What You'll Learn

• How furnace probe placement differs from air handler placement and why those differences matter
• Where to place suction line and liquid line temperature clamps on furnace installations
• Correct supply air and return air temperature probe positioning for accurate temperature rise and split readings
• Where static pressure test ports go on a furnace with an A-coil
• Where to tap for gas manifold pressure measurement
• How the heat exchanger's position in the airflow path changes your measurement locations compared to an air handler

What You'll Need

• Device: iPhone (iOS 15+) or Android phone/tablet (Android 10+) with measureQuick installed
• Account: measureQuick account with probes paired (see Bluetooth Pairing Basics)
• Temperature probes: Pipe clamp temperature probes (2 minimum: suction line, liquid line), air temperature probes (2: supply, return)
• Manometer: Bluetooth-paired manometer for static pressure (Fieldpiece JL3KM2, UEi DPM/SPMKIT, CPS SPM-K1, Testo 510i, or Yellow Jacket YJACK MANO)
• Static pressure tips and tubing: For drilling test ports in the ductwork
• Equipment access: Gas furnace with A-coil or cased coil, running in cooling mode (for refrigerant measurements) or heating mode (for combustion and temperature rise)
• Time: 10 minutes to read; 10-15 minutes to deploy all probes in the field

Furnace vs. Air Handler: Why Placement Differs

If you have read Indoor Probe Placement: Air Handler, you know where probes go on a standard air handler. A furnace changes the layout because of one component: the heat exchanger.

In an air handler, the airflow path is straightforward: return air enters, passes through the blower, passes through the evaporator coil, and exits as supply air.

In an upflow furnace with an A-coil on top, the airflow path is: return air enters, passes through the blower, passes through the heat exchanger, exits the furnace cabinet, enters the A-coil (evaporator), and exits as supply air.

The heat exchanger sits between the blower and the evaporator coil. This changes where you measure static pressure, where supply air temperature is captured, and how you access the gas valve for manifold pressure.

[Diagram] In an air handler, the airflow path is: return air -> filter -> blower -> evaporator coil -> supply plenum. In an upflow furnace, the airflow path is: return air -> filter -> blower -> heat exchanger -> evaporator coil (A-coil on top) -> supply plenum. The key difference is the heat exchanger sitting between the blower and the evaporator coil in a furnace. This changes where supply-side static pressure ports are placed and affects probe access.

Probe-by-Probe Placement

1. Suction Line Temperature Clamp (Indoor)

What it is: The suction line carries low-pressure refrigerant vapor from the evaporator coil back to the outdoor unit. On most furnace installations, the suction line exits the A-coil and runs through or near the furnace cabinet before exiting the building.

Where to place it: Clamp the pipe clamp temperature probe on the suction line as it exits the evaporator coil, before it runs through walls or penetrations to the outdoor unit. Place the clamp 6 or more inches from the coil to avoid reading coil surface temperature rather than refrigerant line temperature.

Furnace-specific note: On upflow furnaces with the A-coil on top, the suction line typically exits from the top of the coil casing and runs down the side of the furnace or through the wall behind it. Access may require removing a panel on the coil casing.

Insulate the probe. The suction line runs cool (typically 45-65F in cooling mode). Ambient air in the furnace closet or utility room influences an uninsulated clamp. Wrap the clamp and the surrounding pipe with insulation tape or foam.

2. Liquid Line Temperature Clamp (Indoor)

What it is: The liquid line carries subcooled liquid refrigerant from the outdoor unit to the metering device (TXV or piston) at the evaporator coil inlet.

Where to place it: Clamp the pipe clamp temperature probe on the liquid line near the evaporator coil inlet, before the metering device. On most A-coil installations, the liquid line enters from the bottom of the coil casing. Place the clamp 6 or more inches before the metering device.

Why this location matters for furnaces: On some furnace installations, the liquid line runs through a hot furnace cabinet (especially on upflow configurations where the furnace generates heat below the coil). In heating mode, the furnace cabinet temperature can warm the liquid line and affect the reading. In cooling mode, the furnace is not firing, so cabinet temperature is not a factor.

Insulate the probe. The liquid line runs warm (typically 80-110F), but insulating prevents ambient interference and is good practice for all pipe clamps.

3. Supply Air Temperature Probe

What it is: Measures the temperature of the air leaving the evaporator coil (cooling mode) or the temperature of the air leaving the furnace and coil assembly (heating mode).

Where to place it: Insert the supply air temperature probe into a supply register, 6 or more inches deep into the register boot. The probe should be in the airstream, not resting against the register frame or the duct wall.

As described in the training walkthrough: "The supply probe needs to go inside the register. This avoids air" mixing from the room that would bias the reading.

Furnace-specific note: On upflow furnaces with an A-coil on top, the supply plenum is above the coil. Supply air has passed through the blower, the heat exchanger (not firing in cooling mode), and the evaporator coil. In heating mode, supply air has passed through the blower and the heat exchanger (firing) and the coil (not active). The supply temperature probe captures the final condition of the air after all components.

Do not place the probe between the furnace and the coil. That location measures air that has passed through the heat exchanger but not yet through the evaporator coil. In cooling mode, this air is warmer than the final supply air. The correct location is after all components, at the supply register.

[Visual Reference] The supply air temperature probe is inserted at least 6 inches deep into a supply register boot so the sensor tip sits fully in the airstream. The probe should not rest against the register frame or the duct wall. Inserting it deep enough prevents room air mixing at the register face from biasing the reading. This location captures the final air temperature after it has passed through the blower, heat exchanger (furnace), and evaporator coil.

4. Return Air Temperature Probe

What it is: Measures the temperature of the air entering the system from the conditioned space.

Where to place it: Lay the return air temperature probe on the return grille. The probe "just simply needs to lay on the grill; it doesn't need to" be pushed deep into the duct. The return grille captures air from the room at room conditions.

Furnace-specific note: On some furnace installations, the return duct connects directly to the bottom of the furnace (upflow) or the side (horizontal). If there is no dedicated return grille accessible from the room, place the probe in the return duct as close to the furnace inlet as practical, upstream of the filter.

The temperature difference between supply and return is the temperature split (cooling mode) or temperature rise (heating mode). Both are key diagnostic values:

• Cooling: Typical split is 14-22F depending on humidity
• Heating: Manufacturer's specified rise (printed on the furnace data plate, typically 15-45F for an 80% furnace, 35-65F for a 90%+ furnace)

[Visual Reference] The return air temperature probe lays flat on the return grille surface. It does not need to be pushed deep into the duct; the grille captures room-condition air entering the system. On furnace installations where no accessible return grille exists (e.g., the return duct connects directly to the bottom or side of the furnace), place the probe in the return duct as close to the furnace inlet as practical, upstream of the filter.

5. Static Pressure Test Ports

Static pressure measurement on a furnace requires two test ports: one on the return side and one on the supply side. The specific locations differ from an air handler because of the heat exchanger.

Return side test port: Between the filter and the blower inlet. This is the same location as on an air handler. Drill a 3/8" hole in the duct or cabinet between the filter rack and the blower compartment.

Supply side test port: After the evaporator coil, in the supply plenum. On an upflow furnace with an A-coil on top, this means above the coil, in the supply plenum or trunk. This captures the total supply-side pressure drop including both the heat exchanger and the evaporator coil.

Key difference from air handler: On an air handler, there is no heat exchanger between the blower and the coil. On a furnace, the heat exchanger adds pressure drop between the blower and the coil. If you drill a test port between the furnace top and the coil bottom, you are measuring the air that has passed through the heat exchanger but not yet through the coil. This is useful for isolating components (see TESP Budget) but is not the correct location for the standard supply-side TESP measurement.

For TESP: return port before blower, supply port after coil.

For component isolation: additional port between furnace outlet and coil inlet.

As described in the gas furnace workflow: "I've got a Fieldpiece manometer down here at the bottom that's measuring my return air static, then I've got one up here that's below the evaporator coil that's measuring my supply-side static."

Diagram of upflow furnace with A-coil showing return and supply static pressure test port locations

6. Gas Manifold Pressure Tap

What it is: The gas valve on the furnace has a pressure tap (1/8" NPT fitting, usually with a plug or cap) that allows you to measure the gas pressure being delivered to the burners.

Where it is: On the gas valve body, accessible from the furnace's burner compartment. Most gas valves have two taps: one for inlet pressure (before the valve) and one for manifold pressure (after the valve). The manifold pressure tap is the one downstream of the valve, measuring the pressure at the burners.

How to connect: Remove the plug from the manifold pressure tap. Connect a rubber or silicone hose from the tap to the negative port on your Bluetooth manometer. The positive port remains open to atmosphere. The manometer reads the differential between manifold pressure and atmospheric pressure.

Expected values:

• Natural gas: 3.5 inWC (standard), check data plate
• LP/propane: 10-11 inWC (standard), check data plate

From the furnace data plate: "the maximum gas supply pressure is seven, the minimum is five, and 3.5" for manifold pressure. Always check the specific furnace's data plate; manufacturer specs take precedence over general guidelines.

Configuration-Specific Notes

Upflow Furnace with A-Coil (Most Common)

This is the standard residential configuration. Airflow goes up: return enters the bottom, passes through the filter, blower pulls air up through the heat exchanger, air exits the furnace cabinet top into the A-coil, and supply exits the coil into the supply plenum.

All placement descriptions above assume this configuration.

Downflow (Counterflow) Furnace

Airflow is reversed. Return enters the top, blower pushes air down through the heat exchanger, and air exits the bottom of the furnace into a coil mounted below. Supply and return static pressure port locations are physically inverted but follow the same principle: return port between filter and blower, supply port after coil.

Horizontal Furnace

Airflow is horizontal. Return enters one side, supply exits the other. The heat exchanger and coil are in sequence horizontally. Test port locations follow the same airflow-path logic, but the physical orientation is different.

Furnace Without a Coil (Heating Only)

If the furnace has no evaporator coil (heating-only system with no central cooling), static pressure is measured across the furnace only: return port before blower, supply port after heat exchanger. No coil drop is included.

Video Walkthrough

• measureQuick Gas Furnace or Boiler Workflow 2022 (31 min): - Complete gas furnace workflow showing probe placement for return/supply static, temperature probes, manifold pressure, and combustion analysis. Covers the full probe deployment on a furnace with A-coil

• Probe Placement (7:43): - Dedicated probe placement walkthrough covering outdoor and indoor positions for both air handlers and furnaces

• Making measureQuick Easy Part 3: New System Commissioning - New system commissioning walkthrough including indoor probe deployment. Covers "suction and liquid line temperature" clamp placement and "the supply probe needs to go inside the register."

• Tuning a Gas Furnace with Testo Smart Probes and the BluFlame Analyzer (19 min, 10K views): - Gas furnace tuning with static pressure, combustion, and temperature measurement. Shows furnace-specific probe placement with Testo Smart Probes

• Fieldpiece Manometers and measureQuick (3:54, 12.7K views): - Fieldpiece manometer setup and pairing with measureQuick, applicable to furnace static pressure measurement

Tips & Common Issues

I cannot access the suction line inside the coil casing

On some A-coil installations, the refrigerant lines are enclosed in a sealed casing with no access panel. In this case, clamp the suction line where it exits the casing and enters the line set running to the outdoor unit. The reading will be slightly different from a measurement at the coil due to heat gain along the exposed pipe, but it is acceptable for field diagnostics if you insulate the clamp thoroughly.

Temperature rise is outside the data plate range

If the measured temperature rise exceeds the maximum on the data plate, airflow is too low. Check the filter, blower speed, and duct system. If temperature rise is below the minimum, airflow is too high or gas pressure is too low. Verify manifold pressure matches the data plate spec before investigating airflow.

Static pressure ports between the furnace and coil

If you drill a test port between the top of the furnace and the bottom of the A-coil, you can isolate the heat exchanger pressure drop from the coil pressure drop. This is useful for the TESP budget (E11) but is not the standard TESP measurement location. For TESP, the supply-side port goes after the coil, not between furnace and coil.

Return probe placement on bottom-return furnaces

Some upflow furnaces have the return duct connecting directly to the bottom of the cabinet, with the filter inside the furnace compartment. In this configuration, place the return air probe at the return grille in the floor or wall, not inside the furnace cabinet. You want to measure room air temperature entering the system, not air temperature after it has been affected by radiant heat from the furnace cabinet.

Gas manifold pressure reads zero or negative

Confirm the manometer hose is connected to the manifold (downstream) tap, not the inlet (upstream) tap. Verify the furnace is firing. If manifold pressure is zero with the furnace calling for heat, the gas valve is not opening - this is a gas valve or control board issue, not a measurement problem.

Heat pump with electric backup in a furnace cabinet

Some systems use a heat pump for primary heating with an electric furnace (strip heat) as backup. These do not have a gas valve or combustion components. Probe placement follows the air handler pattern from F2, not the furnace pattern described here, because there is no heat exchanger in the airflow path - only electric heating elements.

Reference Material

Download: Tests and Probes Quick Reference (PDF)

Related Articles

Prerequisites (complete these first):

• Indoor Probe Placement: Air Handler - General indoor probe placement principles
• Total External Static Pressure - TESP measurement technique and interpretation

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

• Manometer Placement: Furnace - Detailed manometer placement for furnace static pressure
• Combustion Probe Placement - Where to insert the combustion analyzer probe
• Gas Furnace Workflow - Complete guided workflow for gas furnace testing

Related in the same domain:

• Outdoor Probe Placement - Outdoor probe positions for the condensing unit
• Manometer Placement: Air Handler - Manometer placement for air handler comparison
• Static Pressure Measurement - General static pressure measurement technique

Need Help?

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• Check the Related Articles section above
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