Choosing Between HRV and ERV?

HRV/ERVs provide a well‐engineered and efficiently packaged means of meeting the relatively demanding residential mechanical ventilation system requirements of most building codes.  Further, they offer homeowners with a much more affordable, effective and efficient means of ventilating their houses. This article will assist homeowners and renovators  gain a better understanding of heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs). Also learn how these systems support healthy indoor living environments in single family, semi‐detached and row housing, including:

• Why ventilating houses is important
• Residential ventilation building code requirements
• How HRVs and ERVs work
• System design considerations for both new houses and retrofits
• Important operation and maintenance considerations

Why ventilating houses is important

Poor indoor air quality has reported impacts on human health. Impacts can include increased asthma, headaches, and fatigue.  Health Canada publishes Residential Indoor Air Quality Guidelines, which advise on recommended exposure limits for a range of indoor pollutants. Naming some of the pollutants including benzene, carbon monoxide, fine particulate matter, formaldehyde, mould, naphthalene, nitrogen dioxide, ozone, and toluene1  ‐ all of which can be found in homes.  While source control is an essential first step toward limiting exposure to indoor pollutants , adequate ventilation (paired with filtration) is a critical means of establishing and maintaining indoor air quality.

Residential ventilation building code requirements

The new Ontario Building Code amendment has been in effect since January 1, 2017. The Supplementary Standard SB-12 (Energy Efficiency for Housing) mandates the installation of a heat or energy recovery ventilator in newly built homes as homes get increasingly “air tight”.  

How HRVs and ERVs work

Balanced mechanical ventilation systems use fans to simultaneously exhaust stale air and supply outdoor air in equal quantities. It is the preferred, and in many jurisdictions the only allowed, approach to ventilation. A central HRV/ERV or air exchanger continuously exhausts stale air from the bathrooms, kitchen and laundry rooms, and supplies air to bedrooms and living, dining and recreation rooms through dedicated ventilation ducts. Systems that utilize the furnace or air handler for distribution of ventilation air tend to use more energy than ventilation systems that use dedicated supply and exhaust ductwork because of the need to continuously operate the large furnace or air handler fan.

Heating can account for over 50% of annual energy consumption in houses. Since typical ventilation systems introduce unconditioned outdoor air and exhaust conditioned indoor air, there is potential for energy savings by incorporating heat transfer between the two air streams. This can work both during the winter, when warm exhaust air preheats the intake air, and during the summer, when cooler air-conditioned exhaust air pre-cools the intake air.

Heat Recovery Ventilation Systems

HRVs simultaneously supply and exhaust equal quantities of air to and from a house while transferring heat between the two air streams (with minimal mixing of air in the two streams). This reduces the energy consumption associated with heating or cooling ventilation air while providing a balanced ventilation system. Heat recovery also helps condition the incoming outdoor air to temperatures that are more acceptable to the occupants.

HRVs typically consist of the following components

• An airtight insulated case
• Supply and exhaust fans
• Outdoor air inlet from outside (shown with insulated duct connected)
• Outdoor supply air outlet (shown with duct connected)
• Exhaust air inlet (shown with duct connected)
• Exhaust air outlet to outside (shown with insulated duct connected)
• Heat exchanger
• Condensation drain pan connecting to a drain
• Sensors and controls
• Removable /cleanable filters
• In some cases motorized dampers to aid in defrost

The heat exchanger core of an HRV is constructed of a series of parallel plates that separate the exhaust and outdoor air streams. These plates are typically fabricated of metal or plastic.

When heat is transferred from the exhaust to the outdoor air stream during the heating season, condensation can form inside the heat exchange core. For this reason, drain pans are located inside the HRV to collect any water buildup, and the HRV is connected to a drain. In persistently colder winter conditions, the condensation inside the core can freeze and block the exhaust air stream. Some HRVs are designed to protect against freezing and clear the core of ice by going automatically into defrost mode. This is typically accomplished by a damper that closes off the outdoor air supply and allows warm indoor air into the HRV to heat the core and melt any ice on the exhaust side.

The heat exchange process is reversed during the cooling season. Cool air being exhausted from an air conditioned house removes heat from the incoming warm outdoor air. In other words, the HRV pre-cools the outdoor air that is brought into the house. An HRV in a house without air conditioning will have limited ability to pre-cool outdoor air during warm temperature conditions (though the system still provides good indoor air quality by continuously ventilating the space). In either case, as the heat recovery efficiency is not 100%, the outdoor air is never raised to the temperature of the exhaust air during the winter and it is never lowered to the temperature of the exhaust air in the summer. Therefore, careful consideration must be given to the location of supply air diffusers to reduce the chance that the outdoor air results in comfort problems. Many HRV fans can operate at low, medium, or high speeds depending on the ventilation requirements. A common control strategy is to have the HRV run continuously at low or medium speed, and switch to high speed when a higher ventilation rate is needed, such as when the bathroom is in use or during high occupancy periods.

Energy Recovery Ventilation Systems

An energy recovery ventilator (ERV) functions in a similar way to an HRV, but in addition to recovering heat, it also transfers moisture between the exhaust and supply air streams. This can be advantageous when there is a need to maintain indoor relative humidity levels in the winter or to reduce the moisture in the incoming outdoor air in the summer (a concern in warm, humid climates). During the heating season, an ERV will transfer heat from the exhaust to the intake air stream. An ERV can also raise the humidity in the intake air to a more comfortable level by returning a portion of the water vapour from the exhaust air to the incoming supply air. However, in houses that see significant moisture generation, ERVs may re-introduce too much moisture back into the house. In such cases, an HRV may be more appropriate. During the cooling season, an ERV in an air-conditioned house will both dehumidify and pre-cool hot humid outdoor air by transferring heat and moisture from the outdoor air to the cool exhaust air stream. If the building does not have air conditioning these pre-cooling and dehumidification benefits are not fully realized, though the system still provides constant ventilation to the space.

System design considerations for both new houses and retrofits

Airflow Capacity

Airflow rate, (CFM), should match or slightly exceed the specification at design conditions. A good rule of thumb is to select the capacity such that the calculated continuous ventilation rate is a maximum of 60% of the unit capacity, to allow boost capacity for kitchens and bathrooms and a low setting for low occupancy.  An undersized unit may not meet ventilation requirements, while an oversized unit adds unnecessary capital and energy costs. The capacity selected will be impacted by considerations such as whether the unit is providing bathroom ventilation in lieu of bathroom fans.

Sound Performance

Sound data of the unit is reported on the technical data sheet. A quiet unit is critical for house applications (there should be no audible noise at air outlets), although system installation practices also have a significant impact on vibration and sound transfer and airflow noise.  Installing units with vibration isolation mounting, properly sizing ducts, grilles and diffusers, using smooth transitions, reducing number of unnecessary fittings and using short lengths of flexible ducting to connect the ducts to the four ports of the HRV/ERV will minimize sound transfer. Locating HRVs away from sleeping areas is recommended. Example sound data for a high quality HRV unit ranges from 36‐73 dBA (for low to maximum airflow) at the supply air outlet of the unit.

Control Options

Control options determine how the occupants interact with the HRV/ERV system. Units can have varying levels of control at both the unit and at remote manual locations. Examples include manual timed controllers in bathrooms and kitchens that increase the unit operation from low to high mode, and automatic humidistat controls at the unit. Many units have control panels that can be mounted in a central location such as where the thermostat is located. High/low mode may be required by code (particularly if bathrooms or kitchens are connected to the HRV/ERV and no other exhaust is provided).  Control options should be chosen carefully to give occupants the desired level of control, while also ensuring that the overall system operates as intended (maintaining comfort, indoor air quality and appropriate humidity levels).

Inlet and Outlet Locations

Different units may have inlet and outlet ports located on any of the six faces of the unit to suit the particular access and duct routing requirements of the project. Outlet/inlet locations that are matched to the ducting design will minimize duct runs and ensure a smooth airflow pathway (thereby helping to ensure the desired airflow rates are achieved). The heat exchanger configuration (cross‐ flow, counter‐flow, heat wheel, or other) will impact the available port locations.

Evaluate and Select Ventilation System Retrofit Options

Once the ventilation system airflow and air tightness have been determined for a house, then the overall ventilation rate can be calculated.  If the existing ventilation rate has been found to be inadequate, upgrade options can then be considered.  Simpler options, such as cleaning the existing system to re‐establish rated airflows, replacing components (e.g., upgrading a fan, or section of ductwork), or modifying controls (e.g.. automating fan operation to provide more ventilation over a 24 hour period) could contribute to better airflow and indoor air quality.  However, if the building has an airtight enclosure and a balanced system is required, or if simpler options will not solve the problem, an HRV/ERV should be considered.    Many occupants of houses that have an exhaust‐only ventilation system do not use the exhaust fans to improve overall indoor air quality.  If exhaust‐only systems are to be relied upon for ventilation, homeowner education may be required.

Important maintenance and operation considerations

One of the biggest factors in occupant satisfaction with ventilation systems is the occupants’ own understanding of how to operate and maintain their system.  The builder’s or renovator’s representative or service contractor can facilitate this understanding and explain any owner responsibilities at project completion.

Training for New Owners

For new owners, the builder’s representative or installer can explain that the ventilation system is the primary source of outdoor air for the house.  Include the following key points:

• Although windows can be opened at any time of the year, they will not necessarily enhance indoor air quality and in many cases will lead to increased heating and cooling costs.
• The HRV/ERV is intended to operate (at least at low speed) on a continuous basis to remove moisture and pollutants generated by normal human activities and to maintain good indoor air quality.
• Shutting off the HRV/ERV for prolonged periods can lead to a buildup of indoor air pollutants and humidity, and can also potentially void warranties on the system.
• In cases where the HRV/ERV is interconnected with the furnace system, the furnace fan should be set to operate continuously as well.

Below are basic operational topics that should be covered with all new owners:

• Basic operating modes: Units can be specified with a range of operating modes (see inset).  The owner should understand which operational options are available for the system, and what they can control.
• Programming the humidistat: If a central humidistat is used to raise or lower the ventilation rate of an HRV/ERV system, it can programmed and/or manually adjusted to respond to seasonal changes (in climates where humidity control is a concern).  For example, the humidistat can be set to high during warmer months to avoid having the HRV attempt to reduce interior moisture with warmer moist outdoor air. Similarly in the winter, the humidistat can be set lower to avoid having the HRV trying to reduce moisture if not needed. The setting should be based on what the occupants find comfortable, but should always be low enough to prevent condensation from forming on the windows.  The typical range is between 30% and 60% Relative Humidity (RH).
• Scheduling: If a timer is used and programmed to occupants’ schedules, occupants should be shown how to program the timer.  For hourly schedules, daylight savings time will require re programming twice per year.

Modes of Operation

• Manual Operation requires the occupant to turn the ventilation system on and off. High speed operation may be also be initiated by manual controls.
• Automatic Operation uses controls such as timers, humidistats, and occupancy sensors to operate the ventilation system or to operate it temporarily at higher speeds as needed.  The occupant needs to understand which sensor or timer is activating the system.
• Continuous Operation ensures that the house is always ventilated, but may result in over‐ or under‐ ventilation at times. All ventilation systems must include manual controls, even if the occupant installs automatic controls or plans to operate the ventilation system continuously.

HRV & ERV Maintenance

HRV/ERV systems are intended to operate 24/7 and, like all mechanical equipment, will require ongoing preventive maintenance.  Owners may undertake simpler maintenance tasks, and should be trained accordingly.  An annual servicing by a mechanical contractor accredited by HRAI or TECA is recommended for all systems.