AHU Heat Recovery Calculator Excel: Free Download

Heya! Welcome to Crypto To You. Today on this occasion I am going to share AHU Heat Recovery Calculator Excel: Free Download.

 Every building that uses outdoor air for ventilation is throwing energy out the exhaust. In winter, warm exhaust air carries away heat that must be replaced by the heating system. In summer, cool exhaust air carries away cooling energy. Across the year, this waste can equal 20–40% of the building's HVAC energy consumption.

A Heat Recovery Unit (HRU) captures this energy and transfers it to incoming outdoor air—reducing heating and cooling loads dramatically. Modern HRUs achieve 50–85% sensible effectiveness, and with energy recovery wheels, total effectiveness (including latent) can exceed 80%.

The problem? Engineers often underestimate the savings, undersize the HRU, or skip the calculation entirely. A proper heat recovery calculation determines:

• Required HRU capacity (based on airflow and temperature difference)

• Heat recovery efficiency (sensible vs. total)

• Energy savings (kWh/year)

• Payback period (years)

• Impact on heating/cooling equipment sizing

In this guide, you'll learn the complete heat recovery calculation methodology and get access to a free Excel calculator that automates the entire process.

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Heat Recovery Calculation Fundamentals

The Sensible Heat Recovery Formula

The heat recovered by an HRU is:

$${\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">Q</span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">c</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">o</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">v</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">d</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">\eta </span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">s</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">V</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">\rho </span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">c</span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">p</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span><span\; style="font-family:\; verdana;\; text-align:\; justify;">(</span>{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">T</span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">t</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">u</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">n</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">-</span>{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">T</span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">o</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">u</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">t</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">d</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">o</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">o</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">)</span>$$

Where:

• η_s = Sensible effectiveness (0.50–0.85)

• V = Airflow rate (m³/s or CFM)

• ρ = Air density (≈ 1.2 kg/m³)

• c_p = Specific heat (≈ 1.005 kJ/kg·K)

• T_return = Exhaust air temperature

• T_outdoor = Outdoor air temperature

In imperial units:

$${\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">Q</span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">c</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">o</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">v</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">d</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">(</span>\text{<span style="font-family: verdana; text-align: justify;">BTU/hr</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">)</span><span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">1.08</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\text{<span style="font-family: verdana; text-align: justify;">CFM</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">\Delta </span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">T</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">\eta </span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">s</span>}}$$

The Total Heat Recovery Formula

For HRUs that recover both sensible and latent heat (e.g., enthalpy wheels):

$${\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">Q</span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">c</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">o</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">v</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">d</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">\eta </span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">t</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">V</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">\rho </span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span><span\; style="font-family:\; verdana;\; text-align:\; justify;">(</span>{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">h</span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">t</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">u</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">n</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">-</span>{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">h</span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">o</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">u</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">t</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">d</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">o</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">o</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">)</span>$$

Where:

• η_t = Total effectiveness

• h = Enthalpy (kJ/kg)

Energy Savings Calculation

Annual energy savings depend on the climate zone and operating hours:

$$\text{<span style="font-family: verdana; text-align: justify;">Annual Savings (kWh)</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span><span\; style="font-family:\; verdana;\; text-align:\; justify;">\sum </span><span\; style="font-family:\; verdana;\; text-align:\; justify;">(</span>\frac{{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">Q</span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">c</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">o</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">v</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">d</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\text{<span style="font-family: verdana; text-align: justify;">hours</span>}}{\text{<span style="font-family: verdana; text-align: justify;">COP</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">)</span>$$

For heating, use the boiler/heater efficiency. For cooling, use the chiller COP.

Payback Period

$$\text{<span style="font-family: verdana; text-align: justify;">Payback (years)</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\frac{\text{<span style="font-family: verdana; text-align: justify;">HRU Installed Cost</span>}}{\text{<span style="font-family: verdana; text-align: justify;">Annual Energy Savings</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\text{<span style="font-family: verdana; text-align: justify;">Electricity Rate</span>}}$$

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Download Your Free AHU Heat Recovery Calculator

Skip the manual calculations and get instant HRU sizing and savings estimates:

📥 Download Free AHU With Heat Recovery Excel Sheet Calculator

What's Inside:

• ✅ Airflow and temperature input

• ✅ Sensible and total effectiveness selector

• ✅ Heat recovery capacity calculation (kW or BTU/hr)

• ✅ Annual energy savings calculation

• ✅ Payback period calculator

• ✅ Climate zone database

• ✅ Print-ready HRU sizing report

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Step-by-Step Example: Sizing an HRU

Given Data

Parameter	Value
Airflow	5,000 CFM
Exhaust air temperature	24°C (75°F)
Outdoor air temperature (winter)	0°C (32°F)
Sensible effectiveness	70%
Operating hours	3,000 hours/year
Heating fuel	Natural gas (85% efficiency)

Step 1: Calculate Temperature Difference

$$\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">\Delta </span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">T</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">24</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">-</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">0</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">24</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">°</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">C</span>}$$

Step 2: Calculate Recovered Heat (Sensible)

$${\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">Q</span>}}_{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">c</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">o</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">v</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">r</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">e</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">d</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">1.08</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">5000</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">43.2</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">°</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">F</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">0.70</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">163</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">,</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">296</span>}\text{<span style="font-family: verdana; text-align: justify;">\hspace{0.17em}</span>}\text{<span style="font-family: verdana; text-align: justify;">BTU/hr</span>}$$

Convert to kW:

$$\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">163</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">,</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">296</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">÷</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">3412</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">47.9</span>}\text{<span style="font-family: verdana; text-align: justify;">\hspace{0.17em}</span>}\text{<span style="font-family: verdana; text-align: justify;">kW</span>}$$

Step 3: Calculate Annual Energy Savings

$$\text{<span style="font-family: verdana; text-align: justify;">Annual Savings</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">47.9</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">3000</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">143</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">,</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">700</span>}\text{<span style="font-family: verdana; text-align: justify;">\hspace{0.17em}</span>}\text{<span style="font-family: verdana; text-align: justify;">kWh</span>}$$

Convert to gas equivalent (85% efficiency):

$$\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">143</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">,</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">700</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">÷</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">0.85</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">169</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">,</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">059</span>}\text{<span style="font-family: verdana; text-align: justify;">\hspace{0.17em}</span>}\text{<span style="font-family: verdana; text-align: justify;">kWh gas equivalent</span>}$$

Step 4: Calculate Payback

If the HRU installed cost is $25,000 and gas cost is $0.05/kWh:

$$\text{<span style="font-family: verdana; text-align: justify;">Annual Cost Savings</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">169</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">,</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">059</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">\times </span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">0.05</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">\$</span>}\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">8</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">,</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">453</span>}$$$$\text{<span style="font-family: verdana; text-align: justify;">Payback</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\frac{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">25</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">,</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">000</span>}}{\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">8</span>}<span\; style="font-family:\; verdana;\; text-align:\; justify;">,</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">453</span>}}<span\; style="font-family:\; verdana;\; text-align:\; justify;">=</span>\mathrm{<span\; style="font-family:\; verdana;\; text-align:\; justify;">2.96</span>}\text{<span style="font-family: verdana; text-align: justify;">\hspace{0.17em}</span>}\text{<span style="font-family: verdana; text-align: justify;">years</span>}$$

A 3-year payback is excellent for HVAC equipment.

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Common Heat Recovery Calculation Mistakes

❌ Mistake #1: Using Sensible Effectiveness for Total Energy

Fix: Sensible effectiveness only accounts for temperature. For latent recovery, use total effectiveness and enthalpy difference.

❌ Mistake #2: Ignoring Frost Protection

Fix: In cold climates, HRUs need frost protection (preheat or defrost cycles). These reduce effective savings and must be included.

❌ Mistake #3: Using Annual Average Instead of Hourly Analysis

Fix: Heat recovery savings vary hourly with outdoor temperature. Use hourly weather data for accurate annual savings.

❌ Mistake #4: Skipping the Payback Calculation

Fix: Clients want to know the ROI. Always calculate payback—it's the deciding factor for many projects.

❌ Mistake #5: Not Verifying With Manufacturer Data

Fix: Effectiveness varies by airflow and conditions. Always verify with the HRU manufacturer's performance data.

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FAQ

Q1: What is sensible effectiveness?

A: Sensible effectiveness is the percentage of temperature difference that is recovered by the HRU. A 70% sensible effectiveness means 70% of the temperature difference between exhaust and outdoor air is transferred.

Q2: What is the difference between sensible and total effectiveness?

A: Sensible effectiveness only accounts for temperature. Total effectiveness accounts for both temperature and moisture (enthalpy), used for energy recovery wheels.

Q3: What HRU type should I choose?

A: Cross-flow plate HRUs: 50–70% sensible. Rotary wheels: 70–85% total. Runaround coils: 40–60% sensible. Selection depends on climate and budget.

Q4: How do I calculate annual savings?

A: Multiply the recovered heat (kW) by annual operating hours. Convert to fuel/electricity savings using the appropriate efficiency and rate.

Q5: What payback period is acceptable?

A: 2–5 years is typically excellent for HVAC equipment. 5–7 years is acceptable for energy efficiency projects. Above 7 years requires justification.

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Conclusion

Heat recovery is one of the highest-value upgrades in HVAC design. Download the free Excel calculator, apply the formulas explained in this guide, and pair it with professional HVAC training to design systems that save energy and deliver excellent ROI.

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