Sensible Heat Recovery Calculator

Concepts behind this calculation

Diagnose measured sensible temperature recovery, or predict both outlet temperatures and recovered heat using supplied effectiveness and unequal air-side heat-capacity rates.

Measured temperature recovery describes how far the leaving supply temperature has moved from the outdoor inlet toward the entering extract temperature. It is an apparent supply temperature ratio, so sensor location, fan heat, leakage, and controls can influence it. It is not automatically a certified recovery efficiency.

Prediction instead uses sensible exchanger effectiveness and the smaller of the two heat-capacity rates. Each capacity rate combines actual inlet flow, dry-air density, and compatible dry-air-basis specific heat. Both streams exchange equal and opposite sensible heat, but the stream with greater capacity changes temperature less. Equal CFM does not necessarily mean equal capacity.

How to use this calculator

1. Select Sensible recovery method.
2. For diagnosis, enter Outdoor entering temperature, Extract air entering the exchanger, Supply air leaving the exchanger, and Measured supply airflow.
3. For prediction, document Operating point, property and effectiveness reference, confirm Dry sensible recovery basis, and supply both actual inlet airflows, dry-air densities, specific heats, inlet temperatures, and Applicable sensible exchanger effectiveness.
4. Inspect supply and extract temperatures without treating the result as latent recovery or annual savings.

Calculation method and applicability

Measured ratio = (Ts,out−To,in)/(Te,in−To,in). Prediction: Ci = 60 Qi ρdry,i cp,dry,i; qs = ε Cmin (Te,in−To,in); Ts,out = To,in + qs/Cs; Te,out = Te,in − qs/Ce.

C = dry-air sensible capacity rate (BTU/(h·°F)); Q = actual inlet flow (CFM); ρdry = dry-air mass per actual inlet ft³; cp,dry = compatible specific heat (BTU/(lb dry air·°F)); ε = supplied sensible effectiveness, entered as percent and used as a fraction.

Input definitions and units

Sensible recovery method

Keep a measured temperature-ratio diagnostic separate from prediction using independently established sensible exchanger effectiveness and both heat-capacity rates. Choices: Diagnose measured supply temperature recovery; Predict with unequal heat-capacity rates and effectiveness.

Outdoor entering temperature (°F)

Measure the entering outdoor stream before recovery. Applies to Sensible recovery method: Diagnose measured supply temperature recovery.

Extract air entering the exchanger (°F)

This is the room/exhaust stream entering the exchanger, not the exhaust leaving it. Applies to Sensible recovery method: Diagnose measured supply temperature recovery.

Supply air leaving the exchanger (°F)

Locate sensors consistently; fan heat, frost controls and leakage can affect the apparent ratio. Applies to Sensible recovery method: Diagnose measured supply temperature recovery.

Measured supply airflow (CFM)

Standard-air sensible duty assumes density 0.075 lb/ft³ and cp 0.24 BTU/(lb·°F). Applies to Sensible recovery method: Diagnose measured supply temperature recovery.

Operating point, property and effectiveness reference

Required: identify actual inlet-flow states, dry-air density/cp basis and source of effectiveness at this flow ratio/operating point. A catalog balanced-flow rating does not automatically apply to unbalanced flow. Applies to Sensible recovery method: Predict with unequal heat-capacity rates and effectiveness.

Dry sensible recovery basis

Confirm steady recovery with no moisture transfer, condensation/frost, cross-leakage, bypass, fan/motor heat or surrounding heat transfer. No humidity is entered, so this tool cannot check condensation or frost. Choices: Not established; Dry sensible basis, compatible properties and effectiveness established. Applies to Sensible recovery method: Predict with unequal heat-capacity rates and effectiveness.

Supply actual inlet airflow (ACFM at outdoor inlet)

Actual volume at the outdoor inlet state. Use a dry-air density at this same state; SCFM and actual CFM are not interchangeable. Applies to Sensible recovery method: Predict with unequal heat-capacity rates and effectiveness.

Supply inlet dry-air density (lb dry air/ft³ inlet air)

Dry-air mass per volume of inlet moist air, equal to 1/v when v is ft³/lb dry air. This is not total moist-air density. Obtain it independently; the example is a constructed property assumption. Applies to Sensible recovery method: Predict with unequal heat-capacity rates and effectiveness.

Supply specific heat on dry-air basis (BTU/(lb dry air·°F))

Compatible effective constant sensible cp per lb dry air over the temperature change. Do not combine total-air-mass cp with dry-air density. Properties are supplied, not inferred from temperature or altitude. Applies to Sensible recovery method: Predict with unequal heat-capacity rates and effectiveness.

Extract actual inlet airflow (ACFM at extract inlet)

Room/exhaust stream entering the exchanger; positive actual flow at its own entered property state. Equal volume flows need not have equal heat-capacity rates. Applies to Sensible recovery method: Predict with unequal heat-capacity rates and effectiveness.

Extract inlet dry-air density (lb dry air/ft³ inlet air)

Use dry-air mass per actual extract inlet volume with the same dry-air basis as its cp; do not reuse supply density without justification. Applies to Sensible recovery method: Predict with unequal heat-capacity rates and effectiveness.

Extract specific heat on dry-air basis (BTU/(lb dry air·°F))

Independently established effective constant sensible cp per lb dry air for this stream. Different inlet humidity can change the compatible dry-air-basis cp. Applies to Sensible recovery method: Predict with unequal heat-capacity rates and effectiveness.

Outdoor supply inlet temperature (°F)

Entering supply-side dry bulb, before recovery and fan heat. Negative Fahrenheit is valid above absolute zero; a dry noncondensing basis must be established independently. Applies to Sensible recovery method: Predict with unequal heat-capacity rates and effectiveness.

Room extract inlet temperature (°F)

Room/exhaust stream entering the exchanger, not the leaving exhaust. Equal inlet temperatures are valid and give zero sensible transfer. Applies to Sensible recovery method: Predict with unequal heat-capacity rates and effectiveness.

Applicable sensible exchanger effectiveness (%)

ε is actual sensible transfer divided by Cmin times the entering temperature difference. Supply temperature ratio, total/latent effectiveness and certified SRE/ASRE are not interchangeable. Establish ε for these flows and conditions; no type preset is supplied. Applies to Sensible recovery method: Predict with unequal heat-capacity rates and effectiveness.

How to interpret the result

Positive supply heat means outdoor air is warmed; negative heat means it is cooled. Compare each outlet and approach factor on its own side rather than expecting equal temperature changes.

Worked examples

Assumptions and limits

• Measured mode reports an apparent supply temperature ratio, not certified sensible recovery efficiency, total/enthalpy effectiveness or annual performance. Its signed heat uses standard-air 1.08 × CFM × temperature change.
• Measured ratio is withheld below a 1°F inlet separation; outside-0–100% apparent ratios remain visible with a warning for diagnosis. This guard is not a measurement or certification tolerance.
• Prediction uses the named ASHRAE 2016 steady dry sensible minimum heat-capacity-rate method. Both positive actual inlet flows require dry-air density at their own state and compatible constant cp per lb dry air. Actual volume equality does not establish equal mass or heat-capacity rates.
• Supply and extract temperature-approach factors are ε Cmin/C for each side and can differ from ε. At equal entering temperatures, heat is zero; the reported factors are model coefficients, not measured zero-over-zero ratios.
• Effectiveness must be independently applicable at the entered flow ratio and operating conditions. No exchanger-type default, flow-dependent effectiveness curve, NTU/UA model or extrapolation of a balanced-flow rating is provided.
• Prediction assumes no moisture transfer, condensation/frost, cross-leakage, bypass, surrounding heat transfer or fan/motor heat. No humidity is entered, so condensation and frost are not detected. Verify that dry sensible operation applies independently.
• No latent/total ERV recovery, certified SRE/ASRE, net electrical energy, fan pressure penalty, annual savings, product selection, field calibration or current-code compliance is determined.

Sources

• ASHRAE 2016 Systems and Equipment, Chapter 26: Air-to-Air Energy Recovery EquipmentNamed historical sensible-effectiveness and minimum-capacity-rate method, Equations 1–3. Example 1 sensible branch is compared using its rounded 660 lb dry air/min and separately its rounded inlet specific volumes; no latent/total or current certification claim.
• CloudAir: Heat Recovery Efficiency CalculatorDirect peer two-flow/four-temperature form and measured/prediction methods observed. Its predictive mode explicitly assumes balanced flow; no unequal-capacity live numeric output is claimed or copied.
• Engineering ToolBox: Heat Recovery CalculatorActual sensible form executed for 40,000 CFM, 75/90°F and 0.73, returning 473,040 BTU/h. A constructed balanced standard-property case reproduces that heat arithmetic; ambiguous peer temperature labels are not adopted as unbalanced-state or outlet validation.