Providing answers to your plumbing, piping, fire protection and hydronics questions.

Gas Piping Above the Ceiling

Chris Abruzzo, KP Professional Engineering, P.C., writes:
I am a Plumbing Engineer in Long Island, NY, and I have a question regarding gas piping above the ceiling. When is gas piping allowed above the ceiling? Does it matter if it is a drop ceiling or a gyp ceiling? Are the requirements for the gas piping above a ceiling affected by the HVAC hard duct return or a return air plenum?

Julius Ballanco, P.E., CPD, F-ASPE, responds:
A gas pipe can be located above the ceiling. There is really no restriction on the location of the gas piping in most buildings. The limitation is on the connections. For example, a union cannot be located above a ceiling. All unions must be accessible. The same would be true for a gas valve. All gas valves must be accessible.

Some codes, specifically the ICC International Fuel Gas Code, prohibits gas piping from running between units in a townhouse. This limitation would prohibit gas piping to run through a townhouse unit to the adjacent unit.

With regard to a return air ceiling plenum, gas piping is permitted in the plenum, but fuel-fired appliances are not permitted. Some codes restrict gas valves from being located in a plenum as well.

Water Hammer

Larry Fox, CPD, Durkin & Villalta Partners Engineering, writes:
I have had a water hammer problem in the domestic water piping at an eight-story bank building that I designed six years ago. This has been an ongoing problem since day one and I have been unsuccessful in pinpointing the source. I have done everything I know to eliminate the problem but without any success. The water hammer is very severe and only occurs at the top level of the building where the make-up water to the cooling tower is located.  The constant speed booster pump is located in the basement of the bank.  I really could use some help on this one. I am afraid I will get the call one day from the owner saying the piping burst and flooded the upper floors of the bank!

Julius Ballanco responds:
When trying to determine the source of hydraulic shock (water hammer), the first thing you look for is a location where there is a quick change in velocity. It is often assumed that hydraulic shock occurs when a valve or fixture closes instantly. While this will cause a hydraulic shock wave, any time the velocity is lowered will result in hydraulic shock.

For example, if water is flowing in a pipe at 12 feet per second, and suddenly slows to 2 feet per second, you have the same shock wave as if water was flowing at 10 feet per second and instantly stops. The shock wave intensity is based on the change in velocity, not the stopping of flow.

A shock wave occurs anytime you change the velocity; it is simply a matter of intensity which is based on initial pressure, material modulus of elasticity, amount of change in velocity and time to change the velocity.

The key to eliminating the hammering sound is to lower the intensity. This can be accomplished by slowing the rate of change in velocity, lowering the initial velocity (increasing pipe size), changing the piping material or installing a water hammer arrestor. However, in order to install a water hammer arrestor, you need to find the source of the intensity. The water hammer arrestor is only effective when installed within 20 feet of the source of the intensity.

Accessible Stops

Ramdeo Maraj, Trinidad, West Indies, writes:
In Arthur A Bell, Jr.’s book HVAC - Equations, Data, and Rules of Thumb, page 748 Section 49.03, Plumbing Note R says, “Provide all plumbing fixtures and equipment with accessible stops.” What exactly are accessible stops? Is there a specification for these stops? Who are some of the manufacturers of these stops?

Julius Ballanco responds:
Actually, the proper term is a “supply stop that is accessible.” The devices are technically called “supply stops.” These are the shut-off valves that are located immediately upstream from the terminal fixture (faucet). A supply stop is provided at the fixture to shut off the water supply when servicing the fixture.

Supply stops are regulated by ASME A112.18.1. There are many manufacturers of these stops. Many advertise their products in this magazine. There are a wide variety of supply stops including rotating, push-pull and quarter-turn valves. 

Since the purpose of the supply stop is to facilitate service and maintenance of the faucet, it must be accessible. The plumbing code defines accessible as being open and available to use, but you first may have to open a door or access panel. The other term used in the plumbing codes is readily accessible. The difference between the two terms is that readily accessible does not allow a door or access panel to be installed.

An example of this terminology is that a lavatory faucet is readily accessible; the supply stops under the lavatory faucet are accessible. (You have to open the vanity cabinet to gain access.)