Q. Even though I followed the manufacturer's specifications when I installed the 2-gallon thermal expansion tank and check valve for my customers' domestic hot-water system, they still complain of "clicking" noises, mainly during the heating season. Does the location of the expansion tank (3 horizontal feet from the 50-gallon hot-water heater) and check valve (1 1/2 feet from the water heater on the feed side of the water line) have anything to do with this noise?

A. Dave Yates, a plumbing contractor in York, Pa., responds: The placement of the tank and check valve are fine; that ticking sound you hear is coming from thermal stress taking place in the expansion tank as it accepts expanding water.

This problem is likely to be more noticeable during the heating season because that's when there is the greatest difference between inlet and storage temperatures: The greater this difference, the larger the volume that the TXT (a thermal expansion tank rated for 150-psi maximum working pressure and potable water) must accept.

For example, if your customers' incoming water temperature is 40°F (a typical wintertime temperature for most of the country) and the storage temperature is set at 140°F, their 50-gallon water heater will generate as much as .75 gallon of thermal expansion.

A 2-gallon TXT that's been precharged to match an average incoming water pressure of 70 psi would reach a pressure of 112 psi in this case, but pressure spikes in municipal systems and temperature spikes in storage tanks aren't uncommon, so the actual pressure may be even higher (For more on the relationship between temperature and pressure in a domestic hot-water system, see Q&A, 3/06).

To reduce the noise level, you'll need to reduce stress on the tank, its bladder, and the potable water distribution system by increasing the size of the TXT.

Under the same conditions as the example above, the pressure inside a larger 4.5 TXT would be 84 psi, which would allow plenty of extra capacity for spikes in the water system's temperature and pressure.

Spending a few dollars more on the larger expansion tank should solve the problem.