📐 The Formula
HDD = Tbase − Tavg
Where Tbase = 18°C and Tavg = mean outdoor temperature
over 24 hours.
Heating Degree Days (HDD): How We Calculate a Region's Climate
When an engineer designs the heating for a home, the first thing the software asks is not how many square metres the building is, but where exactly it sits on the map.
It is not enough to know that an area is "cold". We must be able to measure that cold with absolute mathematical precision, to calculate how much oil or electricity the boiler will need over an entire year. This unit of measuring cold is called a Heating Degree Day (HDD).
1. How Do the Mathematics of Cold Work?
The Degree Day logic is based on a target temperature (Base Temperature), which in Europe is usually set at 18°C . If a day's average outdoor temperature drops below 18°C, the home will need heating.
🔢 Example
If one January day the mean temperature is 5°C, that day "produces" 13 Degree Days (18 − 5 = 13). If the next day it drops to 0°C, that day produces 18 Degree Days (18 − 0 = 18). At year's end, meteorologists add up all winter day values to give the annual HDD index.
2. The Map of Greece in Degree Days
Greece, due to its extreme terrain (mountains, islands), has enormous differences. It is officially divided into 4 Climate Zones (A, B, C, D). To grasp the consumption gap:
🏝️ Zone A - Heraklion, Crete
Approximately 600 to 800 Degree Days per year.
🏛️ Zone B - Athens
Approximately 1,100 to 1,300 Degree Days per year.
🌊 Zone C - Thessaloniki
Approximately 1,800 to 2,000 Degree Days per year.
🏔️ Zone D - Florina / Karpenisi
Exceeds 2,600 Degree Days! A home in Florina needs over 3 times more energy to heat than an identical home in Heraklion.
3. The 10x10 Model Experiment (The Relocation)
We take our perfectly insulated digital home. It loses 100 Watts of energy for every degree of temperature difference with the environment.
☀️ Scenario A (Crete)
Winter is mild. With only 700 HDD, the heat pump runs at idle. At winter's end, the heating bill comes to just €250. We are overjoyed and tell everyone "the heat pump is a miracle!".
❄️ Scenario B (Florina)
We take the exact same home with the same materials and the same pump, and "plant" it in Florina (2,600 HDD). Temperatures stay below zero for months. The pump works overtime. At winter's end, the bill comes to €900. The machine didn't break, nor did the insulation fail. Simply the "weight" of the climate was triple.
The Final Conclusion: Never compare your heating bill with your cousin's who lives in another city. Heating Degree Days (HDD) are the only fair and scientific way to judge whether your home "burns" a lot or a little, because they capture the harsh reality of local weather!
Related Articles
Why theoretical consumption differs from the bill Case Study: The 10x10 Model in 3 Greek cities How airtightness boosts heat pump efficiency by 20% Calculating ventilation losses in kWh (10x10 Model)
Ventilation, Airtightness & Real Consumption: Climate Control
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