hot water in cold line

Sine the self-install of a short 40 gallon gas water tank.... Everytime I use the cold water in the house, every faucet, I get about 15-20 seconds of hot water. It starts off cold, then warm/hot, then cold. This happens during winter and summer, so I can rule out the furnace warming the pipes.
The copper pipes go directly to the water tank... could I be missing a backflow valve? or something in that effect? The water lines are not touching, nor are they touching the exhaust.
Thanks for any advice.
r3rjr
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r3rjr wrote:

Often, thermal expansion will push hot water out of the tank, into the cold supply pipe. Then, drawing water from the cold pipe for a fixture will introduce that heated water. Most noticeable if the water heater only has a short drop off the main Cold line.
There are special nipples to be installed on the heater inlet/outlet. They are called "Heat Trap" nipples and have a small ball check restrictor built in. Maybe the old tank had them.
Jim
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r3rjr wrote:

Do you have a recirc pump that keeps hot water available for immediate use at faucets far from the heater (upstairs, etc.)? If so it could be pushing hot water through a leaky single stem faucet.
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