Heat and Energy Recovery Ventilators (HRVs & ERVs)

Heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs) are used to provide fresh indoor air to homes, while reducing heating and cooling costs. HRVs and ERVs use exhaust air to preheat or precool incoming fresh air, but do not mix the air streams. They are an efficient option for providing balanced ventilation.

The essential elements of an HRV or ERV are a heat exchanger, fans, controls, and a ducted distribution system to remove exhaust air and supply fresh air (see figure 1). HRVs transfer only sensible heat energy to incoming air, while ERVs transfer latent heat from water vapor, as well as sensible heat energy. Because of this, ERVs have the ability to regulate humidity, as well as the temperature of incoming air.1 Depending on the volume of airflow and the temperature of the outdoor air, HRVs and ERVs typically recapture 60%-85% of the outgoing air's sensible heat, after accounting for the unit's own energy use. This efficiency is termed the sensible recovery efficiency. Total recovery efficiency is typically much lower, especially for HRVs, which cannot transfer latent heat. Ventilating with an ERV or HRV can greatly reduce heating and cooling loads by recovering heat energy before it is exhausted, but does not eliminate the need for a furnace or additional air conditioning.

General Recommendations
An HRV or ERV should be sized to accommodate the necessary air changes per hour, based on home size and the number of occupants. Learn about Green Communities ventilation requirements here. To provide the quantity of fresh air required, ventilators can operate continuously at low speed or they can operate less frequently at proportionately higher speeds. Generally, HRVs and ERVs operate most efficiently at their lowest speed settings. Consumers should seek high recovery efficiency ratings from the independent Home Ventilating Institute (HVI). High-efficiency HRVs and ERVs should have HVI-rated sensible recovery efficiencies of at least 76%. This level of efficiency is required to achieve an ENERGY STAR rating for homes with electric heat.5 The type of controls offered and the unit's energy efficiency compared to other models should also be considered when choosing a recovery ventilator. Energy efficiency can be measured by the unit's fan efficiency. Consumers should seek fan efficiencies greater than 1.5 cfm/watt.

Choosing between an HRV and ERV requires careful consideration of climate and other factors that affect indoor humidity levels. A smaller house with more people, more cooking, high levels of activity, etc., can increase indoor humidity. Building factors, such as the presence of a continuous vapor barrier, tight air sealing, and closed combustion can also increase humidity. In cold climates, high indoor humidity levels can be most effectively controlled in the winter using an HRV. However, for homes that suffer from low humidity levels (<25% RH) in the winter, an ERV is often a better choice. An ERV should also be chosen if it will be used frequently to ventilate an air-conditioned house in the summer.

Figure 1 - HRV/ERV system diagram4

Environmental Context
Combining balanced ventilation via an HRV or ERV with a tight building envelope is the most energy-efficient method of providing fresh air to occupants. Heated or air-conditioned air that would normally escape through cracks and holes in the home's envelope is instead directed through a heat exchanger in the ventilator where energy is extracted and supplied to the incoming fresh air (winter) or exhausted to the outdoors (summer). This reduces heating and cooling energy use.

Notes on Use
Performance issues with HRVs and ERVs are often the result of improper installation, poor placement, and/or lack of maintenance. Installations should be done by experienced, qualified HVAC contractors who will properly test and balance the system. Improperly balanced systems can lead to negatively pressurized homes that backdraft from combustion appliances, drawing carbon monoxide and soil gases, such as radon, into the home. In addition, contractors should be willing to properly educate occupants on the use and maintenance of the HRV or ERV system.

Proper system placement includes locating return vents where they won't become contaminated with grease, lint, or excess moisture. For example, an HRV or ERV return should never be connected to a kitchen exhaust fan. Outside, the exhaust and intake vents should not be located near the ground or other sources of contamination. Unless the HRV or ERV is connected to the HVAC system and runs in conjunction with the furnace, fresh supply air will still be slightly cooler than the indoor air temperature. To avoid comfort issues, care should be taken to avoid blowing fresh air onto occupants. Stairwells and hallways often make more comfortable locations for fresh air vents.

In cold climates, it is important that the recovery ventilator system includes frost prevention, usually via a preheater.2 Alternatively, some models offer a recirculation option that cycles warm house air through the entire heat recovery core, periodically melting any frost build-up.

To learn more about maintenance issues and HRV/ERV controls, click here.

Options and Analysis

HRV/ERV Comparison Chart
Explaining the HRV/ERV comparison chart

alternatives efficiency: sensible/total retail cost heating cost cooling cost total energy cost % design load reduction: heating/cooling estimated annual CO2 production (lbs)*
balanced ventilation - no recovery NA $601.00 $46.00 $647.00 0% / 0% 6,196
avg. efficiency HRV 69% / 20% $600 $512.00 $50.00 $562.00 10% / 9% 5,464
avg. efficiency ERV 69% / 51% $800 $512.00 $48.00 $560.00 10% / 9% 5,430
high efficiency HRV 84% / 13% $850 $493.00 $51.00 $544.00 12% / 10% 5,311
high efficiency ERV 83% / 53% $1100 $495.00 $49.00 $544.00 12% / 10% 5,294

Energy modeling was conducted using REM Rate 12.3 based on an 864sf Minnesota code base house, with wood siding, 15% window-to-floor area, window U-value 0.33 and SHGC 0.3, 80 AFUE furnace, and 10 EER air conditioning.
Energy rates based on 2007 EIA average U.S. residential rates for electricity and gas: $1.30/therm, $0.106/kWh
* Annual C02 production resulting from heating and cooling energy use.

Energy
HRVs and ERVs reduce energy consumption by decreasing the energy spent to heat or cool incoming fresh air. Although ERVs and HRVs use electricity to power ventilation fans, on average they will save 4-5 times more energy than they consume in a cold climate (based on energy model results from above). Average sensible efficiencies for HRVs and ERVs are in the range of 68%-70%. High performance models can achieve sensible efficiencies in the range of 83%-84%.

In cold climates, ventilating with HRVs and ERVs in the summer may actually increase cooling energy use slightly. This is because, even during the summer, outdoor air temperatures are most often lower than the indoor air temperature. During these times, HRVs and ERVs are pre-warming incoming air, rather than allowing outdoor air to naturally cool the house. From an energy-efficiency standpoint, it may be best to turn off the recovery ventilator and provide fresh air with open windows whenever humidity levels and temperatures permit. However, during the hottest times of day, HRVs and ERVs can work in conjunction with air conditioners to save energy by precooling incoming air. In cold climates, increased energy use during the summer is nearly negligible compared to the energy savings afforded by HRVs and ERVs in the winter. Average efficiency units can reduce heating energy consumption by 15%. High efficiency models can reduce heating energy consumption by up to 18%.

Cost
Generally, medium-sized (70-120cfm) recovery ventilators cost between $600 and $1100, not including installation. Costs vary according to quality, capacity, controls, efficiency, and type. ERVs can cost $150 to $200 more than a comparable HRV. High-efficiency models generally cost $250 more than comparably sized average-efficiency units. Heating cost savings pay back the initial cost of the recovery ventilator, often in 6-8 years. Recovery ventilators can also save money by decreasing the maximum heating and cooling loads. This reduces the required capacity of the furnace and air conditioner by 9%-12%, depending on efficiency.

Installation costs vary widely depending on the house's heating and cooling systems and the presence or lack of HVAC ductwork. Typically, installation costs are comparable to other ventilation systems and may be in the range of $1200 to $1500.

IAQ
Both HRVs and ERVs can provide superior indoor air quality. Exhaust air can be drawn mechanically from the most polluted spaces, such as the kitchen and bathroom, and incoming fresh air can be filtered as it passes through the ventilator. Companies typically offer several varieties of filters to meet different needs. HRVs and ERVs also effectively reduce indoor humidity levels, particularly in the winter. Excessive humidity can lead to mold growth, with negative repercussions for occupant health and building durability.

HRV or ERV?
HRVs are often used in cold climates where the primary concern is recovering heat as efficiently as possible during the winter. ERVs are used frequently in hot, humid climates where it is more desirable to preserve the conditioned indoor humidity levels during the summer. However, both ERVs and HRVs can be used effectively in cold climates and the choice between an ERV and HRV can depend on multiple factors. Click here to read more.

Integrated or Direct-Ducted?
Recovery ventilation systems can either be integrated with a forced-air HVAC system, or installed as a stand-alone, direct-ducted ventilation system (see figure 2). The integrated system uses the same HVAC ductwork as the furnace and central air conditioner and is often run in tandem whenever the furnace or air conditioner is on. The direct-ducted system requires a separate network of supply and return ducts and is commonly used for radiant-heated homes without an HVAC system. Click here to read more.

Figure 2 - HRV/ERV ventilation system options4

Other Resources

The Canada Mortgage and Housing Corporation (CHMC) offers a simple 7-step checklist for homeowners to ensure proper maintenance of their HRVs.

Home Ventilating Institute (HVI)

HRV/ERV maintenance chart from Canda's Office of Energy Efficiency.
U.S. EPA, U.S. Department of Energy.

HRV/ERV troubleshooting advice from Canada's Office of Energy Efficiency.
U.S. EPA, U.S. Department of Energy.

1 "Energy and Heat Recovery Ventilators (ERV/HRV)." ToolBase Services web site. 2008. www.toolbase.org/Technology-Inventory/HVAC/energy-recovery-ventilators.
2 Huelman, Pat and Wanda Olson. "Common Questions about Heat and Energy Recovery Ventilators." University of Minnesota Extension Services Web Site. 2008. University of Minnesota. www.extension.umn.edu/distribution/housingandclothing/DK7284.html.
3 "How a Heat Recovery Ventilator Works" Canada Office of Energy Efficiency Website. 7 Feb 2007 Canada. Office of Energy Efficiency. www.oee.nrcan.gc.ca/residential/personal/new-homes/r-2000/standard/how-hrv-works.cfm?attr=4.
4 Heat Recovery Venitlator Website. 28 Jan 2005. Canada. Office of Energy Efficiency. www.oee.nrcan.gc.ca/Publications/infosource/Pub/hrv/hrvsystem.cfm?text=N&printview=N.
5 ENERGY STAR Qualified Homes Equivalent Program Requirements For Use in the State of Minnesota. ENERGY STAR. 1 Jan 2007. www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/MN_Equivalent_Program.pdf.
6 "Whole House Mechanical Ventilation, A South Chicago Case Study." Consortium for Advanced Residential Buildings (CARB) and Steven Winter Associates, Inc. 2005.