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Air-Source Heat Pumps

An air-source heat pump can provide efficient heating and cooling for your home. When properly installed, an air-source heat pump can deliver one-and-a-half to three times more heat energy to a home than the electrical energy it consumes. This is possible because a heat pump moves heat rather than converting it from a fuel like combustion heating systems do.

Air-source heat pumps have been used for many years in nearly all parts of the United States, but until recently they have not been used in areas that experienced extended periods of subfreezing temperatures. However, in recent years, air-source heat pump technology has advanced so that it now offers a legitimate space heating alternative in colder regions.

For example, when entire units are replaced in the Northeast and Mid-Atlantic regions, the Northeast Energy Efficiency Partnerships found that the annual savings when using an air-source heat pump are around 3,000 kWh (or $459) when compared to electric resistance heaters, and 6,200 kWh (or $948) when compared to oil systems. When displacing oil (i.e., the oil system remains, but operates less frequently), the average annual savings are near 3,000 kWh (or about $300).

Types of Air-Source Heat Pumps

The different types of air source heat pumps are described below.

Ductless vs. Ducted vs. Short-Run Ducted

Ductless applications require minimal construction as only a three-inch hole through the wall is required to connect the outdoor condenser and the indoor heads. Ductless systems are often installed in additions.

Ducted systems simply use ductwork. If your home already has a ventilation system or the home will be a new construction, you might consider this system.

Short-run ducted is traditional large ductwork that only runs through a small section of the house.  Short-run ducted is often complemented by other ductless units for the remainder of the house.

Split vs. Packaged

Most heat pumps are split-systems—that is, they have one coil inside and one outside. Supply and return ducts connect to the indoor central fan.

Packaged systems usually have both coils and the fan outdoors. Heated or cooled air is delivered to the interior from ductwork that passes through a wall or roof.

Multi-Zone vs. Single-Zone

Single-zone systems are designed for a single room with one outdoor condenser matched to one indoor head.

Multi-zone installations can have two or more indoor heads connected to one outdoor condenser. Multi-zone indoor heads vary by size and style and each creates its own "zone" of comfort, allowing you to heat or cool individual rooms, hallways, and open spaces. This distinction may also be referred to as "multi-head vs. single-head" and "multi-port vs. single-port."

How They Work

A heat pump's refrigeration system consists of a compressor and two coils made of copper tubing (one indoors and one outside), which are surrounded by aluminum fins to aid heat transfer. In heating mode, liquid refrigerant in the outside coils extracts heat from the air and evaporates into a gas. The indoor coils release heat from the refrigerant as it condenses back into a liquid. A reversing valve, near the compressor, can change the direction of the refrigerant flow for cooling as well as for defrosting the outdoor coils in winter.

The efficiency and performance of today's air-source heat pumps is a result of technical advances such as the following:

  • Thermostatic expansion valves for more precise control of the refrigerant flow to the indoor coil
  • Variable speed blowers, which are more efficient and can compensate for some of the adverse effects of restricted ducts, dirty filters, and dirty coils
  • Improved coil design
  • Improved electric motor and two-speed compressor designs
  • Copper tubing, grooved inside to increase surface area.

Selecting a Heat Pump

Every residential heat pump sold in this country has an EnergyGuide label, which displays the heat pump's heating and cooling efficiency performance rating, comparing it to other available makes and models.

Heating efficiency for air-source electric heat pumps is indicated by the heating season performance factor (HSPF), which is the total space heating required during the heating season, expressed in Btu, divided by the total electrical energy consumed by the heat pump system during the same season, expressed in watt-hours.

Cooling efficiency is indicated by the seasonal energy efficiency ratio (SEER), which is the total heat removed from the conditioned space during the annual cooling season, expressed in Btu, divided by the total electrical energy consumed by the heat pump during the same season, expressed in watt-hours.

The HSPF rates both the efficiency of the compressor and the electric-resistance elements.

The SEER rates a heat pump's cooling efficiency. In general, the higher the SEER, the higher the cost. However, the energy savings can return the higher initial investment several times during the heat pump's life. A new central heat pump replacing a vintage unit will use much less energy, cutting air-conditioning costs substantially. 

To choose an air-source electric heat pump, look for the ENERGY STAR® label. In warmer climates, SEER is more important than HSPF. In colder climates, focus on getting the highest HSPF feasible.

These are some other factors to consider when choosing and installing air-source heat pumps:

  • Select a heat pump with a demand-defrost control. This will minimize the defrost cycles, thereby reducing supplementary and heat pump energy use.
  • Fans and compressors make noise. Locate the outdoor unit away from windows and adjacent buildings, and select a heat pump with an outdoor sound rating of 7.6 bels or lower. You can also reduce this noise by mounting the unit on a noise-absorbing base.
  • The location of the outdoor unit may affect its efficiency. Outdoor units should be protected from high winds, which can cause defrosting problems. You can strategically place a bush or a fence upwind of the coils to block the unit from high winds.

Performance Issues with Heat Pumps

Heat pumps can have problems with low airflow, leaky ducts, and incorrect refrigerant charge. There should be about 400 to 500 cubic feet per minute (cfm) airflow for each ton of the heat pump's air-conditioning capacity. Efficiency and performance deteriorate if airflow is much less than 350 cfm per ton. Technicians can increase the airflow by cleaning the evaporator coil or increasing the fan speed, but often some modification of the ductwork is needed. See Minimizing Energy Losses in Ducts and Insulating Ducts.

Refrigeration systems should be leak-checked at installation and during each service call. Room heat pumps and packaged heat pumps are charged with refrigerant at the factory, and are seldom incorrectly charged. Split-system heat pumps, on the other hand, are charged in the field, which can sometimes result in either too much or too little refrigerant. Split-system heat pumps that have the correct refrigerant charge and airflow usually perform very close to manufacturer's listed SEER and HSPF. Too much or too little refrigerant, however, reduces heat-pump performance and efficiency.

For satisfactory performance and efficiency, a split-system heat pump should be within a few ounces of the correct charge, specified by the manufacturer. The technician must measure airflow prior to checking refrigerant charge because the refrigerant measurements aren't accurate unless airflow is correct. When the charge is correct, specific refrigerant temperatures and pressures listed by the manufacturer will match temperatures and pressures measured by your service technician. Verify these measurements with the technician. If the manufacturer's temperatures and pressures don't match the measured ones, refrigerant should be added or withdrawn, according to standards specified by the EPA.

Advanced Technologies: Reverse Cycle Chillers

One of the more notable innovations in air-source heat pumps is called a reverse cycle chiller (RCC). It offers the advantages of allowing the homeowner to choose from a wide variety of heating and cooling distribution systems, from radiant floor systems to forced air systems with multiple zones. It also offers the potential for lower winter electric bills and hotter air out of the supply vents for greater comfort.

An RCC is especially economical for all-electric homes or in areas where natural gas is not available. Depending on other fuel rates, it may even be the least expensive heating option among the remaining heating fuel choices.

The system consists of a standard single speed, air-source heat pump, sized to the heating load rather than the usual smaller summer cooling load. The heat pump is connected to a large, heavily insulated tank of water that the heat pump heats or cools, depending on the season of the year. Most systems will use a fan coil with ducts, using the stored water to heat or cool the air and distribute it to the house.

The RCC system allows the heat pump to operate at peak efficiency even at low temperatures. This provides greater comfort and economy without the need for electric resistance auxiliary heating coils.

The RCC can also be equipped with a refrigeration heat reclaimer, which is similar to the common desuperheater coil found on the high-end heat pumps and air conditioners. The combined system costs about 25% more than a standard heat pump of similar size, and the simple payback on the additional cost in areas where natural gas is not available is about 2 to 3 years.

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