The heat pump may run for hours in cold weather without any apparent problem, but it still produces less heat than it should. Sometimes it’s a bad defrost sensor not reading correctly. Other times, a dirty outdoor coil, restricted ductwork or an airflow problem puts unnecessary strain on the system.
For HVAC technicians, finding the cause matters more than anything else. This guide explores the field checks that can aid in diagnosing winter performance problems, including heat pump defrost cycle troubleshooting, thermistor testing, defrost operation, compressor amp draw measurement, and external static pressure HVAC testing.
Troubleshooting a Heat Pump Stuck in Defrost Mode When It’s Sub-Freezing?
Frost on an outdoor coil is normal when a heat pump needs to pull heat out of cold, humid air. The problem starts when the defrost cycle does not clear the coil, runs too often, or does not stop when it should.
In a typical reverse-cycle defrost, the reversing valve changes the direction of the refrigerant flow, causing the outdoor coil to become warm and melt the frost. In the process, the outdoor fan may turn off and supplemental heat may come on to keep things comfortable inside. The exact sequence is equipment dependent.
Begin with the defrost sequence
- See what the system is actually doing before you replace a sensor or control board.
- Note the conditions. Record outdoor temperature, frost amount, heating efficiency, and defrost frequency.
- Check out the outside unit. Check for blocked coil surfaces, leaves, snow, bent fins, and tight space around the unit.
- Verify the sensor. Check that the coil thermistor is in the correct position and that the wiring and connector are not damaged.
- Check the input controls. Compare sensor values to manufacturer temperature and resistance specifications. Look for any stored fault codes.
- Observe the defrost operation. Check the reversing valve, fan operation and termination controls. Follow the service procedure as outlined by the manufacturer.
- Confirm recovery. When the defrost is over, verify that the unit resumes heating and that the coil does not develop heavy frost again right away.
- If the coil is heavily iced after the defrost, it needs to be looked into further. Poor drainage can also cause water to freeze around the base, interfering with later operation, even if the control sequence itself is working.
Test the NTC thermistor with a multimeter
The resistance of a negative temperature coefficient (NTC) thermistor varies with temperature. Its resistance decreases with the increase of temperature, but the specific resistance of a certain temperature is a function of the sensor model.
If the thermistor on the outside of the specified curve is drifted, the control board will think the coil is colder or hotter than it really is. This can influence the start and end of defrost.
- Isolate the sensor. Follow the manufacturer’s instructions for electrical safety. Disconnect power and isolate the thermistor from the circuit as required.
- Measure the resistance of the sensor with a suitable multimeter.
- Measure the temperature at the sensor with a contact temperature probe. Allow both readings to settle.
- Compare measured resistance to manufacturer’s resistance-temperature table.
- Check the connection. Look for corrosion, broken insulation, loose terminals, moisture, or poor contact between sensor and coil.
For example, a resistance reading of 10 kΩ doesn’t tell you if a thermistor is good or bad. The technician should be aware of the curve of the sensor as specified and the temperature of the sensor at the time of the test.
If the reading is out of the permitted tolerance, change the sensor to the correct part and repeat the test. Never use a generic thermistor chart when the manufacturer data is available.
What if the sensor is good?
If the Thermistor readings are within spec, proceed to the rest of the defrost circuit. Verify wiring continuity, control board inputs, termination conditions and maximum defrost time, if applicable.
Normal defrost operation can also be prevented by a reversing valve which does not switch. But a bad valve is only one possibility. Bad control voltage, bad wiring and refrigerant circuit trouble can show similar symptoms.
Do not bypass a defrost cycle or a safety control unless the test is specifically allowed by the manufacturer’s approved diagnostic procedure. Set all controls to normal operation before leaving the site.
Measuring the Impact of Outdoor Fin Restricted Heat Transfer on Compressor Amp Draw
The outdoor coil acts as the evaporator in normal heating operation. The refrigerant absorbs heat from the outside air and returns to the compressor.
When the coil is covered with frost, dirt or other obstructions, less air may pass across the fins and heat transfer can decline. This can then cause problems keeping up with heating capacity in cold weather, particularly when restricted outdoor coil airflow prevents the system from absorbing enough heat.
How coil restrictions impact compressor operation
Heavy frost on a coil reduces the amount of heat available to the refrigerant. The evaporating temperature and the suction pressure may be reduced, which reduces the mass flow of refrigerant and the heat delivered indoor.
The pressure ratio of the compressor increases when the suction pressure decreases while the discharge pressure is still relatively high. Depending on the compressor and the operating conditions, this can increase the discharge temperature and shift the system out of its intended operating range.
However, restricted airflow doesn’t always equal higher compressor amp draw. The refrigerant flow reduction produces some conditions of current reduction, and others of electrical loading increase. The clamp meter reading should be read together with pressure, temperature, voltage and manufacturer data.
Field check of compressor loading
1. Check the outdoor coil.
Uneven frost patterns, dirt deposits between the fins, and any physical damage or obstructions around the unit. Also, look at the drainage area for any standing water that may freeze and create recurring ice problems.
2. Measure the compressor current
Use the right clamp meter on the right conductor and use electrical safety practices. Compare the operating voltage and the compressor current with the nameplate data and the manufacturer’s expected operating data.
As a rating for equipment and electrical design. Running load amps (RLA) Not necessarily the current the compressor should draw under all conditions of operation.
A reading below RLA does not mean the compressor is healthy, nor does a high reading mean the compressor is failing. Study the operating conditions before coming to a conclusion.
3. Check refrigerant pressures and temperature.
Where the equipment and service procedure allow, measure the suction and discharge pressures with suitable instruments. Change the pressure readings to refrigerant saturation temperatures, using the correct pressure temperature data.
Compare these readings to outdoor conditions, indoor demand and manufacturer performance information. Low suction pressure can be a symptom of poor heat absorption, but refrigerant undercharge, a restricted metering device and other malfunctions can also cause the same effect.
4. Check the operating limits of the compressor.
Check discharge temperature, current, pressure readings and any active protection trips. If readings are outside the allowable operating range, locate and correct the cause before allowing the system to continue running under those conditions.
Can a restricted coil cause liquid slugging?
Liquid slugging occurs when liquid refrigerant enters the compression chamber of a compressor in a quantity that the compressor cannot safely handle. It can cause serious mechanical damage.
A dirty outdoor coil does NOT always cause liquid slugging. Liquid return can be associated with other faults such as:
- Incorrect refrigerant charge
- Metering problems
- Refrigerant migration
- Abnormal operating conditions
If the compressor is making strange noises after defrost or restart, find out why instead of assuming coil restriction is the cause. Follow the manufacturer’s instructions for diagnosis and stop operating the equipment if it could be damaged by continued operation.
External Static Pressure (ESP) and CFM Metrics for Winter Cycles
If indoor airflow is not enough, even a clean outdoor coil and a working defrost system do not guarantee good heating from a heat pump. Restricted airflow across the indoor coil can create abnormal operating pressures, decrease heat delivery, and cause high pressure protection.
You can feel the air flow at a supply register but measuring it is more accurate. Total external static pressure and actual airflow in cubic feet per minute (CFM) are two useful measurements for winter heat pump maintenance.
Digital manometer for measuring total external static pressure
TESP is the resistance the air-moving system has to work against through the external duct system. A digital manometer and appropriate static pressure probes are utilized by the technicians to quantify the return and supply sides.
Field procedure
- Find the manufacturer’s specified return and supply test points.
- Zero the digital manometer and verify that tubing and probes are properly connected.
- Measure and record return static pressure.
- Measure static pressure and record on the supply side.
- Sum the readings using the correct signed-pressure convention.
- Compare the result to the unit’s rated static pressure and blower performance data.
For example, a return reading of -0.20 in. w.c. and a supply reading of +0.30 in. w.c. will give a TESP of 0.50 in. w.c.
This is an example measurement, not a global target. Some systems are built for different external static pressures, so always check the equipment documentation.
High TESP readings may indicate high resistance in the filter, ducts, coil or other external parts. Remember that a low reading doesn’t always mean there is enough air flowing, you also need to look at the blower setting and the actual fan performance.
What is the correct CFM per ton for a heat pump?
CFM per ton heat pump airflow should be checked against the manufacturer’s specifications. CFM is the volume of air flowing through a system in a minute. A range of 350-400 CFM per ton is often used as a reference for many comfort-cooling applications.
For a 3 ton system, that would be about 1,050 to 1,200 CFM. But the actual air flow is a function of the equipment, the mode of operation, the indoor coil, the blower configuration and the manufacturers’ performance tables.
Do not set the cooling airflow target to winter heating by default. Some heat pumps (like variable-speed ones) have different airflow settings for heating and cooling.
If measured airflow is too low, check the followings:
- Check the air filter. Check for dirt, wrong filter size or too much filter resistance.
- Look for blocked grilles, undersized ducts, closed dampers, and other restrictions.
- Inspect for crushed flex ducts, closed registers and broken duct sections.
- Check for the blower assembly. Check wheel for dirt, check motor, wiring and settings.
- Indoor coils should also be checked. Check for dirt build-up or restrictions that limit airflow.
- Fix the limitation and re-measure. The goal is to know the system is working in its specified airflow and static pressure parameters, not just a number that is generic.
Use a psychrometer to assess heating performance
A digital psychrometer or suitable temperature and humidity instrument can help establish the indoor and outdoor conditions during testing. At stable operating conditions, record relevant return-air, supply-air temperatures and humidity readings.
Difference between supply and return temperatures is useful but does not independently confirm correct airflow or refrigerant charge. For example, low airflow results in a large temperature difference, while other faults may result in a smaller difference.
Readings should be used in conjunction with static pressure, measured airflow, compressor current and manufacturer performance data.
Maintenance Checklist for Heat Pump
Routine maintenance will help identify problems before they become emergency heating calls. These checks are useful for seasonal maintenance and investigating poor winter performance.
Outdoor unit
- Remove leaves, snow and other debris from around the unit.
- Check the coil for dirt and damaged fins and for unusual frost patterns.
- Make sure that the defrost water can run away without freezing again at the bottom.
- Check the outdoor fan, motor and electrical connections.
- Check that the defrost sensor is fitted securely and correctly positioned.
Indoor unit
- Check the air filter and change if necessary.
- Check supply and return ducts for leaks, damage and restrictions.
- TESP should be measured with a digital manometer.
- Check blower settings, check for unusual noise or vibration.
- Record return and supply temperatures when they are steady.
Electrical and refrigerant systems
- Check electrical connections and components according to the manufacturer’s procedure.
- Use an appropriate meter to record compressor voltage and current.
- If symptoms indicate additional testing is needed, check the refrigerant operating conditions.
- Check for fault codes and proper operation of protective controls.
Avoid adding refrigerant simply because suction pressure is low. First establish the operating conditions, check for airflow and coil problems, and follow the manufacturer’s method for confirming refrigerant charge.
Bhai, ye lo proper borders wala table. Isay copy karke directly Word ya Google Docs mein paste kar sakte ho.
Heat Pump Winter Troubleshooting Table
| Symptoms | Possible Technical Causes | Corrective Field Action |
| Heavy ice on outdoor coil | Failed defrost, restricted airflow, sensor fault, or drainage issue | Inspect the coil, check the sensor, verify the defrost sequence, and inspect drainage |
| Unit remains in defrost | Faulty thermistor, wiring issue, control-board fault, or termination problem | Compare resistance with the manufacturer’s curve and test control inputs. |
| Defrost occurs too frequently | Incorrect sensor readings, restricted airflow, or control fault | Check sensor placement, coil condition, and defrost settings. |
| Weak heating output | Low airflow, poor heat transfer, refrigerant fault, or excessive defrost | Measure airflow, inspect both coils, and review operating pressures. |
| High indoor discharge pressure | Restricted indoor airflow, dirty coil, or refrigerant-related problem | Measure TESP, inspect filters and ducts, and verify blower operation. |
| Low suction pressure | Poor heat absorption, low charge, or metering restriction | Inspect the outdoor coil and compare pressure-temperature readings. |
| Compressor current is abnormal | Voltage issue, abnormal refrigerant conditions, or compressor fault | Measure voltage and current and compare with manufacturer data. |
| Outdoor fan does not run | Motor fault, wiring problem, control fault, or normal defrost operation | Confirm operating mode and test the fan circuit. |
| Ice returns soon after defrost | Drainage issue, inadequate defrost, or persistent airflow restriction | Observe defrost termination and investigate rapid refreezing. |
| Compressor makes unusual noise | Mechanical issue or abnormal refrigerant return | Investigate promptly and stop operation if damage is suspected. |
These symptoms are starting points, not final diagnoses. Confirm the cause through measurements before replacing components.
Frequently Asked Questions
How often should you service a heat pump in winter?
Follow the maintenance schedule suggested by the manufacturer, considering the local weather, operating hours, and site conditions. A seasonal inspection can catch dirty coils, restricted airflow, failed sensors and electrical problems before they interrupt heat.
Is it normal to have frost on an outdoor coil?
Yes. In cold and humid weather, light frost can be produced during normal operation. The defrost system should remove the build-up of frost and if there is a constant heavy build-up of ice, or the coil continually fails to defrost this should be investigated.
Why is my heat pump stuck in defrost mode?
A thermistor that is out of its specified resistance curve, a control-board fault, a termination problem, a reversing valve that fails to shift, or damaged wiring can cause this problem. Test the whole sequence before swapping parts.
What is the correct winter flow of air for a heat pump?
The typical ratio for many comfort-cooling uses is 350–400 CFM per ton, but not for every winter heating system. Ensure the airflow is as recommended by the manufacturer for the specific equipment and mode of operation.
Will a dirty outdoor coil hurt the compressor?
If the coil is limited, it may decrease heat transfer and cause abnormal working conditions. If the system is run outside its allowable limits, stress on the compressor may increase. Therefore, the limiting should be corrected and the system retested.
Need more help for heat pump winter maintenance?
If your heat pump is having trouble in the winter, check the defrost system, outdoor coil, compressor, and indoor airflow. The problem is to figure out what part is causing the problem, not to just replace parts based on symptoms.
Start with the complaint, measure under known operating conditions and compare results to the manufacturer’s specifications. This approach improves the accuracy of heat pump winter maintenance, helps to avoid repeat service calls, and provides a clearer basis for repair decisions for technicians.
HVACPool.com is always available for your help regarding heating, ventilation, and air conditioning issues. Do not hesitate to get in touch!
Related HVAC Products
Related Guides
- Annual HVAC Maintenance Checklist by Season
- Furnace Maintenance Guide: What a Tune-Up Covers
- How Often Should You Change Your Air Filter?
Sources
- U.S. Department of Energy — add the specific page used
- ENERGY STAR — add the specific page used
- AHRI Directory / HVI Certified Products Directory — if a rating is cited
Written by [author name]. Technically reviewed by [reviewer name], [credential]. Last reviewed: [date].
