Water column is a unit of measurement used to express gas pressure in residential and commercial systems. Written as “inches of water column” (abbreviated inWC, in. WC, or just WC), it describes the amount of pressure that would push a column of water up by that many inches. Most homes in North America receive natural gas at roughly 7 to 8 inches of water column, which equals about a quarter of a pound per square inch. It’s a small amount of pressure, but it’s the standard that keeps your furnace, water heater, and stove running safely.
How Water Column Measures Pressure
The concept comes from a simple lab instrument called a manometer: a U-shaped tube partially filled with water. When gas pressure pushes on one side of the tube, the water level rises on the other side. The difference in height between the two sides, measured in inches, is the pressure reading. One inch of water column equals about 0.036 PSI, or 249 pascals. That tiny fraction of a PSI is why the gas industry uses water column instead. Saying “7 inches of water column” is far more precise and practical than saying “0.25 PSI.”
Technicians today mostly use digital manometers rather than water-filled tubes, but the unit of measurement remains the same. You’ll see it on appliance spec sheets, gas valve labels, and building codes.
Standard Gas Pressures in Your Home
Natural gas arrives at your home’s meter at a higher pressure, then a regulator steps it down for safe use indoors. In a typical single-family home, the gas pressure inside the piping is about 8 inches of water column (roughly one-third of a PSI). Once the gas reaches an appliance like a furnace, the inlet pressure usually falls between 5.5 and 10.5 inches of water column for natural gas systems. Appliance controls generally need a minimum of 4 to 6 inches of water column to function properly.
Propane systems run at slightly higher pressures. A propane-fired furnace or boiler typically needs an inlet pressure of 11.3 inches of water column, with a maximum around 14 inches.
Inside the appliance, a gas valve further regulates pressure down to what’s called the manifold pressure. On a two-stage natural gas furnace, for example, high fire is typically set around 3.5 inches of water column and low fire around 2.0 inches. For propane equipment, manifold pressure runs higher: roughly 6.7 to 7.3 inches on low fire and 9.7 to 10.3 inches on high fire.
Why These Numbers Matter for Safety
Residential appliances are engineered to burn gas within a narrow pressure range. The National Fuel Gas Code (NFPA 54) requires overpressure protection devices on any piping system where gas is delivered above 2 PSI but serves appliances rated for 14 inches of water column or less. That 14-inch threshold is essentially the safety ceiling for standard home appliances.
When pressure drops too low, you’ll notice it in your appliances before anything else. Common signs include burners that struggle to stay lit, weak or flickering flames, longer times to heat water or warm a room, and delayed ignition that produces a small “puff” when the burner finally catches. If every gas appliance in your home seems underpowered at the same time, the problem is likely with the supply line, regulator, or meter rather than any individual appliance.
Persistent low pressure can also cause incomplete combustion. This shows up as yellow or orange flames instead of the normal blue, soot buildup around burners and vents, and in serious cases, elevated carbon monoxide levels. A single appliance producing yellow flames with soot is a red flag worth addressing promptly, even if the cause turns out to be a dirty burner rather than a pressure problem.
Pressure Too High vs. Too Low
Excess pressure is less common in residential settings because regulators are specifically designed to prevent it, but it can happen if a regulator fails. Overpressure can damage gas valves, overwhelm appliance controls, and create dangerous conditions. That’s why the building code mandates overpressure protection as a backup.
Low pressure is more frequently encountered and has several possible causes: a partially closed valve, a failing regulator, undersized piping for the distance gas has to travel, or high demand on the utility’s distribution system during cold weather. A single appliance struggling while others work fine usually points to a localized issue like a clogged burner orifice or a failing appliance gas valve rather than a supply-side problem.
How Technicians Check Water Column Pressure
During a furnace tune-up or appliance installation, a technician will connect a manometer to a test port on the gas valve. They measure two readings: the inlet pressure (gas coming into the valve from the supply line) and the manifold pressure (gas leaving the valve and heading to the burners). Both readings are taken in inches of water column and compared against the appliance manufacturer’s specifications, which are printed on the rating plate.
If inlet pressure is low, the issue is upstream, somewhere between the meter and the appliance. If inlet pressure is fine but manifold pressure is off, the gas valve itself likely needs adjustment or replacement. These measurements take only a minute or two with a digital manometer, making them a routine part of any gas appliance service call.
Quick Reference for Common Pressures
- Standard residential supply: approximately 8 inches of water column (0.29 PSI)
- Natural gas appliance inlet range: 5.5 to 10.5 inches of water column
- Propane appliance inlet range: 11.3 to 14 inches of water column
- Minimum for appliance controls: 4 to 6 inches of water column
- Overpressure protection required above: 14 inches of water column for standard appliances
- Conversion: 1 inch of water column = 0.036 PSI = 249 pascals