# Expanding your mind with expansion tanks

What law of physics determines that an expansion tank (including all other components in between) 8 inches from a source of expansion (ie water heater) in a closed system and an expansion tank (including all other components in between) 8 feet from a source of expansion in a identical but separate system would be subjected to the same amount of pressure from thermal expansion?

For this theoretical closed system we have Type M copper pipe with CTS fittings in a single family residential home with 1 kitchen and 1 bath.

All pipe is no greater than 1 inch in size nor less than 1/2 in size.

An integrated pressure reducer check valve backflow prevention device is installed at the water meter to prevent cross-connection.

Pressure is factory set at 50 psi

Expansion tank pre-charge is statically balanced.

Water heater consists of a standard efficiency 30 gallon glass lined tank and lp gas as a fuel source.

The temperature of the water heater is set to 120 degrees Fahrenheit.

There are zero fixtures in use and the potable system does not provide water to any home heating system.

The outdoor temperature is 75 degrees Fahrenheit

The ambient indoor temperature is 23 degrees Celsius.

The 2 systems are identical except for the location of the expansion tank.

Extra credit- lol
The year is 1980 something
The Game is eventually rained out.
The Baron is Nagsisimula Na
A Mad Dog slides across the infield.
What day is it?

Key word is thermal expansion. Do you think a pressure gauge on the second floor of the home would read the same as a gauge near the water heater?

Let’s say a 10 story building. Water pressure is increased 5psi in basement. Will water pressure increase 5psi on 10th floor? I say no the pressure increase will not be equal.

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I’m with the Tri-state plumber!

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In the closed system home that I represent? Yes

Although your model is quite different than the one presented I will ascertain a guess that you would not be able to see a 5 psi pressure increase in the basement. Therefore you would not be able to notice a 5 psi increase on the 10th floor.
Also this is why the question is limited to 8 feet.

This is the very reason my test subject is limited to a residential water heater. Thermal expansion over 300 degrees would have additional considerations beyond the law posed in the question.

Thank you for your insight. If I am wrong in my basis for applying the law in question to thermal properties I will.
Wouldn’t this law be the reasoning behind the use of the expansion tank to begin with?

Hint better yet the give away.
In the English system the unit for pressure is PSI
In the Systeme International d’Unites better known as SI system the unit for pressure is named after this law.