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Defrost Cycle of a Heat Pump

By Ben Gromicko and Nick Gromicko
According to the InterNACHI Residential Standards of Practice, a home inspection is a non-invasive, visual examination of a residential dwelling that is designed to identify observed material defects within specific components of that dwelling.  Part of the home inspection includes the inspection, identification and description of the heating system, which includes heat pumps.
The inspector is required to inspect the heating systems using normal operating controls, and describe the energy source and heating method. The inspector’s report shall describe and identify, in written format, the inspected heating system, and shall identify material defects observed.
In order to perform an inspection according to the Standards of Practice, an inspector must apply the knowledge of what he/she understands about the different types of residential heating systems.  To inspect and identify a particular heating system, describe its heating method, and identify any material defects observed, an inspector should be able to explain and discuss with their client:
bulletthe heating system;
bulletits heating method;
bulletits type or identification;
bullethow the heating system operates;
bullethow to maintain it; and
bulletthe common problems that may be found. 

The inspector must be able to thoroughly examine a heating system, understand how a particular heating system operates, and analyze and draw conclusions as to its apparent condition.  An inspector should also be able to justify his/her observations, opinions and recommendations that were written in the inspection report.

Here, we cover some fundamentals of a particular heating system called a heat pump using non-invasive, visual-only inspection techniques.  We also discuss its defrost cycle. It is up to the inspector’s judgment as to how detailed the inspection and report will be. For example, the inspector is not required to determine the capacity or BTU of the inspected heating system, but many inspectors record that detailed information in their reports. 
How it Operates
When a heat pump is operating in the heating mode or heat cycle, the outdoor air is relatively cool and the outdoor coil acts as an evaporator.  Under certain conditions of temperature and relative humidity, frost might form on the surface of the outdoor coil.  The layer of frost will interfere with the operation of the heat pump by making the pump work harder and, therefore, inefficiently.  The frost must be removed.  A heat pump has a cycle called a defrost cycle, which removes the frost from the outdoor coil.
A heat pump unit will defrost regularly when frost conditions occur.  The defrost cycle should be long enough to melt the ice, and short enough to be energy-efficient.
In the defrost cycle, the heat pump is automatically operated in reverse, for a moment, in the cooling cycle.  This action temporarily warms up the outdoor coil and melts the frost from the coil.  In this defrost cycle, the outdoor fan is prevented from turning on when the heat pump switches over, and the temperature rise of the outdoor coil is accelerated and increased.
The heat pump will operate in the defrost cycle until the outdoor coil temperature reaches around 57° F.  The time it takes to melt and remove accumulated frost from an outdoor coil will vary, depending on the amount of frost and the internal timing device of the system.
Interior Heating Element
During this defrost cycle with older heat pumps, the indoor unit might be operating with the fan blowing cool air.  To prevent cool air from being produced and distributed inside the house, an electric heating element can be installed and engaged at the same time as the defrost cycle.  In defrost mode, this heating element will automatically turn on, or the interior blower fan will turn off.  The heating component is wired up to the second stage of a two-stage thermostat.
The Typical Cycle
The components that make up the defrost cycle system includes a thermostat, timer and a relay.  There is a special thermostat or sensor of the defrost cycle system, often referred to as the frost thermostat.  It is located on the bottom of the outdoor coil where it can detect the temperature of the coil.
When the outdoor coil temperature drops to around 32° F, the thermostat closes the circuit and makes the system respond.  This causes an internal timer to start.  Many heat pumps have a generic timer that energizes the defrost relays at certain intervals of time. Some generic timers will energize the defrost cycle every 30, 60 and 90 minutes.
The defrost relays turn on the compressor, switch the reversing valve of the heat pump, turn on the interior electric heating element, and stop the fan at the outdoor coil from spinning.  The unit is now in the defrost cycle.
The unit remains in the defrost cycle (or cooling cycle) until the thermostat on the bottom of the outdoor coil senses that the outdoor coil temperature has reached about 57° F. At that temperature, the outdoor coil should be free of frost.  The frost thermostat opens the circuit, stops the timer, then the defrost cycle stops, the internal heater turns off, the valve reverses, and the unit returns to the heating cycle. A typical defrost cycle might run from 30 seconds to a few minutes.  The defrost cycles should repeat regularly at timed intervals.  An inspector should not observe a rapid cycling of the defrost operation.
In summary, certain conditions can force a heat pump into a defrost cycle (or cooling cycle) where the fan in the outdoor coil is stopped, the indoor fan is stopped or electric heat is turned on, the frost melts and is removed from the outdoor coils.  When the frost thermostat is satisfied or a certain pre-set time period elapses, the outdoor fan comes back on, and the heat pump goes back into the heating cycle.
One problem of many older heat pump systems is that the unit will operate in the defrost cycle regardless of whether ice is present.  On these systems, if it’s cold outside, the defrost cycle might turn on when it is not needed.
If the defrost cycle is not functioning properly, the outdoor coil will appear like a big block of ice, making the unit non-functional.  Damage could result if the heat pump operates without a functional, normal-operating defrost cycle.
Causes of Frost
There are many reasons why an inspector might find frost and ice stuck on an outdoor coil of a heat pump that is not properly defrosting.  The cause of the frost and ice problem may include:
a bad reversing valve;
a damaged outdoor coil;
a wiring problem;
a bad thermostat;
a leak in the refrigerant;
a dirty outdoor coil covered with grass, dirt, debris and/or pet hair;
a fan that won’t turn on;
a fan installed backwards with the blades running in the wrong direction;
a motor operating in the incorrect direction; and/or 
a replacement fan motor spinning at a very low rpm. 
Diagnosing apparent problems with the defrost cycle of a heat pump is beyond the scope of a home inspection. 
The Best Techniques
There are three cycles of a heat pump:  heating, cooling and defrost cycles.  We learned about the defrost cycle of a heat pump. 
To learn how to properly inspect a heat pump system using the best non-invasive, visual-only inspection techniques as required by the Standards of Practice, an inspector should be professionally trained.  


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Last modified: 10/04/21