Gas air heater sizing calculator
Solve for required heating capacity, achievable outlet temperature or required airflow — for direct- or indirect-fired heaters, on GCV or NCV basis — and see gas input power, gas flow rate, running cost, and the seasonal heating-load and modulation range on a chart, using the actual air density at your inlet condition, not a fixed sea-level constant.
How to use this calculator
- Pick what you're solving for — required heating capacity (the usual case), achievable outlet temperature for a heater you already have, or the airflow needed to hit a target temperature rise with a known heat output. The form swaps which field is an input and which is the answer.
- Enter the airflow, inlet temperature and target discharge temperature (or the known heating capacity, if you're solving for temperature or airflow instead), plus altitude if the installation is significantly above sea level.
- Choose the heater type — direct-fired (combustion products mix straight into the airstream, ~100% efficiency, outdoor air only) or indirect-fired (heat exchanger, flue-vented, pick an efficiency band or enter your own).
- Pick the fuel and calorific value basis (GCV/HHV or NCV/LHV) — the gas industry and different countries quote calorific value on different bases, and mixing them up is a common source of error.
- Set a sizing margin (10% by default) to get a recommended nominal heater size on top of the bare calculated requirement — an explicit design allowance, not a regulatory minimum.
- Optional: check the seasonal range. Enter a winter design temperature and a heating cutoff temperature to see how required capacity varies across the season, and enter a specific heater's nominal capacity and turndown ratio to check whether its modulation range actually covers that season — the results panel shows this as a heating-load chart with the normal, cycling and undersized zones marked, plus the matching gas-consumption curve.
- Read the results: required heat output, recommended nominal heater size, gas input power, gas flow rate, and — if you filled in operating hours and price — the estimated annual running cost and CO2.
The physics behind the sizing
Heating air is a straightforward energy balance: the heat added per unit time equals the mass of air moved per unit time, multiplied by its specific heat, multiplied by how much its temperature rises. The same equation just gets rearranged depending on which quantity you're solving for.
Q = ṁ × cp × ΔT solving for required capacity
Tout = Tin + Q / (ṁ × cp) solving for achievable outlet temperature
V̇ = Q / (ρ × cp × ΔT) solving for required airflow
ṁ = V̇ × ρ mass flow, kg/s (V̇ in m³/s, ρ in kg/m³)
cp ≈ 1.005 kJ/(kg·K) specific heat of dry air
ΔT = Tout − Tin
The one detail worth getting right is ρ, the air density — it isn't a fixed number. Cold air and air at altitude are both denser than the 1.204 kg/m³ textbook value at 20°C and sea level, so the same volumetric airflow carries more mass, and needs more heat output to hit the same ΔT. This calculator computes the real density at your stated inlet temperature and altitude using the barometric formula and the ideal gas law — the same method used in CloudAir's Air Density calculator — rather than assuming standard conditions. The same relationship is swept across a range of outdoor temperatures to build the seasonal heating-load chart, using the density at each outdoor temperature in turn.
Direct-fired vs indirect-fired
A direct-fired heater burns fuel directly in the airstream — there's no heat exchanger to lose heat across, so essentially all the combustion energy ends up in the supply air and this calculator treats its efficiency as ~100%. The trade-off is that combustion products (CO2, water vapour, trace NOx and CO) are now part of the delivered air, so direct-fired heaters are only used on 100% outdoor (make-up) air, never on recirculated air, and always with the minimum airflow the manufacturer specifies to keep combustion product concentrations safe.
An indirect-fired heater keeps combustion inside a heat exchanger and vents the products through a flue, so the supply air never mixes with them — that's what allows recirculated air, but it also means real losses (typically 80-95% efficiency depending on the design) that a direct-fired unit doesn't have.
From heat output to gas consumption
The heat output Q is what the air actually receives. The heater has to burn more fuel than that to deliver it, because combustion, heat exchange and flue losses are never 100% efficient (direct-fired excepted, as above):
Gas input power = Q / η
Gas flow rate = Gas input power / calorific value
Calorific value can be quoted on two different bases, and mixing them up is a common source of sizing error: GCV (gross calorific value / HHV) includes the latent heat of the water vapour formed by combustion, and is the basis Regulation (EU) 2016/2281 itself uses for reporting NOx and efficiency figures. NCV (net calorific value / LHV) excludes that latent heat, since a conventional (non-condensing) appliance can't recover it — so NCV figures are consistently a bit lower than GCV for the same fuel. This calculator lets you pick either basis; typical GCV reference values are natural gas around 10.5 kWh/m³, propane around 13.8 kWh/kg, butane around 13.7 kWh/kg — these vary by gas composition and supply, so use your actual supplier's figure for a firm design.
Reading the seasonal heating-load chart
If you enter a winter design temperature and a heating cutoff temperature, the same Q = ṁ×cp×ΔT relationship is swept across that range to show how required capacity falls as the outdoor temperature rises toward the point heating is no longer needed. Add a specific heater's nominal capacity and minimum modulation (turndown ratio) and the chart shades three zones: green where the required load sits inside the heater's normal modulation range, orange where the required load drops below the heater's minimum output — meaning it will cycle on and off rather than modulate smoothly — and red where the required load at the cold end exceeds the heater's nominal capacity altogether. A matching gas consumption chart underneath shows the same range converted through the current efficiency and calorific value.
This is a sizing calculation, not a compliance check. It tells you the heat output and gas consumption a given airflow and temperature rise require — a physical result, true for any heater with the stated efficiency. Whether a specific product's declared ηs,h and NOx meet Regulation (EU) 2016/2281's legal minimum is a separate question, answered by CloudAir's Ecodesign compliance checker.
Frequently asked questions
How do you size a gas-fired air heater?
Required heat output is Q = mass flow × specific heat of air × temperature rise (Q = ṁ × cp × ΔT). Mass flow comes from the volumetric airflow multiplied by the actual air density at the inlet condition, and specific heat of dry air is taken as 1.005 kJ/(kg·K). The gas input power then follows from dividing the heat output by the heater's thermal efficiency.
What temperature rise can a gas air heater achieve?
There is no universal limit — it depends entirely on the specific heater's design and manufacturer rating. Direct-fired and indirect-fired warm air heaters are commonly specified for temperature rises in roughly the 15-45 K range, but always check the manufacturer's rated discharge air temperature limit for the specific unit before finalising a selection.
Does altitude or temperature affect gas heater sizing?
Yes. Colder or higher-altitude air is denser, so the same volumetric airflow carries more air mass and needs more heat output to achieve the same temperature rise. This calculator uses the actual air density at your stated inlet temperature and altitude rather than a fixed sea-level value.
Is this the same as checking Ecodesign compliance?
No. This tool sizes the heat output and gas consumption needed for a given airflow and temperature rise — a physical engineering calculation. Whether a specific product's declared efficiency and NOx meet Regulation (EU) 2016/2281's minimum thresholds is a separate, regulatory question, checked with CloudAir's dedicated Ecodesign compliance checker.
What is the difference between a direct-fired and an indirect-fired gas heater?
A direct-fired heater burns fuel straight into the airstream, so almost all the combustion energy reaches the supply air (~100% efficiency), but the combustion products become part of that air, which limits it to 100% outdoor (make-up) air applications. An indirect-fired heater burns fuel inside a heat exchanger and vents the combustion products through a flue, keeping them separate from the supply air, which allows recirculated air but at a real efficiency loss, typically 80-95%.
What is the difference between GCV and NCV for gas heater sizing?
GCV (gross calorific value, also called HHV) includes the latent heat of the water vapour produced by combustion. NCV (net calorific value, also called LHV) excludes it, since a conventional non-condensing appliance can't recover that latent heat, so NCV figures are always somewhat lower than GCV for the same fuel. Regulation (EU) 2016/2281 uses GCV for its own efficiency and NOx reporting, so it's worth confirming which basis your gas supplier or manufacturer datasheet is quoting before comparing numbers.
What does the heating-load chart's cycling zone mean?
It means the outdoor temperature has risen enough that the required heat output has fallen below the heater's minimum burner output. A modulating burner can't turn down further than its rated minimum, so instead of running continuously at a reduced rate it starts cycling on and off to average out the lower load — normal behaviour, but worth knowing about if continuous, non-cycling operation matters for the application.