We are in a small house on a slab. The plumbing is in the slab. We have a 40 gallon natural gas hot water heater that is approximately 40 feet from the bath tub. There is another half bath (no tub) on the other side of the wall making it about 45 feet from the heater. We have noticed that every once in a while, hot water is instantly available at the tub when the hot water is turned on. This occurs without having recently run hot water from any of the other faucets. Normally, we have to run the water about 30 seconds to get hot water to the tub. We have found no leaks and have not had any plumbing work in two or three years. It happened this morning, and no one had been up for 8 hours. Can someone suggest an explanation.