Gas Pressure Set-Up Numbers Field Card

Why this matters

Manifold pressure is the number that decides how much gas the burners actually get, and it is the reading most often skipped on a furnace start-up because the furnace lights either way. A furnace set 0.5 in w.c. low on natural gas under-fires, runs a low temperature rise, and can condense in a vent that was never meant to see condensate. One set high over-fires, runs hot, cycles on the limit, and pushes combustion toward carbon monoxide. Neither shows at the thermostat. This card owns the inlet and manifold pressure values for the shelf: what they typically are, where you read them, and why the inlet has to be read with the house's gas load running.

Safety first. If you smell gas at any point, stop, get everyone out of the house without operating switches or using a phone inside, and call the gas utility from outside. Every tap you open gets its plug or screw reinstalled and leak-tested with a bubble solution while the valve is pressurized, before you leave the appliance. Wear a personal CO monitor while you work on a firing appliance, and read pressures with the burner compartment door handled as the maker directs, since some furnaces must have it in place to run correctly.

The set-up sheet

This is the shape a set-up record takes for a two-stage natural gas furnace. The rating-plate values are typical of residential natural gas equipment and are shown to teach the reading; the job uses the values on the furnace's own rating plate and in its installation instructions.

GAS PRESSURE SET-UP SHEET  two-stage furnace, NG
Units: inches water column (in w.c.)

FROM RATING PLATE
  Inlet minimum ............... 4.5
  Inlet maximum ............... 10.5
  Manifold, high fire ......... 3.5
  Manifold, low fire .......... 1.7

MEASURED
  1 Inlet static, all gas off . 7.0
  2 Inlet under load .......... 6.3
    (furnace high fire, water
     heater and range on)
  3 Manifold high, as found ... 3.2
    Manifold high, as left .... 3.5
  4 Manifold low, as found .... 1.7
    Manifold low, as left ..... 1.7

Tap plugs reinstalled, bubble
tested at operating pressure: YES

The typical numbers to recognize, always confirmed on the plate:

Two units of pressure appear in gas work and get confused. Inches of water column measure low pressure at the appliance; pounds per square inch measure the service and propane tank side. One psi is about 27.7 in w.c., so a 7 in w.c. natural gas supply is about a quarter psi.

Where the taps are

On a residential combination gas valve, two pressure taps matter:

• The inlet tap, upstream of the valve's regulator, usually marked INLET or IN. It reads what the house piping delivers to the furnace.
• The outlet or manifold tap, downstream of the regulator, marked OUTLET or OUT. It reads what the valve delivers to the burner manifold.

The tap is either a 1/8 inch NPT plug you remove and replace with a barbed fitting, or a test boss with a screw you back out a turn or so and slip the manometer hose over. Shut the gas cock before opening a plugged tap; a test-boss screw can be opened under pressure as the valve maker describes.

A reference port changes the reading on many condensing furnaces. The valve's regulator may be referenced to the pressure inside the sealed burner box rather than to the room. On those furnaces the maker requires manifold pressure to be read as a differential, with the manometer's other port connected to the burner box reference tube. Read it against the room instead and the number is off by whatever pressure the inducer is pulling in the burner box, and the "correction" you make will mis-fire the furnace. The instructions say when this applies and how to tee in.

Why inlet is read under full load

A static inlet reading, with every gas appliance in the house off, tells you the service regulator's setting. It tells you nothing about whether the piping can carry the gas. Pipe that is too small for the total load looks perfect at static and sags the moment the furnace, water heater, range and dryer all fire together.

So inlet is read twice: static, then under load, with the furnace at high fire and the other large gas appliances running. The under-load figure must stay at or above the plate's minimum inlet. A drop of a few tenths from static is normal piping friction. A drop that takes the inlet below the plate minimum is a supply problem: undersized pipe, a kinked appliance connector, a partly closed valve, or a meter regulator that cannot keep up.

Why this order matters for the manifold: the valve's regulator can only reduce pressure. If the inlet under load is 4.0 and the manifold wants 3.5, the regulator has 0.5 in w.c. of room to regulate across, and many valves need more than that to hold a steady outlet. Turning the manifold adjustment up on a starved inlet does nothing useful at full load and over-fires the furnace the day the other appliances are off.

Setting the manifold

With inlet confirmed under load, the manifold is set at the adjustment screw under the regulator cap. On most valves clockwise raises the outlet pressure and counterclockwise lowers it. Two-stage valves have a separate high and low adjustment, and the furnace has to be held at the stage you are setting: high fire for the high screw, low fire for the low one. Modulating furnaces have a maker's procedure for forcing minimum and maximum rate, often through the control board.

Adjust to the plate value, within the tolerance the maker allows, and replace the cap. If the manifold cannot reach the plate value with a good inlet, the regulator or valve is the problem, not the setting. Then confirm the firing rate it produces by clocking the meter (See related: How to Clock a Gas Meter to Check Furnace Input) and confirm combustion (See related: Gas Furnace Combustion Analysis and Tuning). The manifold number sets the gas; the clock and the analyzer confirm the gas did what the number promised.

Worked example: a manifold that read low because the house could not feed it

A two-stage natural gas furnace, rating plate as on the sheet above (the measured column there is a different job): inlet 4.5 to 10.5, manifold high 3.5, low 1.7. Callback complaint: house slow to recover on cold mornings.

Inlet static. Gas cock open, every gas appliance off, manometer on the inlet tap: 7.0 in w.c. Normal.

Manifold, as found, with only the furnace firing. High fire 3.5, low fire 1.7. Both on plate. The furnace looks perfectly set up, and on the original start-up it was.

Inlet under load. The tech fires the furnace at high fire, then has the homeowner start the tank water heater with a hot tap open and light all four range burners. A tankless water heater added last year also fires on the hot tap. Inlet falls from 7.0 to 4.1 in w.c.: 7.0 - 4.1 = 2.9 in w.c. of drop, and 4.1 is below the 4.5 plate minimum.

Manifold at the same moment. High fire reads 3.0 against the 3.5 set point. The regulator cannot hold 3.5 when it has only 4.1 coming in.

What that means. The furnace is under-firing exactly when the house demands the most gas, which is the cold morning when everyone showers. Natural gas flow through a fixed orifice goes roughly with the square root of the pressure across it, so 3.0 in w.c. instead of 3.5 cuts firing rate by a factor of about the square root of 3.0 / 3.5, which is the square root of 0.857, about 0.93. That is roughly 7 percent less input on high fire during the overlap, which fits a slow-recovery complaint.

The wrong fix. Turning the high-fire screw up until the manifold reads 3.5 during the overlap. The rest of the day, with the inlet back at 7.0, the furnace would be over-firing.

The right call. The gas supply cannot carry the house load since the tankless heater was added; that heater's input is large, and the existing pipe was sized before it existed. Gas piping sizing and alteration go to the licensed gas fitter or plumber, where your state or locality licenses that work, and the meter regulator question goes to the utility. The furnace stays at 3.5 high and 1.7 low, the under-load readings go on the ticket, and the customer gets a written explanation of why the furnace was not the problem.

What changes on propane

The sequence is identical; the numbers and the failure points move. Propane arrives through two regulators, a first stage at the tank and a second stage near the house, and the second stage sets the inlet the furnace sees, commonly around 11 in w.c. Inlet under load on propane sags for different reasons: a tank low on liquid on a cold day cannot vaporize fast enough, and a frozen or undersized second-stage regulator starves every appliance at once. The manifold commonly sits near 10 in w.c. and the valve needs the propane spring or conversion parts from the maker's kit to reach it. A natural gas valve turned all the way up does not make a propane setting, and an LP furnace reading 3.5 on the manifold was converted wrong or not at all. See related: How to Convert a Furnace From Natural Gas to Propane.

High altitude

Air thins with elevation, so a furnace burning the sea-level gas rate at altitude runs rich. Makers derate by an orifice change, a manifold reduction, or both, according to their high-altitude table, and that table governs. The commonly cited general guidance in the National Fuel Gas Code (NFPA 54, as adopted in your jurisdiction) is a derate of 4 percent per 1,000 feet above sea level for appliances above 2,000 feet unless the appliance listing says otherwise. Many makers' instructions differ from that, which is why the listing comes first. Some utilities at altitude also deliver gas with a lower heating value, which is part of why makers handle it in a table rather than a formula.

References

• Furnace rating plate and installation instructions: inlet range, manifold settings, reference-port instructions, high-altitude table
• NFPA 54 National Fuel Gas Code, as adopted by your jurisdiction, current edition
• See related: Furnace Rating Plate Read Before Start-Up; How to Set Gas Manifold Pressure on a Furnace
• See related: How to Clock a Gas Meter to Check Furnace Input; Gas Furnace Combustion Analysis and Tuning