Informational Guide

How To Defrost a Heat Pump?

With heat pumps, frost build-up on the outside is a very common issue. Learn how to defrost a heat pump correctly.

by Josh M

Is your heat pump at home freezing up? You’ll need to know how to defrost a heat pump! With how a heat pump works, frost building up on the outside is a very common issue.

You should know how to correct it since it causes the machine to run inefficiently due to blockages.

Frozen HP Outdoor Heat Pump Unit

Your heat pump has an automatic defrost cycle. During this cycle, the pump removes frost from the outside coils. Your unit should go through this cycle regularly to prevent ice buildup.

When the machine senses ice, the reversing valve switches the machine into air conditioning mode.

While this may seem counterintuitive at first, this process causes the outside coils to heat- melting the frost. The defrost cycle only lasts between two and 10 minutes and won’t impact the temperature in your home.

Overall, the heat pump goes into defrost mode when it detects ice. It does this to avoid unnecessary strain on the system.

Learn more on heat pumps versus AC units here.

How Often Do Heat Pumps Defrost?

There are several factors that determine when the heat pump goes into defrost mode. They include:

  • The outdoor temperature and humidity.
    When it’s hot and humid outside, the outer coils make more condensation, which leads to freezing heat pumps. Your unit will always check for these conditions so it knows when to start running.
  • The amount of heating load the system is trying to deliver.
    When producing more heat, the outer pipes become cooler. That means there are more chances for ice to form against the pipes.
  • The condition of the heat pump.
    A pipe that’s in bad condition may have damaged sensors. This could lead to it ignoring the frost building up on its pipe or running when it doesn’t need to.

Most units run for about 35 minutes, every 40 minutes. You don’t want to stop the cycle, as it could lead to unnecessary damages to the machine. If it keeps running a defrost cycle, you’ll want to have it inspected.

What To Do If The Heat Pump Doesn’t Defrost Itself? (Steps for Manual Defrosting)

Defrost problems are caused when your unit has faulty wires, sensors, relays, or controls. The reverse valve might also be damaged. You’ll notice that the heat pump has frost buildup but never seems to run a defrost cycle.

Before doing anything, always make sure you turn off the heat pump! You don’t want to get electrocuted, since most of the fixes involve using water to melt the ice.

If your heat pump isn’t defrosting, here’s what you should do:

  1. Turn the unit off at the circuit breaker.
  2. Use a hose to spray the ice unit it melts.
  3. Turn on the fan and let it run for 60 minutes.

In the winter, you should instead make sure to use warm water, if possible. That way, it doesn’t freeze over the top of your heat pump. Your machine likely has a button to run the defrost cycle as well. You may need professional assistance.

Closeup Photo of Ice Formation Field

How To Check Heat Pump’s Defrost Sensor?

The defrost sensor is usually a tube on top of the condenser. The sensor closes when it gets cold enough to frost, which causes the control board to run a defrost cycle. This part is there to control the defrost functions.

You need to check the defrost sensor when you first notice frost appearing on your machine. Letting it go will only cause the unit to run inefficiently. Here’s how to check your defrost sensor:

  1. Cut power to the condenser.
  2. Find and remove the defrost sensor from the control board.
  3. Put the defrost sensor in a glass of ice water.
  4. Use a multimeter to read- it should say zero ohms.
  5. Remove the sensor and let it warm up in your hands.
  6. Use the multimeter again- it should read infinite ohms.

If any other numbers appear on the multimeter, the sensor needs to be replaced.

How To Change The Heat Pump’s Defrost Timer

The defrost timer is what controls the defrost cycle. When the sensor indicates it’s time to run a cycle, the timer determines how often to run. Most units allow you to choose from 30, 60, and 90 minute settings.

If it’s broken, you can use these steps to change it:

  1. Cut power to the unit.
  2. Find and remove the part from the control board.
  3. Replace the timer.
  4. Restore the unit and turn it back on.

If you replaced the part with a brand new one, it should work now. If not, you’ll have to do some troubleshooting to determine what’s wrong.

People Also Ask (FAQ)

How much is a defrost board for a heat pump?

Replacing the defrost board will likely cost about $350 to $400. However, the price differs depending on the brand and other factors. You’ll want to search the cost for your unit online.

How do I tell if my unit is in the defrost cycle?

There will be no heat entering your home, temporarily. The unit might also blink or have a light turn on. You might also be able to tell by looking at your thermostat.

How can I reduce defrosting of my heat pump unit?

You can reduce the defrosting by running the fan mode on your unit every day for 60 minutes. This process melts small amounts of frost on the heat pump.

Conclusion

Overall, there are a few different things you can do to defrost your heat pump manually. It’s important to keep it free of frost, so the machine can still run at its best. And if you’re in the market for a new heat pump, make sure to check out our guide on the:

Best heat pumps for cold weather

Best window heat pumps

Josh M

My name is Josh and I am obsessed with the HVAC industry. I created this website to help HVAC techs of all levels get the best out of their heating & cooling systems. I have spent thousands of hours studying air conditioners, heaters and home air products so you can learn & buy with confidence. Learn more about the team here.

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