An air-source heat pump system provides efficient heating and cooling for your home, especially if you live in a warm climate. When properly installed, an air-source heat pump system in Southeast Texas can deliver three times more heat energy to a home than electric heat strips, for the same electrical wattage used. “Air-source” is the typical heat pump system with an outside unit, not a water-source or geothermal heat pump system. A heat pump system efficiently transfers heat in the cool mode just like a system with a cooling-only outside unit, by absorbing heat in the indoor coil, which cools the house, and then transferring that heat to the outside coil to be released outside. However, a heat pump system (as opposed to electric heat strips or gas heating with a cooling-only outside unit) also works in the heat mode by reversing the flow of refrigerant in the winter, heating the house by absorbing heat from outside and efficiently transferring that heat inside.
Any air conditioning system consists of a compressor and two coils made of copper tubing (one indoor and one outside), which are surrounded by aluminum fins to aid heat transfer. Each coil has a blower or fan to move air past the coil. A heat pump outside unit has the same components as a cooling-only outside unit, but also has a refrigerant flow reversing valve and an extra expansion valve in the outside unit.
HEAT PUMP & ELECTRIC HEAT STRIPS STAGING AND OPERATION
Air-source heat pump systems have electric heat strips installed in the inside unit to provide extra heat if the heat pump outdoor unit cannot maintain the indoor heating set point, due to cold weather or if the thermostat heating set point is manually turned up more than 1-1/2 to 2 degrees. In Southeast Texas, heat pumps can typically heat your house with little need for the electric heat strips to turn on, which is why they are often called “auxiliary heat strips.” When the heat pump outdoor unit needs to defrost its coils, the electric heat strips turn on to warm up the inside air coming out of the air grilles. More on this defrost cycle below.
A typical heat pump system has one or two stages of cooling and two or three stages of heating. The extra stage for heating is for operation of the electric heat strips. If the heat pump compressor is a single stage compressor (full capacity only), there is one stage of cooling and two stages of heating: 1st – the heat pump; 2nd – the electric heat strips. If the heat pump compressor is a two-stage compressor (second stage with full cooling or heat pump heating capacity, first stage around 67% of full cooling or heat pump heating capacity), there are two stages of cooling and three stages of heating: 1st – 67% of full heat pump heating capacity; 2nd – full heat pump heating capacity; 3rd – electric heat strips.
An AC & Heating system’s operation is automatically controlled by the thermostat, with each stage of cooling or heating normally around ½ to 1 degree apart from each other. For example, if the heat pump system is a two-stage system and in the Heat or Auto Mode, the first stage of heating (67% of full heat capacity) will turn on when the indoor temperature at the thermostat is around ½ degree below the heating set point. It will turn off when the indoor temperature is around ½ degree above the heating set point. IF while the first stage of heating is operating, the indoor temperature falls to around 1 to 1-1/2 degrees below the heating set point, the second stage of heating (full heat capacity from the heat pump compressor) turns on. It turns off when the indoor temperature gets close to the heating set point. IF while first and second stages of heating are operating, the indoor temperature falls to around 2 – 2-1/2 degrees below the heating set point, the third stage of heating, which is the electric heat strips, turns on. Third stage electric heat strips operate until the indoor temperature rises to around 1 degree below the heating set point to allow the more efficient heat pump heat to continue to heat the house. Therefore, the thermostat allows electric heat strip operation only when the heat pump itself cannot heat the house as desired.
A cooling-only outside unit with either gas heat or electric heat strips inside, will have four thermostat system modes on most digital thermostats – OFF, COOL, AUTO, HEAT. AUTO mode allows separate heating and cooling set points so that the AC & heating system can automatically switch between cooling and heating. A heat pump system will have a fifth thermostat system mode – EMERGENCY HEAT. EMERGENCY HEAT mode is ONLY to be used when the heat pump system outside unit is broken or not performing well. When the thermostat is in the EMERGENCY HEAT mode, the heat pump system outside unit will NOT operate, and ALL heating is by the electric heat strips until the outside unit can be repaired.
DEFROST CYCLE OF AN AIR-SOURCE HEAT PUMP
In weather less than 40 or so degrees, and especially during very humid or rainy conditions in weather less than 45 or so degrees, the outside coil of a heat pump will frost and often ice up, due to the outside unit’s coil temperatures being well below freezing. The outside unit’s coil needs to be around 20 degrees colder than the outside temperature in order to absorb heat from outside, and transfer that heat to inside. The heat pump automatically defrosts itself in order to remove excessive frost or ice and maintain the heat pump’s efficiency and capacity to heat.
Less efficient heat pumps have a defrost cycle set to operate on a timed basis, usually around every 90 minutes of heat pump operation, with the defrost cycle lasting no more than 10 minutes. In more efficient heat pumps, the defrost cycle starts and terminates based on the actual need to defrost the outside coils, and defrosts just long enough to clear the outside coils of excessive frost and ice. Water around the heat pump in the winter is normal and due to frost and ice being thawed off the outside coils during the defrost cycle.
When the defrost cycle begins, the switchover valve in the outside unit will switch positions, making a “swish” sound. The outside unit fan will stop turning. Now the heat pump is defrosting itself by operating in the cooling mode and taking heat from inside the house, transferring that heat to the outside coils, warming them up enough to thaw frost and ice. During the defrost cycle, indoor air coming off the indoor coil will be around 20 degrees colder than the house temperature. Therefore, the defrost controls turn on the electric heat strips and warm the cooler air back up to around house temperatures. During this defrost cycle, only the compressor in the outside unit will run, since the outdoor fan has stopped. Water vapor steam will rise out of the outside unit (is not smoke!) and thawing of ice may crackle, all of which is normal.
When the heat pump outside unit defrost control senses that the outside coils are thawed, the entire system returns to heating mode. The switchover valve will “swish” into heating mode, the outdoor unit’s fan will come on, water vapor steam will blow into the air, and the indoor electric heat strips will turn off.
EFFICIENCY OF TODAY’S HEAT PUMPS
The efficiency and performance of today’s air-source heat pumps is 1½ – 2 times greater than those available 30 years ago, as a result of technical advances such as:
- Thermostatic expansion valves on more efficient heat pumps for more precise control of the refrigerant flow to the indoor and outdoor coils.
- Variable speed indoor blowers, which are more efficient and can compensate for some of the adverse effects of restricted ducts, dirty filters, and dirty coils.
- Very high efficient heat pumps have variable speed outdoor blowers.
- Improved coil design with copper tubing grooved inside, to increase surface area for more efficient heat transfer in both cooling and heating modes.
Two-stage compressor and variable speed compressor designs increase efficiency and improve dehumidification in the cooling mode.
- Demand-defrost control minimizes the defrost cycles and reduces the run time of the defrost cycle and indoor auxiliary electric heat strips.
Heating efficiency for air-source heat pumps is indicated by the heating season performance factor (HSPF), which is the total heating required for the conditioned space during an average annual 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.) The HSPF rates both the efficiency of the whole heat pump system and the use of electric heat strips. The most efficient heat pumps have an HSPF of between 8 and 10.
Cooling efficiency is indicated by the seasonal energy efficiency ratio (SEER), which is the total heat removed from the conditioned space during an average 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 SEER rates a heat pump’s cooling efficiency. The most efficient heat pump systems have SEERs of between 16 and 24. A heat pump with a SEER of 20 cools a house for 50% of the electrical costs of an older heat pump with a SEER of 10.
Two stage and variable speed heat pumps are actually more efficient than the rated SEER value. The average heat pump or cooling-only two stage unit will operate in first stage mode for 90 plus percent of the time, since the majority of cooling hours in Southeast Texas are below 90 degrees, and systems in Southeast Texas should be designed for maximum cooling capacity at 95 degrees outdoors, per ACCA standards. In first stage cooling operation on a two stage outside unit (67% of full capacity), the outside unit has increased outside coil surface area per ton of capacity which increases efficiency by 15 – 20% over second stage full capacity cooling operation. The new variable speed heat pumps or variable speed cooling-only units are even more efficient, since their capacities range from 30% – 100%.
HEAT PUMP DESIGN CRITERIA
In Southeast Texas, a heat pump system should be sized based on the home’s cooling load, just as a cooling-only system would be sized. Efficient Systems designs the airflow (cfm) per room based on the cooling airflow needed, since cooling is used almost four times as much as heating. We perform an ACCA Manual J whole-house cooling load to determine exact cooling tonnage needed, and a room-by-room cooling load if installing new ductwork or balancing airflow per room.
We install supply air grilles that have adjustable louvers so that the airflow can be adjusted vertically for personal preference and comfort, regarding feeling the air blowing from the grilles. The temperature of the air flowing out the supply grilles during a heat pump’s heating mode is around 90 to a max of 100 degrees, and 65 – 70 degrees during the defrost cycle, so best NOT to feel the air blowing out the grilles in the heating mode. Compare this to a gas furnace producing air temperatures of 120 – 130 degrees.
Rain is not an issue for a cooling-only outside unit, but can create excessive ice build up on a heat pump outside unit when outside temperatures are below 40 – 45 degrees. Therefore, if a heat pump is installed where rain flows off the roof onto the outside unit, guttering will keep excessive roof water off the outside unit, or the outside unit can be installed further away from the house.