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Where is the heat pump defrost sensor, and how do I check it?

Views: 1898     Author: Site Editor     Publish Time: 2023-03-24      Origin: Site


Heat pumps are energy-efficient machines that may be used in any home to provide both heating and cooling. However, in colder climates, they can become inefficient because of ice formation on the exterior coils. To avoid this, a heat pump must enter defrost mode to clear the ice. In this post, we'll show you where to find the defrost sensor on your heat pump and how to test it if your gadget isn't working properly.

A defrost sensor is a heat pump system component that senses ice or frost on the machine's outdoor coils and triggers the appropriate process to rectify the situation. Heat pumps transport heat from the exterior air into the house during the colder months, helping make a house ambient. However, if the exterior coils become coated in ice or frost, they can't draw heat properly, resulting in an inefficient and probably damaged system.

How does a heat pump defrost sensor work?

The defrost sensor operates by keeping track of the exterior coils' temperature. It is typically made up of a timer and a temperature sensor. Once the temperature sensor perceives that the exterior coils have reached a particular temperature, usually about 32 degrees Fahrenheit, the timer is triggered to start the defrost cycle.

During the defrost phase, the heat pump simply alters the refrigerant stream, warming the outdoor coils and melting any accumulated ice or frost. The cycle normally lasts a couple of minutes before the heat pump returns to the heating cycle.

A defrost sensor is an essential component of a heat pump unit since it helps ensure the system runs reliably and effectively even in freezing conditions. A heat pump with a faulty or missing defrost sensor may struggle to meet demand in cold conditions or experience decreased performance and greater wear and tear.

Some modern heat pumps even use smart defrost sensors that factor humidity and external temperature into account to enhance the defrost cycle and boost efficiency. These defrost sensors may employ more advanced algorithms to decide when and how long the defrost cycle should stay active.

Where is the heat pump defrost sensor located?

The defrost sensor is often found on the heat pump's outdoor coil, close to the exterior fan motor, where frost or ice is likely to build up during the cold season. It is usually contained in a plastic casing and comprises a timer and temperature sensor.

The sensor is a metal tube or rod attached to the heat pump's defrost control board. It might be visible or protected by a protective cover. When the exterior coils reach a specified temperature, the sensor triggers the defrost control board to initiate the defrost cycle.

It is critical to understand the location of the defrost sensor so that you can inspect it on a regular basis for signs of malfunction or damage and ensure that your system operates efficiently during the cold season.

Certain heat pumps may have numerous defrost sensors positioned at various points on the exterior coil to ensure precise detection of ice deposits. The precise location of the sensor varies depending on the heat pump model and manufacturer.  If you need help finding the defrost sensor on your heat pump unit, check the owner's manual or reach out to the manufacturer for assistance.

What are the signs of a faulty defrost sensor in a heat pump unit?

If your heat pump's defrost sensor fails, you may observe many signs, including:

● Ice formation on the outside coil

● A continuously or periodically running outdoor fan

● A defrost period that is longer than usual.

● Inability to achieve the desired temperature

● The outside unit is making loud noises.

● Cycling on and off frequently

● Running continuously but not generating any heat.

Checking the Heat Pump Defrost Sensor

Once you've identified your heat pump's defrost sensor, you can start checking it to make sure it's working properly. There are several ways to examine the defrost sensor. Here’s a breakdown:

Check for continuity.

The first step is to use a multimeter to check for continuity. Continuity simply means the circuit is complete when the defrost sensor closes. To do this, turn off the electricity to the heat pump, then take off the outdoor unit's electrical cover.

After locating the defrost sensor, verify for continuity using the multimeter. If you don't detect continuity, the sensor is most likely malfunctioning and must be replaced.

Measure the outdoor coil temperature.

You can also check the temperature of the unit's outdoor coil to determine if the defrost sensor is working properly. You'll need a special thermometer to measure the temperature of a heat pump's outdoor coil. Locate the heat pump's outdoor unit and detach the access panel to access it. Switch on the heat pump for at least 10 minutes to bring it to a stable temperature.

Take a temperature reading by placing the thermometer probe on the coil's surface. Take several readings across the coil to guarantee a reliable average temperature estimate.

Remember that the temperature of the outdoor coil will fluctuate based on the operation of the heat pump, the ambient temperature, and many other factors. Use the manufacturer's suggested operating temperature to determine if the outdoor coil works properly. If you suspect an issue with the outdoor coil, call an HVAC expert for additional inspection and repairs. You may only need to buy a replacement defrost sensor.

Examine the sensor wiring.

You should also check the sensor's wiring. Verify that the wiring is intact and that there is no evidence of breaks or damage. If the wire is broken or damaged, replace it immediately.

Examine the sensor connections.

After you've verified the wiring, double-check the sensor connections. Check the connections for tightness and evidence of corrosion or other deterioration. Change the connections if they show evidence of deterioration or corrosion.

Check if the sensor is opening and closing correctly.

These operations require lowering or raising the sensor’s temperature and taking multimeter readings. Lowering the temperature is as simple as immersing the sensor in cold water, whereas raising the temperature requires hot water or a heating gun. Don't use an oxy-acetylene torch, or you'll damage your sensor.

For example, if the sensor is designed to close at 30°F and the outside temperature is 42°F, merely immersing it in cold water should cause it to close, and the ohms across the terminals should read between 0.3 and 0.7 ohms. If the resistance is outside this range, replace the sensor.

Then, take the sensor out and rewarm it using a heat gun or some warm water. Again, ohm readings across the terminals should read 0L (open). Any other reading shows that the sensor is defective.

What are some crucial to remember when diagnosing and repairing heat pump systems defrost sensors?

There are a few crucial things to keep in mind when diagnosing and repairing heat pump systems' defrost sensors:

● Don't be scared off by heat pumps; once you understand how they work, you can diagnose them accurately and fix problems.

● When a heat pump operates normally in the heating mode, ensure the defrost sensor is open unless the coil is frosted.

● The unit's defrost sensor should close when the heat pump enters defrost mode. This will switch the reversing valve into cooling mode and shut the condenser fan motor off. However, the blower motor should stay on and activate the auxiliary heat strips to help maintain the interior temperature during defrosting.

● There are several ways to diagnose a defrost sensor. Start by checking the wiring for signs of damage, then unplug it from the control board and place it in cold and warm water to see if it closes and opens properly.

Take the time to examine your heat pump's defrost sensor properly. Investing this time and effort can help ensure your heat pump functions efficiently and minimize wear and tear. If you suspect an issue with the outdoor coil, call an HVAC expert for additional inspection and repairs.


The defrost sensor is an important component of a heat pump that helps trigger the defrost cycle when necessary. It is usually located close to the outdoor fan motor and connected to the unit's control board. There are several ways to diagnose a heat pump's defrost sensor, including the use of a multimeter or a specialized diagnostic device. Replace faulty defrost sensors immediately to reduce inefficiencies and avoid overworking your heat pump.

If you're searching for a high-quality heat pump provider, look no further than SPRSUN. We are a reputable heat pump manufacturer with over 20 years in the industry. We provide a variety of heat pumps, including air source heat pumps, DC inverter heat pumps, and swimming pool heat pumps, all of which are manufactured per the EN14511 requirement and have CCC, CB, CE, KEYMARK, SAA, and ERP certifications. We also work with industry leaders like Panasonic, Grundfos, Copeland, CAREL, Mitsubishi, and Sanyo to guarantee high-quality products. Call us to find out more.

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