Types of Heat Pumps: (Air & Ground-Source Explained)

Heat pumps do not create heat in the same manner as a traditional furnace. A heat pump needs electrical power to work, but it can supply more heat than the electricity used. Although, just like any device, a heat pump reaches an equilibrium where it cannot make any more heat. 

When the ambient temperature drops below the heat pump’s balance point, the heat pump will not be able to hold a steady indoor temperature. This varies depending on the design, but it is valid for all air-source heat pumps (a significant concern if you live in an extreme climate).

Let’s dig into all the details and learn about the different types of heat pumps.

Residential homes use two primary heat pumps for space heating and cooling:

  • Ground-source (or geothermal)
  • Air-source

Ground-Source Heat Pumps

ground source heat pump diagram using a well

Ground-source heat pumps can extract heat from the earth, well water, or a pond when used to heat a home using forced air or hydronic systems. 

In addition, ground-source heat pumps can heat water for domestic household use.

Ground-source can be designed as closed-loop or open-loop systems.

Open-loop Systems

Open-loop ground source systems typically use water from a pond or well to move heat. The pump will extract the heat, then dump it back into the reservoir. These types of systems are typically less expensive than closed-loop.

Closed-loop Systems

Plastic tubes filled with water or antifreeze are buried underground, dropped into holes, or placed in ponds.

Horizontal-loop systems require large amounts of land and are usually not feasible in urban areas; they’re most common in rural settings.

Vertical-loop systems are more expensive but fit better in tight spaces.

The direct-exchange system is a variation on the closed system that utilizes a copper loop that is buried and filled with circulating refrigerant rather than a plastic loop that is filled with water or antifreeze solution.

Ground-Source: Expensive But Efficient

Ground-source heat pumps can be incredibly efficient yet cost more than most other heating and cooling systems. The highest COP (Coefficient of Performance) numbers claimed by the most effective systems are 5, although these figures are erroneous.

An ideal installation would result in a COP of 3. 

What is COP: The efficiency of several forms of heating, cooling, and refrigeration equipment, including heat pumps, is measured by COP. It is the proportion of energy input to usable energy output (such as heating or cooling).

For instance, a heat pump with a COP of 2 produces twice as much energy as it consumes. Therefore, a system with a greater COP is more effective. COP is calculated by dividing the energy efficiency ratio (EER) by 3.415.

Desuperheaters are auxiliary heat exchangers that heat household water and can be added to ground-source heat pumps.

An installation of a ground-source heat pump typically costs $20,000 to $50,000. That’s a lot of money, especially considering that a ductless minisplit system can be installed for a lot less money and function almost as efficiently.

Current models of ductless mini-split air-source heat pumps from China, South Korea, and Japan can heat a home even when the outside ambient temperature is -17°F.

System Sizing is Critical

Given the high equipment cost required for a ground-source heat pump system, careful load assessments are essential.

Correctly sizing the pumps is a considerable part of the system’s efficiency. Since ground-source heat pumps usually need to move large amounts of water- it can cost thousands of dollars more than necessary if a system is oversized.

Ground-Source Better in Northern Climates

Air-to-air heat pumps are effective in providing both heating and cooling requirements. However, they frequently need to be supplemented by an expensive dual-stage compressor or an ineffective electric resistance heating element in cold climates. 

Since ground-source heat pumps’ efficiency is independent of outside air temperatures, they may be preferable in northern climates.

Air-Source Heat Pumps: Cost Less & Smaller

diagram that explains air source heat pumps

Air-source heat pumps are like window air conditioners running backward. However, because they are compact, affordable, and can offer cooling, air-source heat pumps are an excellent choice for tightly constructed and well-insulated homes.

Most air-source heat pumps work well in milder climates where heating and cooling are required, but winters are not harsh. 

Changing from electric resistance heat to an air-source heat pump can save energy expenses by up to 40%, according to the U.S. Department of Energy (partnership link). 

Their flaw is that they have significantly reduced heating efficiency at low temperatures. So when air temperatures drop to about 40°F, most air-source heat pumps switch to backup heating, which is often electric resistance coils. 

There are few exceptions; for instance, some systems have a gas-fired furnace as a backup, and more sophisticated air-source pumps, such as the Hallowell cold-climate heat pump, are effective down to 0°F (PDF of measure performance at low temperatures).

Even in freezing climates, net-zero energy home designers are starting to demand air-source heat pumps. A Mitsubishi ductless mini-split heat pump is typically installed at a total cost of about $6,000. The system also includes air conditioning as a bonus to heating.

Caution: It’s Not All Good (Refrigerant)

The weak link may be the refrigerant. In the past, refrigerants had substances that damaged the earth’s ozone layer; these substances have gradually been replaced by a series of more recent formulations that are less harmful.

For example, some manufacturers have already transitioned to R-410A, an R-22 substitute that doesn’t deplete the ozone layer. R-410A is slated to be phased out starting in 2010.

Air-Source Heat Pumps Can Be Noisy

Typically, heat pumps feature powerful fans and compressors. It can be very annoying when nearby masonry foundations and retaining walls reverberate off the vibration of concrete mounting pads. 

In mechanical rooms, additional pumps may also generate heat and vibration. Try to keep mechanical devices away from living spaces, or think about soundproofing the walls in between.

Either System Needs Proper Design

A well-designed heat pump system will hardly heat the house on the coldest day of the year. The system may even be sized to handle less than the intended heat load if the system designer is willing to rely on electric resistance heat during severely cold weather. 

The home’s distribution system must be flawless to ensure that the priceless heat the system has collected is not lost. 

The duct system in a house with a forced-air heating and cooling system needs to be well-designed and as airtight as feasible. Mastic should seal any duct seams, and the HVAC installer should use a Duct Blaster test to ensure that the ducts are tight.

What Building Codes Say About Heat Pumps

Sections 1401 and 1403 of the 2006 IRC are the main sections that deal with heat pumps. 

Equipment should be sized using ACCA Manual J or another recognized calculating technique (1401.3). 

It is vital to locate the equipment so it may be maintained and replaced as needed (1401.2). 

Except in flood-prone areas, which must be installed above the design flood elevation shown in Table 301.2, the outdoor components must be installed on a firm, level base at least 3 inches above grade (1403.2). (1). 

A minimum of 6 square inches should be allocated to the combined area of the exterior and return ducts for every 1000 Btu of output.

Heat pumps with supplemental electric-resistance heat are required to have automatic controls to stop resistance heating when the heat pump compressor can handle the heating demand per IRC Section 1103.1.1. 

In Section M2105.1 of the IRC, requirements for ground-source heat pump pipes are listed. Ground loops must undergo a 30-minute test at 100 psi.

Are Heat Pumps “Green”?

Although heat pump manufacturers advertise incredibly high efficiency, reliable monitoring information is unavailable on completed homes with ground-source heat pumps. 

Because the calculations might not consider the electricity needed to run all of the pumps and blowers needed by a ground-source heat pump system, some COP numbers that are promoted may be inflated. 

The high expense of installing ground-source heat pumps is contention for many energy consultants, who argue that the money would be better spent on envelope upgrades like improved air sealing, thicker insulation, and triple-glazed windows.

The degree to which heat pumps are environmentally friendly depends on your electrical supply.

For example, coal power plants are the primary source of mercury contamination (PDF), waste 70% of the BTU in coal (PDF), and produce a sizable amount of greenhouse gases. Therefore, heat pumps might be a great choice if your electricity comes from environmentally friendly sources (solar energy is an excellent option).

The Future of Heat Pumps (Absorption)

Absorption heat pumps, which are an adaptation of the air-source heat pump, are powered by heat sources other than electricity. Examples of sources include natural gas, propane, solar hot water, or geothermal-heated water.

Both heating and cooling are accomplished using an ammonia-water absorption cycle. 

Although they are being developed for residential usage, absorption heat pumps are typically used in commercial and industrial environments.

Their benefit is that they don’t require power and can use any heat source.

Points for Being Environmentally Friendly

If your home build is going after a green certification, here are the points you can get for using a heat pump:

  • LEED-H: Depending on the heat pump’s efficiency rating under EA6, up to 4 points (Energy & Atmosphere). 
  • Up to 12 points under NGBS Chapter 7 — Energy Efficiency, based on system effectiveness and climate (703.4.4).