Heat recovery sounds like free money. It is not.
A fresh air system with a heat recovery exchanger can cut ventilation heating and cooling costs by 30–60% in many buildings. But the real number depends on climate, exchanger type, effectiveness, fan power, and how the system is controlled. In some cases, the savings are smaller than the sales sheet claims. In others, they are larger.
Here is the honest math.
Quick answer
Cold climates: An HRV or ERV can recover 50–70% of the heat in exhaust air. Heating savings often reach 40–60% of the ventilation load.
Hot, humid climates: An ERV can recover 40–60% of the total cooling and dehumidification load. An HRV saves less because it does not transfer moisture.
Mild climates: Savings are real but smaller — often 20–40% of ventilation energy.
Fan power penalty: The extra pressure drop from the exchanger adds fan energy. Net savings are typically 10–20% lower than the gross heat recovery number.
Payback: Usually 2–7 years, depending on energy prices, operating hours, and climate.
1. HRV vs ERV: what are you actually recovering?
Heat Recovery Ventilator (HRV)
Transfers sensible heat only. It warms incoming fresh air in winter and cools it in summer. It does not transfer moisture.
Energy Recovery Ventilator (ERV)
Transfers sensible heat and latent heat (moisture). It preconditions both temperature and humidity.
Which saves more?
| Climate | Best choice | Why |
|---|---|---|
| Cold, dry | HRV | Moisture transfer is not needed; HRV is simpler and cheaper |
| Cold, humid | ERV | Avoids frosting and keeps indoor humidity balanced |
| Hot, humid | ERV | Reduces dehumidification load, which is often the biggest cost |
| Hot, dry | HRV or ERV | Sensible cooling dominates; ERV adds little |
| Mild | Either | Savings are moderate; choose based on humidity control |
If you are in Miami, Houston, or Singapore, an ERV usually wins. If you are in Minneapolis, Chicago, or Toronto, an HRV often wins. If you are in Seattle or London, the difference is smaller.
2. Effectiveness: the number that drives savings
Effectiveness is the percentage of heat (or moisture) transferred from the exhaust air to the incoming fresh air.
Typical ranges:
HRV sensible effectiveness: 50–80%
ERV sensible effectiveness: 50–75%
ERV latent effectiveness: 40–65%
ERV total effectiveness: 50–70%
A higher effectiveness means more savings, but it also means more surface area, higher pressure drop, and higher cost.
Do not confuse “effectiveness” with “efficiency.” Effectiveness is a temperature ratio. Efficiency is energy out divided by energy in.
3. The energy math (simple version)
You do not need a full energy model to estimate savings. Use this:
Imperial:
Q = 1.08 × CFM × ΔT × effectiveness
Where:
Q = recovered energy in Btu/h
CFM = fresh air flow
ΔT = difference between outdoor air and exhaust air temperature (°F)
effectiveness = decimal (e.g., 0.7 for 70%)
Metric:
Q = 1.2 × L/s × ΔT × effectiveness
Where:
Q = recovered energy in watts
L/s = fresh air flow
ΔT = temperature difference in °C
effectiveness = decimal
Example:
A school brings in 5,000 CFM of fresh air. Outdoor air is 10°F. Exhaust air is 70°F. ΔT = 60°F. HRV effectiveness = 70%.
Q = 1.08 × 5,000 × 60 × 0.7 = 226,800 Btu/h
That is about 18.9 tons of heating capacity recovered. Over a heating season, that adds up.
Now subtract fan power. If the HRV adds 0.5 in. w.c. of pressure drop and the fan is 65% efficient, the fan penalty might be 2–5 kW. Over 3,000 hours, that is 6,000–15,000 kWh. At $0.12/kWh, that is $720–$1,800. The heating savings might be $3,000–$6,000. Net savings are still positive, but smaller than the gross number.
4. Climate makes the biggest difference
| Climate | Ventilation load | HRV savings | ERV savings |
|---|---|---|---|
| Cold (Minneapolis) | High heating | 50–70% of heating | 50–70% of heating, plus moisture control |
| Hot-humid (Miami) | High cooling + dehumidification | 20–35% of cooling | 40–60% of cooling + dehumidification |
| Hot-dry (Phoenix) | High sensible cooling | 30–45% of cooling | 30–45% of cooling |
| Mild (San Francisco) | Low | 15–30% | 15–30% |
| Cold-humid (London) | Moderate heating | 40–60% of heating | 45–65% of heating + moisture balance |
The same exchanger that saves 60% in Minnesota might save 20% in San Diego. Climate is not a detail. It is the main variable.
5. The fan power penalty
Every heat recovery exchanger adds pressure drop. That pressure drop costs fan energy.
Plate heat exchangers: 0.4–1.0 in. w.c. (100–250 Pa)
Rotary enthalpy wheels: 0.5–1.2 in. w.c. (125–300 Pa)
Heat pipes: 0.3–0.8 in. w.c. (75–200 Pa)
Runaround loops: 0.8–1.5 in. w.c. (200–375 Pa) plus pump energy
A high-effectiveness exchanger often has higher pressure drop. The net savings can be lower than a medium-effectiveness unit with lower pressure drop. Always compare net savings, not just effectiveness.
6. Real-world savings examples
Office building, Chicago
10,000 CFM fresh air, HRV 70% effective, 3,000 heating hours.
Heating savings: $4,000–$7,000 per year.
Fan penalty: $1,000–$2,000.
Net: $3,000–$5,000 per year.
Payback: 3–5 years.
School, Houston
15,000 CFM fresh air, ERV 60% total effective, 2,500 cooling hours.
Cooling and dehumidification savings: $6,000–$10,000 per year.
Fan penalty: $1,500–$3,000.
Net: $4,500–$7,000 per year.
Payback: 3–6 years.
Home, Vancouver
200 CFM HRV, 70% effective, 4,000 heating hours.
Heating savings: $200–$400 per year.
Fan penalty: $50–$100.
Net: $150–$300 per year.
Payback: 5–10 years (longer for residential).
Residential payback is longer because energy prices are lower and operating hours are shorter. Commercial and institutional buildings usually see faster payback.
7. What kills the savings
Wrong exchanger for the climate. HRV in Miami wastes dehumidification energy.
Oversizing the fresh air flow. More air means more energy, even with recovery.
High pressure drop. Fan power eats the savings.
Poor installation. Leaky ducts, bypass dampers, and unbalanced flows cut effectiveness.
No commissioning. If the system is not balanced, the exchanger does not perform.
Dirty filters and fouled cores. Pressure drop rises, effectiveness drops.
Frost control that dumps energy. Some defrost strategies reduce net savings.
No controls. Without demand-based ventilation, you recover heat from air you did not need.
8. Selection checklist
Climate: Heating-dominated, cooling-dominated, or balanced?
Humidity: Do you need latent recovery? If yes, choose ERV.
Fresh air flow: Size for actual occupancy, not maximum occupancy.
Effectiveness: 60–70% is a good target. Higher costs more.
Pressure drop: Compare net savings, not just effectiveness.
Frost control: Required in cold climates. Check the strategy.
Maintenance: Can you change filters and clean the core?
Controls: Demand-based ventilation, CO2 sensors, and scheduling.
Space: Plate units are compact. Rotary wheels need access.
Payback: Use real energy prices and operating hours.
FAQ
How much energy does an HRV save?
Typically 50–70% of the ventilation heating load in cold climates. Net savings after fan power are often 40–60%.
How much energy does an ERV save?
In hot, humid climates, an ERV can save 40–60% of the cooling and dehumidification load. In cold climates, it saves heating energy and improves moisture balance.
Is an ERV always better than an HRV?
No. In cold, dry climates, an HRV is simpler and cheaper. In hot, humid climates, an ERV usually wins.
What effectiveness should I look for?
60–70% is a practical target for most commercial systems. Higher effectiveness costs more and may add pressure drop.
Do heat recovery exchangers really pay back?
In commercial buildings with long operating hours and high energy prices, yes — often 2–7 years. In residential applications, payback can be 5–10 years or longer.
Does fan power cancel out the savings?
It reduces them, but rarely cancels them. A well-designed system still saves net energy. A poorly designed one may not.
How often should I maintain the exchanger?
Inspect filters monthly. Clean the core or wheel according to the manufacturer. Check for frost, dirt, and seal wear.