Thermal Expansion Tank, CSV, Well water system...

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Ted M

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I use well water, pressure set to 30-50 PSI.
Captive air tank has a drawdown of 10.7 gallons.

Water heater is electric, 40 gallons, set at 120 degrees.
Never any problems with T&P valve, etc.

Over the next few months I want to install a CSV system.
I already bought a 4.4 gallon tank, a Cycle Sensor unit, and I am gathering other parts little by little.

At present, any thermal expansion in the water heater goes back into the captive air tank. With 10.7 gallons drawdown, that seems like plenty of room for expansion.

With a 4.4 gallon captive air tank with about a one gallon drawdown, would that be enough volume for any thermal expansion from the water heater?

OR, will it be necessary to install a separate TET?

Thank you.
Ted
 

Valveman

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That 4.4 gallon size tank is plenty for thermal expansion. Many people use a much smaller tank than that. Thermal expansion will only cause an increase by a couple of thimbles full of water. But if you don't have a little tank for those thimbles of water to expand into, the pressure gets high very quickly.
 

Valveman

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That is pretty cool but I doubt a 50 gallon heater has expansion of .55 of a gallon as that calculator says. But it also says a 2.1 gallon size tank has an acceptance volume of 1.2 gallons, which is over twice the size needed. So a 4.5 gallon size tank would be 4X larger than really needed for expansion.
 

Ted M

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I see where some of these calculators show about 1/2 gallon expansion, depending on conditions, like temperature.
My water in temperature is about 45 degrees. Heat up to 120 degrees; 40 gallons.

What happens when the 4.4 gallon tank is already full, though (say 60 psi)?

THEN the water heater comes on and needs to heat almost the entire tank.
For example, after taking a long shower.

Will the pressure go as high as 75 and activate the water pressure relief valve?

Ted
 

Valveman

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Here is another calculator you might find handy. http://www.cyclestopvalves.com/runtime_app.php

It shows the 4.5 gallon tank has a draw down of 1.18 gallons at 40/60. It also shows a drawdown of 1.72 gallons with a 40/75 setting. So if it where full at 60 PSI and you had .55 gallons of expansion, you would be just at 75 PSI. Although I have never actually seen the pressure increase from expansion with the systems I have. I think that is a worst case scenario.
 

Reach4

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Pressure relief is normally at 125 to 150 PSI.

Your shower would not let the water in the tank go down to the supply temperature. Once the water temperature is down to 60 or so, you would have probably terminated the shower. It is only after you stop using water that the pressure can rise due to temperature.
 

Ted M

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Pressure relief is normally at 125 to 150 PSI.

Your shower would not let the water in the tank go down to the supply temperature. Once the water temperature is down to 60 or so, you would have probably terminated the shower. It is only after you stop using water that the pressure can rise due to temperature.


On my water heater the T&P valve is 150 psi and 210 degrees.
However, there is also a 75 psi pressure relief valve in the piping.
It came with the house, although I put a new one in a few years ago.

The T&P valve, therefore, would theoretically never trigger, as the 75 psi relief valve would dump pressure first.
I do test it once a month though, and let a quart or two of water out.
Also drain a quart or two out of the bottom of the water heater.

Good point on taking a shower!

Ted
 
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