Heat Pumps 101

What is a Heat Pump?

Heat pumps are an energy-efficient alternative to furnaces and air conditioners for all climates. That's right; heat pumps can heat AND cool your building or home. Like your refrigerator, heat pumps use electricity to transfer heat from a colder space to a warmer space rather than generating heat.  

  • In winter, heat pumps gather heat from the outdoors and move it into your building or home to warm it. 
  • In summer, heat pumps collect heat from your home and transfer it outside to cool your building or home. 
  • Heat pump water heaters collect heat from the area outside of the unit and use that heat to heat water.

Since heat pumps move heat instead of creating it, they achieve 200-300% efficiency. While natural gas is currently less expensive than electricity, more efficient heat pumps use 2-3 times less energy than gas heating systems.

 

Heat Pump Types

There are five main types of heat pumps:  

  • Air Source Heat Pump and Cold Climate Air Source Heat Pumps (ASHP/ccASHP) 
  • Mini-split Heat Pump and Cold Climate Mini-split Heat Pumps (MSHP/ccMSHP)  
  • Ground Source Heat Pump (GSHP)  
  • Heat Pump Rooftop Unit (RTU)  
  • Heat Pump Water Heater

Air Source Heat Pump

an air source heat pump

A ducted air source heat pump system has four components:

  • An outdoor heat pump unit.
  • An indoor air handler.
  • Ductwork that distributes hot and cold air throughout the home.
  • A thermostat.

This equipment is most suitable for buildings that have an existing central air conditioner or furnace, where you can continue using the existing ductwork and air handlers. These units will often replace both the furnace and air conditioner, or may be compatible with the existing furnace in certain conditions.

If you already have a furnace, it might be compatible with a heat pump addition. The existing furnace should not be near the end of life (20+ years old) and should have a variable speed motor or electronically communicated motor (ECM). If these conditions apply, a heat pump might be compatible with the existing furnace, but an HVAC contractor must confirm.   

Cold climate models can operate in heat pump mode for most of the year, reducing the use of less efficient backup heat (natural gas or electric resistance). A cold climate model can eliminate the need for backup heating source for some homes. 

Cutaway view of a two-story home showing an outdoor heat pump unit connected to an indoor air handler by refrigerant lines. Ductwork extends from the air handler to all areas of the home and provides conditioned and heated supply air. A thermostat is shown on a wall of the home.

Image from: https://www.ecomfort.com/stories/1482-How-Central-Heat-Pumps-Work.html

Mini-Split Heat Pump

A mini-split (non-ducted) system has three components: 

  • An outdoor heat pump unit
  • Indoor head(s) that deliver hot or cold air
  • A thermostat

Refrigerant lines connect the two units to transfer heat between them. During the winter, a mini-split extracts heat from the outside air and transfers it to the indoor head(s) via the refrigerant. The indoor head(s) then distributes heat throughout the building. During the summer, the process is reversed: the heat pump extracts heat from the indoor air and releases it outside.

the outdoor portion of a mini-split heat pump unit

A mini-split is a great option to add heating/cooling to a single space in a building. Often mini-split heat pumps are ductless, which means they need no ductwork to deliver heating or cooling to the building. Instead the indoor heads have fans and directly deliver the heating or cooling. They are often mounted on a wall or ceiling.

the indoor portion of a minisplit heat pump unit

A mini-split may be the most cost-effective heat pump option for buildings without ductwork or where adding ductwork is cost-prohibitive. Because each head is controlled individually, mini-splits allow you to keep different spaces at different temperatures. They are suitable for infrequently used rooms. 

Single zone MSHPs are best suited for spaces less than 1,500 sq. ft. and lend themselves to open concept spaces. An MSHP will not effectively condition rooms sectioned off by doors or bends. You may need an additional MSHP to maintain the desired temperature. 

Ground Source Heat Pump

A ground source heat pump system has an internal heat pump unit that can serve forced air or hydronic systems. The heat pump is connected to a piped heat exchanger underground or in a body of water.  

  • GSHPs transfer heat from the ground, maintaining a more constant temperature than outside air. They offer high efficiency and capacity year-round. GSHPs are the heat pump technology with the lowest operating costs.   
  • The drilling and bore field required for GSHPs makes them the most expensive heat pump technology to install. Local geological characteristics can limit GSHPs' feasibility. 
  • GSHPs are the most likely system not to require backup heat. 

Alt Text suggestion: A cutaway of a home is shown. A Ground Source Heat Pump unit is shown in the basement of the home. Several options are shown for the collector piping connected to the Ground Source Heat Pump. 1) Vertical Collector exchanging heat with the ground 2) Looped collector exchanging heat with the ground 3) horizontal collector exchanging heat with the ground 4) looped collector exchanging heat with a pond.

Heat Pump Rooftop Unit

A heat pump rooftop unit is a type of Heating, Ventilation, and Air Conditioning (HVAC) system. They are often installed on the roof of a commercial or industrial building, but they can also be ground mounted.

a heat pump rooftop unit on top of a commercial building roof.

These units are most commonly used for larger commercial or multifamily buildings. They provide both heating and cooling to the building using heat pump technology. They extract heat from the outside air during the winter and release heat outside during the summer. Rooftop units usually either have a gas or electric resistance backup heat source. Heat pump rooftop units typically contain:

  • A compressor
  • A reversing valve
  • A fan
  • A heating and cooling coil
  • A back-up heat system
  • Controls that regulate the temperature and airflow

Heat Pump Water Heater

A heat pump water heater pulls heat from the surrounding air and uses it to heat water in a storage tank. They use the same technology as heat pumps that heat air and apply it to water heating instead. 

A heat pump water heater

Glossary

Auxiliary Heat or Supplemental Heat

An extra heating source that works together with a heat pump to provide more heat. You may need this when the outdoor temperature gets too low for the heat pump to work well. You will know it's too cold because the temperature is below the heat pump’s balance point. This extra heat usually comes from electric resistance.

In Denver, having a cold climate heat pump can often avoid the need for auxiliary heat. But it can be helpful in some situations.

Auxiliary heat or Supplemental Heat Activation Temperature

The outdoor temperature that will trigger the use of auxiliary heat. The heating system designer chooses this temperature. Single-stage auxiliary heat will activate at 5° F above the balance point temperature. Two-stage auxiliary heating equipment uses many set points. This helps to achieve optimal efficiency and comfort.

Backup Heat

A secondary heat source sized to serve the building’s total heating load. You will use backup heat when the heat pump is off either because:

  • The outdoor temperature is too low for the heat pump to work
  • The heat pump has broken down, but the power is still on.

Backup systems are often electric resistance systems. Most buildings with heat pumps don’t have a backup system because electric resistance heat is not very efficient. Additionally, it won’t work if the power goes out.

It's better to use a cold climate heat pump and limit electric resistance heat to auxiliary heat.

Balance Point Temperature

The lowest outdoor temperature at which the heat pump can keep the building warm by itself. When the temperature drops below this point, you will need a supplemental heat source. Cold climate heat pumps have much lower balance points. This means they will heat your building in colder temperatures.

Capacity Ratio or Capacity Maintenance

A heat pump’s heat output at 5° F compared to its output at 47° F. This is reported as a percentage. Cold climate heat pumps have capacity ratios between 70% and 100%. ENERGY STAR cold climate certified heat pumps must have a minimum ratio of 70%.

Cold Climate Heat Pump

Cold climate heat pumps use technology that helps them work well in very cold weather. Most work well down to -22°F. Some can even work at -30°F.

These heat pumps adjust how much heat or cool air they produce based on how much the building needs. This helps keep the building comfortable in all temperatures. It also saves energy during both winter and summer.

When correctly designed, they can avoid the need for auxiliary heating systems.

Reducing or eliminating electric resistance heat helps lower the building's peak electricity use and overall energy consumption. This is better for the building occupants  and aligns with Denver's electrification goals.

Unsure if your Heat Pump is Cold Climate certified?  You can search by model numbers on the lists below to see if your equipment is certified as Cold Climate.

Cut-off Temperature

The lowest outdoor temperature that a heat pump can operate in. If the temperature drops below this point, the heat pump will shut off. Cold climate heat pumps typically operate down to -22° F. For this reason, we recommend cold climate heat pumps for Denver's climate.

The highest cut-off temperature we recommend considering in Denver is for is -13° F for cold climate heat pumps. Standard heat pumps often have cut-off temperatures at 10° F or higher. This may mean you will need backup heat for Denver’s outdoor design temperature of 3° F.

Integrated Systems Approach

A holistic building method for new construction or retrofits. This method looks at how all parts of the building work together and affect each other. All parts of a building need to work well together for the building to work efficiently. To get the best results from making homes more efficient and using electric power, it's essential to see the building as one complete system. This includes looking at how well it's sealed and insulated.

Maximum Heating Capacity at Outdoor Design Temperature

The heat pump’s maximum heating capacity at the heating design outdoor temperature. In Denver, that is set at 3° F.

Most manufacturers provide the maximum capacity at 5°, 17°, and 47° F so you will have to extrapolate the capacity at 3° F.

Balance point charts are a useful tool to extrapolate the capacity at 3° F.

Outdoor Design Temperature

Defined by ACCA Manual J as the outdoor temperature at which:

  • 99% of the heating season is warmer
    • This is 3° F for Denver
  • 99% of the cooling season is cooler
    • This is 92° F for Denver

These values guide load calculations and system sizing. Using this data ensures reliable and efficient operation under local climate conditions.

Percent of Heating Load Served

The percent of the total annual heating load served by the primary heat pump. This is heating without auxiliary or supplementary heat. The goal is to match the heat pump's heating capacity with the building’s total heating load. When you minimize excess, you will optimize the equipment size and selection.

SEER/EER/HSPF and SEER2/EER2/HSPF2

SEER - Seasonal Energy Efficiency Ratio

  • An indicator of cooling energy efficiency throughout a cooling season. A higher value indicates better efficiency.

EER - Energy Efficiency Ratio

  • A snapshot of cooling efficiency at 95° outdoor air temperature.

HSPF - Heating Seasonal Performance Factor

  • An indicator of heating efficiency throughout a heating season. A higher value indicates better efficiency.

In 2023 the Department of Energy created new standards. These standards more closely mimic real-world conditions. This change should result in tested efficiencies that are closer to actual performance. These are SEER2/EER2/HSPF2.

Simultaneous Operation

The condition when the primary heat pump and the secondary heat source are operating at the same time. This occurs when the outdoor temperature is below the balance point temperature. The system will move to backup heat if the temperature goes below the cut off temperature.

Switchover or Changeover Temperature

See Cut-off Temperature.