Pressure Increase Formula

Hilly

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.

Jamie Hall

If you are on a well, you should have a hydropneumatic tank on the system, too -- and the only check valve should be on the drop pipe (if it is a submersible) in the well. If that is so, and the hydro tank isn't waterlogged, then there should be very little pressure change from the water heater cycling as the hydro tank should take care of it. (Note that if one is on city water, and there is a pressure reducing valve and no hydro tank, you can get very high pressures!). Now if there are other check valves or pressure reducing valves in your system between the hot water heater and the main hydro tank, you could be seeing some real pressure jumps from the water heater.

Your 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.

Hilly

My water softener has checks in it. So I know me personally that the hydro tank isn't helping or harming my situation. This stems from my personally noted experience from above but also on Sunday my uncle in the city called and said his new water heater had the relief valve open. At a family dinner last night I gave him my gauge and told him to go out it on a hose bibb. He called and said it was 95psi and this was at about 700/730 pm. I told him to leave it on. I'm going to head over and install a PRV on the incomining water line regardless. But I got my to thinking whether the pressure would build in the hot water tank enough to reach the 150psi opening. After he gets home today i am sure he'll call with an updated psi on the bump gauge. I am pretty curious to see where it'll reach with the line pressure gets a little more static in low use times of day.

Zman

I am not sure I am following you exactly.

I 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.

Jamie Hall

Bottom line: there MUST be an expansion tank on the same side of all check or pressure reducing valves as the water heater.

If 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...

Hilly

I understand the need a requirement. Personally this is a new house so I am just going to install the exp tank when the mech room gets reorganized. I do understand the need a theory behind it all but I was curious in lieu of a proper installation how much pressure could build in a system.

Jamie Hall

As you can see from all the above jabber, it's really hard to give an exact number. In a truly rigid system, a 10 degree rise would produce several thousand psi. This, obviously, doesn't happen! The biggest flex in most systems will be in the tank itself, and mostly in deformation of the bottom closure.