My home is built on a slab foundation. When I turn on the hot water valve I get no hot water. Instead it backs up from the floor drain under my washing machine. One person told me I had to re-plumb my home. This doesn't make sense to me. I can use cold water all day long with no problem in shower, toilet, washing machine, sinks, etc. How would turning on the hot water cause the drain to back up? I am a 69 year old single woman and don't know if I am getting the correct information. A total re-plumb is very costly.
One way this could be occurring is if the PT valve on the hot water heater is open and if the water through it is directed into your santitary sewer. If this is the case, it will be very easy to fix.
You will feel and hear water flowing through this overflow; it comes out of the side of your hot water tank. I assume you have turned off the cold water supply valve in the cold water supply to the water heater. Turn on this valve and listen and feel the overflow line. Turn it off quickly so you don't get water backing up in your floor drain. If the PT relief valve will not stay closed, you will have to call a plumber. There is a toggle handle on the PT relief valve. Cycle it and see if it will close.