Manometer Basics

Gas pressure and draft are measured in inches of water column (often abbreviated “WC, in. WC, or just “w.c.”). This is a small unit — 27.7 inches of water column equals one PSI — chosen specifically because residential gas pressures and draft pressures are small and a small unit gives useful resolution.

You’ll measure two kinds of pressure on a service call: gas-side pressures at the gas valve (inlet and outlet), and air-side pressures at pressure switches and inducer ports (usually slightly negative — representing draft). A digital manometer handles both.

Digital vs. U-tube

U-tube manometers — literally a U-shaped glass or plastic tube half-filled with water — are the original. Pressure on one side pushes water down and up on the other side; you read the height difference. Accurate and requiring no calibration, but slow to set up and awkward to carry.

Digital manometers (Fieldpiece SDMN6, Testo 510, UEi EM200, etc.) have largely replaced U-tubes for field work. They’re fast, auto-zero, have useful features like min/max capture, and most read both positive and negative pressures. The tradeoff is battery dependence and the occasional calibration drift — but for everyday service, digital wins.

Taking a gas pressure reading

Gas inlet / manifold pressure measurement

procedure

1. Turn off the appliance. Locate the pressure taps on the gas valve — typically two screw plugs labeled “inlet” (upstream of the regulator) and “outlet” or “manifold” (downstream).
2. Back out the appropriate plug by one or two turns. A small amount of gas will start to escape — don’t back it all the way out yet.
3. Thread the manometer’s barb-fitting adapter into the tap. Different manufacturers use different thread types; the common ones are 1/8” NPT.
4. Attach the manometer’s hose and turn the meter on. Zero it to atmospheric if needed.
5. Turn the appliance on and call for heat. Watch the pressure during the ignition trial and into steady firing.
6. Record the reading. Compare to spec for the fuel type.
7. When done, turn the appliance off. Disconnect the hose. Re-tighten the tap plug firmly. Leak-check the plug with soap solution.

Inlet sagging under load

One of the most diagnostic gas-pressure measurements is comparing inlet pressure at idle to inlet pressure at firing. A healthy gas supply holds pressure under load. A marginal gas supply sags when the appliance fires — static pressure looks fine, but as soon as gas is flowing, pressure at the inlet drops because something upstream (partial valve, undersized line, meter regulator) can’t keep up with the flow.

Symptom: static 7.0 in. WC, firing 4.2 in. WC. The 2.8 in. WC drop under load is the diagnostic signature. Something upstream is restricting flow.

Taking a draft / pressure switch reading

Pressure switch draft measurement

procedure

1. Locate the pressure switch on the furnace. There will be one or more clear silicone or rubber hoses running from the switch to the inducer or to a sensing port on the heat exchanger.
2. Disconnect the hose at the pressure switch.
3. Attach a T-fitting or the manometer’s probe directly into the hose end.
4. Turn on the appliance; call for heat.
5. Once the inducer is running, read the draft value on the manometer. It should be a negative number (vacuum), typically between −0.5 and −1.5 in. WC depending on the appliance.
6. Compare to the trip spec printed on the pressure switch body (e.g., “0.65 in. WC”). The actual draft must exceed the switch’s spec for the switch to close.
7. Reconnect the hose securely and verify switch operation.

Low-fire manifold on two-stage systems

On two-stage furnaces and boilers, manifold pressure varies between low-fire and high-fire. A complete diagnostic checks both:

• Call for low-fire (often by calling W1 without W2). Measure manifold. Should match the low-fire spec on the data plate (often 1.5–2.0 in. WC on NG).
• Call for high-fire (both W1 and W2). Measure manifold. Should match high-fire spec (~3.5 in. WC on NG).

Drift between the two stages, or one stage not hitting spec, often indicates a pressure-regulation issue inside the gas valve or a failed staging solenoid.

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Check your understanding

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01You measure natural gas inlet pressure: 7.2 in. WC at idle, 3.5 in. WC while the furnace is firing. Is this diagnostic of a problem?

ANo — inlet pressure varies normallyBYes — 3.7 in. WC sag under load means gas supply can't maintain pressure during firing. Investigate upstream restrictions (partially-closed valve, undersized pipe, meter regulator) before further diagnosisCThis is normal for LP systemsDInlet should always match manifold

02A pressure switch is spec'd to close at 0.65 in. WC. You measure actual draft at the sensing port: 0.85 in. WC. The switch isn't closing (continuity stays open). What's your diagnosis?

ADraft is insufficient — need to clear the flueBThe switch itself is failed — draft is above the trip spec but the mechanism isn't responding. Replace the switch.CThe manometer is reading wrongDThe trip spec must be higher than printed

03Why does NG have a lower manifold pressure spec (~3.5 in. WC) than LP (~10 in. WC) on residential equipment?

ARegulatory requirementBNG and LP have different energy densities and flame speeds; the different manifold pressure produces appropriate flame characteristics for each fuel at the same burner geometryCLP is a heavier gasDThe pressures should actually be the same

Before you close the chapter

You should now be able to measure gas inlet and manifold pressure on a residential appliance, measure pressure-switch draft, and interpret readings against fuel-type specs and switch trip points. The next chapter (Ch. 20) covers meter safety and CAT ratings — the often-ignored but genuinely important topic of keeping yourself alive when working on energized equipment.