I'm trying to figure out what might be wrong with my water heater. We had a new 40 gallon tank installed in August 2006. For the last 6 months or so, we've noticed that when we run the water for a bath to bathe our kids, we have to turn off the cold water before the tub is filled because the water coming from the faucet has cooled off. It starts out hot, then after about 5 minutes it cools off, so eventually just the hot is on but lukewarm water is coming out. This is the case whether we've done laundry all day or we've been out of the house all day using no hot water. We had the utility company (who installed it) come out once & they assured us the tank is working fine. It's a gas fired water tank. We certainly aren't getting 40 gallons of hot water; we're not even getting enough to fill a tub. We have the temp setting on "A", about 120 degrees. What's going on here?