Final answer:
Many natural gas heating units do operate at a manifold pressure of 3.5 inches of water column, which is a common standard for these appliances in the U.S.
Step-by-step explanation:
Many natural gases fired heating units operate at a manifold pressure of 3.5" WC which is considered standard for natural gas appliances. This value represents the pressure needed to push the gas from the manifold into the burners. The "WC" stands for inches of water column, a unit of pressure commonly used in the United States to measure the pressure of natural gas. A pressure of 3.5" WC is equivalent to about 0.125 psi, and it provides the correct flow rate for the combustion process in most residential and commercial natural gas heaters and furnaces.