What You'll Learn

• Where to place return air dry bulb and wet bulb probes on the indoor unit
• Where to place supply air dry bulb and wet bulb probes downstream of the evaporator coil
• Why probe location matters for temperature split, airflow verification, and target superheat calculation
• How indoor probes contribute to the Vitals Score probe channel count
• How measureQuick uses return air wet bulb to calculate target superheat on piston systems
• How to avoid common placement mistakes that produce inaccurate readings

What You'll Need

• Device: iPhone (iOS 15+) or Android phone/tablet (Android 10+) with measureQuick installed
• Probes: At least two temperature probes (pipe clamp or air temperature type) for return and supply dry bulb. A psychrometer probe for wet bulb (Fieldpiece JL3RH, UEi WHP1, or Testo 605i).
• Bluetooth: Probes paired and streaming live data (see Bluetooth Pairing Basics)
• Access: Indoor air handler or furnace with return and supply ductwork accessible
• Time: 5 minutes to read; 5-10 minutes to place probes in the field

Step-by-Step Guide

Step 1: Understand What Indoor Probes Measure

Indoor probes capture the air temperatures entering and leaving your air handler or furnace. measureQuick uses these readings to calculate temperature split, verify airflow, and (on piston systems) determine the target superheat.

Four measurement channels are relevant to the indoor unit:

Not every test requires all four channels. For a basic cooling diagnostic, return dry bulb and supply dry bulb are the minimum. Adding wet bulb probes enables target superheat calculation and more complete diagnostics.

Step 2: Place the Return Air Probes

The return air probes measure the air entering the system before any conditioning occurs.

Location: In the return duct or return air plenum, upstream of the air filter.

Placement rules:

1. Insert the probe through the return grille or drill a small access hole in the return duct, close to the air handler. The return air probe can be placed at the face of the return grille (unlike the supply probe, which must go inside the register).
2. Position the sensor tip in the center of the airstream, not against the duct wall. Air temperature near duct walls is affected by conduction from surrounding surfaces.
3. Keep the probe at least 6 inches from the filter face. Too close to the filter can pick up turbulence and temperature stratification.
4. If using a psychrometer for wet bulb, place it in the same general area as the dry bulb probe. The wet bulb reading depends on the same air conditions. A psychrometer probe measures both temperature and relative humidity, then calculates wet bulb.

Field tip - thermostat vs. return air comparison: Compare the thermostat temperature reading to the return air probe reading. A significant difference between the two indicates a potential return air leak from an unconditioned space (attic, crawlspace) or air leakage behind the thermostat through the wiring chase. This is a free diagnostic that takes no additional time.

The app's guidance: When you tap "Deploy Indoor Probes" in the workflow checklist, measureQuick displays an instructional diagram showing probe positions on both furnace and air handler configurations. The diagram labels "Before the coil" and "After filter" positions for the return side.

Deploy Indoor Probes instructional diagram from the workflow, showing furnace and air handler probe positions

Why return air wet bulb matters: For piston (fixed orifice) metering devices, measureQuick calculates target superheat from three inputs: outdoor ambient temperature, return air wet bulb temperature, and AHRI standard conditions. Without a return air wet bulb reading, the app cannot calculate the condition-specific target. It will either use a default value or leave the target blank. See Superheat & Subcooling for full detail on target superheat calculation.

Step 3: Place the Supply Air Probes

The supply air probes measure conditioned air leaving the evaporator coil.

Location: In the supply plenum, downstream of the evaporator coil, before the air reaches any branch ductwork.

Placement rules:

1. Position the probe in the supply plenum as close to the coil as practical, but at least 12-18 inches downstream. Directly at the coil face, air temperature varies significantly across the surface. A short distance downstream allows the air to mix. If the probe is too close to the evaporator, it reads coil surface temperature rather than mixed air temperature, which produces misleadingly low supply readings.
2. Insert the sensor tip into the center of the airstream. Avoid touching the plenum wall or any metal surface.
3. If you cannot access the supply plenum, the supply register closest to the air handler is an acceptable alternative. The supply probe must be inserted INTO the register, not held at the face. At the register face, room air mixes with the supply air (air entrainment), diluting the reading and producing an artificially warm temperature. This is one of the most common placement errors for indoor probes.
4. For supply wet bulb, place the psychrometer in the same supply plenum location as the dry bulb probe.

The app's guidance: The Deploy Indoor Probes diagram shows "After the coil" for the supply side position. It also illustrates that the probe at the supply grille should be inserted into the grille, not held in front of it. Room air mixing at the grille face dilutes the reading.

Deploy Indoor Probes diagram detail showing Probe is IN the Supply Grill with probe inserted into the grille

Step 4: Verify Live Readings in the App

After placing probes, confirm that measureQuick is receiving live data.

1. Check the workflow screen or gauges view. Return air and supply air temperature fields should show live, updating values.
2. Confirm readings are reasonable. In cooling mode, return air dry bulb is typically 70-80F (indoor conditions). Supply air dry bulb is typically 48-62F.
3. If a reading shows "---" or a value that does not change, the probe may have lost its Bluetooth connection or may not be assigned to the correct channel. Open the Probe Manager to verify assignment (see Bluetooth Pairing Basics).

Step 5: Confirm Temperature Split

Temperature split is a quick field check for cooling performance:

Temperature split = Return air dry bulb - Supply air dry bulb

Temperature split alone does not diagnose the problem. It is a screening check. A split outside the typical range tells you something needs further investigation with pressure and additional temperature data.

How Indoor Probes Feed the Vitals Score

The HVAC Vitals Score requires a minimum of 9 physical probe channels for cooling and heating tests, or 7 for gas furnace tests. Indoor probes contribute 3-4 of those channels:

• Return air dry bulb (1 channel)
• Return air wet bulb / psychrometer (1 channel)
• Supply air dry bulb (1 channel)
• Supply air wet bulb (1 channel, when connected)

The remaining channels come from outdoor probes (suction line, liquid line, discharge line, outdoor ambient) and static pressure instruments. If you connect only outdoor probes and skip indoor placement, you will fall short of the 9-channel minimum and the app will not generate a Vitals Score.

measureQuick counts physical probe channels, not calculated values. The probe_count_physical field excludes CALCULATED and WEATHER sources. Only instruments that are physically connected and streaming data count toward the threshold.

Tips & Common Issues

Probe on the outside of the return grille

Placing a temperature probe on the face of the return grille, exposed to the room, measures room temperature, not return air temperature. The probe must be inside the grille or in the return duct. Room air at the grille face mixes with unconditioned air from the surrounding space.

Supply probe too close to the coil

Directly at the coil face, air temperature is not uniform. Some areas of the coil surface are colder than others, especially if the coil is partially frosted or has uneven refrigerant distribution. Move the probe 12-18 inches downstream to allow mixing.

Supply probe at a distant register

The farther the supply register is from the air handler, the more duct gain (warming) affects the reading. A register 30 feet from the air handler in an unconditioned attic can read 5-10F warmer than the actual supply plenum temperature. Use the closest register or, better, access the supply plenum directly.

Wet bulb probe blocked or stagnant

Psychrometer probes need airflow across the sensor to measure wet bulb accurately. If the probe is in a dead-air pocket (behind an obstruction, in a sealed cavity), the humidity reading drifts. Position the psychrometer where it has exposure to the moving airstream.

Return air probe at the register vs. in the duct

The existing Zoho help desk article "Proper Wireless Probe Placement for Accurate Diagnostics in measureQuick" confirms: the return air probe can be placed at the face of the return grille, but the supply probe must go inside the register. This asymmetry exists because the return grille draws air inward (room air is what you want to measure), while the supply grille blows air outward and mixes with room air at the face.

Probes out of Bluetooth range between indoor and outdoor units

On split systems, you may not be able to stand in one spot and receive Bluetooth signals from both indoor and outdoor probes simultaneously. measureQuick has a "Hold" button at the top right of the Indoor Measurements and Outdoor Measurements screens. Tap Hold to lock readings for that page, then walk to the other unit. Held values persist until you release them. Verify accuracy before saving a test snapshot, since held readings do not update with changing conditions.

Confusing connected vs. physical probe count

The app's probe_count_connected includes all channels, including CALCULATED and WEATHER sources. For Vitals Score thresholds and data quality, probe_count_physical is the correct metric. If you see a high connected count but a low physical count, check whether some channels are derived rather than measured.

Video Walkthrough

• Why all 9 probes matter: (1:42, 5.8K views, 152 likes) - Jim Bergmann explains the measurement channels required for a complete diagnostic

• Probe Placement: (7:43) - Dedicated walkthrough covering both indoor and outdoor probe positions

• Full cooling commissioning with probe placement: (80 min, 13K views) - Complete walkthrough including indoor probe deployment. Transcript note: Jim Bergmann specifically addresses supply probe placement being "too close to the evaporator and seeing evaporator coil" as a common error

• New System Commissioning: - Covers probe deployment with the specific guidance: "the supply probe needs to go inside the register" to avoid air entrainment at the face

• AC commissioning workflow: - End-to-end service workflow with probe placement in context (45 min, 15.7K views)

• Static pressure probe deployment: - Detailed walkthrough of indoor static pressure probe placement (39 min, 3.4K views)

Reference Material

Download: Tests and Probes Quick Reference (PDF)

Related Articles

Prerequisites (complete these first):

• Bluetooth Pairing Basics
• Fieldpiece Probe Pairing (if using Fieldpiece JobLink probes)

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

• Outdoor Probe Placement
• Manometer Placement for Static Pressure
• Capturing Measurements
• Superheat & Subcooling (how the app uses return air wet bulb for target superheat)

Related in the same domain:

• Heat Pump Heating Diagnostics (additional indoor probe requirements for heating mode)
• Combustion Probe Placement (indoor probes for gas furnace stack temperature) Related in other domains:
• Smart Tool Overview (compatible probe manufacturers and models)
• TrueFlow Grid Setup (airflow measurement using indoor probe data)
• A/C Service Workflow (indoor probe placement in context of a complete diagnostic)

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
• Contact measureQuick support: support@measurequick.com