Aquarion Water Co. is conducting tests to figure out why a major pipe broke last month, shutting down water or lowering pressure to much of the town for hours. The 20-inch cast-iron main, which ruptured Jan. 14, was installed in the 1920s and is the primary water main for Hull, according to Aquarion vice president John Walsh. While the company at first thought the damage was caused by extreme fluctuating temperatures, the utility decided to conduct a comprehensive study to see “whether a problem exists with only that one piece of main that broke, or with the full length of our 20-inch water main in Nantasket Avenue,” Walsh said in an e-mail. The work includes sandblasting a segment of the broken main to allow for inspection for pitting and cracking, and crushing samples of the pipe to determine its strength, he said. “While testing like this is not common in the water industry, we believe that in this instance it’s necessary so that we can make informed decisions about the infrastructure going forward,” he said.