Saturday, October 24, 2009

Air-Source Heat Pump (ASHP) Auxiliary-Supplemental Heat Sizing

by Phil Rains

Most ASHPs will have electric heaters installed within the “air handler” (where the supply air duct work originates). Outdoor temperatures lower than the structure “balance point” requires an additional heat source since the ASHP refrigeration cycle alone will not produce enough heat for indoor comfort needs during winter months. This additional heat is often called either auxiliary or supplemental heat.

Typically, around 30 - 40 degrees the ASHP will generally be able to produce the same amount of heat with the compressor and refrigerant cycle alone that the structure is losing (this is considered the structure "balance point”).Every structure has a distinctive “balance point” which can be calculated.

If the ASHP is located where the heating requirements are very much greater than the cooling requirements, there will be a need for this additional heat to help meet the load of the structure during heating. This is required as it is harder to extract heat from colder air.

Since the indoor coil of an "all electric heat pump" can be located upstream of the electric heating elements used as auxiliary or supplemental heat and emergency heat, the ASHP and the backup heating can operate simultaneously in the heating mode. These heaters, of course, will not, and should not, energize during the cooling mode. If additional heat is required due to unexpected heat loss above the “balance point,” the heaters energize briefly, controlled by the thermostat.

Note in the diagram shown that the RA (return air) enters the indoor coil, not the heated air from the heating elements. An ASHP is always sized for the cooling load of the structure so that it has a long run cycle and can properly dehumidify the space.
When installing an ASHP, choosing the right auxiliary/supplemental heating system is almost as important as proper heat pump sizing.

Unfortunately, in the past and even now, it has become common practice by some contractors and/or technicians to install an ASHP with larger electric heat strips than necessary. But remember, any time the temperature is below the structure “balance point” the auxiliary/supplemental heat may come on, called for by the “second heating stage” of the indoor thermostat.

Most ASHPs operating in the heating mode around these temperatures will run most of the time. This is actually normal operation, At or below the “balance point” the heat is leaving the structure faster than the ASHP compressor can transfer it into the conditioned space. This is a function of 1) the temperature difference between indoors and outdoors, 2) the insulating value of the structure envelope, and 3) the amount of air leakage into or out of the structure. Without supplemental heating, the indoor temperature will fall below the thermostat setting.

You should always calculate the proper size of the heaters to meet the entire structure heat loss in Btu/h from the indoor design temperature (say 70 degrees) down to the typical outdoor design temperature for the locale (for example 0 degrees). This will provide enough electric heat to meet the indoor thermostat setting alone without the compressor. However, remember that the electric heaters operating alone are much less efficient than the heat pump compressor during heat pump heating. The heat pump should be configured to provide heating throughout the heating needs (say from 65 degrees outside down to the outdoor design temperature) to achieve the efficiency of the electric heat pump compressor, with or without the heaters. Basically, always allow the compressor to provide heat during the heating cycle and never wire the system to turn the compressor off at some specific outdoor temperature above the outdoor design except if combined with a gas furnace (an add-on or dual fuel configuration).

The best way to provide effective auxiliary/supplemental heating is to have multiple levels of heat if necessary. These banks of heat then energize as necessary. Most manufacturers provide heaters in sizes that meet most structure heating needs (5, 10, 15, 20 kW banks, etc.).

Emergency heat is always necessary with ASHPs if the need arises. This heating is actually provided by the auxiliary/supplemental heaters as well. This allows the occupant to energize the heaters and the indoor blower to meet the heating requirements of the particular structure as a backup if malfunctions occur with the ASHP refrigerant cycle. Homeowners can energize the emergency heat mode typically with a switch on the indoor thermostat. In fact, most ASHP thermostats are now electronic and equipped with these selector switches or other means of energizing emergency heat. Always study the particular thermostat specifications for the model you use to assure the emergency heat can operate properly if necessary.

Emergency heat relays should always be used in conjunction with outdoor thermostats to allow electric heat operation in the event the heat pump compressor and refrigerant cycle become inoperative. An outdoor thermostat disallows the second stage (if provided) of electric heat above a selected outdoor temperature. If the outdoor temperature falls below the setting on the outdoor thermostat, this additional heater stage will come on. When the outdoor air temperature rises, and the outdoor thermostat set point is reached, the system will revert back to first stage electric heating.

Codes are very strict concerning electric heaters in ASHPs. The code governs where the electric heaters are to be installed, how they must be wired, and, in some cases, how much additional heat must be added to the ASHP to compensate for capacity loss during the heating mode. If the ASHP fails, the code specifies how much heat must be added to serve as both supplemental heat and emergency heat. It is imperative that you as the technician check the governing codes and follow them exactly.

Copyright © Phil Rains

About the Author: Phil Rains is Master Trainer/Technical Developer for He has over 35 years of HVAC and Refrigeration experience in installation, service, and training. He is NATE-certified in 5 areas, a member of ASHRAE and RSES, and ACCA EPIC-Certified in Residential and Commercial Design. He also holds a Universal Classification in EPA 608.