Heat pumps outsold gas furnaces for the first time in the U.S. last year. This growth was spurred, in part, by federal tax credits — up to $2,000 off installation of these systems, which serve as alternatives to furnaces and air conditioners in all climates. While early adoption is largely in the residential market, the technology is also widely applicable for commercial and industrial buildings. 

Conditioning indoor air is the single largest consumer of energy in residential and commercial buildings. Almost 25 percent of energy produced worldwide is used for heating and cooling purposes. Heat pumps can offer an elegant solution to this huge problem. 

The environmental benefits of heat pumps are twofold. First, they are electric, which means they can be powered by renewable generation sources. Second, they are highly energy efficient, which reduces overall energy dependence and demand on the grid. Recent research from RMI has shown that replacing a gas furnace with a heat pump reduces carbon emissions in the first year of installation across all 48 continental states. 

Another impressive metric is that heat pumps can create three to four times more energy in the form of heat than the electricity that goes into them. This energy efficiency is possible because the electricity they consume is not used to directly create heat. Instead the electricity powers pumps, condensers and fans in the system that move a refrigerant whose chemical properties absorb and release heat. That heat can be moved in both directions, pumping heat out of a building to make it cooler or pumping heat into the building to make it hotter. 

So, heat pumps are getting their well-deserved time in the spotlight as a key solution for decarbonizing buildings. However, if heat pumps aren’t properly deployed, their efficiency can be dramatically eroded. Professionals in the building industry need to focus on how this critical decarbonization technology is deployed. The following compendium covers key considerations for designing and installing heat pumps in buildings. 

Take note of the envelope

The exterior surfaces of a building make up its envelope — a necessary barrier between the outdoor elements and the climate-controlled indoors. An ideal building envelope is like the walls of an ice chest: They minimize the movement of heat, moisture and air from entering or leaving, keeping the temperature in the chest consistent for as long as possible.

To see this principle in action, check out the icebox challenge, which shows just how incredible the difference between a poor and highly effective envelope can make. Installing a heat pump without addressing the building’s envelope is like putting your cold beer in a paper bag instead of an ice chest. 

The right size does matter

Heat pumps come in different sizes depending on the amount of heating and cooling they need to provide (referred to as the "load"). A typical single family home may use a 3- to 5-ton system, while office buildings get into 30 to 50 tons of heating and cooling capacity. 

It’s important to size heat pumps properly. Choose one that is too small, and the equipment will not be able to keep up with the demand for heating and cooling — it will run continuously without reaching the desired temperature. Opt for one that is too large, and the equipment will short cycle, meaning it will run for a short period of time and shut off, continually on a loop.

Short cycling is highly energy intensive, because a heat pump uses the most energy right when it starts. Properly sized equipment runs at a low speed continually, maintaining the desired temperature and comfort as energy efficiently as possible. 

Check the rating

Heat pumps come in different efficiencies. For residential equipment, the cooling efficiency is measured by its seasonal energy efficiency ratio (SEER) and its heating efficiency is its heating seasonal performance factor (HSPF). In both cases, a higher number is better. For the sake of a baseline, in California the new code minimum is SEER 15 and 8.8 HSPF. Higher efficiency equipment comes with an upfront cost premium but lowers ongoing utility costs.

An energy model is the best tool to evaluate the tradeoff between the increase in initial cost and reduction in operating costs from higher efficiency models. An energy model simulates the energy consumption of a building so buyers can make informed decisions about how equipment will perform. In addition to calculating the savings gained from higher efficiency systems, a model can later be used to compare the actual performance of deployed systems to modeled performance. 

Pay attention to speed controls

Heat pumps come with different speed settings, but the best of the bunch come with variable speed motors. This allows the equipment to dial in the exact speed of air needed to provide the desired temperature.

The speed of a heat pump relates to the rationale about proper sizing. Heat pumps are most efficient when they run consistently at a low speed — the more speed options a system offers, the more agile it can be at hitting that sweet spot.

In my personal experience, I love my variable speed heat pump be