Many people who face this kind of price tag wonder if they can get away with patching their pipes. But if the sewer line is old there are almost certainly running toilet other bad spots that will give you problems now or in the near future. A series of patch jobs isn't cheap. In the long run, it just makes more sense to replace the entire line.
When you get to the house stand if front of the toilet and locate the water shut off valve. Should be down below the toilet on the wall. Just turn it off (righty-tighty) completely and flush the toilet. All the water should have left the tank, well at least most of it. If there is remaining water line repair get it out with paper towels or a rag. The important thing is that no more water came in. If it did then your valve is not shut all the way or not working and you need additional help.
Another obvious indicator is if the water you get is close to cold as ice, then your water heating device has already reached its maximum level of service. If it can talk, it might have been shouting at the top of its voice telling you it needs replacement.
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Check that your plug holes and waste outlets are clear of hair and solidified soap also check the "U" bends or traps under your sinks for potential blockages. Occasionally pour boiling water line repair down your sinks and shower wastes not forgetting the washing machine trap. Look outside too for any weeds or leaves that may be blocking gutters, gully's and waste outlets.
When doing the install on a water heater, keep an eye out for unconnected pipes. A commonly missed pipe is the one that leads out from the area for drainage. It is probably a recirculation pipe; that helps your water remain heated without wasting too much water.
The plumber you hire does not normally do heating but will do hot water heating or in floor heating in the house. Some places you can use the water heater for both domestic hot water and for in floor heating if the heating is a secondary heat. Check with your local plumbing codes on this to be clear.