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Turning toasty exhaust air into usable heat—that’s sustainability

Exhaust air heat pump recovery can offer 400 percent efficiency

Exhaust fans atop a building—part of the exhaust air heat pump recovery system.

By Tariq Amlani, P.Eng, Principal (Victoria, BC)

When a mechanical engineer starts design on a new institutional project they are thinking about solving three big questions:

  1. “How are we going to heat all of this fresh air?”
  2. “How are can we best meet the domestic hot water demand?”
  3. “How are we going to provide heat to offset all of this envelope loss?”

As technology and design practice has evolved, engineers around the world have come up with many valid solutions to the challenge of providing heat and hot water, some more energy efficient and sustainable than others. But today I’d like to investigate one of our favorite options, exhaust air heat pump heat recovery. To put it simply, this design strategy uses the hot air a building exhales to heat the building.

Buildings use a lot of air. Building mechanical systems push vast quantities of air, every minute. Depending on the occupancy type, the mechanical system could be required to push between 1 and 4 cubic feet of air, each minute, for each square foot of the building. That’s a lot of hot air!

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Healthcare facilities can be particularly demanding when it comes to air needs. A single 750-square-foot operating room can require upwards of 150,000 cubic feet of fresh air per hour to satisfy minimum air change requirements for building code and infection control procedures.

Depending on the type of ventilation system the building has, this air can all be fresh outdoor air or it could be a mix of fresh outdoor air and recirculated air. Either way, for every cubic foot of outdoor air that is introduced to the building per minute, the same amount of air needs to be exhausted from the building.

There’s something special about that air. All that exhaust air is usually at a nice and toasty temperature, about 72 F (22 C) year-round, regardless of outdoor air conditions.

Exhaust air heat recovery is a well-known strategy for reducing energy consumption. In fact, in many regions the energy code mandates that you take advantage of the opportunity. But, the type of heat recovery used can have a huge impact on the amount of recoverable energy.

How much efficiency do you want?

There are several ways to accomplish exhaust heat recovery: through a circulating loop of a glycol/water mix, across a conductive plate that has exhaust air on one side and fresh air on the other, or by using a wheel that warms up in the exhaust air stream and then rotates around into the fresh air stream to release the heat. Overall system efficiencies can vary between 35 to 40 percent efficient for glycol/water run-around loops to 80 to 95 percent for plate-and-wheel type heat recovery systems. Exhaust air heat pump heat recovery solutions, however, can achieve 350 to 400 percent efficiency.

What is exhaust air heat pump heat recovery? It’s somewhat counterintuitive, but it involves air conditioning. That’s right, to heat a building in the winter, we air condition the exhaust air.

By installing a cooling coil in the exhaust air, and running an electric chiller to air condition that air before it leaves the building, we can extract useful energy. A typical heat pump system can extract 3½ to 4 units of thermal for each unit of electrical energy used to run the chiller (hence the 350 to 400 percent efficiencies).

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Exhaust air heat pumps are also flexible. While other exhaust air heat recovery systems are only able to offset heating loads for air handling units, exhaust air heat pump heat recovery systems can be used to offset heating loads anywhere that hot water is used in a building. This includes air handling unit loads, as well as zone loads associated with room thermostats, perimeter heating systems, and even domestic hot water preheating.

In healthcare facilities, exhaust air heat pumps enable us to recover heat where traditional heat recovery fails due to cross contamination issues—isolation rooms and labs. In campus situations, where existing buildings expel a great deal of exhaust air without heat recovery, a pair of interconnecting pipes can bring all that recoverable heat back into a new build.

At Stantec, we are using exhaust air heat recovery systems on the North Island Hospitals project in Comox and Campbell River on Vancouver Island, British Columbia. For North Island, the use of the exhaust air heat pump heat recovery system is projected to deliver more than $275,000 in energy cost savings annually, earning the project over $1.7 million in energy incentive funding from local utility companies. As designed, the heat recovery system will result in an annual greenhouse gas savings of over 750 tons of carbon dioxide per year, versus traditional gas boiler heat generation. There are numerous examples of exhaust air heat recovery systems that are so successful that for parts of the year the system recovers more heat than needed and excess heat is shared with neighboring buildings.

Not every building will have such success, but with the availability of exhaust air heat recovery there’s no good reason to let your building’s warm air exhaust go to waste.

Tariq, P.Eng., LEED AP, is a seasoned mechanical engineer who is an active volunteer in the buildings engineering industry.

What is exhaust air heat pump heat recovery? It involves air conditioning. That’s right, to heat a building in the winter, we air condition the exhaust air.

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