How to Check Furnace Gas Pressure: Inlet and Manifold with a Manometer

Gas pressure that's too low or too high causes weak heat, ignition problems, sooting, overheating and high CO. Checking it takes a manometer, a few fittings, and about 15 minutes.

Safety: You're opening the gas train. Turn the gas off before connecting or removing fittings, leak check every plug you remove and reinstall, and measure CO and draft after any adjustment. If you smell gas or can't stop a leak, shut the gas off and leave.

What you need

• A digital manometer (or a U-tube) that reads inches of water column (in. w.c.)
• The pressure tap fitting or adapter for the gas valve (some valves have a screw you back out; others need a hose barb in a 1/8" NPT tap)
• The furnace's rating plate and installation manual for the specified pressures
• Leak detection solution

Typical values

These are typical; the rating plate and manual are what count.

Manufacturers commonly allow only small changes from the nominal manifold pressure (often ±0.3 in. w.c.). Bigger changes in input are made with different orifices, not the regulator.

Measuring inlet pressure

1. Turn off the gas at the furnace's manual shutoff.
2. Connect the manometer to the gas valve's inlet pressure tap, or to a fitting at the drip leg.
3. Turn the gas on and run the furnace. Also turn on the other gas appliances on the same line (water heater, range, dryer) to see the pressure under full load.
4. Read the pressure with the burners firing. It must be inside the range on the rating plate.
5. Turn the gas off, remove the manometer, reinstall the plug, and leak check it.

Low inlet pressure points to an undersized gas line, a long run, a failing meter or tank regulator, or too many appliances on one line. That's a supply problem, not something to fix at the furnace regulator. High inlet pressure can damage the gas valve; check the service regulator.

Measuring and setting manifold pressure

1. Turn off the gas and power.
2. Connect the manometer to the outlet (manifold) pressure tap on the gas valve.
3. Restore gas and power and start the furnace. On two-stage furnaces, run it at the stage you're checking.
4. Read the pressure with the burners firing.
5. Adjust if needed: remove the regulator cap and turn the adjustment screw (usually clockwise to increase, counterclockwise to decrease; check the valve's markings). Make small changes and let it settle.
6. Replace the regulator cap before the final reading; some valves read differently with it off.
7. Shut down, remove the manometer, reinstall the plug and leak check.

Clocking the gas meter (natural gas)

Clocking checks the actual input against the rating plate:

1. Turn off every other gas appliance.
2. With the furnace at full fire, time how many seconds the meter takes to pass 1 cubic foot (use the smallest dial).
3. Input (BTU/h) = heating value × 3,600 ÷ seconds per cubic foot. Get the heating value from the gas utility; it's often around 1,000–1,050 BTU per cubic foot.

Example: 1,030 BTU/ft³ and 45 seconds per cubic foot: 1,030 × 3,600 ÷ 45 = 82,400 BTU/h, close to an 80,000 BTU/h input furnace. Measured input must never be higher than the rating plate.

After any change

• Measure temperature rise and confirm it's within the range on the rating plate. Use the furnace airflow from temperature rise calculator.
• Run a combustion test and record CO and O₂ on the combustion analysis log.
• Watch the flame: steady, blue, centered in the heat exchanger openings, with no lifting or rollout.

What the readings point to

Propane conversions

A furnace converted to propane needs the manufacturer's LP kit: a different regulator spring or valve setting and smaller orifices. Running propane through natural gas orifices badly overfires the furnace. After a conversion, mark the furnace with the conversion label, then check inlet and manifold pressure and run a combustion test.

Common mistakes

• Reading pressure without the burners firing. Static (lock-up) pressure is higher than operating pressure.
• Adjusting the regulator to fix a supply problem. If inlet pressure is low, the furnace regulator can't make it up.
• Big regulator changes to fix a wrong input. Change orifices instead.
• Forgetting the regulator cap before the final reading.
• Not leak checking the pressure tap plug afterward. It's the most common leak after a gas pressure check.

FAQ

Sources

• Goodman MVC8 Installation Instructions: gas supply and manifold pressure
• Goodman CPG series: inlet gas pressure table
• Johnson Controls: inlet gas pressure range and pressure ports