Calculator
Example Data Table
| Case | Indoor | Outdoor | Area | U-value | ACH | Typical Result |
|---|---|---|---|---|---|---|
| Small insulated room | 21°C | 0°C | 30 m² | 0.35 | 0.4 | Lower heating load |
| Average apartment | 21°C | -5°C | 80 m² | 0.55 | 0.6 | Moderate heating load |
| Leaky older space | 21°C | -10°C | 120 m² | 0.85 | 1.2 | High heating load |
Formula Used
Temperature difference: ΔT = indoor temperature − outdoor temperature.
Conduction loss: Qc = U × A × ΔT.
Infiltration loss: Qi = 0.33 × ACH × V × ΔT.
Net load: Qn = max(0, Qc + Qi − internal gains).
Design capacity: Qd = Qn × (1 + reserve percentage ÷ 100).
Input power: Pin = Qd ÷ system efficiency.
Daily energy: kWh = Pin ÷ 1000 × operating hours.
How to Use This Calculator
Choose the unit system first. Enter the indoor set temperature and the cold outdoor design temperature.
Add the floor area and ceiling height. The calculator uses these values to estimate room volume.
Select an insulation level or enter a custom U-value. Use a lower U-value for better insulation.
Enter air changes per hour. Tight rooms use lower values. Leaky rooms use higher values.
Add internal heat gains, reserve percentage, efficiency, daily runtime, and energy price. Press calculate.
Use the CSV or PDF buttons to save the current result after entering the same values.
Heating Load Planning With Outdoor Temperature
A heating requirement calculator connects weather, room size, insulation, leakage, and system efficiency. Outdoor temperature is important because heat moves from warm indoor air toward colder outside air. The larger the temperature difference, the faster heat leaves the space. This tool converts that difference into a design heating load.
Why Temperature Difference Matters
Heat loss rises almost in direct proportion to indoor minus outdoor temperature. A room held at 21°C needs much more heat on a 0°C night than on a 12°C afternoon. Designers often choose a local design outdoor temperature. It represents a cold condition that the heater should handle without constant oversizing.
Envelope And Air Leakage Effects
The building envelope includes walls, roof, floor, windows, and doors. A lower U-value means better insulation and slower heat flow. Air leakage also matters. Cold outdoor air enters through cracks, vents, and door openings. The calculator estimates infiltration from air changes per hour and room volume. This makes the result more realistic than area alone.
Efficiency And Practical Capacity
A heater must deliver enough output heat to match the design load. Real equipment also has efficiency losses. Electric resistance heat is near unity at the room, while combustion and heat pump systems vary by model and conditions. The calculator separates useful heat, equipment input, daily energy, and estimated cost. This helps compare different systems fairly.
Using Results Safely
The output is an estimate, not a stamped load calculation. It is useful for early sizing, quick comparisons, and checking whether a heater seems too small. Use conservative outdoor temperatures for bedrooms, nurseries, clinics, or critical work areas. Avoid extreme oversizing, because it can cause short cycling, noise, uneven temperatures, and poor humidity control.
Improving Heating Demand
Lower demand first. Seal air leaks, insulate exposed surfaces, close gaps around pipes, and improve window coverings. Small improvements can reduce both conduction and infiltration loads. After changes, run the calculator again with better U-values or lower air changes. This shows the physics benefit in watts, BTU per hour, and cost.
Seasonal Interpretation
For seasonal planning, test several outdoor temperatures. Mild, design, and extreme cases reveal how demand changes across winter, helping owners budget energy and choose backup heat wisely.
FAQs
What outdoor temperature should I enter?
Use the cold design temperature for your area. For a quick estimate, enter the lowest common winter temperature. Avoid using rare record lows unless the space needs critical heating.
What is a U-value?
U-value measures heat flow through a surface. A lower value means better insulation. Walls, roofs, windows, and doors all have different U-values.
Why does air leakage affect heating load?
Leaking air brings cold outdoor air inside. The heater must warm that incoming air. More air changes per hour raise the infiltration load.
What is the envelope exposure factor?
It estimates exposed heat loss area from floor area. Higher values represent more outside walls, roof exposure, floor exposure, windows, or corner rooms.
Should I include internal heat gains?
Yes, if people, lights, appliances, or equipment add heat. These gains reduce the net heating demand. Use conservative values for design sizing.
Why add capacity reserve?
A reserve helps cover colder weather, wind, startup recovery, and input uncertainty. Keep it reasonable. Too much reserve can oversize the heater.
Can this replace a professional load calculation?
No. It is a planning calculator. Detailed building design should include room-by-room loads, window data, local codes, ventilation, and equipment performance curves.
Why are watts and BTU per hour both shown?
Watts are standard for physics calculations. BTU per hour is common in heating equipment labels. Showing both makes equipment comparison easier.