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Degree Days: Understand What These Are and How They Affect Your Home Comfort

The National Weather Service (NWS) keeps track of degree days for a number of reasons. A degree day is a term that refers to how much heating or cooling will be needed to maintain comfort. For homeowners and HVAC professionals this information is vital for anticipating energy usage and cost.  

Calculating Degree Days

The baseline temperature is set at 65 degrees Fahrenheit, a point at which the NWS assumes no heating or cooling is needed, although this can be different for each home or building. The simplest description of the way they make the calculation is by averaging the difference between the high and low and subtracting it from 65. For example, if it was 50 degrees F for a half day and 30 degrees for a half day, you would add half the difference between 50 and 65 (7.5 degrees) to half the difference between 30 and 65 (17.5 degrees), to get 25 heating degree days. For cooling degree days, a similar process is used. 

In reality, the calculation is more complex, involving weighted averages on a daily basis of the temperature and the length of time the air temperature stayed the same. It makes a difference if it's 70 degrees for two hours or four, or if the low temperature was 40 degrees for 20 minutes rather than 90 minutes. Computers make these kinds of data modeling for temperatures easier. 

Purpose of Degree Days

This kind of data is useful for manufacturers of HVAC equipment and contractors, along with building engineers and those who need to plan high-efficiency systems, homes and buildings. It's useful for homeowners, too, since these degree days can help them anticipate heating and cooling needs, as well as select the right equipment to maintain comfort year-round.

In 2013, the degree day data for O'Hare Airport in Chicago indicated that there were 1,044 cooling and 7,274 heating degree days. Although the weather varies annually, with some summers warmer than others and winters colder than normal, this data suggests that people in northern Illinois and nearby Michigan as well as Indiana will need to rely more on their heating systems than the A/C. 

When you invest in a new heating system, seeking a high-efficiency heating system instead of opting for a mid-efficiency furnace will help you save money as soon as you turn it on. You can use the EnergyGuide label or look for the Energy Star logo to sort out those with better efficiency. Combustion furnaces have AFUE (annual fuel utilization efficiency) ratings that range from 78 to 99 percent. In the northern states of Illinois, Michigan and Indiana, those with AFUE ratings over 95 percent should qualify for the Energy Star designation. The EnergyGuide label indicates the AFUE or other applicable ratings.  

Shortcomings of Degree Days

Something that the degree day calculations don't take into account is the humidity, however. This may not be a big deal during the winter time in Chicagoland, but in the summer, the humidity makes the air feel much warmer than it really is. When choosing a cooling system, your HVAC contractor will employ the industry-standard software protocol, Manual J, to determine the cooling load calculation for your home. This information helps determine the proper size of a cooling system for your house. (A similar process is used to find a correctly sized furnace.)

The sister software is called Manual S and it uses the findings of Manual J to match the best system for your home, based partly on the need for humidity removal. The "S" stands for selection, and while your home's cooling load may be similar to that in an arid climate, the selection may end up being entirely different because of the need for more humidity removal. 

Air conditioners and heat pumps have to deal with two kinds of heat. The first is sensible heat, which is air temperature. It's the heat your thermostat senses. The second measure is the latent heat, which is the temperature plus the water vapor in the air. A cooling system that can't manage the latent heat won't be as efficient or comfortable during the humid summer climate. 

Latent heat isn't much of an issue when heating your home, since cold air holds much less humidity and your indoor air tends to dry out over the course of a heating season. If you aren't doing a lot of cooking or deliberately humidifying the indoor air, it could be a sign of a furnace problem if excess humidity develops indoors and you notice condensation on the windows. 

To learn more about degree days and their impact on your home, contact today. We serve the Chicago area, northwest Indiana or southwest Michigan. 

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