Choosing a Base Temperature for Degree Days
This reference point is important when using degree days for energy consumption analysis
Degree days are a combination of time and temperature. We looked at their uses and where they come from in Part 1 of this series, and now it's time to go a little deeper.
The temperature enters as a temperature difference, ΔT (delta T), but it's not the ΔT between inside and outside of the building. It's the difference between the outdoor temperature and the base temperature. But what is this thing called base temperature?
Base temperature defined
Let's start with the most general definition. The base temperature is the outdoor temperature that separates when the building needs heating or cooling from when it doesn't. If the outdoor temperature is higher than the base temperature, the building's heating system shouldn't have to operate to maintain the desired indoor temperature. If the outdoor temperature is at or below the base temperature, the building needs heat.
That's pretty straightforward. The base temperature is a balance pointBalance point is the outdoor temperature at which the amount of heating provided by an air source heat pump just equals the amount of heat lost from the house. Below this point, supplementary heat (typically inefficient electric resistance heat or “strip heat”) is required. Typical balance point temperatures are in the range of 27 - 35 degrees Fahrenheit. and has a nice, clear, conceptual meaning. It just doesn't correspond to an exact, constant number, which is how it's usually presented.
Factors that affect base temperature
I used 65°F as the base temperature in the previous article, but let's examine that choice. Why is it the baseline most often used in the U.S. for heating degree days? According to the definition above, that would be the threshold at which you need to start adding heat to keep the building at a setpoint of about 68°F. (More on that discrepancy between base temperature and setpoint below.)
But in what building? With what kind of air and thermal control layers? How well is it shielded from changes in the outdoor environment? 65°F is probably about right for a building with a relatively weak level of air sealing and insulation. Maybe. Even a leaky, poorly insulated building won't need heating at 65°F if it's got a lot of internal heating loads (e.g., people or appliances). And a PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. building may be fine with just the internal loads down into the 30s or 40s Fahrenheit, perhaps even lower.
Here are a few of the factors that affect what the actual base temperature will be:
- Indoor temperature. The warmer you keep it inside, the higher the base temperature will be.
- Internal heat gains. Lights, appliances, and people add heat to the inside of the building. The more they add, the less your heating system needs to supply in winter and the lower the actual base temperature.
- Rate of heat loss or gain. A building that loses heat faster in winter will have a higher base temperature than one that holds onto the heat.
- Season. Solar gains change throughout the year. Wind patterns change. How a building is used changes. Indoor setpoints change, too, because of the effects above but also because of varying mean radiant temperatureMean radiant temperature (MRT) is roughly the average temperature of all the objects or surfaces that a person "sees" inside a building, with the surface temperatures being weighted by their area. A surface or object's contribution to MRT is also based on its temperature in comparison to the person (temperature difference or differential) and the viewing angle between the person and the surface.. (Naked people need building science, you know.)
One interesting relationship came up when my colleagure Jeffrey Sauls and I were discussing this topic. When you make a building enclosure more energy-efficient with better insulation and air sealing, you reduce the heating base temperature. Jeffrey pointed out, however, that if you make the building more energy-efficient by using more efficient lights and appliances, the heating base temperature goes up because the heating system has to do more of the work. If you do it right, the building enclosure improvements trump the light and appliance improvements by a lot.
We should probably be talking about two different base temperatures: actual and standard. The actual base temperature is the one defined above. The standard base temperature is the one commonly used, and it's not the same everywhere. In the U.S., we mostly use 65°F for heating degree days. In the U.K., 15.5°C (60°F) is the standard base temperature.
Difference between base and indoor temperatures
I was a little confused when I first started thinking about degree days. As I showed in my previous article on the topic, you can put DD right into the heat flow equation (Q = U x A x ΔT), but in that equation, ΔT is the temperature difference between indoors and outdoors. The ΔT used in degree days is the temperature difference between outdoors and the base temperature.
In the U.S., we use 65°F as our base temperature, but we typically keep the indoor temperature at 70°F. Why don't we account for the other 5 degrees when we use degree days in the calculation?
The answer goes back to the definition of base temperature. Recall, it's the balance point between needing the heating or cooling system to operate or not. If the temperature in heating season is above the base temperature but below the setpoint, the house is still losing heat through the building enclosure. It just doesn't need the heating system to make up for the heat loss because the internal gains (lights, appliances, and people) add enough heat to keep the house comfortable.
So, there is indeed a discrepancy between the two calculations. Using degree days, you get the amount of heat the heating system needs to add. Using ΔT as the temperature difference between indoors and outdoors, you get the amount of heat loss through the building enclosure, which is made up by both the heating system and the internal gains.
What are you using degree days for?
Despite these problems, having a uniform baseline is still good for some things. Being able to compare one climate to another is the main one. Since I know both numbers use the same baseline of 65°F, I can tell right away that the 14,000 HDDThe difference between the 24-hour average (daily) temperature and the base temperature for one year for each day that the average is below the base temperature. For heating degree days, the base is usually 65 degrees Fahrenheit. For example, if the average temperature for December 1, 2001 was 30 degrees Fahrenheit, then the number of heating degrees for that day was 35. of Fairbanks, Alaska, is much too cold a place for me. Give me Atlanta's 3000 HDD... with a few excursions to the 7,000 to 9,000 HDD skiing climates!
If you're doing calculations with degree days to determine the effectiveness of energy-efficiency measures, it's probably worthwhile to, you need to choose a baseline appropriate to the building. Energy Lens, a U.K. firm that makes energy management software, has a great article about the intricacies of degree days. They also have one of the best sites for finding degree days for a lot of locations. It's called DegreeDays.net, and it allows you to generate degree days for whatever baseline you want to use.
Resolving the base temperature problem
As I mentioned in the previous section, if you're using degree days to compare climates, you don't need to do anything except make sure the numbers you're using all have the same base temperature. If you're doing degree day calculations, you want to use a base temperature as close to the actual base temperature as possible. With data loggers or even just paying a little bit of attention to when the heating and cooling system come on, you can do better than using the standard base temperatures.
The more important thing, though, is to keep in mind that you're not getting results to nearest BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. . Degree day calculations won't give you results to the nearest 100 BTU and probably not even to the nearest 1,000 BTU no matter how good a job you do choosing the base temperature. It's an approximate calculation, and it's the relative changes that matter more than the numbers of British Thermal Units.
That doesn't mean it's not useful and can't be immensely helpful. It just means you need to keep it in perspective and understand the limits.
Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.
- Hans Splinter, from flickr.com
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