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Every house that uses fuel for heating loses some of its heat through exhaust airflows. A heat recovery ventilator (HRV) uses a heat exchanger to heat or cool incoming fresh air, which can reduce the energy consumption of a ventilating system by more than half. This
makes effective use of heat energy that would usually be wasted.
An HRV that also exchanges moisture between indoor and outdoor air is called an energy recovery ventilator (ERV). This technology takes moisture from incoming humid air and puts it in the exhaust air to reduce the humidity level indoors. An ERV is best suited for use in climates with both high heat and high humidity.
Both HRVs and ERVs use fans to circulate indoor air. They create a slow but steady flow of fresh air into the heating, ventilation, and air conditioning (HVAC) system, and draw stale exhaust air out. An HRV or ERV with one exhaust fan relies on displacement to bring in
a supply of fresh air; a system with two fans is called balanced, and the fans are meant to provide equal flow on exhaust and intake.
How an HRV Works
At the core of each HRV is a heat exchanger, which is made of materials that conduct heat quickly and efficiently. The heat exchanger has two ducts—one for incoming fresh air, the other for outgoing exhaust air.
The two ducts remain separate, so while heat is transferred from one to the other, the air streams do not mix. Instead, heat from the exhaust air is used to raise or lower the temperature of the incoming air.
The incoming air, once it has been heated or cooled, is routed to the existing HVAC system. There, it can be filtered and further heated or cooled, then mixed with other indoor air to provide a steady flow through the house. Most experts recommend a complete change of
room air every three hours. With the help of an HRV, this can be accomplished with lower fuel costs because the fresh air requires less heating or cooling.
Differences between HRV and ERV
An HRV uses a recovery core that exchanges heat between incoming and outgoing airflows. The core is usually made of several aluminum or plastic plates. An ERV also uses a series of desiccants or permeable plates to transfer moisture between the two airflows. In other words, an ERV works in the same way as an HRV, except that it also transfers moisture from the incoming air to the outgoing air.
This helps control the humidity of indoor air, which can have as great an effect on personal comfort as the temperature. While ERVs are most often recommended for use in hot, humid locations during warmer months, they can also help prevent dry indoor air in cold climates during the winter.
Benefits in Comfort and Health
A regular flow of fresh air helps make a house more comfortable— and healthier. Newer houses, built to be virtually airtight, need active moisture control to prevent the growth of mildew and mold. These organisms can spread throughout a house and contribute to respiratory problems for the people in it. Opening a window or running an exhaust fan in the kitchen or bathroom will help circulate air for a moment, but these do little to manage the indoor climate over the long term.
Installation Options
Heat recovery ventilators depend on airflow, so they are most often used with forced-air systems, not hydronic or radiant systems. While it’s possible to add an HRV to a house without a forced-air system, the HRV then needs one or more fans to circulate incoming and outgoing air. It must also depend on the difference between denser, cooler air and warmer, less-dense air—just like the gravity furnaces of a century ago.
Because it is designed to affect all the air that flows through a system, an HRV is most often installed as a whole-house unit. Some models of HRV or ERV are made for placement in a window or on a wall, but these are best for specific applications that are restricted to one room or area. For example, the exhaust fan from a bathroom or kitchen may also draw significant amounts of heat out of the room, and therefore the house. If the main HRV system is too far removed from one of these locations and the heat loss is great enough to require one, a separate HRV may be worth installing.
Reasons to Use an HRV
In new construction and in remodeling, one goal for builders is to create a “tight envelope.” That means sealing the indoor space effectively from all outside elements. Many products mentioned in this book, such as spray-foam insulation and structural insulated panels, are valued for their ability to keep spaces isolated from unplanned airflow.
A tight envelope is desirable because it gives you greater control over the indoor climate, keeping heat in and moisture out—if that’s where they are already. But if you need to change the temperature or humidity of an indoor space and it has no natural airflow, you need mechanical ventilation. In fact, building codes require it for most new houses.
As long as you need to have a method of cycling your indoor air, an HRV or ERV brings numerous benefits for little extra money. Adding an HRV to a new forced-air system will increase the cost by $2000 to $3000; working one into an existing system, of course, will
require more time, effort and expense. If you’re building a new house and are not ready for an HRV yet, you can have the ductwork roughed in for use later on.
Finding the Right Model
The models of HRV and ERV units available to you will depend on the brands carried by your local distributors. And each model may offer a range of configurations, which can make choosing more difficult. For example, the same basic models may offer optional air filters, speed controls, or air quality sensors; in colder climates, they may also have defrost controls or preheaters.
To compare the specifications of different models, get information from the manufacturers—but be sure to look for independent test results as well. The Home Ventilation Institute (HVI) provides performance ratings on HRV and ERV systems, including airflow capacity, recovery efficiency, and sound levels.
Before you decide what kind of HRV to install, talk with HVAC contractors in your area. They should know which systems are appropriate for the climate you live in, and whether the added control over moisture makes an ERV a good investment. Another source of information is a distributor of HRV equipment, who will have broader knowledge of the
industry, and may be able to recommend the best qualified installers. Also, your local building inspections department will have an HVAC specialist, whose advice can be both valuable and free.
Before You Install an HRV
Professional HVAC installers know what will affect the success of a new system. They recommend you take the following factors into consideration as you make your plan:
- Install the fan component in an accessible location for easier cleaning and service
- Make sure the fresh air intake is well away from chimneys, exhaust vents, and driveways
- Provide a separate supply inlet for each bedroom, plus one for each shared space such as a living room or study
- Provide a separate return outlet in each room with high humidity, such as a bathroom, kitchen, or laundry room
- Place each return outlet near the ceiling, and well away from any range or cooktop
- As with any air-handling system, use smooth, round ducting and the shortest routes possible
- Where ducts pass through uninsulated spaces, insulate the ducts themselves
- Seal any joints where ducts meet plenums, tees, wyes, registers, and so on
- If the system is likely to collect condensation, install a drain to draw moisture away from the heat exchanger
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