I have a utility sink in the garage that has a hot and cold water valve - the faucet of this is connected to a spray nozzle. So, when the valve(s) are turned on , no water runs, until the spray nozzle is depressed. Once depressed, water comes out, and the temperature of the water is adjusted by using each of the cold and hot water valves. Everything works fine. Here is the problem... If the hot water valve is open on this utility sink (and water is NOT running), I will not get hot water (or at least, only a little warm water) from any other faucet in my house. If I go out the the garage and turn off the hot water valve in this utility sink, then the hot water runs fine in the other faucets. What could be causing this issue?

You need check valves on the supply piping at the utility sink(or shut the valves off when not in use). Cold water is entering the hot water piping through the utility sink faucet and the cold water is tempering the hot water in the rest of the house.

If you have a faucet and supply hoses like a kitchen or bathroom sink normally have, these check valves would work.
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Edited 1 times.

ditto

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