Proper equipment sizing may require considering supplementary electric heaters for those times when the heating load exceeds the capacity of the heat pump. The designer should determine the proper use of supplemental or backup heat for each installation.
Here are some things to consider:
- Use of the system for emergency heat,
- its quick recovery from setback temperatures, and
- its active part of the overall system.
There are various approaches, which include:
- Sizing the heat pump to handle the entire heat load and use the backup heat for emergency only,
- Sizing the heat pump to handle only a percentage of the heat load, perhaps 80 percent, and use supplemental heat for the rest.
The second alternative is common in areas where the heat pump is sized for the cooling load; it allows a smaller system with a lower installed and operating cost.
Where supplemental heat is used, there are several typical approaches:
- Duct heaters in the heat pump supply plenum for heating or subzone reheat,
- Duct heaters at all or selected outlet grilles, or
- A central boiler package.
Duct heaters should be UL listed for zero clearance and meet all requirements of the National Electric Code, and be serviceable without removing the heater.
Duct heat packages should include a hinged control panel covering fusing, magnetic contactors, and a blower relay. Typical packages range from 5 to 15 kW at 230 volts.