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Community and Q&A

Impact of Microclimates on Sizing a Heat Pump System

Andrew3GP | Posted in General Questions on

Hi, inexpert homeowner here.  We are replacing oil burner with an ASHP system.  Climate zone 6A in NY.  The winter outdoor temps at our house consistently are 5F more than the Man. J outdoor design temps at nearest weather station.  Not a fluke – influenced by various local factors.  Thoughts?

Thank you

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  1. GBA Editor
    Martin Holladay | | #1

    Why "consistently"? The design temperature is an annual minimum. It has nothing to do with day-to-day variations, so where does "consistently" enter the picture?

    As Allison Bailes explained in one of his blogs, "Winter design temperature [is] defined as the temperature that a location stays above a certain percentage of the hours in a year. The 99% design temperature is the one usually used. That means a place stays above the 99% design temperature 99% of the hours in a year. The total number of hours in a year is 365 x 24 = 8760. Ninety-nine percent of that is 8672 hours, which means that if your 99% deign temperature is 0° F as it is in Fort Collins, Colorado, your temperature will be below 0° F for only 88 hours per year, on average."

  2. Andrew3GP | | #2

    Martin, thanks for your response. "Consistently" in the sense that if it is -10F at the weather station, it is -5F here; if it is 1F at the weather station, it is 6F here. Not exactly, of course, and not every hour of every day, but let's say "almost always."

    So, if I am understanding correctly, at the weather station the winter temp is below the 99% design temperature for only 88 hours per year; but at our house the winter temp is below that 99% design temperature for far fewer hours.

    My practical concern is whether using the Man. J winter outdoor design temp without adjusting for the average (~5F) winter temperature difference between the weather station and our house will skew the outcome toward an oversized system?

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