Heating degree-days

What are heating degree-days and what are the advantages and limitations of that conept? Generally speaking, heating degree-days (HDD) represent a sensible measure in order to estimate how much energy must be provided for heating purposes.

It´s cold outside, you turn on the heating. The colder it is, the more you have to heat, if you want to keep the room temperature at a convenient level. As a consequence, you need more energy, if the outside temperatures are lower. Thus the temperature difference between inside and outside to a large extent determines how much oil, gas, wood or electricty you need in order to keep your place cosy and warm.

But this is not the only parameter having an impact on your heating bill. Another factor of crucial importance is the numer of days you have to keep the heating running in the first place. Wintry weather conditions and their duration can vary considerably from one year to another. Last year, at the beginning of November, outside temperatures in northern Europe were already below 0° C. This year, however, in the same region the thermometer has hardly ever touched the freezing point, thus saving a lot of energy costs.

So we see that two crucial parameters determine the heating effort: the temperature difference between living room and outside on the one hand and the duration of the period when the heating is on.

Formally speaking, following the definition used by Eurostat, HDD may be defined as follows:

HDD = (18° C – Tm)*d,  if Tm <= 15° C  or

HDD = 0,  if Tm > 15° C

In this formula, d represents the number of days when heating is considered to be required and Tm is the mean outside temperature defined as Tm = (Tmin + Tmax)/2. Thus, Tm is an average value of minimum and maximum temperatures during a certain period. But when is the heating actually on? According to Eurostat the heating is on when Tm <= 15° C, whereas for Tm > 15° C it is off and then HDD = 0.

In this way, we have obtained an important indicator for the amount of energy which is needed in order to keep our living or working space at an agreable level.

However, HDD in itself is not sufficient to determine or even estimate the actual amount of energy necessary for heating purposes. To that end, more input is needed. In particular, we need to know how big the energy flow from our living and/or working premises to the outside world is. Clearly the heat flow is directly proportional to the difference in temperatures as indicated in the formula for HDD. Yet, the amount of heat passing from the cosy appartment to the cold and sometimes frosty environment also depends on the insulation we use in order to reduce the loss of heat. The insulation in turn is closely linked to the construction materials used.

Fig. 1 gives us an overview over HDD in the EU-27 and some selected countries. The raw data for this have been taken from Eurostat.

Fig. 1 HDD in EU-27 and selected countries, 1980-2009

Apparently, there is a clear distinction between several countries, depending on their geographical location. HDD for Germany and UK are closely following the EU average. The northern countries Sweden and Finland are placed well above that average, whereas the southern Member States Spain and Portugal find themselves well below that value. The mean deviation from the EU average amounts in the case of Sweden and Finland to 67% and 79%, respectively. Spain and Portugal, as the antipodes in HDD,  are as far as 43% and 60% below the European mean value, respectively.

HDD reflects the climatic conditions of each country. Average temperatures are considerably lower in Europe´s northern periphery and in the southern part. Therefore the difference in HDD between Finland (5800 on average) and Portugal (1300) is easily explained. Taking HDD as the only reference, Finland would need more than 4 times as much energy for heating than its couterpart. However, comparing these figures with the energy consumption per capita for both countries (Finland 5.3 ktoe and Portugal 2.2 ktoe, annual average for 1991-2010) yields a clear indication that there must be some features which tend to soften the sharp discrepancies. Among these are the standards for heat insulation (which can vary between different countries), the number of cooling degree days (having an opposite north-south tendency) and the level of industrialization.

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