Watch air flow through the heating and cooling coils of an AHU in real time, and visualize the temperature/humidity change, the moving state point on a psychrometric chart, and the sensible / latent / total loads.
Parameters
Airflow
m³/min
Air volume through the AHU.
Inlet dry-bulb
°C
Air temperature at the coil inlet (after mixing).
Inlet relative humidity
%
Relative humidity of inlet air. Drives the latent load.
Heating coil surface temp
°C
Preheat coil surface temperature. At or below inlet temp the heating coil is inactive.
Cooling coil ADP
°C
Cooling coil apparatus dew point (ADP). At 30 °C or above the cooling coil is inactive.
Bypass factor
-
Fraction of air that does not fully contact the coil (ADP).
Results
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Supply dry-bulb
—
Supply RH
—
Sensible load Qs
—
Latent load Ql
—
Total load Qt
—
Mass airflow ṁ
AHU air-flow animation
Air flows left to right; as it passes the heating coil (red) and cooling coil (blue), its color (temperature) and particle whiteness (humidity) change. The arrows at each coil show the heat exchanged.
$\dot m=\rho\,\dot V$ is the mass airflow (kg/s, $\rho$=1.2 kg/m³), $c_p$=1.006 kJ/kgK, $h_{fg}$=2501 kJ/kg, and $w$ is the humidity ratio (kg/kg). In engineering coefficient form this equals $Q_s[\text{W}]=1.21\,\dot V[\text{L/s}]\,\Delta T$ and $Q_l[\text{W}]=3010\,\dot V[\text{L/s}]\,\Delta w$. When the cooling coil surface (ADP) is below the air dew point, condensation occurs and a latent load $Q_l$ appears.
What is AHU heat balance
An AHU (air handling unit) mixes outdoor and return air and passes it through filters, heating and cooling coils, a humidifier, and a fan to condition the temperature and humidity of the supply air delivered to a space. Its heat balance determines the outlet (supply) state from the inlet air state (temperature and humidity) and the heat the coils add to or remove from the air.
The heat carried by air splits into two kinds: sensible heat that changes the dry-bulb temperature, and latent heat that changes the moisture content (humidity ratio). A heating coil usually delivers sensible heat only, while a cooling coil whose surface is below the dew point condenses moisture, lowering temperature (sensible) and removing water (latent) at the same time.
How to read this animation
In the air-flow animation, air entering from the left passes the heating coil (red band) and the cooling coil (blue band) in sequence. Particle color shows temperature (blue = cold / red = warm), and the brightness of the white dot inside each particle shows humidity (brighter = more moist). When the cooling coil condenses moisture, droplets fall below the blue band to indicate dehumidification.
On the psychrometric chart, the state point moves from inlet (I) → after heating (H) → outlet (O). The horizontal axis is dry-bulb temperature and the vertical axis is humidity ratio: the outlet point drops for dehumidification, moves right for heating, and moves left for cooling. The load bars place the sensible (orange) and latent (cyan) magnitudes side by side, so the share of total (the sensible heat factor, SHF) is obvious at a glance.
Physical model and key equations
The tool uses the bypass-factor method. The cooling-coil outlet dry-bulb is $T_{out}=T_{adp}+BF\,(T_{in}-T_{adp})$, so a smaller BF brings the air closer to the ADP (apparatus dew point). Dehumidification is approximated by the outlet humidity ratio $w_{out}=w_{adp}+BF\,(w_{in}-w_{adp})$, where $w_{adp}$ is the saturation humidity ratio at the ADP. A heating coil raises temperature only, as $T_{out}=T_{surf}-BF\,(T_{surf}-T_{in})$, leaving the humidity ratio unchanged.
The loads are $Q_s=\dot m\,c_p\,(T_{in}-T_{out})$ and $Q_l=\dot m\,h_{fg}\,(w_{in}-w_{out})$, with the total $Q_t=Q_s+Q_l$. The sign convention treats heat removed from the air (cooling / dehumidification) as positive. The mass airflow is $\dot m=1.2\times\dot V/60$ (kg/s, $\dot V$ in m³/min). For example, 120 m³/min, 30 °C / 60% RH, ADP 10 °C, BF 0.12 gives Qs ≈ 42 kW, Ql ≈ 44 kW, Qt ≈ 87 kW — the latent load is as large as the sensible one.
Learn AHU heat balance by dialogue
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I get that a cooling coil lowers the temperature, but why does the humidity (latent heat) change too?
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In a nutshell, when the coil surface is colder than the air dew point, water vapor condenses on the surface and drips off — those are the droplets you see falling below the blue band. The air loses that water, so its humidity ratio drops, and the latent load Ql is added on top of the cooling load. If the surface is warmer than the dew point, only the temperature drops and dehumidification is zero.
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There's a bypass-factor slider — what is it actually changing?
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Think of it as the fraction of air that slips past without touching the coil. At BF 0 all the air is cooled to the ADP (coil surface temp); at BF 0.3 about 30% stays at the inlet state. That's why $T_{out}=T_{adp}+BF(T_{in}-T_{adp})$. In real units, too few coil rows, a high face velocity, or a clogged filter that skews the airflow all raise BF, and the coil can no longer dehumidify enough.
🙋
On the load bars the sensible and latent loads were about equal. Is that normal?
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When you deeply cool and dehumidify humid summer outdoor air (say 30 °C / 60% RH), the latent load often equals or exceeds the sensible load — that's a sensible heat factor SHF = Qs/Qt around 0.5. Data centers and kitchens are sensible-dominated with SHF near 0.9, while theaters and pools get a lot of latent load from people and water and run a low SHF. Checking that ratio on the bars points you toward the coil and reheat strategy.
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Can I use this result directly for design?
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It's solid for a first-pass review. It's great for sweeping BF and ADP to see how much margin the outlet temperature and humidity keep. But it fixes the constants (density 1.2, cp, hfg), so confirm the final figures against the manufacturer's selection software, measured temperatures and humidity, and standards. Watching where the state point lands on the psychrometric chart is a good sanity check too.
Real-world applications
First-pass AHU coil capacity checks for office buildings and tenant spaces. Estimate sensible, latent, and total load at peak summer conditions to provisionally select the chiller and coil rows.
Diagnosing insufficient supply temperature or humidity on an existing AHU. Raise BF to reproduce dehumidification shortfalls caused by filter fouling or excessive airflow, and weigh cleaning or damper adjustment.
Outdoor-air units (OAU) and high-latent uses such as kitchens or theaters, where checking the sensible heat factor (SHF) tells you whether reheat or a dedicated dehumidification mode is needed.
Common misconceptions and cautions
Cooling is not the same as dehumidification. If the coil surface (ADP) is above the air dew point, only the temperature drops and the latent load is zero. To dehumidify you must set the ADP below the dew point.
The bypass factor is not a fixed value; it changes with operating state. Increasing airflow raises the face velocity and worsens BF, which can leave dehumidification short. Airflow and dehumidification are a trade-off.
Loads are proportional to mass airflow ṁ. Even at the same volumetric airflow (m³/min), a different air density (high outdoor temperature, high altitude, etc.) changes the load. This tool fixes density at 1.2 kg/m³, so correct it for extreme conditions.
FAQ
Sensible load changes the dry-bulb temperature, Qs=ṁ·cp·ΔT, while latent load changes the moisture (humidity), Ql=ṁ·hfg·Δw. A heating coil is usually sensible only; a cooling coil adds latent load when its surface temperature is below the air dew point and condensation occurs. The total load is the sum, Qt=Qs+Ql.
The bypass factor is the fraction of air that slips past the coil without reaching the coil surface temperature (the ADP for cooling). The outlet dry-bulb temperature is Tout=Tadp+BF·(Tin-Tadp); a larger BF leaves the outlet closer to the inlet, so cooling and dehumidification fall short. BF depends on coil rows, face velocity, and fin pitch, and rises with fouling or excessive airflow.
Every load term is proportional to mass airflow, so airflow scales both sensible and latent load equally. Inlet dry-bulb mainly drives the sensible term through ΔT, while inlet humidity drives the latent term through Δw. Reading the sensible-to-latent ratio (SHF) on the load bars shows which input dominates.
It is a first-pass approximation using the bypass factor and the apparatus dew point (ADP); it does not capture nonlinear coil heat transfer or detailed partial-condensation profiles. It assumes air density 1.2 kg/m³, cp=1.006 kJ/kgK, and hfg=2501 kJ/kg. Confirm final designs against vendor selection software, measured temperatures and humidity, and standards.
How to Use
Enter airflow rate in m³/min (typical range 50–120 m³/min for commercial AHU)
Set the inlet dry-bulb temperature and relative humidity of the outdoor or mixed air, and enter the cooling coil ADP (effective coil surface temperature). To study heating, set the heating coil surface temperature above the inlet temperature
Enter the bypass factor (0.02–0.60) to reflect coil contact; supply temperature/humidity and the sensible, latent, and total loads update automatically. Use the presets (Heating / Cooling / Dehumidification) to load representative conditions at once
Worked Example
For dehumidification at inlet 30 °C / 60% RH, 120 m³/min, cooling coil ADP 10 °C, and bypass factor 0.12: supply dry-bulb ≈ 12.4 °C, sensible load ≈ 42 kW, latent load ≈ 44 kW, and total load ≈ 87 kW — the latent load is as large as the sensible one. Increasing the bypass factor to 0.25 raises the supply temperature to about 15 °C and reduces dehumidification, lowering the latent load. This reflects reduced coil contact.
Practical Notes
Condensation / dehumidification: cooling the supply below the dew point (typically 12–15 °C) creates a large latent load. If the supply ends up too cold, a reheat coil is needed to raise the supply temperature
Coil fouling: a clogged filter that skews the airflow raises the bypass factor (lowers contact efficiency), so build a regular cleaning schedule (every 3–6 months)
Winter heating: when using a hot-water coil, set the heating coil surface temperature near the supply-water value (45–55 °C) and also check for freeze protection and over-humidification