Wednesday, December 14, 2022

Air Source Heat Pumps


from DOE

Heat pumps transfer the heat from a heat source to a heat sink using mechanical power. Different sources such as water, air and earth can be utilized as heat source for heat pump cycles. Air source heat pumps have been widely used in the south and mid-Atlantic states for decades. Warmer air leads to improving the performance of the heat pump cycle on air source heat pumps.  One of the main challenges using air source heat pumps is the effects of variations in climate and weather conditions on the operation of the heat pump cycles.

Air-source heat pumps take heat from the outside air, run it through a refrigeration cycle to step up the temperature, and deliver warm air to a building in winter and use the reverse cycle to cool the building in summer. One benefit of air source heat pumps in less extreme climates is that they can provide both heating and cooling, with the simple switch of a valve. Though, my friend insists that the air blown into the room is too cold. Though you do not get the hot blast of a gas furnace, I don’t experience cold air and think she needs to insulate her ducts.

Though the optimal operating point at maximum efficiency depends on geometric, thermodynamic and physical properties of the system components like the ducting and working fluids; in general. the colder it is outside the less efficient the heating cycle is. For years heat pump design has undergone improvements and changes to improve their performance in more extreme weather. Today, there are air source heat pumps available that will operate in more extreme conditions using less energy than the air source heat pumps of the past. 

Improvements continue. Research is on going is Alaska, Montana and Washington state to look at how the efficiency of a heat pump drops in colder temperatures, and how to appropriately size a heat pump for a home. The air-source heat pumps available today can keep your home warm even amid bone-chilling cold, using less energy than other types of heating systems. Or so the researchers say. One of the options to address heat loss in a duct system or avoid the cost of installing ducting is to utilize ductless min-split heat pumps in a home or addition.  

Ten years ago when I replace the failed air source heat pump in my house, I altered the size, changed the ducting configuration and materials, insulated the ducts, and bought a much more efficient heat pump with a variable speed. I ended up with a more comfortable home and a lower electric bill. I have been really happy with my current system, but heat pumps last only 8-12 years (in general) so I have been looking at the options on the market today.

There’s no official “cold climate” standard for heat pumps yet, but next month the U.S. Department of Energy, Energy Star program will introduce a cold-climate standard for air-source heat pumps. These heat pump will include a certification for air source heat pumps with an “acceptable” level of low-temperature performance and efficiency. 

For now, you will have to depend on the Energy Guide label which displays the heat pump's heating and cooling efficiency performance rating. Every residential heat pump sold in the United States has an Energy Guide label, look for it.

Heating efficiency for air-source electric heat pumps is indicated by the heating season performance factor (HSPF), which is a measure over an average heating season of the total heat provided to the conditioned space, expressed in Btu, divided by the total electrical energy consumed by the heat pump system, expressed in watt-hours. This is measured in set environmental conditions which is not going to occur outside your house on a regular basis. It is only a measurement for comparison like miles per gallon. Understand it as a measurement tool.

Cooling efficiency is measured by the seasonal energy efficiency ratio (SEER), which is a measure over an average cooling season of the total heat removed from the conditioned space, expressed in Btu, divided by the total electrical energy consumed by the heat pump, expressed in watt-hours. Once more this is measured in set environmental conditions.

The higher the HSPF the more effective the heat pump is on cold days at heating and the higher the SEER, the more effective the heat pump is at cooling on hot days. In general, the higher the HSPF and SEER, the higher the cost of the unit. However, the energy savings can return the higher initial investment during the heat pump's life. As I found when I replaced my first heat pump the new heat pump used less energy, substantially reducing air-conditioning and heating costs, but was also able to cool my house on the hottest of days and keep it warm in the winter. In our 4-season climate, getting the highest SEER and HSPF really paid off in overall satisfaction and operating cost savings.

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