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# Pressure Increase Formula

For the life of me I cannot find my math on this. I know it's pretty simple but I cannot easily find it on the internet or in my head. I am trying to figure out the pressure increase in a hot water. I know that is the science method. I did preform real world calculation thought too. I was curious about it all and I put a bump gauge on my water heater for approximately a month. I am on a drilled well with a 40/60 switch in place. My system is then 'closed' due to the softener and filters in place. My spike needle measured around 90PSI. So is there a formula to enter incoming psi, water temp in, water temp hi-limit and water volume to determine the potential pressure increase on the system? I believe the major of water heater failures where I come from (over 95% electrical tank style) are due to pressure build up on the tank and anode rods not being maintained. I am on a well but most are on a regional water supply that has good water quality.

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## Comments

8,895MemberYour 90 psi spike just might be a water hammer.

All that said, then there is your original question: how to calculate the pressure change from the water heater, assuming a truly closed system. And yes, there are scientific -- or more accurately, engineering -- ways to calculate it. However... it's complex. On the one hand, you have the expansion of water with temperature. No sweat -- so and so many gallons of water with a temperature change of x will try to expand by y percent. Note the "try to expand". Now when that happens, there are two other factors which need to be incorporated. The easy one is the compressibility of water (it does, a little!) which pressure change. If the container were absolutely rigid, the problem would still be easy -- you know what the attempted expansion was, that was step one up there, and so you need to find the pressure increase which will compress the water back to its original volume. Which is also easy. Unfortunately, the container isn't rigid. You have the tank itself, and the tank shell can expand somewhat with the change in the internal pressure, as well as the top and bottom domes flexing. You also have the piping, which will expand somewhat with the change in pressure, so you need to know how much, and what size and material, all the piping is which forms the overall enclosure.

For what it's worth, the compressibility of water with pressure is about 6 orders of magnitude less than the expansion of water with temperature, which means simply that in a perfectly rigid container, the pressure change from thermal expansion is truly astronomical -- in the real world what you will see will be a pressure change related to the expansion of the container.

At which point I, at least, would have a headache and go and check my pressure relief valve... and make sure my hydro tank wasn't waterlogged.

Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch

399Member4,402MemberI assume you have a check valve on the incoming water line to the house. When the water enters the house it will become warmer. The cold side will warm a bit just from indoor air. The hot will warm significantly more for obvious reasons. This warming will result in expansion.

Exactly how much it will expand and how much pressure increase you will see depends on a whole bunch of factors. Factors like temp rise and volume are easy to calc. The tough one would be how much "give" the system has. PEX will expand more that copper for example, this will effect your pressure measurement.

None of this is all that important as long as you install a properly sized domestic water expansion tank. This really important as not only will the pressure effect your water tank, but it can cause failures in the cheap water valves found in Icemaker, dishwashers, washers ect.

Yes, the expansion will cause the water heaters T&P to release.

The manufactures all have simple calculators to assist with the sizing.

Albert Einstein

8,895MemberIf there are odd checks or reducing valve in the system, this tank must be connected to the water heater with no valves of any kind between it and the tank.

The system expansion tank for the well can be used, but ONLY if there are no valves between it and the water heater.

Otherwise, as you have noted, the water heater's T&P will open -- or at least you hope it will...

Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch

399Member8,895MemberBuilding superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch