Why does hot water get colder right before it gets hot?

[MarioD]

Hi,
I'm not sure if this is in the right section but here it goes.
Ok for the sake of this question let's assume that the water hasn't run for a few hours
Whenever I first turn on the hot water, the water first runs either cold or room temperature (depending on the amount of time since the last use of course) then RIGHT before it turns hot it gets a little colder than it had been seconds before, no matter how cold it was running before
Is this a physics question or simply a plumbing question? :uhh:
Thanks

 

[russ_watters]

It's a plumbing question, and it depends on the routing of the piping (ie, through the ground=cold, through your house=cool). For a typical house with a basement, the water at the tap will start at room temperature and the little bit of water that is in the pipes in the basement will be a few degrees cooler.

 

[charng]

I can provide an explanation for why hot water may get colder right before it gets hot. This phenomenon is known as the "cold water sandwich effect" and is commonly observed in residential plumbing systems.

The reason for this effect lies in the design and function of the hot water heater. When hot water is not being used for some time, the water in the hot water heater tank will cool down to match the temperature of the surrounding environment. When hot water is then turned on, the cold water from the main supply is pushed into the hot water heater to refill the tank. This cold water then mixes with the remaining hot water in the tank, resulting in a temporary drop in temperature.

Additionally, the distance between the hot water heater and the faucet can also play a role in the cold water sandwich effect. The longer the distance, the more cold water that can mix with the hot water before reaching the faucet, resulting in a larger drop in temperature.

So, to summarize, the cold water sandwich effect is a result of the mixing of cold water from the main supply with the remaining hot water in the tank, as well as the distance between the hot water heater and the faucet. This phenomenon is not an indication of any issue with the plumbing or the hot water heater, but rather a natural occurrence in residential plumbing systems.