Water heater building codes?

My boyfriend and I recently moved in together. We rented a townhouse duplex that seemed perfect. It is older, Built in 1987. Our landlords are our neighbors and live in the connected unit. We have lived here 4 months and everything has been going great. Recently I clogged the garbage disposal and went next door to ask my landlord where the water shutoff was. She got very defensive and told me that I did not need to turn the water off. After the sink was fixed my Boyfriend and I realized that we have not noticed a hot water heater in the house. and We have alot of issues with keeping the shower hot. I am wondering if we share a hot water heater with them... is that possible? Is it against some sort of building code? We have searched the crawl space under the house and the attic... Nothing. If anyone knows anything about building codes please feel free to give some input. Thanks!