Air Source Heat Pump Buying Guide - Save on Energy

How do air source heat pumps work?

Air source heat pumps are the most common heat pump choice for residential heating and cooling. Here’s how they work.

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A typical air source heat pump is made up of an outdoor unit connected to an indoor unit by a refrigerant line. In the winter, the outdoor unit draws in air, absorbs its heat and transfers it to a refrigerant fluid. A combination of temperature and pressure transforms this fluid into a gas. The gas then passes through a compressor to increase its temperature further, and the resulting heat is transferred into your home, either directly from the unit or through ductwork.

This process reverses in the summer. The heat pump absorbs heat from inside your home and transfers it to the outdoors, becoming a high-efficiency air conditioner and dehumidifier.

What’s involved in making the switch?

Installing a heat pump is more complex than simply replacing your existing furnace, boiler, baseboards, or other HVAC system. It may involve modifying ductwork and wiring, as well as finding space for an outdoor condensing unit.

If you live in an older home, you should install other energy-efficiency measures first to reduce your heating load. This could include new insulation, air sealing and more. By limiting heat loss from air leaks or thin insulation, you’ll get the most out of your heat pump and may even be able to opt for a smaller, more affordable unit.

Ducted or ductless?

A ducted heat pump works in a similar way to a central furnace. It’s connected to interior ductwork and controlled by a central thermostat. Warm or cool air travels through the ducts and into your home.

A ductless heat pump emits warm or cool air directly from an indoor unit mounted to the wall or floor. Depending on the size and layout of your home, you may need more than one indoor unit.

You might want to opt for a ductless heat pump if you:

• Live in a home with no existing ductwork.
• Want to have flexibility in heating and cooling in multiple areas of your home.
• Need supplemental heating and cooling in a particular area, such as a home addition.

Standalone or hybrid?

As the outside air temperature drops, the heat pump has to work harder, gradually reducing its efficiency and heating capacity. Some homeowners opt for hybrid systems, with a conventional furnace or boiler acting as a backup to a primary air source heat pump. Some choose to install cold climate air source heat pumps, which are specially designed to work in low outdoor temperatures.

The choice of whether to select a standalone or hybrid HVAC system will be dependent on several factors, such as climate, energy costs, heat loss and home type. A licensed HVAC contractor can help assess your home and identify the most energy-efficient and cost-effective solution for you.

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Single stage, two stage or variable stage?

A single-stage heat pump is the most basic system available. It uses a simple on/off cycle that is triggered when the temperature dips below a set point. It tends to be the most costly type of heat pump to run, as the only setting is full speed.

A two-stage heat pump has two settings—high and low—allowing for slightly more precise temperature control.

A variable-stage heat pump is the most advanced system available. It is able to monitor and adjust to a full range of speeds to maintain the desired indoor temperature. Since it only uses as much power as it needs, it is the most energy-efficient option.

How to find the right contractor

Replacing your home's HVAC system is a big undertaking. If you’ve decided a heat pump is the right choice for you, the next step is finding a qualified, licensed contractor. They can help you make the best purchasing decision for your home to meet your needs. In Ontario, HVAC contractors must be registered with the Technical Standards and Safety Authority.

Heat pump water heaters require installation in locations that remain in the 40º–90ºF (4.4º–32.2ºC) range year-round and provide at least 1,000 cubic feet (28.3 cubic meters) of air space around the water heater. Air passing over the evaporator can be exhausted to the room or outdoors.

Heat pump water heaters will not operate efficiently in a cold space since they tend to cool the space they are in.  Installing them in a space with excess heat, such as a furnace room, will increase their efficiency.

You can also install an air-source heat pump system that combines heating, cooling, and water heating. These combination systems pull their heat indoors from the outside air in the winter and from the inside air in the summer. Because they remove heat from the air, any type of air-source heat pump system works more efficiently in a warm climate.

Homeowners primarily install geothermal heat pumps -- which draw heat from the ground during the winter and from the indoor air during the summer -- for heating and cooling their homes. For water heating, you can add a desuperheater to a geothermal heat pump system. A desuperheater is a small, auxiliary heat exchanger that uses superheated gases from the heat pump's compressor to heat water. This hot water then circulates through a pipe to the storage water heater tank in the house.

Desuperheaters are also available for tankless or demand-type water heaters. In the summer, the desuperheater uses the excess heat that would otherwise be expelled to the ground. With frequent operation during the summer, the geothermal heat pump may provide the majority of your hot water needs.  

During the fall, winter, and spring -- when the desuperheater isn't producing as much excess heat -- you'll need to rely more on your storage or demand water heater. Some manufacturers also offer triple-function geothermal heat pump systems, which provide heating, cooling, and hot water. They use a separate heat exchanger to meet all of a household's hot water needs.

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