Manometer Gauge: Reading Radon, HVAC and Gas Pressure

We may earn commission from links on this page. Lead-form submissions are forwarded to local inspector partners. How we research and review.

Most homeowners meet a manometer gauge without knowing its name. It is the small U-shaped tube of colored liquid clipped to the white PVC pipe of a radon mitigation system, usually in a basement, garage or utility room. The same kind of instrument, often in digital form, is used by HVAC technicians to check duct pressure, by gas fitters to test appliance and line pressure, and by energy auditors running a blower-door test. In every case it does one job: it compares two pressures and shows the difference, usually in very small units. This guide explains how liquid and digital manometers work, the units they report, how to read the gauge on a radon system, what a level or “zero” reading usually means, when to call the installer, and the other places the tool shows up in a house. It is general guidance for Colorado Front Range homes based on EPA and CDC material current as of 2026; radon systems and gas equipment should be serviced by qualified professionals, and a retest is the only way to confirm radon levels.

Part of our Home Inspection Tools guide — start there for the full picture →

What a Manometer Gauge Measures

A manometer measures pressure difference. One side of the instrument connects to the space being measured, such as the inside of a vent pipe or a duct, and the other side is open to a reference, usually the room air. The reading shows how much higher or lower the measured pressure is compared with the reference.

Units: inches of water column and pascals

The pressures involved in homes are tiny compared with tire or water-line pressure, so manometers use small units:

• Inches of water column (in. w.c. or “WC): the height difference, in inches, that a given pressure would push a column of water. This unit is common on U-tube radon gauges, gas appliance specifications and HVAC equipment data in the United States.
• Pascals (Pa): the metric unit of pressure. One inch of water column is roughly 249 pascals. Blower-door testing and many digital manometers use pascals because building pressure differences are often only a few pascals to a few dozen.
• Other units such as millibars, inches of mercury or psi appear on some instruments, but for household diagnostics, in. w.c. and Pa are the ones most people encounter.

Because these are pressure differences, the sign matters. A negative reading means the measured space is at lower pressure than the reference, which is what a radon fan is supposed to create under a slab. A positive reading means higher pressure, which is normal in a supply duct downstream of a furnace blower.

U-Tube Versus Digital Manometers

Both styles answer the same question. They differ in precision, convenience and cost.

How a liquid U-tube works

A U-tube manometer is a clear tube bent into a U and partly filled with colored liquid. With equal pressure on both ends, the liquid sits level in both legs. When one leg connects to a space at lower pressure, the liquid is drawn up on that side and pushed down on the other. The difference in height between the two liquid surfaces is the pressure difference, read against a printed scale. Radon systems use small, inexpensive U-tubes because they need no power, have no electronics to fail, and can be read at a glance by anyone walking past.

U-tubes have limits. They are not very precise at very small differences, they must hang plumb to read correctly, the liquid can evaporate or discolor over years, and a loose or cracked tube can give a false reading.

How a digital manometer works

A digital manometer uses an electronic pressure sensor and shows the reading on a screen. Many have two ports so they can read the difference between any two points, switch units, hold readings, average fluctuating values, and log data. HVAC technicians, gas fitters and energy auditors rely on digital models because they can resolve small differences precisely and record results. Some radon systems use an electronic pressure switch or alarm instead of, or in addition to, a U-tube; it sounds or lights when suction drops below a set point.

Digital tools require batteries, periodic calibration checks and a little training to set up correctly. For a homeowner who only wants to glance at a radon system, the U-tube is usually all that is needed. The inspection tools hub covers how manometers fit alongside other diagnostic instruments, and the home inspection tools pillar compares tool categories more broadly.

The Manometer on a Radon Mitigation System

The most common household manometer sits on an active soil depressurization system. CDC describes this as the most-used radon reduction method: a vent pipe system and fan that pull radon from beneath the house and vent it outside (CDC, Reducing Radon Levels in Your Home). The fan creates suction under the slab, and the manometer, connected to a small hole in the vent pipe below the fan, shows whether that suction is present.

EPA’s Citizen’s Guide notes that most soil suction radon reduction systems include a monitor that will indicate whether the system is operating properly (EPA, A Citizen’s Guide to Radon). On most residential systems that monitor is the U-tube manometer. For how the whole system is laid out, see the guide to a sub-slab radon mitigation system.

What the liquid levels mean

When the fan is running and the system is sealed, the pipe below the fan is under negative pressure. That pulls the liquid up on the pipe side of the U-tube and down on the room side. The two liquid surfaces end up at different heights, and the gap is the suction reading. Many installers mark the reading on the tube or on a label at commissioning so later readings can be compared.

There is no single “correct” number. Each system’s reading depends on the fan model, pipe size and length, how readily air moves through the soil or gravel under the slab, and how well the slab and sump are sealed. A tight clay soil may give a higher reading than a gravel base, and both can be working well. What matters most is that the reading is clearly not level and that it stays fairly close to the value recorded when the system was installed.

What a level or “zero” reading means

If the liquid in both legs is level, the gauge is showing no pressure difference. On an active radon system that almost always means the fan is not moving air. Common causes include:

• The fan is switched off, unplugged, or on a tripped breaker or GFCI outlet.
• The fan motor has failed or seized.
• The pipe has disconnected, cracked or come apart at a coupling, so the fan is pulling room air instead of soil gas.
• The manometer tubing has slipped off its port or cracked.

A reading that has risen sharply compared with the installation mark can also signal a problem, such as a blockage from ice, debris or water in the pipe, or a change below the slab. A reading that has dropped a lot, without going all the way to zero, may point to a leak in the pipe or a weakening fan.

Reading table for a radon system manometer

Seasonal swings are normal to a degree. Frozen ground, very wet soil and heavy snow can change airflow under the slab, so a reading may move a little through the year. Large, sudden changes are the ones worth acting on.

When to call the installer

A manometer only confirms that the fan is producing suction; it does not measure radon. CDC advises testing the home again after installing a radon reduction system to make sure it is working, considering a retest every two years, and retesting after any remodeling (CDC). Call the installer or another qualified radon professional when:

• The gauge reads level and power to the fan is confirmed.
• The reading has moved far from the commissioning mark and stays there.
• The fan is noisy, vibrating or cycling, or there are signs of water in the pipe.
• A follow-up radon test comes back elevated, even if the gauge looks normal.
• Remodeling, a new sump pit, or slab work has opened new paths for soil gas.

Fan replacement and system adjustments are covered in the guide to radon fan installation, and typical repair and system pricing is covered under radon mitigation cost. If a sump pit is part of the system, a loose or missing lid can weaken suction; the guide to a radon sump pump cover explains what a proper seal looks like.

Why it matters on the Front Range

Radon is common in many Colorado homes, and basements are often where levels are highest; the guide to radon levels in a basement explains why. CDC calls radon the second leading cause of lung cancer deaths in the United States after cigarette smoking (CDC, About Radon), and EPA and CDC both recommend fixing homes at or above 4 pCi/L. A fan that quietly stopped months ago leaves a household believing it is protected when it may not be. A five-second glance at the gauge, every time someone passes it, is one of the cheapest safety checks in a house.

Checking the gauge when buying or selling

Many Front Range home sales involve a house that already has a radon system. During a showing or inspection, a quick look at the gauge tells a buyer whether the fan is running at that moment. It does not reveal how long the system has been working, whether the fan is near the end of its life, or what the current radon level is. Useful questions for the seller include when the system was installed, who installed it, whether there is a commissioning reading on the label, and when the most recent post-mitigation radon test was done. If no recent test exists, a new test during the inspection period gives a current answer. Sellers can help by keeping the installer’s paperwork and the latest test result with the house documents, and by making sure the gauge is visible and not blocked by stored items.

Other Uses: HVAC, Gas and Blower-Door Testing

Outside radon systems, manometers are mostly professional instruments. It still helps to know what a technician is measuring and why.

HVAC static pressure

HVAC technicians use a digital manometer to measure static pressure in the ductwork, usually by inserting probes into small test holes on the supply and return sides of the air handler. The combined reading, called total external static pressure, is compared with the equipment maker’s rating. Readings that are too high often point to restrictive ductwork, dirty filters, closed or blocked registers, or an undersized return. That can reduce airflow, comfort and equipment life. Homeowners do not need to take these readings, but asking a technician whether static pressure was measured is a reasonable question during a service call or before a system replacement.

Gas pressure: professionals only

Gas appliances are designed to run at specific inlet and manifold pressures, which are usually stated on the appliance rating plate in inches of water column. Gas fitters and HVAC technicians use manometers to confirm supply pressure and to adjust appliance regulators, and pressure tests are used to check new or modified gas piping for leaks. This is work for licensed professionals. Opening test ports on gas valves or piping without training can release gas and create a fire or explosion hazard.

For homeowners, the relevant tools are detection rather than measurement. A combustible gas detector can help locate a suspected leak at fittings, as described in the guide to a gas leak detector. If there is a smell of gas, leave the house and call the gas utility or 911 from outside rather than testing anything.

Combustion and draft

Technicians also use manometers to check draft in a flue, confirming that a water heater or furnace is pulling exhaust up the vent rather than spilling it into the room. This pairs with a combustion analyzer, which measures flue gases such as carbon monoxide and oxygen; the guide to a combustion analyzer explains that tool. ENERGY STAR notes that sealing a home or its ductwork can, in some situations, cause combustion appliances like water heaters to backdraft, and that inspection and testing by a home energy professional can identify combustion safety issues to fix before air sealing (ENERGY STAR).

Blower-door testing

A blower-door test uses a calibrated fan mounted in an exterior doorway to depressurize the house, while a digital manometer measures the pressure difference between inside and outside and the airflow through the fan. The result shows how leaky the building envelope is, and with the house under pressure, auditors can find specific leaks with smoke or a thermal camera. The guide to the blower door test walks through what happens during one and how results are reported. Manometers are also used during these tests to check that the radon system and other exhaust appliances behave as expected when the house is depressurized.

Reading and Caring for a Manometer

A gauge is only useful if it is trustworthy. A few habits keep it that way.

Reading a U-tube correctly

• Look at the liquid at eye level. Viewing from above or below can shift the apparent height.
• Read the bottom of the curved liquid surface (the meniscus) on each side, or follow the scale instructions printed on the gauge.
• Most radon gauges have a sliding scale or a zero marker. The reading is the total height difference between the two sides.
• Note the reading and date on the label or in a phone note. A simple log makes changes easy to spot.

Simple care

• Keep the gauge plumb and secured to the pipe. A tilted tube reads incorrectly.
• Check that the small tubing to the pipe port is attached and not kinked or cracked.
• If the liquid has faded or dropped noticeably, ask the installer about refilling or replacing the gauge rather than adding an unknown liquid.
• Do not drill extra holes in the vent pipe or disconnect tubing to “test” it.
• Keep stored items away from the gauge so it is not bumped or hidden.

Digital meters

For professionals and serious DIY users with a digital manometer, zeroing the instrument before each use, keeping both ports clean, checking the battery, and having calibration verified on the maker’s schedule all help keep readings dependable. Hose length and kinks can slow the response, so many users wait a few seconds for the reading to settle and use averaging in gusty or fluctuating conditions.

Choosing a Manometer Gauge for Home Use

Most homeowners do not need to buy one. The radon gauge comes with the system, and HVAC or gas measurements belong to technicians. The exceptions are owners of older radon systems with a damaged gauge, people doing a lot of their own building science work, and home inspectors.

• Replacement radon gauge: ask the installer for the model that matches the system, or a qualified radon professional for a compatible U-tube or electronic alarm. A gauge with a clear, adjustable scale makes comparisons easier.
• Digital manometer for diagnostics: look for dual ports, selectable units including in. w.c. and Pa, a hold function, adequate resolution for low-pressure work, and replaceable hoses.
• For inspectors: a digital manometer can document radon system suction and help with draft and building pressure observations, but readings should be reported with their limits and with a recommendation for retesting radon when appropriate.

Buying a better instrument does not change the bottom line on radon. The gauge shows the fan is working; only a radon test shows the air is safe. EPA’s Citizen’s Guide also recommends retesting every two years to be sure radon levels remain low (EPA).

References

• A Citizen’s Guide to Radon (PDF) — U.S. Environmental Protection Agency
• Reducing Radon Levels in Your Home — Centers for Disease Control and Prevention
• About Radon — Centers for Disease Control and Prevention
• Identify Problems You Want to Fix — ENERGY STAR, U.S. Environmental Protection Agency

Radon gauge sitting level, or a reading that looks very different from the installer’s mark? Send us a note and we can point you to a qualified Front Range radon professional to check the system.