Mar 15 2016

“Degree days” are a measure of how much outdoor temperatures vary over time from a constant base temperature. Engineers and heating and air conditioning contractors use degrees days as a key factor in calculating heating and cooling loads for houses and commercial buildings. If you have a basic understanding of what a degree day is, you will be able to communicate informatively with your HVAC professionals as they determine the right heating and air conditioning systems for your Sarasota home. You can also use your understanding of degree days to compare your home energy use from year to year to see if your efforts to improve home energy efficiency have been effective.

Base Temperature

Sixty-five degrees Fahrenheit is the standard base temperature used in the United States for determining heating and cooling degree days. If you prefer to keep your home or other building at a temperature much higher or lower than the mid-60s you can calculate degree days using any base temperature you prefer. For example, an assisted-living facility might require indoor temperatures close to 80 degrees for the health and comfort of its residents, so a base temperature near 80 would give a more accurate indication of the building’s actual heating and cooling loads. If you operate a fresh produce warehouse, you might need to keep your indoor temperature at 50 degrees or lower, so you would need to use a base temperature much lower than 65 for calculating your cooling-degree days.

One Degree Day

Using the 65-degree base temperature, one heating degree day occurs if the outdoor temperature stays a constant 64 degrees for 24 hours; or one cooling degree day occurs if the outdoor temperature stays at 66 degrees for 24 hours. While meteorologists and research engineers might concern themselves with temperature changes as they occur minute-by-minute throughout the year, for HVAC design purposes degree days calculated using historical daily average temperatures are sufficiently accurate. You can get daily average temperature data as well as historical degree day calculations from the National Weather Service.

Here is a simple example of a degree day calculation for a day with a low temperature of 5 degrees (F) and a high temperature of 39 degrees (F). If you add the high and low temperatures and divide by 2, you get a temperature averaged over 24 hours for the day of 22 degrees. If you subtract 22 degrees from the base temperature of 65 degrees, you get 43 degrees, which is the number of heating degree days for our sample day. On days when the average temperature is exactly 65 degrees, you would have zero degree days, and on days when the average temperature is above 65 degrees, you would have cooling degree days.

Putting it to Use

If you add up all of the cooling and heating degree days for a year, you will have key information that you need to calculate the energy required to maintain your building at a constant temperature that is comfortable for its occupants. Other factors that your HVAC contractor will take into account when designing your heating and cooling systems include insulation, total square footage, number of windows, expected building uses and occupancy levels, and external environmental factors such as building orientation and shading from adjacent structures.

You can use degree day information as a tool in your efforts to assess and reduce your home energy consumption. If you spent a lot of time and money last year upgrading insulation or even upgrading your HVAC system in order to save energy, you can use degree day data to see how effective your efforts have been. Look at the monthly statement you get from your utility company. It probably has information comparing your energy use and the month’s average outdoor temperature to your usage and the average outdoor temperature for the same period a year ago.

For more information about degree days and how you can improve cooling efficiency in your home or business, please contact All American Heating & Cooling today at 941-479-6060.

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