E7 · HVAC Equipment and Components

Foundation lesson · 18 minute study session

Recover the available stream energy.

Distinguish sensible recovery from total-energy recovery and preserve the entering-stream basis.

By the end of this lesson

Apply supplied sensible effectiveness to balanced dry-air streams.
Separate temperature recovery from moisture and enthalpy recovery.
Recognize fan energy and operating limits beyond gross recovered duty.

The exhaust stream is the reference opportunity.

A recovery device transfers energy between exhaust and outdoor streams. During summer sensible cooling, outdoor supply air moves toward the cooler exhaust inlet temperature. During winter it moves toward the warmer exhaust inlet. Effectiveness describes the stated device and stream conditions, not the fraction of whole-building energy saved.

Sensible and total recovery have different outputs.

A sensible device exchanges heat without intended moisture transfer. A total-energy device can also exchange moisture, so enthalpy or humidity effectiveness may be specified. Do not apply sensible effectiveness to an enthalpy difference without explicit authorization. Unequal stream capacity rates require the appropriate minimum-capacity relationship.

Gross transfer is not net savings.

Pressure drop adds fan input, frost protection can change operation, and leakage or carryover can matter to air quality. A problem may impose an operating lockout or economizer bypass. Preserve the stated boundary before converting recovered duty to an annual cost saving.

A decision map for this lesson

Recovery must fit the stream and operating objective

A gross thermal benefit is not the same as a net electrical saving.

Choose the governing relationship and catch the wrong route
Condition or question	Governing decision	Error to avoid
Sensible effectiveness is supplied	Use the stated inlet temperature difference and stream capacity-rate basis.	Applying balanced-stream formulas to unequal streams without checking the model.
Latent or total-energy recovery is claimed	Check the vapor-transfer mechanism or the stated enthalpy-effectiveness definition.	Inferring latent recovery from a sensible-only rating.
Net electrical benefit is requested	Convert avoided thermal duty through the displaced plant COP, then subtract added input power.	Subtracting electrical kW directly from thermal kW.
Free cooling, frost or contaminated exhaust is present	Evaluate bypass, frost protection and leakage/carryover against the stated objective.	Assuming the summer rating establishes winter operation or contaminant separation.

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

Relationships and their limits

Balanced-stream sensible effectiveness

\varepsilon_s=\frac{T_{oa}-T_{s,out}}{T_{oa}-T_{exh}};\quad T_{s,out}=T_{oa}-\varepsilon_s(T_{oa}-T_{exh})

Equal heat-capacity rates, stated sensible-only model and negligible leakage. Signs also describe winter operation.

Sensible recovered rate

\dot Q_s=C_s\dot V(T_{oa}-T_{s,out})

For stated standard sea-level flow, use $C_s$ = 1.10 Btu/(h·cfm·°F). Change the basis when required.

General capacity-rate limit

|\dot Q|=\varepsilon C_{min}|T_{oa}-T_{exh}|

Sensible exchanger model with appropriate effectiveness definition; $C_{\mathrm{min}}$ is the smaller stream capacity rate.

Symbols and unit basis
Symbol	Meaning	Units or basis
$\varepsilon_s$	Supplied sensible effectiveness	dimensionless
$T_{oa}$	Outdoor entering dry-bulb temperature	°F
$T_{exh}$	Exhaust entering dry-bulb temperature	°F
$T_{s,out}$	Supply-side leaving temperature	°F
$C_{min}$	Smaller heat-capacity rate	Btu/(h·°F)
$C_s$	Stated sensible airflow coefficient	Btu/(h·cfm·°F)
$\dot V$	Airflow on stated basis	cfm
$\dot Q_s$	Gross sensible transfer rate	Btu/h
$\varepsilon$	Effectiveness for stated sensible model	dimensionless

One decision, fully worked

Apply the method.

A sensible-only recovery device has equal stream capacity rates and effectiveness 0.70. Outdoor air enters at 95°F and exhaust at 75°F. Outdoor flow is 2,000 cfm on the stated standard sea-level basis. Use $C_s$ = 1.10; neglect leakage and fan energy.

Apply effectiveness to the 20°F available entering temperature difference.
Subtract the recovered 14°F from outdoor inlet temperature.
Calculate gross sensible transfer on the supplied airflow basis.

T_{s,out}=95-0.70(95-75)=81^{\circ}\mathrm F

\dot Q_s=1.10(2000)(95-81)=30{,}800\ \mathrm{Btu/h}

Decision

Supply-side leaving temperature is 81°F and gross sensible recovery is 30,800 Btu/h. The result says nothing about latent recovery or annual net energy savings.

Independent check

The outlet lies between the two inlet temperatures. Full sensible effectiveness would produce a 44,000 Btu/h upper limit, and the calculated transfer is 70% of it.

Specialist extension

Compare net electrical savings after recovery penalties

A heat-recovery device can reduce thermal coil duty while adding fan or pump power. Compare both on the requested energy boundary. For unequal stream capacity rates, a supplied effectiveness relation uses the smaller capacity rate for maximum possible exchange, while the receiving-stream temperature rise uses its own capacity rate. Leakage, bypass and pressure penalties require their stated definitions and data.

A new applied case

A recovery device avoids 30 kW of thermal cooling. Supplied cooling COP is 3.0 and added fan input is 1.5 kW. Other effects are unchanged.

P_{avoided}=30/3.0=10\ \mathrm{kW}

P_{net\ saved}=10-1.5=8.5\ \mathrm{kW}

Decision. Net electrical savings are 8.5 kW at the stated operating point.

Cross-check. Subtracting fan power directly from thermal kW would combine unlike input/output boundaries.

Unit basis. Thermal duty and electrical input are both expressed in kW but describe different boundaries; COP is dimensionless.

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

Open targeted practice in the study studio →

Transfer the method

HVAC Equipment and Components · HV-LC-E7

Try a new decision.

Select one

A sensible-only energy-recovery device has equal stream heat-capacity rates and supplied effectiveness 0.60. Outdoor air enters at 20°F; exhaust enters at 70°F. Neglect leakage and all other transfers. The supply-side leaving temperature is most nearly:

A50°FB30°FC−10°FD50°F with 60% total building energy savings

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

Catch the common errors

Interpreting 70% effectiveness as 70% reduction of total building energy.
Using sensible effectiveness to calculate a moisture transfer.
Ignoring stream imbalance or fan energy when the problem includes it.

Handbook navigation drill

Find it, then explain it.

Search: Heat Recovery Ventilators; Energy Recovery

Compare the handbook sensible and total-energy recovery sections. Identify which property difference appears in each effectiveness definition.

Check your navigation

Sections 9.2.7 and 9.2.8 distinguish recovery methods. The stated effectiveness must match the property and stream balance being calculated.

Reference path: §9.2.7–9.2.8, pp. 482–485. These are printed handbook page numbers; section names and search terms remain the primary navigation method. See technical review status.

Deeper application · Decision Lab

Recover energy without crossing the prohibited air path

Select a recovery mechanism from the actual air-quality and layout constraints, then ask whether its added electrical load outweighs the benefit.

Choose the method, predict the changed-condition result and inspect the complete cross-check. Sign in before starting to keep your first response and later reflection in the separate reasoning notebook.

Open the original workshop →

Theory sources

NCEES PE Mechanical Reference Handbook v2.1: §9.2.7–9.2.8, pp. 482–485
Mechanical and Electrical Equipment for Buildings, 12th ed.: mechanical systems and energy principles. Supporting theory; not an examination reference.

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