Base outdoor temperature: the key parameter for sizing

Understanding the base temperature and its role in sizing

Simple definition

The base temperature (or base outdoor temperature) is a conventional value used to calculate a building's heat loss and size the heating system. It represents a "reasonably severe" cold spell for your zone, not the historical record. It's used to set the temperature gap between the target indoor temperature and the design outdoor temperature.

Avoiding confusion

• Average temperature. This describes the typical climate over a long period. It doesn't size a power output.
• Cold snap. This is a short, sometimes extreme episode. If you size for it, you often end up oversizing.

Why it changes your calculations

The lower the base temperature, the higher the required heating power. This affects the choice of heat generator, water flow rate, emitter size, and the ability to hold the setpoint without short cycling. The result: more stable comfort and fewer disappointed customers when winter "bites."

Where to find the right base temperature for your climate zone

2026 benchmarks and field cases

To choose a reliable base temperature, start from the reference tables used in heating sizing (EN 12831 standard) and the official climate zoning by department. In practice, always cross-reference the job's municipality with the zone (H1, H2, H3) and then check altitude. In the mountains, a cold valley can be several degrees off from the county town, while a sunny hillside or a windy coastal area often needs a finer adjustment. If the site is above 800 m, or is very sheltered, switch to a corrected value rather than staying on the zone average. A classic mistake: extrapolating from the neighbouring town or an airport weather station, which smooths out terrain effects and skews heat pump power. Keep local, documented data consistent with the site's exposure.

Field method: factoring the base temperature into your sizing

Step by step

Start by estimating heat loss room by room (surfaces, insulation, ventilation). Then relate the heating need to the local base temperature, and plot the load curve. You position the bivalence point. That's the outdoor temperature at which the backup takes over. From that you derive the power to be delivered. Aim for a power that covers the base need, without inflating it beyond reason.

Impact on emitters

• Radiators. Check the power at the planned water temperature. Water that's too hot lowers efficiency.
• Underfloor heating. Ideal at low temperature, it smooths out variations and limits backup use.
• Fan coil units. Useful in renovation when responsiveness is needed, provided noise is controlled.

Checking consistency

Keep a safety margin of 10% to 15%. Watch for short cycling. If the unit starts too often, you're oversizing or the controls are poorly set. Conversely, too much backup use increases consumption and masks an emitter shortfall.

Heat pumps: fine-tuning sizing with the base temperature

Manufacturer curves, system strategy and caution in cold zones

To size a heat pump, start from your zone's base temperature. Then read the manufacturer's curves at that outdoor temperature. Look at the delivered power (not the electrical power) and the COP, which drops when it's cold and when the flow temperature rises. If the delivered power at base temperature just covers the need, the backup will kick in often. On strategy, a mono-split targets one zone, a bi-split distributes better but complicates balancing. Air/water suits low-temperature emitters well. A hybrid system secures cold peaks thanks to the backup boiler. In cold zones, plan for defrosting. It uses power and briefly interrupts heating. Allow a reasonable margin, check the installation location (wind, snow, air recirculation) and configure a stable heating curve.

Insulation and base temperature: how renovation changes the picture

After insulating the loft, walls or ground floor, heat loss drops. So you don't keep the old calculations. Redo the assessment based on the local base temperature, with the new thermal resistances, treated thermal bridges, and often-improved airtightness. In phased renovation, the trap is well known. Sizing the heating system on the "intermediate" state can lead to an oversized unit once the envelope is finished, with short cycling and falling efficiency. Best practice is to target the final power, and secure the in-between phase with fine-tuned controls, temporary backup, or emitters capable of modulating. Finally, ventilation isn't a detail. Flow rates, balancing and infiltration drive part of the losses. A properly adjusted mechanical ventilation system avoids "ventilating away heat" and stabilises comfort.

Checks and documents to produce to secure your pricing in 2026

Data to archive

To avoid disputes at the end of a job, keep a record of the base temperature used, the location, the altitude if it changes the picture, and the calculation method (heat loss, emitter power, heating curve). Archive your assumptions, comfort setpoints, air renewal, surfaces and insulation materials, then the results. Power at the cold point, margin, distribution by zone, planned settings. With these elements, your pricing stays clear and defensible.

Consistency across audit, DPE and RGE

In 2026, the classic trap is inconsistency. An energy audit, a DPE and a sizing note must tell the same story: same surfaces, same work scenario, same reference temperatures. This also protects your grant applications and your RGE inspections.

Talking points for customers

For the customer, explain it simply: the base temperature is used to size for the coldest days, without oversizing for the rest of the year. Show the "why" with an example: comfort, savings, fewer cycles and a longer-lasting unit.