Defrost Cycle Explained
Have you ever wondered why your heat pump has frost on it in the winter time? Has your heat
pump ever made a loud noise that makes you worry? There is no need to fear or call your heating and
air company right away. Your unit may just be going through a perfectly normal defrost cycle. During the
winter, when it is cold outside and there is a considerable amount of humidity in the air your unit will
run a defrost cycle as needed to keep the unit running efficiently. When your system is running in heat
mode the coil outside is cold and the coil inside is hot. There is a fan that pulls air through each coil both
outside and inside. The refrigerant in the outdoor coil is normally about 20 degrees colder than the
outdoor ambient temperature. During colder high humidity conditions, the moisture in the air your
system is pulling across the outdoor coil will collect and freeze on the coil, causing a layer of frost to
build up. Again, this is completely normal. This is where defrost mode comes into play.
Your systems controls will monitor the temperature of the outdoor coil, and once it drops below
freezing, a run timer of usually about 90 minutes will begin. When the unit has run this amount of time a
defrost cycle is started. That’s right! It is perfectly normal for the unit to run for over an hour after the
coil begins to freeze but before a defrost cycle starts. Once the defrost cycle starts the outdoor fan
motor will stop turning, the compressor will continue to run and the system reverses the direction of
refrigerant flow which causes the unit to run in cooling mode. The sudden change in pressure usually
causes a loud swoosh sound followed by a loud grinding noise from the compressor. Next, the outdoor
coil begins to warm up and melt the frost that previously built up. During a defrost cycle, in place of the
warm air, colder air may come through your vents into your home…making it feel like your heat is not
working. Most units have electric heat strips that will kick in at this time, to make up for the cold air.
The defrost cycle can last for up to ten minutes depending on conditions outside. Once the
unit’s controls see that the outdoor coil has warmed up enough that the ice should have melted off, the
system will switch back to heat mode, again causing the swoosh and grinding noises mentioned earlier.
Then, the outdoor fan begins to turn again usually sending up a cloud of steam due to the moisture
evaporating off of the hot coil and refrigerant piping in the unit. There is no need to call your heating
and air conditioning company when you see this process happening. However, if you notice that there is
several inches of ice built up on your outdoor unit coils, and it doesn’t ever seem to melt, then you may
want to give your heating and air company a call and have them check to be sure your unit is running
properly and efficiently.
Please note that in extreme cold and wet weather conditions your system will go into defrost
more frequently but still may not be able to melt all of the ice from around the unit. However, keep in
mind that there are times when the conditions are just too nasty for a unit to stay defrosted like it
should and the only thing anyone can do about that is wait for the weather to get warmer.
Broom Blog 01 Humidifiers
How comfortable are you right now? By comfortable I do not mean your financial, political or social
comfort, I mean your personable body comfort. Are you hot? Cold? Just right? If my question caught you
off guard my guess is that things are pretty good. Most people do not think about their physical comfort
until they are uncomfortable and when they are uncomfortable, comfort quickly becomes the hot topic
(pun intended). But what’s comfortable to me may not be comfortable to you. After 26 years of
marriage and having raised two daughters all under one roof, I am a certified expert that people do not
always agree as to what’s comfortable. You might be surprised to learn that comfort is not always about
hot or cold, but often about humidity.
The air we breathe consists of two main ingredients: Dry air and water vapor. Of course we have
nitrogen and oxygen (and a few other things) in the air, but comfort is achieved by controlling the dry air
and water vapor. Even though water vapor is only ½ to 1 ½% of the total make up of air, it is this portion
of air that is the most challenging for us to balance to achieve ideal comfort. When heat our homes in
the winter, we reduce the water vapor in the air. Depending upon where we live and the type of heat
we use, we can dry the air out to the point where people experience cracking in their homes woodwork
and nosebleeds. Humidifiers can be used to add moisture back to the air when needed to get the air to a
comfortable point again.
There are three basic types of humidifiers that can be added to your current heating and air system:
steam, evaporative, and spray. Each of these should be controlled by a humidistat. A humidistat is
mounted on your wall like a thermostat but instead of measuring temperature, it measures relative
humidity and turns the humidifier on or off depending upon the setting. A steam humidifier is the most
affective and safest type of humidifier. Water is heated in a vessel outside of you homes duct system.
Once the steam is created, it is piped to directly into the duct for delivery in your home. The steam
evaporates quickly thus raising the humidity level quickly. An evaporative humidifier has a media
inserted into your duct system that holds water that evaporates into the air. This media is often similar
to a sponge. This method of delivery takes longer due to its dependence on the temperature of the
surrounding air to evaporate the water in the sponge. The third type of humidifier is the water spray or
mist humidifier. With the advent of steam and evaporative humidifiers, spray humidifiers are pretty
hard to find today. These humidifiers actually spray a mist of water into the duct to add moisture in to
In the southeast United States from where I write, humidity levels are usually higher humidifiers are not
often needed. In a later blog I will discuss methods of dehumidification. The term “its not the heat it’s
the humidity” is more true than you might think. Until then, stay cool.
Are you interested in knowing the condition of the air in your home? Broom Heating and Air can
perform an Indoor Air Quality test in your home. This useful report will not only show you the relative
humidity level in your home, but also the number of air born particles and chemicals in the air. Also, the
report provides solutions for anything item that needs to be addressed. Call us today at 803-754-9409 or
visit us at www.broomheating.com to schedule an in home test.