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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:
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:
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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