Choosing Between HRVs and ERVs
Understand which type of whole-house ventilation system will create an efficient and well-ventilated home depending on certain factors.
Synopsis: When it comes the choice bewteen heat-recovery ventilators (HRVs) and energy-recovery ventilators (ERVs), there are a multitude of factors to consider, including the climate you’re building in, the size and tightness of the home, and the number of occupants and their living habits. Scott Gibson, contributing writer for Fine Homebuilding and Green Building Advisor, gives a rundown on the differences between HRVs and ERVs and which is more applicable to meet your home’s demands.
Jack Hébert, founder of the Cold Climate Housing Research Center, is credited with a phrase that’s becoming increasingly familiar to high-performance builders: “Build tight, ventilate right.” In short, as houses get tighter and better insulated, the need for well-designed mechanical ventilation gets more important. At its simplest, this means using kitchen and bathroom fans to remove moist or particulate-laden air. In this exhaust-only approach, outside air finds its way into the building via gaps in the building enclosure. Supply-only ventilation works the other way: Fans bring fresh air into the house, but there’s no dedicated path for stale indoor air to leave. Both of these approaches are economical if you’re concerned about up-front cost, but they lack the long-term performance and efficiency of a balanced ventilation system that includes an HRV or ERV.
A balanced ventilation system intelligently combines exhaust- and supply-air ventilation to maintain air pressure without drastically increasing overall energy consumption. That energy consumption includes both electricity to run the HRV or ERV, as well as energy required to condition supply air. A heat-recovery ventilator, or HRV, and energy-recovery ventilator, or ERV, are similar in that they have a core through which both incoming and outgoing air travel to transfer heat energy and, in the case of ERVs, moisture.
Deciding which system is right for your project depends on several factors, including the climate you’re building in, the size and tightness of the home, and the number of occupants and their living habits. Coupled with an efficient distribution system, balanced ventilation will improve the comfort and indoor-air quality of your home and minimize the energy expenditure to heat and cool it.
Unfortunately, there is no one-size-fits-all solution to mechanical ventilation, so we contacted leading ventilation and indoor-air-quality experts to help you understand HRV- and ERV-based ventilation systems, as well as help narrow down options for an HRV or ERV system that will meet your home’s unique demands.
How much air is needed?
Determining how much fresh air a ventilation system should be capable of providing is dependent on many factors and can give even experts a headache. The International Residential Code (IRC) requires buildings with air-leakage rates of less than 5 ACH50 to have whole-house mechanical ventilation, but standards on exactly how much ventilation is needed are evolving.
The benchmark for mechanical ventilation is ASHRAE Standard 62.2. In pre-2013 versions, it required supply air of 0.01 cfm (cubic ft. per minute) of ventilation air per sq. ft. plus 7.5 cfm per occupant; on the exhaust side, it called for 25 cfm of continuous ventilation in kitchens (100 cfm supplied intermittently), and 20 cfm in the bathroom for fans run continuously (50 cfm for intermittent operation). In versions of 62.2 published after 2013, supply-air requirements tripled to 0.03 cfm of ventilation air per sq. ft. (plus 7.5 cfm per occupant), while exhaust air minimums followed a new schedule in the kitchen, depending on whether a range hood was used. Passive House requirements are even more demanding.
Building scientists are divided on whether the changes were a good idea. Some experts argue that even the old requirements were too high because they tended to result in high indoor moisture in humid climates, which is a big concern.
“It depends a lot on what’s going on in that building and what the occupants are doing,” said Cramer Silkworth of Baukraft Engineering. “It’s hard to nail down any one specific formula for maintaining good air quality. More fresh air is better, especially now with all of the Covid concerns, but there’s the energy expense and especially humidity control you have to add to those systems. So, until we have free dehumidification and heating and cooling and filtration on all these systems it’s kind of a battle between those two factors—you need enough but not too much, and it’s really hard to say what those levels are.”
The code-required ventilation rates in the jurisdiction you’re building in are simply the minimum requirements for ventilation, but an HRV or ERV should be considered as part of an entire HVAC system, including dehumidification.
Most often, an ERV is the right choice
When deciding between an ERV or an HRV for whole-house ventilation, an ERV is the best option in most cases and climates across North America. However, several factors play a role in the decision, including the rest of the home’s mechanical systems. The primary consideration should be the moisture-control needs of the home, especially when you want to manage some degree of relative humidity. In both winter and summer, ERVs generally do a better job of keeping indoor humidity levels comfortable because of the latent heat recovery that takes place.
That being said, the more people in your home, the more moisture they generate. An ERV is better at managing moisture when the goal isn’t simply eliminating excess humidity. Oftentimes a central or standalone dehumidifier is needed during humid summer months. Likewise, in an airtight home, humidity and moisture is often a more pressing concern than heat loss, and an ERV is the right choice when you want to address both.
How HRVs and ERVs work
HRVs and ERVs both exploit the second law of thermodynamics, in that heat transfer occurs spontaneously from higher- to lower-temperature bodies, but never spontaneously in the reverse direction. An HRV or ERV is a metal box that contains fans (either single, multispeed, or variable speed) and four ports. Inside is a heat-exchanger core that looks like stacks of corrugated material that enables heat transfer and allows incoming and outgoing air to cross paths without actually mixing (so that any stale or particulate-filled air won’t be pulled back into the home). A less common design swaps the heat-exchanger core for a heat pump, but the function (transferring heat energy between separate air streams) is the same.
During the heating season, an HRV transfers some of the heat in the stream of exhaust air to the incoming air. This takes place in the HRV’s heat exchanger. Thus, your furnace or heat pump doesn’t have to work as hard to warm up the air to the inside temperature. It’s not a one-to-one energy transfer, but it’s more efficient (even though you’re using electricity to power the HRV) than just bringing in cold air. The heat transfer is reversed during cooling season. This energy transfer is what engineers call “sensible heat recovery.”
ERVs get more complicated. In an ERV, there is an exchange of sensible heat but also an exchange of moisture. Engineers refer to this moisture exchange as “latent heat recovery.” Instead of a vapor-impermeable aluminum or polypropylene heat exchanger, an ERV typically has a core made of a vapor-permeable polymer. The polymer core permits the passage of some moisture between exhaust and makeup air, although it’s still air-impregnable so the air streams don’t mix.
To meet the ASHRAE ventilation requirements mentioned earlier, a three-bedroom, 2000-sq.-ft. house would typically need a system rated at 90 to 100 cfm, according to Brian Ault, a senior design engineer with Positive Energy, a mechanical systems consulting firm. Sensible heat recovery in an HRV averages about 70% (but can be as high as 95%). In an ERV, a certain amount of sensible heat recovery is taking place (but somewhat less recovery than an HRV, though the exact number will vary by manufacturer and model) and in addition the “latent heat” recovery—which is a measure of moisture transfer—is usually between 40% and 60%, so about half the moisture difference in the two airstreams will be transferred through the core.
HRV and ERV systems should be tested and adjusted after they have been installed to make sure they’re performing as designed, and filters should be changed on a regular basis. ERV cores eventually get clogged with oil, skin cells, hair, and dust and should be replaced every four to 12 years. A less complicated HRV core should last for a couple of decades without any major maintenance. When it gets dusty, a homeowner can just clean it off with compressed air. Enrico Bonilauri of EMU, a Passive House consulting firm, also mentions that the only moving parts in an HRV or ERV are the fans, so there’s not much to wear out. Cores can last for decades provided filters are changed on schedule.
When should you consider an HRV?
An HRV can be a good option for homes in mixed climates or with cold winters, but there are always other factors to consider. In addition to where you live, it’s also about how you live. An HRV will be most effective at removing excess indoor humidity efficiently in cold winter climates, because as an HRV pulls stale air out of the home, humidity within that air is removed too.
Excess indoor humidity, especially in winter months, may be the result of the number of people and types of activities taking place within the home. Large, active families can produce a lot of moisture as a result of exercising, cooking, and showering. Or excess humidity might relate to the fact that modern homes tend to be more airtight and therefore hold moisture in. Older homes are usually more leaky with a tendency to dry faster, which means an ERV is likely a better option for managing indoor humidity levels.
More and more, ERVs fit the bill
The decision to choose an HRV or ERV is largely driven by where you live. Historically, HRVs were more common in houses in colder climates because the chief concern was the amount of heat energy that could be saved, and humidity was seen as a secondary problem. The most efficient units on the market capture more than 90% of the heat in the outgoing air stream and transfer it to incoming air—in really cold environments, that’s a big plus. ERVs were typically specified in places with hot, humid summers because an ERV brings in less moisture from the outdoors than an HRV.
“It is an appreciable amount of latent energy when you have a legitimate difference between your indoor moisture levels and your outdoor moisture levels,” Ault said. “Up north, it’s dry as a bone outside for four or five months of the year in the winter. In the summer, sometimes it’s about the same as inside the house. Farther south, that’s flipped.”
In humid parts of the country—the southeast U.S.—running an ERV during the summer does not lower indoor humidity. An ERV will actually increase indoor relative humidity because the outdoor air doesn’t shed all of its moisture on its way indoors. But the problem would be worse if an HRV were installed because there is zero moisture transfer in an HRV. Even though moisture levels will go up when running the ERV, stale indoor air is being exhausted and fresh outdoor air is being introduced. In a well-designed HVAC system, a dehumidifier or an air conditioner can deal with excess humidity.
Clearly, there is nothing simple about this debate. A small house with very little air leakage and lots of inhabitants might have high indoor humidity that can be a problem in the winter. An HRV can make it more comfortable. But a large, leaky house in a cold climate may already be very dry during the winter, so an ERV will help prevent it from becoming too dry by retaining what little moisture is in the air.
Additionally, ERVs have been more expensive than HRVs in the past, but the gap in price between the two is narrowing, and designers are recognizing the importance of capturing humidity in the winter, giving ERVs a lift even in areas where HRVs once ruled. Installation of an HRV is also complicated by the fact that it must be oriented in a specific way so that the condensate drains work properly. Alternatively, an ERV can be installed at any orientation. And, in a cold climate, HRVs are more likely to freeze up, requiring a defrost cycle.
When energy usage is a primary concern in choosing between an HRV and ERV, a home’s other mechanical HVAC systems need to be assessed. “An HRV is used to save energy,” shares Nick Agopian, vice president for sales and marketing at RenewAire. “But an ERV is also used to downsize the capital costs of [air conditioning or dehumidification] equipment. If the humidity stays the same and you still have to dehumidify, you can’t downsize [your heating and cooling equipment]. All you’re doing is saving a portion of that energy, but your capital equipment still needs to be the same size and it has to work hard to dehumidify that air.”
Heat transfer in an ERV typically isn’t quite as good as it is with an HRV, he says. And as the latent energy performance of an ERV is increased by making the polymer core thicker, the thermal performance goes down. More and more, though, the tide seems to be turning toward ERVs in most instances.
“Deciding between an ERV and an HRV should land on ERV most of the time,” says Allison Bailes III, a Georgia-based energy consultant. “In a warm, humid climate, an ERV brings in less outdoor humidity than an HRV. (An ERV isn’t a dehumidifier. It does still add to the latent load in the house.) In a hot, dry climate, an HRV will make your already dry air even drier. In a cold climate, bringing in outdoor air without moisture exchange can result in extremely low humidity in winter. Only in mild climates like the West Coast of North America do HRVs make sense … sometimes.”
Bailes shared that occupancy is another factor to consider. “The higher the density of people in a space, the more you might need to lower indoor humidity and dry out the air with an HRV,” he said. “A small, airtight apartment or condo with two or three people in it, for example, may be too humid indoors with an ERV.
“Another reason people choose HRVs is they’re more efficient at transferring heat than are ERVs. What good is it to have high-efficiency ventilation, though, if you end up growing mold? The primary way to choose between an ERV and an HRV is to understand the moisture-control needs of the space being ventilated.”
Standalone ducting stands out
There are two primary options for ducting an ERV or HRV. You can use existing ductwork to integrate into the forced-air system. But if you don’t have existing ductwork or your main goal is efficiency, you can install dedicated ducting specifically for the HRV or ERV unit.
While both systems work to exhaust stale air and supply fresh air, a dedicated ducting system is considered the best option for overall balanced ventilation with minimal energy usage. Tying into an existing system requires interlock wiring between the HRV or ERV and the furnace to keep the furnace fan running to distribute air throughout the home. The result sacrifices efficiency for whole-house ventilation. An alternative to interlock wiring is a motorized damper that closes when the furnace fan shuts off, bypassing the furnace and directing fresh air from the HRV or ERV into the home through the return-air vent. This is a more energy-efficient approach, but fresh air is not being distributed throughout the home while the furnace is bypassed, making the standalone-ducting option ideal.
What you need to know about distribution
Air-distribution systems for HRVs and ERVs can vary. At one end of the scale are systems that include dedicated supply and exhaust ducts to key rooms in the house, separate from any other HVAC ducting. That ensures a constant and well-distributed source of clean outdoor air but at a relatively high material and installation cost. Other systems are less complex and may even use existing HVAC ducting to distribute outdoor air. Ductless HRVs or ERVs are another option, especially useful if you don’t have any existing ductwork. They’re sold in pairs that communicate and switch between bringing in outdoor air or exhausting indoor air, and installation is usually a single hole for each unit through an exterior wall. They don’t require interior ductwork, though you’ll need to run wiring for electricity to power the units. You’ll also need multiple units for a whole house, as they’re typically sized to provide ventilation for smaller areas.
Costs for installing a ducted system in a typical 2000-sq.-ft. house range from about $3000 for a basic system using existing ductwork to nearly $10,000 for a high-end system with more complex dedicated ducting. HRVs range from $600 to $1800 for the unit, while ERV units range from $800 to $2000 or more.
The simplest way to distribute air through a house is with a single-point system, with one supply duct and one exhaust duct. For example, you can exhaust stale indoor air from a hallway and supply fresh air to the primary bedroom. When there is no central air handler available, this type of system is inexpensive, but it doesn’t ensure ventilated air will be distributed evenly around the house. Spot ventilation would still be required in bathrooms and kitchens.
In a multipoint system, fresh air is distributed to bedrooms and main living areas while stale air is drawn from hallways, the kitchen (to avoid fouling the filter and heat exchanger with grease, exhausting the cooking area with an HRV or ERV is not recommended), and bathrooms. These fully ducted systems represent best practice according to Building Science Corp. They are the most efficient but also the most expensive. These systems are also effective where there is no central air handler available.
An air handler can be part of the distribution system. Fresh air is routed through the HRV or ERV and into the supply side of the air handler. Returns to the air handler go through a filter, and other ducts pull stale air from indoors and direct it to the HRV or ERV. These systems achieve whole-house distribution and come with moderate cost. A variation is to draw the exhaust air for the HRV or ERV directly from the air handler’s return trunk while supplying all fresh air through the air handler’s ducts.
Ultimately, it’s a question of budget for systems that support overall indoor-air quality. When spec’ing a new home or remodel, many homeowners budget for luxury appliances and the kitchen to match, but proper ventilation often takes a back seat. Awareness about indoor-air quality is increasing as the result of the ongoing pandemic, and investment in systems to positively impact air quality within the home is growing, though slowly. Agopian calls ventilation “preventive medicine” and makes it seem like an easy decision and investment for something that can have a huge impact on overall comfort and health.
Finding the right mechanicals
The Home Ventilating Institute, an industry trade group, maintains an extensive list of manufacturers that offer ERVs or HRVs (or both) to buyers in the U.S. These include familiar ventilation brands such as Broan, Fantech, Panasonic, and RenewAire, along with companies that may be a little less familiar to U.S. homeowners, such as Zehnder, a Swiss company that makes the high-end ComfoAir systems.
Scott Gibson is a contributing writer for Fine Homebuilding and Green Building Advisor. Drawings by Christopher Mills.
A version of this article originally appeared in Green Building Advisor
From Fine Homebuilding #310