Summer cooling coil selection: mixed air, coil leaving air, and whether you need reheat

We solve the cooling coils of two zones with the same total load on the psychrometric chart. When the latent share rises, the coil line stops meeting the saturation curve, and the numbers show why reheat becomes necessary.

Published 2026-10-22

Meeting room process (outdoor 32 °C / 60 %, room 26 °C / 50 %, 30 % outdoor air, SHF 0.70, supply ΔT 10 K). Dotted = OA-RA mixing line, long dashes = room SHF line, bold = coil M → C, orange = reheat C → S, fine dots = coil line extended to the ADP.

The situation

Say one floor has an open office and a large meeting room, each served by its own air handling unit, and you are picking the cooling coils. Both zones have the same 20 kW room load, but the meeting room is packed with people, so far more of its load is moisture. It is tempting to think the same load means the same coil. Plot it, though, and at a 10 K supply ΔT the meeting room cannot hold its humidity with a coil alone: it needs reheat. The conditions below are a made-up example for illustration. A real design has to be redone with the building's own outdoor design conditions and load calculation.

Given

Item	Office	Meeting room
Outdoor air (OA)	32 °C dry bulb, 60 % RH
Room (RA)	26 °C dry bulb, 50 % RH
Outdoor air fraction	30 % (by dry air mass)
Room sensible · latent	16 kW · 4 kW	14 kW · 6 kW
Room SHF	0.80	0.70
Supply temperature difference	10 K (supply at 16 °C)
Barometric pressure	101.325 kPa (sea level)

Plotting order on the chart

Plot the outdoor point OA (32 °C, 60 %) and the room point RA (26 °C, 50 %).
Join OA and RA with a straight line and mark the mixed point M at 30 % of the way from RA towards OA.
Draw the room SHF line through RA (read the slope off the protractor and slide it across).
The point on that line at 16 °C dry bulb is the supply point S.
Extend the line from M through S (the coil line) until it hits the saturation curve: that is the ADP. If it never hits, or the ADP is too low, the coil cannot produce S on its own.
In that case, cool and dehumidify to point C, at the humidity ratio of S and 90 % RH, then reheat horizontally (sensible only) from C to S.

Step-by-step calculation (meeting room)

1) Room and outdoor states. RA: humidity ratio W_r = 10.50 g/kg, enthalpy h_r = 52.91 kJ/kg, dew point 14.8 °C. OA: W_o = 18.03 g/kg, h_o = 78.37 kJ/kg.

2) SHF. SHF = q_s / (q_s + q_l) = 14 / 20 = 0.70. For the office it is 16 / 20 = 0.80.

3) Supply point S. Supply dry bulb t_s = 26 − 10 = 16.0 °C. Solving for the humidity that makes the supply-to-room process have an SHF of 0.70 gives W_s = 8.78 g/kg (77.5 % RH) and h_s = 38.31 kJ/kg. The office, at the same 16.0 °C, needs W_s = 9.49 g/kg. The meeting room's supply air has to be drier.

4) Supply airflow. ṁ = q_t / (h_r − h_s) = 20 / 14.61 = 1.369 kg/s = 4,930 kg/h. Multiplying by the supply specific volume, 0.831 m³/kg, gives about 4,095 m³/h. The office needs 4,679 m³/h.

5) Mixed point M. Mixing 30 % outdoor with 70 % return air gives M = 27.82 °C, W_m = 12.76 g/kg, h_m = 60.55 kJ/kg. The outdoor air brings in 10.46 kW.

6) ADP check. Extending the coil line M → S, it never meets the saturation curve. The same construction for the office gives an ADP of 10.8 °C (BF 0.31), above 7 °C, so its coil leaving air is simply the supply point, with no reheat. A coil surface cannot get colder than the chilled water feeding it, so a very low ADP cannot be produced by a real chilled water coil. This site's calculator treats 7 °C as that lower limit.

7) Coil leaving point C and reheat. The meeting room coil cools and dehumidifies to C = 13.69 °C, where W_s = 8.78 g/kg sits at 90 % RH. The coil line M → C then has an ADP of 10.4 °C and BF = (t_c − t_ADP) / (t_m − t_ADP) = 0.19. Reheat from C to S is 3.23 kW, all sensible.

8) Coil load. Coil load = ṁ·(h_m − h_c) = 33.69 kW (sensible 19.92, latent 13.77 kW), with about 19.6 L/h of condensate. The office coil is 31.93 kW with 18.4 L/h.

What the result means in practice

Zone	Supply RH	Supply m³/h	ADP (coil line)	Reheat	Coil load kW
Office (SHF 0.80)	83.8 %	4,679	10.8 °C	none	31.93
Meeting room (SHF 0.70)	77.5 %	4,095	no intersection	3.23 kW	33.69

Both rooms carry 20 kW, yet the meeting room coil is 1.76 kW larger and needs another 3.23 kW of reheat on top. Every kilowatt of reheat is heat the coil had to remove first, so it costs energy twice. For a low-SHF zone like this one, the first question is not how much reheat to add but whether you can avoid it.

Here is the same meeting room with only the supply temperature difference changed.

Supply ΔT	Supply RH	Supply m³/h	ADP (coil line)	Reheat	Coil load kW
10 K (supply 16.0 °C)	77.5 %	4,095	no intersection	3.23 kW	33.69
12 K (supply 14.0 °C)	84.8 %	3,389	7.3 °C	none	28.72
14 K (supply 12.0 °C)	92.8 %	2,885	9.4 °C	none	27.48

At 12 K the coil line ADP is 7.3 °C, only just above 7 °C. Reheat drops out, but there is almost no margin. At 14 K the ADP rises to 9.4 °C and the airflow falls to 2,885 m³/h, so the coil, ducts and fan all get smaller. The catch is that from 12 K on, the supply air (14.0 °C) is already below the room dew point of 14.8 °C, and at 14 K it drops to 12.0 °C. A 92.8 % RH supply with a BF of 0.14 also means a deep coil with many rows. You then have to check for condensation on diffusers and duct surfaces and for cold drafts, and choose diffusers and duct insulation suited to low-temperature supply. Either way, the key question is where the coil line meets the saturation curve.

Common mistakes

Sizing the coil on total load alone. With the same 20 kW, moving the SHF from 0.80 to 0.70 changes the supply humidity, the ADP and the need for reheat.
Locating the mixed point by volume (m³/h). Mixing works by dry air mass. The point shifts by the difference in specific volume between outdoor and room air. The error is small here but grows with very hot or very cold outdoor air.
Drawing the SHF line through the mixed point. The room SHF line passes through the room point RA. A line through M is the coil's SHF, which is lower because it includes the outdoor air load.
Forcing an ADP reading when the coil line misses the saturation curve. If the extended line never reaches the saturation curve, that supply point cannot be produced by a coil on its own.
Subtracting reheat from the coil load. Reheat does not shrink the coil. The coil still cools to C, and the reheater separately warms C to S.

Try this calculation on the site

Open the calculator with the meeting room inputs Open the calculator with the office inputs

Each link opens the calculator with the outdoor and room points and the cooling panel filled in as above. Change the outdoor air fraction, the supply temperature difference or the sensible and latent loads and watch where reheat switches on. If you do not have the room loads yet, start with the cooling load calculator. The plotting steps are explained in how to read a psychrometric chart, and similar exam-style problems are in the practice problems.

References

ASHRAE Handbook Fundamentals 2017 (SI), chapter 1 Psychrometrics: moist air state equations, adiabatic mixing, sensible and latent processes. Every number here was computed with this site's engine, which implements those equations at 101.325 kPa.
The 7 °C ADP limit, the 7 to 9 °C caution band and the 90 % RH coil leaving point for reheat are this site's cooling calculator rules. Confirm a real coil selection against the manufacturer's selection data and your chilled water conditions.

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