Definition of degree days
When it comes to assessing the energy efficiency of an existing building, it often pays to compare energy consumption over different periods. A large part of energy consumption in buildings depends on the prevailing external climate, which is not exactly the same from year to year. Degree days must be used to eliminate the differences.
One degree day expresses a 1 °C difference between the estimated internal temperature and the average outdoor air temperature over a 24 hour period. If, for example, the average outdoor temperature over a 24 hour period is 2 °C, then the number of degree days over a 24 hour period (1 twenty-four hour period) is 17 – 2 = 15 (°C×d).
Why is the estimated internal air temperature taken lower than it actually is? The reason is that part of the heating need is covered through so-called free heat in the form of heat emanating from electrical devices, people, etc., and solar radiation. For example, an analysis of the thermal behaviour of our typical five-storey apartment buildings has shown that, on average, the internal temperature rises by approx. 3-4 °C as a result of free heat.
Key regions for designating degree days
The number of degree days within different regions in Estonia are not comparable due to climatic differences. On the basis of a survey [1] conducted by Tallinn University of Technology, it has been proposed that Estonia be divided into six regions based on the number of degree days. These key regions and centres for degree days, the outdoor air temperature of which is used to designate degree days for the region, are as follows:
I Jõhvi
II Tartu
III Tallinn
IV Valga
V Pärnu
VI Ristna
Recommended boundaries for key regions can be seen in the drawing below.
In simplified calculations, degree days for Tallinn can be viewed as the average for Estonia.
Data on degree days
Data is published by the 7th day of each month, at the latest.
Degree day data for Estonia’s different key regions at different bivalent temperatures and years:
I Jõhvi IV Valga
II Tartu V Pärnu
III Tallinn VI Ristna
Compensating for the impact of outdoor temperatures in different years.
An important area of use for degree days is in eliminating the effect of outdoor temperature on the consumption of heat in different years. In order to eliminate the effect of outdoor temperatures in different years, the actual annual consumption of heat is carried over to a comparable, so-called normal, consumption year, using the association:
where: QN – consumption of heat in a normal year, MWh;
Qteg – actual consumption of heat in a year, MWh;
SN – number of degree days (simple degree days, selected based on the bivalent temperature tB in a building) in a normal year;
Steg – actual number of degree days during the year (selected at the same bivalent temperature tB, as SN);
C – consumption of heat, in MWh, regardless of the number of degree days.
Some of the heat consumed in the buildings is weakly associated with the outdoor air temperature, and therefore practically independent of the number of degree days. This is characterised by C in the equation provided above. The main part of the consumption of heat, independent of the number of degree days, is comprised of heat used to produce domestic hot water. The consumption of heat, independent of the number of degree days, includes losses from hot domestic water piping, heat provided to buildings through hand dryers, which may also include heat losses from heating piping located in non-heated rooms (for example, unheated attics).
Bivalent temperatures
Indoor air temperature in the building develops as a result of heating and free heat. The sources of the latter are people, electrical equipment, electric lighting, and solar radiation. Figuratively speaking, heating covers losses from the temperature of the outside air to bivalent temperature tB. Heat losses from the bivalent temperature to a room’s indoor temperature tS are covered by free heat. In the study [2] conducted by Tallinn University of Technology, bivalent temperatures for more important buildings were assigned for different types of buildings, the use of which is recommended on degree days.
Residential buildings (tS=21°C)
As a result of the surveys, the following approximate bivalent temperatures have been obtained for determining annual heating needs:
| Description of the building |
Bivalent temperature |
| old-style unrenovated apartment buildings (natural ventilation) |
17°C |
| old-style renovated apartment buildings (natural ventilation) |
13°C |
| apartment buildings (mechanical ventilation) |
15°C |
| new apartment buildings (mechanical ventilation) |
12°C |
When determining the heating needs for specific months, it is recommended that somewhat different bivalent temperatures be used in order to attain the best results. In colder months (October, November, December, January, February, March, and April) they could be the following:
| Description of the building |
Bivalent temperature |
| old-style unrenovated apartment buildings (natural ventilation) |
18°C |
| old-style renovated apartment buildings (natural ventilation) |
14°C |
| apartment buildings (mechanical ventilation) |
16°C |
| new apartment buildings (mechanical ventilation) |
13°C |
In warmer months, (September, May, etc.) it is recommended that the following bivalent temperatures be used in the analysis of heat consumption:
| Description of the building |
Bivalent temperature |
| old-style unrenovated apartment buildings (natural ventilation) |
15°C |
| old-style renovated apartment buildings (natural ventilation) |
11°C |
| apartment buildings (mechanical ventilation) |
13°C |
| new apartment buildings (mechanical ventilation) |
10°C |
Schools (tS=21°C)
| Description of the building |
Bivalent temperature |
| in a standard school building with renovated windows and unrenovated ventilation |
15°C |
| in a standard school building with renovated building envelopes and unrenovated ventilation |
13°C |
| in a standard school building with renovated building envelopes and mechanical exhaust |
15°C |
| in a school building with renovated building envelopes and mechanical ventilation based on heat recovery units |
13°C |
Office buildings (tS=21°C)
The thermal performance of office buildings is much more complicated. A significant role in the process of the development of internal temperature is played by actual free heat, as well as engineering systems and their level of automation. Therefore, only approximate calculated bivalent temperature (24 hour averages) intervals can be recommended, depending on the enclosed net area per worker:
| Enclosed net area per worker in an office building |
Bivalent temperature |
| up to 16 m2/person |
7..11°C |
| 16 to 30 m2/person |
11..13°C |
| over 30 m2/person |
13..15°C |
Surveys
In order to obtain a more detailed overview of the subject matter associated with the use of degree days, we recommend that you review the following referenced surveys.
[1] Estonia degree days and instructions on their use: Stage I. TalTech Department of Environmental Technology, 2003.
[2] Estonia degree days, TalTech Department of Environmental Technology, 2006.
We also recommend that you review the calculation examples presented in studies on the transferring of actual annual heat consumption to a normal year (for example [2], part 3.1).