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Air flow calculation based on heat gains
Power calculation based on air flow
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Supply air temperature calculation based on power and air flow
Air flow, heater power and supply temperature calculator
Three related engineering calculators for sizing a ventilation unit and its heating coil. The flow calculator returns the airflow required to deliver a given heating power across a given temperature rise. The power calculator returns the heating-coil power needed to heat a known airflow by a given Δt. The supply-temperature calculator shows what temperature the air leaves the heater at, optionally accounting for an upstream heat recovery exchanger. A tool for designers of ventilation, air-conditioning and supply-and-extract air handling units.
Three calculators on one page
Air flow V — enter the heater power [kW] and the required temperature rise Δt [K] (how many degrees the heater should lift the air). Result is given in m³/h and dm³/s.
Heater power P — enter the airflow V [m³/h or dm³/s] and Δt [K]. The result is the required heating-coil thermal power in kilowatts.
Supply temperature Tsup — enter the outdoor temperature Tout, the heater power and the airflow. You can also enable the "heat recovery" toggle and provide the exchanger efficiency together with the exhaust air temperature — the calculator first computes the temperature behind the recovery exchanger and only then adds the heater effect on top of it.
Base equations — sensible heat balance of ventilation air
All three calculators use the same heat-balance equation for an air stream at constant pressure (steady flow, ideal gas):
P = V · ρ · cp · Δt
where: P — heating-coil power [W], V — volumetric airflow [m³/s], ρ — air density (≈ 1.2 kg/m³ at 20 °C and 1013 hPa), cp — specific heat of air at constant pressure (≈ 1005 J/(kg·K)), Δt — temperature difference between inlet and outlet of the heater [K]. The product ρ · cp ≈ 1.2 kJ/(m³·K) is often used as a simplified constant for quick power estimates.
The supply temperature follows from rearranging the heat balance equation. For systems without heat recovery: Tsup = Tout + P / (V · ρ · cp), where Tout is the outdoor temperature taken from the intake. For systems with heat recovery the calculation is two-step: first the temperature behind the recovery exchanger is computed as Trec = Tout + η · (Texh − Tout), and then the post-heater raises it to the supply temperature Tsup = Trec + P / (V · ρ · cp). The calculator handles both cases — the "heat recovery" toggle decides which equation is used.
Typical design values for ventilation in Polish climate
The values below are starting points for quick calculations and follow Polish design practice and the standards used for comfort buildings:
- Comfort supply temperature: 18–22 °C in winter (the Tsup − Troom difference should not exceed −3 K for ceiling-level diffusers, to avoid the cold-jet effect).
- Heater temperature rise Δt: 30–45 K for an electric or hot-water coil running without heat recovery; 10–25 K when the system has a heat recovery exchanger that pre-heats the air.
- Winter design outdoor temperature: depends on the Polish climate zone (PN-EN 12831-1) — from −16 °C (zone I, Pomerania, Greater Poland) to −24 °C (zone V, Suwałki region). For most of the country −20 °C is assumed.
- Summer design outdoor temperature: 30–35 °C (typically 32 °C for zone III) at 45–55% relative humidity.
- Air density ρ ≈ 1.20 kg/m³ at 20 °C and 1013 hPa — the value falls with temperature, rises with pressure and drops by about 1% per 100 m of altitude.
- Specific heat of air cp ≈ 1005 J/(kg·K) for dry air; slightly higher for humid air (about 1010–1020 J/(kg·K) for typical room humidities).
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