What You'll Learn

• How the refrigerant circuit reverses in heating mode, swapping evaporator and condenser roles
• Where to place suction line, liquid line, and vapor line temperature clamps in heating mode vs. cooling mode
• Why mQ needs an additional temperature clamp on the vapor line for heat pump systems
• How to identify the true suction line in heating mode
• How indoor and outdoor probe positions change when the reversing valve shifts

What You'll Need

• Device: iPhone (iOS 15+) or Android phone/tablet (Android 10+) with measureQuick installed
• Probes: Minimum three pipe clamp temperature probes (suction line, liquid line, vapor line), plus return and supply air temperature probes. A psychrometer for wet bulb (Fieldpiece JL3RH, Testo 605i, or UEi equivalent).
• Manifold or pressure probes: For refrigerant pressure readings (high side and low side)
• Bluetooth: Probes paired and streaming live data (see Bluetooth Pairing Basics)
• Access: Both indoor and outdoor units accessible with panels removed
• Time: 10 minutes to read; 15-20 minutes for full probe deployment in the field

Background: Why Heating Mode Is Different

In cooling mode, refrigerant flows in one direction: the outdoor coil is the condenser (rejects heat), and the indoor coil is the evaporator (absorbs heat). The suction line runs from the indoor evaporator back to the compressor, carrying cool, low-pressure vapor.

In heating mode, the reversing valve shifts the flow. The outdoor coil becomes the evaporator (absorbs heat from outdoor air), and the indoor coil becomes the condenser (delivers heat to the space). The same copper lines connect the two units, but the refrigerant direction and temperature profile change.

This matters for probe placement because the measurement channels in measureQuick are tied to refrigerant function (suction, liquid, discharge), not to physical location. If you place probes based on cooling-mode habits, you will assign warm lines to cold channels and produce incorrect superheat and subcooling calculations.

Heat pumps represent a growing share of installations. From the V12 data: heat pumps reached 47% market share in full-year 2025. Getting heating mode diagnostics right is increasingly important.

Step-by-Step Guide

Step 1: Understand the Reversing Valve

The reversing valve is a 4-way valve near the compressor that redirects refrigerant flow. Most residential heat pumps energize the reversing valve solenoid in cooling mode (B-type, the most common configuration). In heating mode, the solenoid de-energizes, and the valve shifts to send hot discharge gas to the indoor coil instead of the outdoor coil.

The result:

The liquid line (small copper line) carries liquid refrigerant in both modes. Its temperature changes, but its function stays the same. The vapor line (large copper line) is where the confusion happens: in cooling mode it carries cool suction gas, and in heating mode it carries hot discharge gas.

Step 2: Identify the True Suction Line in Heating Mode

In heating mode, the true suction line is at the outdoor unit. Refrigerant enters the outdoor coil as a low-pressure liquid/vapor mix, absorbs heat from the outdoor air, and exits the coil as low-pressure vapor. That vapor travels a short distance to the compressor suction port.

The key identification: in heating mode, the true suction line is the pipe between the outdoor coil outlet and the compressor inlet. It will be cool to the touch (close to outdoor ambient or slightly below, depending on conditions). On many packaged or split systems, this is a short run entirely within the outdoor unit cabinet.

Do not confuse the vapor line with the suction line in heating mode. The vapor line (large line running between indoor and outdoor units) carries hot discharge gas in heating mode. It will be warm or hot to the touch. Clamping a suction line temperature probe onto this line produces a grossly incorrect superheat reading.

[Diagram] Heat pump line roles reverse between cooling and heating mode:

| Line | Cooling Mode | Heating Mode | |------|-------------|--------------| | Vapor line (large copper) | Cool suction gas, indoor to outdoor | Hot discharge gas, outdoor to indoor | | Liquid line (small copper) | Warm subcooled liquid, outdoor to indoor | Warm subcooled liquid, indoor to outdoor | | True suction line | At outdoor unit (compressor inlet) | At outdoor unit (between outdoor coil outlet and compressor inlet) | | Discharge line | At outdoor unit (compressor outlet) | At outdoor unit (compressor outlet), vapor line carries it indoors |

In heating mode, the vapor line is hot (carrying discharge gas to the indoor condenser). Do not clamp a suction temperature probe on it; that produces a grossly incorrect superheat reading.

Step 3: Deploy Outdoor Probes for Heating Mode

At the outdoor unit:

1. Suction line clamp - Place on the pipe between the outdoor coil outlet and the compressor inlet. This is the true suction line in heating mode. The pipe will be cool. Insulate the clamp from wind if possible; outdoor conditions affect pipe surface temperature.
2. Liquid line clamp - Place on the liquid line (small copper line) near the outdoor unit, before the outdoor metering device. In heating mode, this carries subcooled liquid from the indoor condenser. It will be warm.
3. Outdoor ambient temperature probe - Place in the shade, away from the condenser discharge air. This measures the air entering the outdoor coil (which is now the evaporator).
4. Pressure probes or manifold - Connect to the service valves. Low-side (suction) pressure is at the larger service valve (vapor line service valve), and high-side (discharge) pressure is measured at the liquid line service valve or discharge port, depending on your manifold setup.

Tip: On many heat pumps, the suction line in heating mode is a short section of pipe entirely inside the outdoor unit cabinet. You may need to remove the service panel to access it. Look for frost or condensation on the pipe as a visual indicator of the suction line in cold weather.

Technician at outdoor unit with panel removed, placing pipe clamp probes on the refrigerant lines while viewing measureQuick on a phone

Step 4: Deploy Indoor Probes for Heating Mode

At the indoor unit (air handler):

1. Return air dry bulb - Same position as cooling mode: in the return duct or plenum, upstream of the filter.
2. Supply air dry bulb - Same position as cooling mode: in the supply plenum, downstream of the coil. In heating mode, supply air will be warm (typically 90-120F depending on system capacity and conditions).
3. Vapor line clamp - This is the additional probe needed for heat pump heating mode. Place a temperature clamp on the vapor line (large copper line) where it enters the indoor unit. In heating mode, this line carries hot discharge gas from the compressor to the indoor coil. measureQuick uses this reading along with high-side pressure to calculate discharge superheat and condenser performance.

The vapor line temperature clamp is what distinguishes heat pump heating probe deployment from cooling mode deployment. In cooling mode, the vapor line carries cool suction gas and is typically measured at the outdoor unit. In heating mode, measuring the vapor line at the indoor unit captures the discharge gas temperature before it enters the indoor condenser.

[Visual Reference] At the indoor air handler in heating mode, three probes are deployed: (1) a temperature clamp on the vapor line (large copper line) where it enters the unit, reading hot discharge gas (typically 120-180F), (2) a return air dry bulb probe in the return duct upstream of the filter, and (3) a supply air dry bulb probe in the supply plenum downstream of the coil, reading warm supply air (typically 90-120F). The vapor line clamp is what distinguishes heat pump heating mode deployment from cooling mode.

Step 5: Assign Channels in measureQuick

After deploying probes, verify channel assignments in the Probe Manager:

If you previously ran a cooling mode test and left probes in place, you must reassign channels. A suction line probe that was on the indoor coil outlet in cooling mode is now on the indoor condenser outlet in heating mode. The physical location did not change, but the refrigerant function did.

Step 6: Select the Heat Pump Heating Workflow

In measureQuick, select the heat pump heating workflow for your test. The app adjusts its diagnostic targets and calculations for heating mode:

• Superheat is calculated from suction pressure and suction line temperature at the outdoor unit
• Subcooling is calculated from liquid line pressure and liquid line temperature
• The app uses outdoor ambient temperature for determining condenser temperature difference (which is now at the indoor coil)
• Target values shift based on heating mode standards

Confirm that the workflow shows "Heat Pump - Heating" or equivalent. Running a cooling workflow in heating mode produces incorrect targets and misleading pass/fail results.

measureQuick workflow selection screen showing Heat Pump - Heating selected

Heating Mode vs. Cooling Mode: Quick Reference

Video Walkthrough

• Probe placement for heat pump heating mode: (1:41, 22.0K views, 207 likes) - Short, focused video showing exactly where probes go in heating mode

• Why commissioning in heating mode is different: (29:46) - Jim Bergmann explains the limitations and considerations for heating mode commissioning

• Heat Pump Commissioning: Applying The Standards: (1:27:15, 1.7K views, 48 likes) - Complete heat pump commissioning walkthrough covering both modes, including probe placement discussion

• Heat Pump Commissioning: Taming The Standards: (1:18:40) - Deep dive into heat pump standards, including heating mode measurement requirements

• Heat Pump Commissioning (third-party): (HVAC Design Partners, 80 min) - Covers TXV bulb diagnostics in heating mode and outdoor probe placement

Tips & Common Issues

I placed probes the same as cooling mode and got strange readings

The most common mistake. In heating mode, the large vapor line is hot (discharge gas), not cool (suction gas). If your "suction line" reading is 130F+, you have the probe on the vapor line and assigned it to the wrong channel. Reassign the channel or move the probe to the actual suction line at the outdoor unit.

Superheat reads extremely high or negative

Check that the suction line probe is on the correct pipe. In heating mode, suction pressure and suction temperature must come from the same side of the system (outdoor). If pressure is from the outdoor service valve but temperature is from the indoor coil outlet, the readings are mismatched and the calculation is meaningless.

The outdoor coil is frosted - is that normal?

In heating mode, the outdoor coil is the evaporator. It absorbs heat from outdoor air and can operate below freezing. Light frost buildup is normal, especially in humid conditions below 40F. The system runs defrost cycles to clear the frost. Heavy ice buildup (solid sheet of ice across the coil) indicates a defrost control problem, low charge, or restricted airflow across the outdoor coil. Do not confuse normal frost with a system fault.

When should I use the Hold button?

On split systems, you often cannot receive Bluetooth signals from both indoor and outdoor probes simultaneously. Capture readings at one unit, tap Hold, walk to the other unit, and capture those readings. This is especially common in heating mode because you need the suction line reading at the outdoor unit and the vapor line reading at the indoor unit.

Can I commission a heat pump in heating mode?

Heat pump commissioning is most accurate in cooling mode, where outdoor ambient conditions are more predictable and measurement standards are better defined. Heating mode testing is valid for diagnostics (identifying faults, checking charge indirectly, verifying airflow), but the industry standard commissioning procedure per ACCA Standard 5 and ASHRAE 37 is performed in cooling mode. Jim Bergmann covers this in detail in the video "You Can't Commission A Heat Pump In Heating Mode - Here's Why."

Reference Material

Download: Tests and Probes Quick Reference (PDF)

Related Articles

Prerequisites (complete these first):

• Outdoor Probe Placement
• Indoor Probe Placement: Air Handler

Follow-up articles (next steps after this one):

• Holding Measurements
• Superheat & Subcooling
• A/C Service Workflow (cooling mode commissioning)

Related in the same domain:

• Capturing Measurements

Related in other domains:

• Bluetooth Pairing Basics
• AI System Profiler (setting up heat pump profile)
• Refrigeration Cycle Basics

Need Help?

If you get stuck or this article does not answer your question:

• Check the Related Articles section above
• Watch the video walkthroughs linked above for visual guidance on probe placement
• Contact measureQuick support: support@measurequick.com