Every commercial building throws heat away. Exhaust air leaves at room temperature in the middle of winter, chillers reject heat to cooling towers while perimeter zones call for heating, and refrigeration systems dump condenser heat outdoors. Heat recovery captures some of that energy and puts it back to work, reducing the heating plant's load and often its size.
In Canada, where heating seasons are long and outdoor air is very cold, heat recovery is one of the most effective energy measures available. The NECB requires exhaust air heat recovery on many air systems, and most net-zero and low-carbon strategies depend on it. This guide surveys the main approaches and the design issues that determine whether they deliver.
Where the recoverable heat is
| Heat source | Typical recovery method | Typical use of recovered heat |
|---|---|---|
| General and toilet exhaust air | Energy recovery wheels, plate exchangers, heat pipes | Preheat ventilation air |
| Laboratory or contaminated exhaust | Run-around glycol loops | Preheat outdoor air without cross-contamination |
| Chilled water return (cooling loads) | Heat recovery chillers, water-to-water heat pumps | Heating hot water, domestic hot water |
| Refrigeration condensers | Desuperheaters, condenser heat recovery | Space heating, underfloor heating in arenas, domestic hot water |
| Data and server rooms | Heat recovery chillers, water loops | Building or district heating |
| Drain water and sewage | Drain water heat exchangers, wastewater heat pumps | Domestic hot water preheat |
| Flue gas | Condensing heat exchangers, economizers | Boiler feedwater or return water preheat |
Exhaust air energy recovery
Recovering heat from exhaust air to preheat incoming outdoor air is the most common strategy. The main devices differ in effectiveness, cross-leakage, moisture transfer and pressure drop.
Rotary wheels
Enthalpy wheels transfer both sensible heat and moisture; sensible wheels transfer heat only. Wheels achieve high effectiveness and the moisture transfer helps keep indoor humidity up in winter. Some exhaust air can carry over into the supply air, so wheels are generally not used with laboratory, hospital isolation or other hazardous exhaust. Purge sections and correct fan arrangement limit carryover.
Fixed plate exchangers
Plate exchangers keep the airstreams separate, with low leakage when well built. Effectiveness is good. They are prone to frosting on the exhaust side in cold weather and need frost control. Counterflow designs achieve higher effectiveness than cross-flow designs.
Heat pipes
Heat pipes use sealed tubes of refrigerant that evaporate on the warm side and condense on the cold side. They have no moving parts and low cross-contamination, but the supply and exhaust ducts must be adjacent.
Run-around loops
A pumped glycol loop connects a coil in the exhaust airstream to a coil in the supply airstream. Effectiveness is lower than wheels or plates, but the airstreams can be far apart and completely separated. This makes run-around loops the usual choice for labs, hospitals and retrofits where ducts cannot be brought together.
For a detailed comparison of ventilation devices, see energy recovery ventilators and heat recovery ventilators.
Frost control in Canadian climates
Frost forms when warm, humid exhaust air cools below its dew point and the surface is below freezing. In Edmonton or Winnipeg winters, unchecked frost can block an exchanger within hours. Common frost control methods:
- Outdoor air preheat ahead of the exchanger.
- Bypass of part of the outdoor air around the exchanger.
- Variable wheel speed to reduce effectiveness during frosting conditions.
- Periodic defrost cycles, recirculation or exhaust-only operation.
- Enthalpy wheels, which frost at lower temperatures than sensible devices because they transfer moisture.
Each method sacrifices some recovery in the coldest hours. Designs should be evaluated with local weather data rather than at a single rating point.
Water-side and refrigerant heat recovery
Heat recovery chillers
Many buildings need cooling and heating at the same time: interior zones, server rooms and process loads need cooling while perimeter zones and ventilation air need heating. A heat recovery chiller, or water-to-water heat pump, cools the chilled water loop and delivers the rejected heat to the heating loop at a useful temperature. Each unit of electricity effectively produces cooling and heating together. Hot water temperatures from these machines are usually lower than conventional boiler temperatures, so the heating system must be designed or reset to suit. See heat recovery chillers.
Refrigeration heat recovery
Supermarkets, ice arenas and cold storage facilities run refrigeration systems that reject large amounts of heat. Desuperheaters capture the high-temperature portion of the condenser heat for domestic hot water, while full condenser heat recovery can serve space heating, ice resurfacing water or under-slab heating to prevent frost heave.
Data centre heat reuse
Server heat is low grade in air-cooled facilities but becomes much more useful with liquid cooling, where return water temperatures are higher. Heat pumps can raise it further for building or district heating. Our guide to data center heat reuse covers this in detail.
Designing heat recovery that works
Heat recovery systems frequently underperform because of design or control errors rather than equipment faults. Practical points:
- Size the recovery for annual energy, not only peak. Evaluate effectiveness and pressure drop across the full range of operating conditions.
- Account for added fan power. The pressure drop of an exchanger runs whenever the fan runs, including in mild weather when recovery is small.
- Provide bypass or economizer control so the exchanger does not add heat when the building needs cooling.
- Specify frost control and verify it during cold weather.
- Balance supply and exhaust flows; recovery effectiveness falls when flows are mismatched.
- Include filters on both airstreams to keep surfaces clean.
- Commission and trend the system: wheel rotation, bypass dampers and loop pumps are common points of failure.
Frequently asked questions
Is heat recovery required by code?
The NECB requires exhaust air heat recovery for many systems based on outdoor air quantity and climate, with minimum effectiveness values. Provincial adoption varies, so check the edition in force. Some building types have exemptions, such as kitchen hood exhaust with grease.
Can heat recovery be added to an existing air handler?
Often yes, although space is the main constraint. Run-around loops are frequently used in retrofits because the coils can fit into existing ductwork separately.
Does heat recovery reduce equipment size?
It can, provided the recovery device and its frost control are reliable at design conditions. Designers often size heating plants assuming some recovery is available, but must consider the effect of a wheel or loop failure.
Key takeaways
- Exhaust air, cooling loads, refrigeration and wastewater are the main sources of recoverable heat.
- Wheels, plates, heat pipes and run-around loops trade effectiveness against separation and space.
- Frost control is essential in Canadian climates and reduces recovery in the coldest hours.
- Heat recovery chillers turn simultaneous heating and cooling loads into a major efficiency gain.
- Fan power, bypass control, balancing and commissioning determine whether recovery pays off.
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