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What Is a Draft Gauge?

A draft gauge is an instrument that measures small pressure differences, usually the slight negative pressure that moves combustion exhaust up a flue or chimney. In HVAC work the reading is normally given in inches of water column (inWC). The device is a type of manometer, which is a general instrument for measuring pressure. This guide explains what a draft gauge is, how the underlying principle works, where technicians use it, and what the sources do and do not say about it.

Quick answer

A draft gauge is a manometer that reads the small pressure in a furnace flue or vent, usually in inches of water column (inWC). Technicians use it to check that exhaust is moving in the right direction and to test furnace pressure switches. The sources support those uses. The specific draft values an appliance should show come from its manufacturer.

How we chose these sources

We used manufacturer pages and manufacturer-published articles from Fieldpiece, which makes the SDMN6 manometer discussed here, plus one NIST page on low-pressure measurement. We opened each page and quoted only what it states. Where a source did not cover a point, we say so in “What we could not verify.”

The short answer

Fieldpiece’s article on manometers describes a manometer as “an instrument used to measure and indicate pressure.” A draft gauge is the everyday name for a manometer used to read the pressure in a furnace flue, vent, or combustion area. The sources in this guide describe manometers in general and a dual-port Fieldpiece unit used for furnace work. They do not give a separate standard definition of “draft gauge” as a product category, so the description here follows how the term is used in HVAC practice and in those sources.

Why it is measured in inches of water column

The same Fieldpiece article states that “Inches of water column is the most common form of measurement in the HVAC industry when measuring static or gas pressure.” Draft pressures in a furnace are very small compared with a tire or a water main. Using inches of water column gives a scale that matches the size of the numbers technicians need to read. The article notes that other units are also used worldwide.

The SDMN6 product page lists its pressure units as inWC, mmWC, mbar, and psi. It gives a gas pressure range up to 60 inWC with 0.1 inWC resolution, and a static pressure mode up to 2 inWC with 0.010 inWC resolution. Those figures are specific to that model and show why the unit matters: a static mode with finer resolution suits small air-side readings, while a higher gas range suits fuel supply checks.

How a manometer reads pressure

The simplest manometer is a U-tube. The sources in this guide do not include a diagram, but the principle they describe is consistent with the classic design: a tube bent into a U holds a liquid, usually water. When a pressure is applied to one side, the liquid moves, and the difference in height between the two columns shows the pressure. Height difference in inches of water is the reading.

Modern draft gauges are often digital. They use an electronic pressure sensor and show the value on a display, with zero adjustment and units selection. The Fieldpiece SDMN6 page describes a zero adjust for different ambient pressures, a hold button for freezing a reading, and an auto power off that can be disabled. Digital units still rely on the same idea: a pressure difference is read against a reference, usually the surrounding air.

NIST’s low-pressure page covers primary standards built around liquid-column manometers. It describes NIST’s primary standard covering pressures from 0.01 Pa to 360 kPa, and notes that the instruments depend on accurate measurement of temperature, density, gravity, and column height. The page is about laboratory standards rather than field gauges, but it confirms that liquid-column manometers are the reference method for low pressure.

Single port and dual port readings

The SDMN6 page describes a dual-port design. It shows the gauge reading for each port and continuously displays the difference between them, P1 minus P2, in the lower right corner of the LCD. That difference reading is useful for comparing two points in an air system, for example before and after a filter or across a coil.

A single-port use is a comparison with ambient air. One hose connects to the point of interest, the other side is open to the room, and the gauge reads the pressure relative to the surrounding air. Zeroing the gauge at ambient pressure before the test is a standard step, and the SDMN6 page notes its zero adjust is there for that purpose.

Where a draft gauge is used

The clearest use in the sources is combustion and furnace checks. Fieldpiece’s pressure switch primer explains that the furnace pressure switch “opens and closes to prevent harmful combustion gases from entering the living space.” When the furnace runs correctly, the draft inducer creates enough pressure to close the switch so the furnace can ignite. If the pressure falls outside the manufacturer’s specification, the switch opens and the furnace shuts down.

The primer recommends the SDMN6 manometer with a pressure switch tester for testing a switch in place. The SDMN6 page states the unit can “Test pressure switches” and says it is “Great for diagnosing furnace performance.” A technician can read draft at the flue, check the switch, and then compare readings to the manufacturer’s specification.

The Fieldpiece manometer article also covers balancing and diagnosing furnace and A/C systems more broadly. Static pressure, the resistance to airflow in ductwork and components, is one of the main readings in that work. The article describes static pressure as “the amount of pressure a fan pushes or pulls against.”

Steps for a basic draft reading

The sources support a general order of work. Confirm the appliance is safe to test and that a CO detector is in place, as the primer recommends before starting work on a furnace. Connect the gauge to the test point with the hose in the correct port. Zero the gauge at ambient pressure. Run the appliance to its normal operating state, then read the value. Compare the reading to the manufacturer’s specification, not to a number from a forum or retailer page.

If the reading is outside specification, the primer lists causes to check: a clogged hose port, a flue blockage, corrosion or debris on the switch, a weak spring, or a switch that is old and cycling erratically. Each of those is a physical check that the gauge cannot fix on its own.

Common mistakes

The most common mistake is comparing a reading to the wrong specification. A draft value for one appliance does not apply to another. A second mistake is skipping the zero step. Because draft values are small, a gauge that is not zeroed at the start can make a normal reading look wrong. A third mistake is treating the gauge as a complete combustion test. The SDMN6 page describes pressure, zeroing, and pressure switch functions. It does not describe oxygen or carbon monoxide measurement, so a draft gauge does not replace a combustion analyzer.

What we could not verify

The sources opened for this guide do not confirm the following:

• Acceptable draft values for any specific furnace, boiler, or water heater.
• Draft measurement method in a flue, such as the exact probe or tap location. The Fieldpiece article on manometers does not discuss furnace flue draft, and the primer does not give draft values.
• The accuracy class or certification of any specific draft gauge other than the SDMN6 accuracy figure of ±1.5% FS at 0 to 50°C listed on its page.
• Any CAT rating or electrical safety standard for a specific draft gauge.

For those points, use the appliance installation instructions, the gauge manual, and your local code or licensing body.

Summary

A draft gauge is a manometer used to read small pressures in HVAC and combustion work, most often the slight negative pressure in a furnace flue. Readings are commonly given in inches of water column. Digital and dual-port designs can zero against ambient air and show pressure differences. In furnace service, draft readings help confirm that exhaust is leaving the building, and pressure switch checks confirm that the safety device controlling ignition is working. Specific limits come from the appliance documentation, not from the gauge itself.

Frequently asked questions

Is a draft gauge the same as a manometer?

A draft gauge is a manometer used for small pressures such as furnace flue draft. The Fieldpiece article describes a manometer as “an instrument used to measure and indicate pressure.”

What units does a draft gauge use?

HVAC work usually uses inches of water column. The SDMN6 page also lists mmWC, mbar and psi.

Can it test a furnace pressure switch?

The SDMN6 page says the unit can “Test pressure switches.” For a broader look at meters used around this work, see our guide to HVAC clamp meters and our home-use multimeter guide.

Sources

• Fieldpiece, “How Manometers Help HVACR Professionals Diagnose and Balance Furnace and A/C Systems”, https://www.fieldpiece.com/news-articles/how-manometers-help-hvacr-professionals-diagnose-and-balance-furnace-and-a-c-systems/ (opened 2026-10-28)
• Fieldpiece, SDMN6 Dual Port Manometer with Pressure Switch Tester, https://www.fieldpiece.com/product/sdmn6-dual-port-manometer-and-pressure-switch-tester/ (opened 2026-10-28)
• Fieldpiece, “A Quick Primer on Furnace Pressure Switches”, https://www.fieldpiece.com/news-articles/a-quick-primer-on-furnace-pressure-switches/ (opened 2026-10-28)
• NIST, “Low Pressure Gauges”, https://www.nist.gov/laboratories/tools-instruments/low-pressure-gauges (opened 2026-10-28)