Heat and Energy Recovery Ventilators (HRVs & ERVs)
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 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 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
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.
Options and Analysis
|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.
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%.
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.
Figure 2 - HRV/ERV ventilation system options4
"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.