Part II: Conservation FirstChapter 2. Find the Leaks

Heating Degree Days Explained

What degree days measure, where to look them up, and how they scale a heat-loss estimate.

Difficulty: Beginner
Cost: Free

published

A heating degree day is a measure of how far, and for how long, the outdoor temperature sat below a base temperature. It is the usual way to say whether last winter was harsh, and the usual way to turn a heat-loss rate into a rough yearly total. It is not a property of your house. Two houses in the same town share the degree days and still burn different amounts of fuel.

Look the number up for your town and for the base temperature you are using. A figure from a book written for another city will scale every later calculation by the wrong weather. The bill baseline is the reality check. The heat-loss sketch supplies the rate this page multiplies.

Difficulty

Level: Beginner
Time: An hour once you have found a local degree-day table
Skills: Multiplying a rate by a weather number and not over-trusting the product

Cost estimate

Free

Weather services and public degree-day tables are enough. The calculator on this page runs in the browser.

Materials

Item	Qty	Notes
A degree-day total for your town	1 year	Note the base temperature. 65 degrees F is the common published base in the United States. It is a convention.
A heat-loss rate	From the sketch	BTU per hour at a stated temperature difference.
The base your thermostat actually suggests	Optional	A tight house with internal gains may track a lower base than 65.

The definition

For one day, subtract the average outdoor temperature from the base. If the result is negative, that day contributes zero. If the base is 65 degrees F and the average outdoor temperature is 30, that day contributes 35 degree days. Add the days for the heating season, or for a calendar year, and you have the annual heating degree days.

A cold climate can land near 7,000 or more at base 65. A mild coastal climate can land near 2,000 or below. Your town is a lookup, not a guess from this paragraph.

The base is trying to represent the outdoor temperature at which the house no longer needs heat, because the sun, the people, and the appliances cover the loss. Older leaky houses often line up near 65. A tight house kept at 68, with ordinary internal gains, may be closer to a base of 60 or even lower. If your lookup tool offers both, run the page twice. The rank order of projects rarely flips. The annual total does move.

Degree days are the area between the base and the outdoor temperature, on the days the outdoors is colder.

From a cold-night rate to a year

The heat-loss sketch gives BTU per hour at one temperature difference. Divide that rate by the temperature difference and you have BTU per hour per degree, sometimes called a UA. Multiply by 24 hours and by the degree days.

Yearly heat, in BTU, is about that UA times 24 times the heating degree days.

This assumes the loss is proportional to the temperature difference, which conduction and a simple infiltration estimate are. It ignores the sun through south glass, which is heat you gained, and it ignores people and appliances. The number comes out high for a real occupied house. Use it to compare an insulation job with a collector, not to promise a fuel delivery.

Then divide by the heating equipment. A 90 percent furnace needs more fuel BTU than the heat delivered. A heat pump needs less electricity than a resistance heater for the same heat, by about its coefficient of performance. The calculator below stops at the heat. The note under the result is where the equipment belongs.

Using the number without fooling yourself

Compare two winters by their degree days before you praise a project. If this winter had 20 percent more degree days, a bill that only rose 10 percent may mean the work helped. A bill that fell in a warm winter may mean the weather helped. Divide fuel use by degree days and look at that ratio across years. It is a better scoreboard than the dollar total.

Do not use a degree-day total to size a furnace to the last thousand BTU. Design-night temperature does that job, and a safety margin belongs to the installer. Degree days are an energy total, not a peak.

Look up the local number
Write the town, the year or the climate normal, and the base temperature on the same line.
Bring a heat-loss rate
From the sketch, with the temperature difference you used. Divide to get BTU per hour per degree.
Multiply by 24 and by the degree days
That is the high estimate of annual heat. Run the calculator so the arithmetic is not the interesting part.
Apply the equipment you have
Furnace efficiency or heat-pump COP. Then compare with the space-heat remainder on the bills. If they are wildly different, believe the bills and revisit the sketch.

Heating Degree Days Explained

Difficulty

Cost estimate

Materials

The definition

From a cold-night rate to a year

Yearly heat from degree days

Using the number without fooling yourself

Look up the local number

Bring a heat-loss rate

Multiply by 24 and by the degree days

Apply the equipment you have

Further reading