Defrost Mode On A Heat Pump

Defrost Mode On A Heat Pump

What Happens in Defrost Mode on a Heat Pump?


There’s something mystical when it comes to the heat pump system.  We know it runs like a normal air conditioner in the cooling season, but when we get into heating season, some extra components come into play and we get confused or lose track of the sequence of operation for a heat pump.

So let’s go over some basics, that even I have to review from time to time because heat pumps are not my strongest suit.  I have a video called Basic Heat Pump Operation that you might want to refer to if you need an even more stripped-down version of heat pump operation.  This one’s going to focus on the defrost function and what we should be checking to diagnose a heat pump that’s not working or might be frozen over in heating mode on a cold day.

So let’s quickly review some things we learned in the last video.  In cooling mode, the heat pump works just like an air conditioner.  The refrigerant cycles through the system and basically makes the indoor evaporator coil, a cold coil, and the outdoor unit’s coil the hot coil.  We remove the heat from inside the house at the outdoor unit and pull it out to the outdoor coil to be released into the atmosphere.

In heating mode, a reversing valve reverses the flow of refrigerant to make the indoor coil the hot coil and the outdoor coil the cold coil.  So, we’re trying to extract heat from the outside and bring it inside, which can be done down to a certain outdoor temperature.  After that, there is very little heat in the air to extract, so heat strips will kick in to supplement that effort.  

A regular occurrence with a heat pump in the heating season is for the outdoor unit to go through a defrost cycle.  You can imagine that cold outdoor coil interacting with the cold outdoor temperatures can cause some freezing.  Anytime that outdoor coil gets below 40° or so, the outdoor coil being the cold coil develops frost on it.  It can’t keep operating this way, or that frost will develop into a straight-up ice block!

So, we want to melt this frost by essentially switching back into cooling mode.  Because remember, in cooling mode, the outdoor coil becomes the hot coil.

You’ll notice when you wire in the low voltage on a heat pump, you’re not just wiring in two wires like on a normal AC condenser.  Single stage heat pumps need five wires running outside to them.  Red for “24 volts”, Blue for “Common” which can be labeled B or C on defrost boards, Orange for the “Reversing Valve” or the O terminal,  Yellow for “1st stage compressor” or the Y terminal, and something like a black or brown for the X2 terminal or “Emergency Heat.”  

Notice I didn’t say “Y for cooling” because the same Y terminal is energized whether we’re in cooling or heating mode.  We’re essentially energizing the compressor and fan on the outdoor unit.  Whether we want to be in cooling or heating is up to the “O” terminal being energized or not.

Heat Pump Wiring

Remember, these wires can be any color coming from the indoor air handler to the outdoor heat pump.  All wires are copper inside.  So, for the Y terminal at the heat pump defrost board, if a wire with purple sheathing leaves the Y terminal at the indoor air handler, then the other end of that purple wire should be tied into the Y terminal at the heat pump.  It doesn’t matter what color that wire is.

The reason we have so many wires coming to the outdoor unit is to relay signals given from the heat pump to the air handler when it goes into defrost.

The defrost board is the quarterback for this whole play too.  For the defrost cycle to begin, two things have to happen.  A sensor attached to the outdoor refrigerant coil (the copper coil)(or aluminum) has to get down to 26° F, and a second requirement is that the defrost board has to agree that the compressor has run the required amount of time.  On the equipment I usually work on, it’s either 45 or 90 minutes.  

When those two requirements have been met, a contact on the defrost board closes, completing a circuit to read 24 volts at X2 so the heat strips at the air handler will come on.   Inside at the air handler, the fan still blows, which means there is cold air coming out of the ducts.  But the air handler’s heat strips come on to neutralize the cold air.  

That same circuit closing causes the O terminal to have 24 volts which reverses the flow of refrigerant to cooling mode.  You’ll hear when that happens too because the reversing valve makes a pretty noticeable whooshing sound when the change in directions happens.  We explain more about the reversing valve in another video.

The third thing that happens when that circuit completes is a set of contacts open to stop the outdoor fan motor.   This is to help warm the coils up faster.  Because if we were drawing cold air across the outdoor coils when we were trying to warm them up, it would be counterproductive.

You would think the reversing valve would energize to go into heating mode, but on 90% of the systems out there, not having 24 volts to the reversing valve causes the system to default to heating mode.  In most parts of the country, having heat is more important than having cool air, so the reversing valve on a heat pump defaults to heating mode.  Here in California during the summer, we would strongly debate that.

So what have we done here?  What voltages should we be reading at their respective terminals as the board triggers the defrost cycle?

  • 24 volts can be read between C and R on the defrost board.
  • 24 volts can be read between C and O.
  • 24 volts can be read between C and Y.
  • 24 volts can be read between C and X2 or whatever the emergency heat terminals happen to be labeled on your equipment.
  • Also, the high voltage wires (usually labeled D1 and D2) on the defrost board leading to the outdoor fan motor, will only be sending 120 to the motor instead of 240.  So, one of those terminals will have 120 to ground and the other will have 0 volts to ground.

What needs to happen for the demand defrost cycle to complete?  When the liquid temperature leaving the outdoor coil reaches about 50 degrees, the defrost termination relay on the defrost board opens.  If the temperature doesn’t rise to that point after 10 minutes, an override switch will open, and de-energize the relay which will terminate the cycle.

One last time the reversing valve makes a big whooshing sound and switches the flow of refrigerant back to heating mode, the outdoor fan turns on, the heat strips inside turn off, and the indoor coil becomes the hot coil again.

When defrost has completed and the system has gone back into heating mode, here are the voltages you’ll read back at those same terminals from earlier.

  • 24 volts can be read between C and R on the defrost board.
  • 0 volts can be read between C and O.
  • 24 volts can be read between C and Y.
  • 0 volts can be read between C and X2 or whatever the emergency heat terminals happen to be labeled on your equipment.
  • Also, the high voltage wires on the defrost board leading to the outdoor fan motor, will be reading 120 to ground on each terminal. 

If you find that the outdoor heat pump is turning into a giant ice ball, there are a few things to check before condemning the defrost board.  After the system has been turned off a while and the ice has melted, let’s make sure the coils are clean.  Restricted airflow across the indoor coil or the outdoor coil can cause the ice build-up.


If the coils are clean, we need to check the refrigerant levels.  If those are good, then something’s going wrong with the defrost operation.  It could be the refrigerant line or ambient sensors, the actual board itself, or the reversing valve that is malfunctioning. 

Most of the time the temperature sensors are permanently attached to the defrost board, so if they’re not reading correctly, the whole board would have to be replaced.  Installation guides have tables that show the resistance the sensors should be reading at certain temperatures.  Using your meter and some super thin leads will help you determine the readings.

Remember, the defrost board sends 24 volts to the reversing valve at the O terminal.  Is that 24 volts reaching the solenoid on the reversing valve?  No? Then check the wire connections.  If they’re good. Then the defrost board itself is likely bad.  

Yes, you do have 24 volts?  Then something is going wrong with that solenoid and or the valve itself.  But the defrost board is doing its job.

Just like with control boards on a furnace, if the board is giving the proper voltage to the motor and the motor isn’t working, it’s not the board.  If the board isn’t giving the proper voltage, then it’s the board or something else upstream of it.

See!  Defrost boards aren’t that hard, huh?

Thanks for checking in on our blog.  See you next week!


Don’t miss our video on this topic:

Air Source Heat Pump Basics

Air Source Heat Pump Basics

Heat pumps and air conditioners are very similar. I want to share my experience with heat pumps and how they operate to give you cooling in the summer and heating in the winter.

Continue reading

Should I Go to HVAC School or Get Hired as an HVAC Apprentice?

Trade School or Apprentice

People entering the trades question whether they should start their journey out by going to an HVAC trade school or by trying to get hired on with a company as an apprentice.  It doesn’t matter where you are in the world.  The answer to that will differ based on the company you’re trying to get on with, and what YOU want as a future employee. 

I think after reading this blog post you’ll have the confidence to start your way into the trades by figuring out this question.


My intention for this post is not to suggest whether you
should or should not go to trade or vocational school to start learning your
trade.  There are a lot of my audience reading
this in a classroom right now.  And
really, there’s nothing preventing you from doing both.  You can never have too much training and
education.  In fact, my company offers
continuous training on a weekly basis. 
New information, best practices, and advanced technology are constantly
updating in this field.

But if you’re reading this, you’ve decided that working in
an office setting is not for you.  You’ve
decided you want to work in a different setting.  One that changes on a daily basis.

An HVAC technician that learns installation and service is
really diversified and becomes proficient at more than just HVAC skills, but
plumbing, gas, electrical, construction, framing, aerodynamics, thermodynamics,
roofing, structural engineering, etc. 
Where are you going to learn all of this?

My Story

I was a mechanic in the air force after graduating high
school, but I don’t think that really played much of part in my first company
hiring me, other than I was manageable. 
I went to a job fair they had and listened to their job descriptions and
everything they had to say.  I deciphered
that they really needed install helpers, so that’s what I told them I wanted to

The next day they called me for an interview.  I went in with a polo shirt, jeans and some
clean black boots, and breezed right through an interview that basically was held
just to see how I spoke for myself.  It
was more of a conversation to determine what kind of personality I had.  And, they hired me with no HVAC knowledge at

I started learning how to install HVAC equipment, run gas
pipes, line sets, handle high and low voltage, frame out a new return can, and
how to run ductwork properly – all while getting paid, and learning some
valuable fundamentals for later on in my career as a service technician.  I got some good overtime hours, and pretty
much doubled my starting pay within a year.

But is it that easy for everyone?  It can be.

Going the Apprentice Route

Most local companies in your area provide either residential
or commercial HVAC services.   Some companies do both.   It really seems like companies who value
their employees have no problem training them, or paying for them to go to
training after they’re hired.  The
benefit to you as a person looking to work somewhere is, these companies get
someone trained up the way they need them to be.  It’s also more efficient for you because it
saves you a lot of money and you get trained for the job you’ll be performing.  So, you get your training in the classroom
either at the shop you work at, or at a school they send you to.

I found most HVAC company owners I speak with would rather
take on a new employee who doesn’t have any experience, but has a great
personality, than taking on a skilled technician who has no class, can’t hold a
conversation, or has no teamwork mentality. 
The reason is that they can be trained in the way that the company wants
them to be trained.

Early Spring

Bigger companies typically hire more techs per year than
smaller companies.  So, if you’re looking
to get on with a company, I tell people to start with the bigger companies
around town.  Also, February to April, (early
spring) is the best time to try because companies are looking to ramp up their
staff to get ready for the busy summer season. 
And that’s a great way to get on to prove yourself to the company that
hires you. 

But I’ve heard that some of those big companies will also
lay off folks when it slows back down after the summer.  And that really saddens me.  It’s got to be frustrating for those
techs.  But I think everyone makes their
own way where they work. 

My company and the company I worked at before don’t practice laying people off.  Some techs might lose some hours because their employer is slow and didn’t have the work for them, but either way, when it did slow down (like every HVAC business does during the off-season) my employer kept me busy because I was out there proving myself worthy of being on the clock every day.

During that first couple of years as an installer, working
hard, staying busy, and getting the job done in a timely manner kept me busy
all year.  That’s what employers like to

The Facts About Trade School

There’s no such thing as too much education.  Look at the people who check out channels
like mine.  It’s because they’re seeking
more input about the HVAC field.

An apprenticeship you’ve been given can be shortened
significantly with a degree you earn at a trade school.  Completing an organized class dedicated to the
HVAC field is a huge help.  The teachers
of those classes are typically seasoned veterans who have been out there and
done that for years.  Getting the
opportunity to learn from these experts is a great opportunity for you to learn
and pick up some really good knowledge.

Getting Hands-On Training

Taking classes after you’ve been hired on deepens your
understanding of the HVAC industry even more.  And going to a trade school puts you in a
setting unlike a university because you’re not sitting in a big lecture
hall.  Typically, these training centers
have air conditioners, furnaces, heat pumps, ductwork, and other HVAC equipment
already set up, so you get hands-on-training while going to class.  It won’t be the same intensity as learning it
out in the field, but it’s a great start.

A lot of trade schools have connections with HVAC companies in
your area, too.  So, it’s nice to have
that in your back pocket as you approach graduation.  That’s the point of it all anyway, to have a
job when you get out of school.

At a trade school, you get your degree faster than going to
a university.  They will likely require
that you take classes that may not have much to do with the HVAC field.  A trade school can get you in and out in
about 6 months, which means you have a good start to finding your first HVAC

Entering the Real World

Once you do have your diploma, its time to go out and face
the music.  Which is what you could have
done rather than going to a school anyway. 
I’m only saying that because the company that gave me my chance
literally taught me everything I knew before going out on my own as an HVAC
contractor.  But if you choose the route
of getting hired on as an apprentice somewhere or if you graduate from school,
you have to go out and find those companies. 
You have to take the step to go face to face with the companies you want
to work for.  Whether you choose to face
them with a diploma in your hand or not is up to you.

Finding a Company

But there are definitely HVAC companies who will hire you
right now, with no experience.  It just
depends on where they are in their demand for technicians at the time you’re
trying to get on with them.  If they’re
not hiring, they’re not hiring and that has nothing to with you having a
diploma in your hand or wanting to earn your way on with an
apprenticeship.  I have had to turn down
good people just because I didn’t have room on my team at the time.

Find some companies in your area that have a good
reputation.  You can find them by looking
at their reviews online.  Try these
companies first, because they are doing something right.  They obviously take pride in their company’s
practices, so they very likely care about their employees.

My Road to Success

I wanted a real job.  Being
an HVAC technician has been the avenue which has gotten me to where I am
today.  A husband, father, homeowner, and
someone who can afford to go out and do the things I want to do with my free
time.  I’m able to save money for my
retirement and take care of my medical needs with the insurance I’ve been
provided.  It’s a long way from the previous
jobs I had which really didn’t offer these extras that a real job provides. 


Hopefully this has helped you on your way to deciding
whether to go to a trade school or to try and go straight for an apprenticeship
with your local company.

Please leave your comments down below and tell us if you went to school or not.  If you could, tell us how that went for you so those who are reading this can learn from your experiences.  See you on the next post!

Don’t miss our video on this topic: