Why does it take so much longer for hot water to enter a faucet than cold, vice versa?
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The faucet and piles leading up to it are already filled with room temperature water at all times, and the metal itself is cool too. Hot water has to displace all the cool water first and also warm up the faucet to come out at its full temperature.
Depending on the system installed, the hot water is held in a tank somewhere else. In a standard American home, normally this is in the garage.
The hot water pipe from the faucet is connected to the tank, but the pipe runs a distance from the main hot water tank. The water is just sitting in the pipe when not in use, and loses temperature to the air. The time between the tap opening and hot water flowing from it is the time it takes for the water in the pipe between the water heater and the tap to flow out and be replaced with hot water.
There are newer systems to prevent this like systems that keep the water constantly recirculating in the pipes but there are downsides that there is now a constant flow of warm water warming the house. Though they can be insulated.
Cold water is generally the same temperature throughout the pipes, it’s not cooled at any point.
Hot water needs to travel from the water heater to the faucet it’s coming out of. In the process of that, all the “room temperature” water is being expelled from the pipe. If the faucet has been used for hot recently it’ll happen faster.
Cold and hot come from two different pipes, so with hot your basically waiting for he temperature to equalize across the pipe.
The “time” you’re measuring is the distance the hot water has to travel from the hot water tank to the faucet. If you go to a bathroom closer to the hot water tank, you’ll notice the time it takes for the water to get hot is less.
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