Are you talking about pipes under a slab here? If so you probably have a hot water line under the slab leaking that is heating the water in the cold line.
Tub, shower, toilet, hot water tank, urinal, sink, lavatory, dishwasher
Hot water tank, hose bibb, kitchen sink, toilet, bathroom vanity sink and a tub/shower.
Supply lines- Hot and cold water for each sink, bathtub, Jacuzzi tub, and shower. Cold water for each toilet. Drain lines- one for each sink, bathtub, Jacuzzi tub, shower, and toilet.
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There are temperature balanced shower bodies that will adjust the hot water flow when it senses that the cold water flow drops due to flushing the toilet.
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Turn on your bathtub or shower hot then turn it off at last turn on your hot water for your sink. Hope this helps!
The hot water under the sink is typically on the left side.
actually, hot water rises.
Replace the faucet.
Guess - unlagged hot pipes under the floor. Or under-floor heating!!