L2 · HVAC Loads and Psychrometrics

Foundation lesson · 25 minute study session

Separate sensible, latent and equipment loads

A coil can meet a temperature target and still fail its moisture duty. Separate sensible, latent and total heat before sizing airflow or equipment.

By the end of this lesson

Separate dry-bulb change from moisture removal.
Match load coefficients to elevation and flow basis.
Locate lights, people, fan and motor heat within the chosen boundary.

Sensible and latent duty

Sensible heat changes temperature. Latent duty accompanies moisture addition or removal. People and outside air may contribute both; lighting is principally sensible. A ventilation rate is a volume rate, but an energy balance requires mass flow and the appropriate inlet and outlet properties. Keep $\dot Q_s$ , $\dot Q_t$ and $SHR$ on the same boundary.

Use the handbook coefficient

The supplied v2.1 handbook uses a sea-level sensible coefficient of 1.10 Btu/(h·cfm·°F) in §9.1.7. Other references commonly use rounded constants such as 1.08. Keep a calculation consistent with the stated problem data and the examination reference instead of mixing constants.

Keep the loads distinct

A room load does not automatically equal a cooling-coil load. Outdoor-air conditioning, fan heat, duct gains and simultaneous load diversity may change the coil duty. When using an enthalpy difference for total air-side duty, do not add the same sensible duty a second time.

A decision map for this lesson

Keep the load and the boundary together

A temperature target, a moisture target and a coil selection answer different questions.

A realistic cutaway air handler shows a two-inlet mixing plenum, cooling coil with drain pan, and downstream supply fan. — **Room load and coil duty use different boundaries**Connect the equipment to the calculation

Outdoor and return air enter a mixing box, then a cooling coil, supply fan and room. The room has 33000 Btu per hour sensible and 10000 latent; outdoor-air conditioning and fan heat require a separate coil/system balance. — **Room load and coil duty use different boundaries**Connect the equipment to the calculation

Choose the governing relationship and catch the wrong route
Condition or question	Governing decision	Error to avoid
Room sensible duty	Use the room-to-supply dry-bulb difference with $\dot Q_s=C_s\dot V\Delta T$ .	Using total load to calculate sensible supply airflow.
Room latent duty	Track the water-vapor balance and the stated latent-energy model.	Assuming that meeting dry bulb also meets moisture removal.
Sensible heat ratio	Use $SHR=\dot Q_s/(\dot Q_s+\dot Q_l)$ on one boundary.	Dividing sensible duty by latent duty, or mixing room and coil quantities.
Coil duty	Include the air states and energy streams crossing the coil/system boundary.	Equating coil duty with room load when outdoor air or fan heat adds a separate contribution.

On a narrow screen, scroll across the table to compare the decision and the error.

Relationships and their limits

Sensible airflow duty

\dot Q_s=C_s\dot V\Delta T

Use $C_s$ = 1.10 for the handbook sea-level approximation. Confirm whether flow is actual or standard.

Total air-side duty

\dot Q_t=\dot m_{da}\Delta h\approx 4.5\dot V\Delta h

The approximation uses the handbook standard sea-level air basis. Enthalpy is in Btu/lb of dry air.

Sensible heat ratio

SHR=\dot Q_s/\dot Q_t

Use the same system boundary and time basis for both loads.

Symbols and unit basis
Symbol	Meaning	Units or basis
$\dot Q_s$	Sensible duty	Btu/h
$\dot Q_t$	Total duty on same boundary	Btu/h
$C_s$	Stated sensible air coefficient	Btu/(h·cfm·°F)
$\dot V$	Airflow on stated basis	cfm
$h$	Moist-air enthalpy	Btu/lb dry air
$SHR$	Sensible heat ratio	dimensionless
$\dot m_{da}$	Dry-air mass flow	lb dry air/h
$\Delta T$	Room-to-supply temperature difference	°F difference

One decision, fully worked

Apply the method.

A sea-level room has a 33,000 Btu/h sensible load and a 10,000 Btu/h latent load. Room air is 75°F and supply air is 55°F. Use the handbook sensible coefficient of 1.10. Find required supply airflow and room SHR.

Size supply airflow from the sensible duty and dry-bulb difference.
Add sensible and latent loads to obtain total room load.
Divide sensible load by total load to obtain SHR.
Treat moisture control as a separate coil/process requirement.

\dot V=33{,}000/[1.10(75-55)]=1{,}500\ \mathrm{cfm}

SHR=33{,}000/(33{,}000+10{,}000)=0.767

Decision

Supply 1,500 cfm to meet the stated sensible duty; the room SHR is about 0.77. A suitable supply humidity ratio must also satisfy the latent duty.

Independent check

Using total load in the sensible equation would give about 1,955 cfm and would not establish humidity control.

Specialist extension

Use the actual energy boundary for motor gains

Motor loss and driven-device dissipation can enter different spaces. Start from real electrical input, subtract work or heat that leaves the requested room boundary, and account for storage only when the problem requires it. Motor efficiency and power factor describe different relationships. A nameplate apparent-power rating is not automatically the room heat gain.

A new applied case

A motor receives 12 kW real power at 90% efficiency. All shaft work leaves the room and all motor losses enter it. Use 3,412 Btu/h per kW.

P_{shaft}=12(0.90)=10.8\ \mathrm{kW}

\dot Q_{room}=12-10.8=1.2\ \mathrm{kW}=4094.4\ \mathrm{Btu/h}

Decision. Assign 4,094.4 Btu/h to the room under the supplied boundary.

Cross-check. If the shaft work instead dissipated inside this room, the steady room gain would be the full 12 kW.

Unit basis. P is real power in kW; efficiency is dimensionless; heat rate is kW or Btu/h.

NCEES PE Mechanical Reference Handbook v2.1; see the module reference path.

Open targeted practice in the study studio →

Transfer the method

HVAC Loads and Psychrometrics · HV-LC-L2

Try a new decision.

Select one

A single room has a 24,000 Btu/h sensible load and 8,000 Btu/h latent load at the same design condition. Supply air is 16°F below room temperature. Use $C_s$ = 1.10 Btu/(h·cfm·°F). The required sensible airflow and room sensible heat ratio, respectively are most nearly:

A1,364 cfm; SHR = 0.75B1,818 cfm; SHR = 0.75C455 cfm; SHR = 0.25D1,364 cfm; SHR = 3.00

A lesson check supports practice, not a readiness score. Your response stays in this page session and clears on reload.

Catch the common errors

Using total load to calculate sensible-only airflow.
Adding sensible duty twice after using an enthalpy difference.
Ignoring whether motor losses and shaft work enter the conditioned space.

Handbook navigation drill

Find it, then explain it.

Search: Heat Gain Calculations Using Standard Air

Locate sensible and total air heat relationships, including their stated elevation factors.

Check your navigation

You can identify 1.10 and 4.5 and state the units of each required property.

Reference path: §9.1.1–9.1.8, pp. 446–456; §9.1.16, p. 477. These are printed handbook page numbers; section names and search terms remain the primary navigation method. See technical review status.

Theory sources

NCEES handbook v2.1, §§9.1.1–9.1.8.
Grondzik and Kwok, Mechanical and Electrical Equipment for Buildings, 12th ed., chs. 4, 7 and 12.
Lechner, Heating, Cooling, Lighting, 4th ed., ch. 3.

Worked examples and lesson prose are original ReasonSpan^™ material. Obtain authorized publications separately; this preview does not reproduce the supplied books or handbook.