In today’s energy-conscious homes, proper ventilation is not just a luxury – it is a necessity. As building codes evolve, and construction becomes more airtight, mechanical ventilation systems have taken on a central role in maintaining healthy indoor environments. Enter HRVs (Heat Recovery Ventilators) and ERVs (Energy Recovery Ventilators) – systems designed not only to bring fresh air into your home, but to do it efficiently, quietly, and intelligently.
Why HRVs and ERVs Exist
Back in the 1980s, researchers with the National Research Council of Canada discovered that homes could significantly reduce energy waste by sealing up unintended air leaks. However, sealing a home tight introduced a new challenge: how to bring in fresh air and remove stale air without giving up the energy savings.
The solution was mechanical ventilation – systems that provide the required airflow while recovering heat (and in some cases, moisture) from the outgoing air. That is exactly what HRVs and ERVs do.
What are HRVs and ERVs?
Both HRVs and ERVs – sometimes grouped as H/ERVs – are balanced ventilation systems. They work by bringing fresh air in and pushing stale air out in equal measure. Inside the unit, a core allows heat (and sometimes moisture) to transfer from the outgoing air to the incoming air, which reduces heating or cooling loss.
Main components include:
- A supply fan (pulls in outdoor air)
- An exhaust fan (removes indoor air)
- A heat or energy recovery core
- Filters, controls, and sometimes dampers or heaters
- An insulated cabinet to prevent heat loss and reduce sound
Ducts connect the unit to the outside and inside of the home:
- Cold side ducts (to and from outdoors) are insulated to prevent condensation.
- Warm side ducts (to and from indoor spaces) don’t require insulation.
HRVs have a drain connection for condensation, while ERVs may or may not need one, depending on the model.
How Do They Work?
The main difference between an HRV and an ERV lies in the core material and function:
- HRVs transfer heat only, using solid materials like aluminum or plastic.
- ERVs transfer both heat and moisture using permeable membranes.
Though they function slightly differently, both systems are designed to meet the same ventilation requirements under building codes. They provide balanced ventilation, meaning the same volume of air is moved in and out, preventing pressure differences that can draw moisture into walls – a major cause of mold and rot in tightly built homes.
Fan Speeds and Control Modes
The amount of air moved is measured in CFM (cubic feet per minute). Building codes specify the minimum required CFM, but most H/ERV units are equipped with multi-speed fans that allow users to:
- Boost airflow during high-activity periods such as cooking or entertaining
- Reduce airflow during low-use times or when the home is unoccupied
- Use standby mode, where fans only run based on a timer or humidity sensor
- Activate recirculation mode, where air circulates inside the home without bringing in outdoor air
These functions are managed through wall-mounted controls, often placed near the home’s thermostat. Some systems also support remote timers (in bathrooms or laundry rooms) or include built-in humidity sensors.
Note: HRVs and ERVs do not humidify or dehumidify your home – but ERVs can help maintain indoor humidity by transferring moisture when outdoor air is very dry or very humid.
Automatic Operations
Modern HRVs and ERVs also include automated functions to keep them working efficiently:
Furnace Interlock
When H/ERVs are connected to a home’s forced-air heating or cooling system, they may rely on the furnace fan to help distribute fresh air throughout the house. If the furnace fan is off, the HRV/ERV will automatically turn it on – a process called furnace interlock – to ensure proper air distribution. This allows the fresh air brought in by the ventilation system to travel through the home’s ductwork, reaching all rooms evenly. Without the interlock, ventilation air could stay confined to one area, reducing system effectiveness.
Defrost Mode
In cold climates, frost can form on the core when moist indoor air meets freezing outdoor temperatures. To prevent ice buildup, which reduces airflow and efficiency, most systems include a defrost cycle:
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- Some use a small electric heater to warm the incoming air.
- Others cycle the supply fan off temporarily, allowing warm exhaust air to defrost the core.
- More advanced models use recirculation defrost, redirecting warm indoor air through the core.
- Some feature a fifth duct that draws in ambient indoor air from the mechanical room during defrost.
- Some use a small electric heater to warm the incoming air.
Fan speeds may increase automatically during defrost cycles to maintain ventilation.
Maintenance Requirements
Regular maintenance is simple but essential:
Clean filters every few months, following the manufacturer’s instructions. Most are washable, while some need replacement.- Clean the core once or twice a year. HRV cores can typically be washed with water, while many ERV cores require vacuuming only.
Following the manufacturer’s maintenance schedule ensures efficient operation and extends the life of the system.
HRVs and ERVs are more than just mechanical add-ons – they are central to a home’s long-term health, comfort, and energy performance. They quietly manage airflow, protect your structure from moisture damage, and help create a consistently fresh indoor environment. Whether you are designing a new build or retrofitting an older home, understanding how these systems work is key to doing the job right!
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