Energy recovery ventilators and heat recovery ventilators

Every building that ventilates throws conditioned air away. In a Canadian winter, that exhaust air may be 20 °C (68 °F) or warmer while the outdoor air replacing it is -25 °C (-13 °F) or colder. Heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs) capture much of that energy and hand it to the incoming air, cutting heating and cooling loads and often allowing smaller central plant equipment.

This guide covers how each device works, the main exchanger types, frost control, ratings and the practical points that decide whether a unit performs as promised.

HRV versus ERV

The difference is moisture.

• An HRV transfers sensible heat only. The two airstreams are separated by a solid surface such as aluminium or plastic, so moisture stays in its own stream.
• An ERV transfers both sensible heat and moisture (latent energy). It uses a hygroscopic wheel coating or a vapour-permeable membrane so water vapour moves from the more humid stream to the drier one.

In winter, an ERV returns some indoor moisture to the dry incoming air, which can reduce humidification load and dry-air complaints. In summer, it moves moisture from humid outdoor air to the exhaust, reducing the latent load on the cooling coil. An HRV does neither, which is an advantage where indoor moisture must be removed, for example in buildings with high internal moisture gains or where winter condensation on windows is a concern.

Exchanger types

Effectiveness values above are typical ranges at balanced flow and rated conditions; actual values depend on face velocity, airflow balance and the manufacturer's test data. Run-around loops and heat pipes keep airstreams completely apart, which is why they are favoured for laboratory and hospital exhaust where recirculation of contaminants is unacceptable.

Ratings and standards

In Canada, packaged residential and light commercial HRVs and ERVs are tested to CSA C439, which reports sensible recovery efficiency and, for ERVs, total recovery efficiency at both 0 °C and -25 °C test conditions. The cold-temperature rating is the more meaningful number for most Canadian locations. Commercial air-to-air exchangers are commonly certified under AHRI Standard 1060, which reports effectiveness at defined heating and cooling conditions.

Energy codes also matter. NECB 2017 and 2020 and ASHRAE 90.1 require exhaust heat recovery on systems above certain outdoor air or exhaust thresholds, with minimum effectiveness requirements and exemptions for some exhaust types such as kitchen hood exhaust. Thresholds vary by edition and climate zone, so check the version adopted by your province. The NECB 2020 and HVAC compliance guide summarises the main HVAC provisions.

Frost control in cold climates

When warm, moist exhaust air is cooled below freezing inside the exchanger, condensate can freeze and block the core. Below roughly -5 to -10 °C (14 to 23 °F) outdoors, most plate-type HRVs need some form of frost protection. Enthalpy wheels and membrane ERVs frost at lower temperatures because they transfer some moisture away from the exhaust, but they still need protection in Prairie and northern winters.

Common methods:

1. Recirculation defrost: the unit temporarily stops bringing in outdoor air and recirculates exhaust air through the core. Simple, but ventilation is interrupted.
2. Exhaust-only defrost or supply fan cycling: the supply fan slows or stops while warm exhaust melts the frost, which depressurizes the building for the defrost period.
3. Face and bypass dampers: some outdoor air bypasses the core, reducing heat transfer and keeping exhaust temperatures above freezing.
4. Preheat: an electric or hydronic coil warms incoming outdoor air before it reaches the core. Reliable and continuous, at the cost of preheat energy.
5. Wheel speed control: slowing an enthalpy wheel reduces effectiveness and frost risk.

For buildings that must ventilate continuously, such as healthcare, labs or schools during occupancy, preheat or bypass is usually preferred over recirculation defrost. Whatever the method, account for it in energy calculations because frost control reduces annual recovery.

Cross-contamination and leakage

Rotary wheels carry a small amount of exhaust air into the supply stream as the wheel rotates (carryover) and allow leakage across the seals. Good practice is to:

• Arrange fans so the supply side is at higher pressure than the exhaust side at the wheel, so leakage flows toward the exhaust
• Fit a purge section to clear exhaust air from the wheel before it enters the supply side
• Account for the exhaust air transfer ratio (EATR) and outdoor air correction factor (OACF) published under AHRI 1060 when sizing outdoor airflow

ASHRAE 62.1 classifies exhaust air by contamination level and limits how much of each class can be recirculated or transferred, which in practice limits wheels on some exhaust streams. Fume hood exhaust, isolation room exhaust and other hazardous streams generally require a recovery device with no cross-leakage, such as a run-around loop.

Design and installation tips

• Balance airflows. Effectiveness ratings assume balanced supply and exhaust. Large imbalances reduce recovery and can create pressurization problems. See building pressurization and infiltration control.
• Filter both airstreams. Dirty cores lose effectiveness and increase pressure drop. MERV 8 or better upstream of the core on both sides is common.
• Include a bypass for economizer operation. In shoulder seasons, recovering heat is counterproductive when free cooling is available. A bypass damper or wheel stop avoids overheating.
• Drain condensate. Plate exchangers produce condensate in winter, and drain pans and traps must be protected from freezing.
• Account for fan energy. Each exchanger adds pressure drop on both airstreams, typically 100 to 250 Pa (0.4 to 1.0 in. w.c.) per side depending on type and face velocity. Fan energy should be weighed against recovered energy.
• Plan access. Wheels, cores, belts and filters need regular cleaning and inspection.

After installation, airflows must be measured and balanced on both streams, and frost control and bypass sequences should be tested. Field measurement of entering and leaving temperatures under cold conditions is the best confirmation that a unit delivers its rated recovery.

Frequently asked questions

Should I choose an HRV or an ERV for an office in Calgary?

Either can work. An ERV helps retain moisture in a very dry winter climate and lowers humidification load, while an HRV is simpler if humidification is not planned. Frost performance at your local design temperature should drive the final selection.

Can heat recovery replace a heating coil?

Rarely. Even a high-effectiveness core leaves the supply air below room temperature in cold weather, and frost control further reduces recovery. A tempering coil is usually still needed.

Our air balancing teams regularly verify recovery unit airflows during start-up and commissioning.

Key takeaways

• HRVs recover sensible heat only; ERVs recover heat and moisture.
• Exchanger type affects effectiveness, cross-leakage and suitability for contaminated exhaust.
• In Canada, CSA C439 and AHRI 1060 are the main rating standards, and NECB sets recovery requirements above certain thresholds.
• Frost control is essential in Canadian winters and reduces annual recovery, so include it in energy calculations.
• Balanced airflow, clean filters, economizer bypass and good access are needed to achieve rated performance.