Feb 4 2015
Heat Pumps for Energy Efficiency.
For the Treasure Coast in general and Martin County in particular, heat pumps are an excellent energy-efficient alternative to furnaces and air conditioners. Because Martin County seldom sees extended periods of sub-freezing temperatures, its residents can take advantage of heat pump technology.
Similar to a refrigerator, a heat pump uses electricity to move heat from a cool space into warm space. During the heating season, heat pumps move heat from the cool outdoors into your warm house; during the cooling season, heat pumps move heat from your cool house into the warmer outdoors.
Because they move heat rather than generate heat, heat pumps can provide up to four times the amount of energy they consume. If you heat with electricity, a heat pump can trim usage by as much as 40 percent. High efficiency heat pumps with variable speed air handlers can also dehumidify better than standard single capacity units, resulting in less energy usage and more cooling comfort in summer.
The energy efficiency rating for a heat pump has two values. The SEER (seasonal energy efficiency ratio) indicates how efficiently the appliance will cool your home. The second is the HSPF (heating season performance factor) that describes its seasonal heating efficiency.
Heat pumps with higher SEER and HSPF ratings will save more energy. The Treasure Coast’s climate is sub-tropical, so models with a variable-speed air handler will remove more humidity in the summer, run more quietly, and reduce summertime energy bills. Since our heating season is short it’s better to select a higher SEER than a higher HSPF, if purchase cost is an issue.
Right Sizing for Heating and Cooling Equipment.
Getting the right size heat pump for your home is a very important process. It makes as much difference to the comfort of your home and the durability of the system as getting the right heat pump model or finding the electrician for the job.
Fixed-capacity direct exchange (DX) space conditioning systems, whether heat pumps or A/C systems, cycle on and off to meet space cooling loads. During system start up, there is a several-minute period during which the cooling coil becomes progressively colder. Sensible capacity recovers to 90% in about one minute while latent capacity takes longer to recover. Therefore, during the first several minutes of each ON cycle, both capacity and efficiency are diminished. During the OFF period, sensible and latent cooling energy stored in the air handler unit (AHU) and ductwork is lost to ambient surroundings.
The number of ON/OFF cycles per day and the proportion of the time that the system is OFF then affects system efficiency. Right-sizing reduces the number of cycles. Over-sizing increases the number of cycles per day and shortens the length of each ON cycle. Therefore, the percentage of the time that the system is operating at full capacity and efficiency is decreased. While a FC system may cycle on and off 50-75 times per day, a VC system may cycle on and off only 10-15 times per day.
Research performed in a South Florida lab house found that right-sizing fixed capacity (FC) heat pumps produces seasonal and peak energy savings while over-sizing variable-capacity (VC) heat pumps produces seasonal and peak energy savings, for both cooling and heating periods. These results support current state codes and primary standards such as Air Conditioning Contractors of America (ACCA) Manual S to right-size fixed capacity systems, but challenges them to allow over-sizing variable-capacity systems. ACCA currently allows up to 15% over-sizing for fixed and variable-capacity units. They have recently begun a review process to allow variable capacity systems to be over-sized by no more than 30%, however there is evidence presented that supports considering a much higher limit of perhaps 100%.