU results in 109,188 BTU consumed per average day. Assuming that the vast majority of this usage is during the waking 12 hours of the day leads to the conclusion that we were expending 9,099BTU/H during those 12 hours.

  1. Is this approach accurate enough to base heater sizing decisions on?
  2. Do these numbers meet the giggle test for an older house (read poorely insulated with lots of uninsulated glass) in heating zone 5? I.e. is a heating input of ~10,000BTU/H reasonable for the [broad] conditions stated?

BTW: I do intend to replace all the glass and reinsulate the sidewalls so I would expect the current wintertime heat loss to go down by roughly half when that project is complete.

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  1. GBA Editor
    Martin Holladay | | #1

    There are a lot of uncertainties and assumptions in your calculations, especially since you are only comparing one month to another month. I think that you need to perform an old-fashioned heat loss calculation or a Manual J if you really want to know the answer to your question.

    Here are a few ways you might have gone astray:

    1. Your assumption that you used no electricity for heating in November may be wrong.

    2. December isn't over -- so how come you already have an electricity bill for December? What weeks does it cover?

    3. Your assumption that December 2016 is a typical month for your winter heating load may be erroneous. Perhaps the weather in December 2017 will be different, and you will need twice as many BTUs for space heathing as needed in December 2016.

    4. Your assumption that you don't use any energy for space heating at night -- but you only use energy for space heating during the day -- is almost certainly erroneous.

  2. Expert Member
    Dana Dorsett | | #2

    Without both the indoor & outdoor temperatures during the days between meter readings there's not much you can glean from the bill. You can find daily degree-day history data from a nearby weatherstation on, but you still have to estimate your average indoor temp to figure out your degree-day base (which will be 5-10F cooler than your indoor temp, assuming it's an occupied house.)

    For examples of how that works, see this:

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