By Roger Desrosiers
Two main types of split system air source heat pumps are; "All Electric" and "Add On". If a heat pump is added on to a fossil fuel furnace, the coil is installed in the supply air plenum, which is downstream of the heat exchanger with respect to air flow. It is therefore impossible to run both the heat pump and the backup heat simultaneously.
The mild 105 ºF heat from the indoor coil could certainly not be rejected into a 150 ºF air stream from the heat exchanger of the fossil fuel furnace. Therefore, anytime that supplemental heat is required, the heat pump must shut off and rest while the space is brought up to temperature by the more expensively fueled back up heat. This is why "add on" heat pumps are not as energy efficient as an all electric system.
Since the indoor coil of an "all electric heat pump" can be located upstream of the electric heating elements, there is no problem running the heat pump and the backup heating simultaneously. Note in the diagram how the RA (Return Air) is what enters the indoor coil, not the heated air from the electric elements.
A heat pump is sized to the air conditioning load so that it has a long run cycle, and can properly dehumidify the space. If it is located in a climate where the heating requirements are very much greater than the cooling requirements, there will obviously be a need for additional heat.
This additional heat has several names, but they all mean the same thing; back up heat, auxiliary heat, AUX heat, supplemental heat. Further complicating the situation is the fact that it is harder to extract heat from colder air. (There is less heat content to be extracted). So as heat is required more, it is less available from the heat pump, and there is a greater reliance on back up heat. Whereas, the add on heat pump must shut off every time back up heat is required, the all electric heat pump can continue to provide some portion of the heat at a more energy efficient rate than the fossil fuel system. This layout where electric back up heat is located downstream of the indoor coil is also what is used in packaged heat pump systems.
One might ask, "why not simply locate the indoor coil upstream of a heat exchanger and, then one could have an 'add on heat pump' that could also run back up heat simultaneously with the heat pump"? The reason is a bit illusive. One must look to the cooling mode for the answer. If the indoor coil was upstream of a heat exchanger, then the heat exchanger would become quite chilled by the evaporator outlet air in the air conditioning mode.
Humidity could condense on the heat exchanger, which would promote corrosion and possibly leaks. Leaks are not allowable in fossil fuel heat exchangers, because combustion products contain CO (Carbon Monoxide) which is poisonous. Since the heat exchanger is located in the air stream supplying the conditioned space, codes do not allow such an arrangement.
Heat Pump Controls
A heat pump is a multi-stage heating and cooling system, and therefore multi-stage T-Stats are required to operate them. Manufacturers of commercial package heat pumps sometimes build in latching circuits in their control system design, and those systems therefore require a "standard" multi-stage T-Stat to operate them.
Residential split systems require Heat Pump T-Stats. A Heat Pump T-Stat will allow use of AUX heat simultaneously with 1st stage Heat Pump heat. They are therefore intended for use with "All Electric" Heat Pump applications and not with "Add On" heat pump systems.
An "Add On" heat pump to a fossil fuel furnace requires special controls, because you can not allow simultaneous operation of both heating stages. Years ago the contractor was responsible for designing a control circuit to disable 1st stage heat, and satisfy the load by 2nd stage heat any time 2nd stage called. There are now commercially available controls that provide this function.
The simplest type is called a "Fossil Fuel T-Stat". It disables 1st stage heat any time 2nd stage heat calls. However, no defrost tempering is provided with this type of control system. In other words, any time the heat pump goes into defrost mode, cold air is discharged into the conditioned space for the duration of the defrost causing uncomfortable conditions.
Co-efficient Of Performance is a way of describing a heat pump's efficiency. It is the ratio of heat produced to the amount of energy required to run the system. The COP is calculated by dividing the total heating capacity provided by the heat pump, including circulating fan heat but excluding supplementary resistance heat (Btu's per hour), by the total electrical input (watts) x 3.412. Another rating given to heat pumps is HSPF. Typical COPs for an air source heat pump under optimum conditions are 3 to 1. In other words, for one dollar's worth of energy input you receive 3 dollars worth of energy output. However, conditions are not always optimum.
As outdoor temperatures drop so does the COP. At a COP of 2:1 you would still be receiving twice the heat output compared to straight electric resistance heating elements. If electric elements were rated by a COP, they would rate 1:1. No matter how cold it gets outside the COP of an air source heat pump never gets any worse than 1:1.
However, it is not wise to torture the expensive heat pump under these conditions, when the same output efficiency can be achieved by other means. The annual energy savings attributable to a heat pump are a result of the sum totals of all the individual COPs the system operated under for the entire heating season. Most of the energy savings occur in the milder portions of the heating season, when little or no back up heat is required, and the bulk of the heating requirements are being met primarily by the energy efficient heat pump.
It is possible to relocate heat from the ground or water with a heat pump. Most of the time ground and water temperatures are higher than winter air temperatures, so they are more efficient to use as heat sources. For example, there can be a raging snow storm with air temperatures in the 0ºF range, yet the ground temperature 6 feet down might be 40ºF. However, those mechanical systems are a lot more complicated than air source heat pumps, and although COPs of 4:1 or higher can be achieved, the much greater installation costs, increased maintenance and repair costs should be carefully considered compared to the expected extra energy savings.
It is important that the indoor air handler and duct distribution system are capable of moving an adequate quantity of air to satisfy the air flow requirements of the indoor coil when it is in the condenser mode. A typical rule of thumb for air flow requirements is 400 CFM per Ton (cubic feet per minute) for air conditioning systems, and 450 CFM per ton for heat pump systems. Inadequate condenser air causes high head pressures which lead to compressor failures.
These are some other factors to consider when choosing and installing an air-source heat pump.
Select a heat pump with a demand-defrost control. This will minimize the defrost cycles, thereby reducing supplementary and heat pump energy use.
If you're adding a heat pump to an electric furnace, the heat pump coil should usually be placed on the cold (upstream) side of the furnace for greatest efficiency.
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.
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.
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