Interesting Expansion Tank Discovery

[go_hercules]

Some of you may find this interesting, others may not, so here goes. We all know to set the initial pressure on the expansion tank "around" the same as static water pressure. You read some arguments that it should be a little higher or a little lower, etc. So, after a long shower, I monitored the house water pressure while the water heater ran. It started at 60 psi. Almost immediately, it started rising slowly. By the time the water heater shut back off, the static water pressure had risen to about 72 psi. Acceptable for sure, but why the difference. I read all sorts of theories (too low initial pressure = low acceptance volume, too high initial pressure = initial rise until pressures equalize, etc.) So here is what I did. I did the whole test over the next day, but this time just after the water heater shut off and the pressure had risen to about 72 psi, I released just a little air from the expansion tank until the static water pressure fell to 60 psi. I did the whole routine over, and this time the pressure stayed at 60 psi even after the water heater cycled. So I think I found the "magic" setting for initial air pressure. When I get a chance, maybe tomorrow, I will drain down the hot water so I can check what actual air pressure I ended up with. Definitely BELOW the water pressure, but not sure how much. I think in most situations, anywhere reasonably close to static water pressure will work good enough to protect pipes and prevent T/P tripping, but this just seems to be more accurate.

 

[Reach4]

I suspect that the ice maker turned on, or some other water use happened, during that stay-60psi result.

I doubt that you have a humidifier, but something....

 

[Sponsor]

 

[LLigetfa]

Maybe your air pressure gauge doesn't read the same as your water pressure gauge so the precharge was wrong to begin with.

 

[go_hercules]

No, the icemaker was off, no water usage at all. Why would you guess something was in use?

 

[go_hercules]

Very good point about the possible pressure gauge discrepancy. Without somehow calibrating the water gauge exactly to the air gauge, my approach bypasses this altogether. The final result is that the system now works exactly as you would expect it to.

 

[Reach4]

No, the icemaker was off, no water usage at all. Why would you guess something was in use?

When you heat water, water expands. If there is no place to go other than the expansion tank, the pressure rises.

How about a dripping faucet that was closed more tightly when you did your first phase test?

 

[go_hercules]

You're missing the point. All I did was adjusted the air pressure VERY slightly to fine tune it. Now I did this in an unconventional way, but slightly adjusting the air pressure is all I did. And that alone allowed the expansion to take place and be more completely accomodated for by the expansion tank.

 

[Reach4]

You're missing the point. All I did was adjusted the air pressure VERY slightly to fine tune it. Now I did this in an unconventional way, but slightly adjusting the air pressure is all I did. And that alone allowed the expansion to take place and be more completely accomodated for by the expansion tank.

The point I took is that you think that reducing your air precharge a bit on the thermal expansion tank caused the water pressure to not rise delectably despite the water heater heating the water.

I further believe that you think this will be repeatable.

We can agree that having the air precharge set to 62 will cause a quick rise to 62 during heating followed by some more rise , but where we disagree is that setting the precharge to 59 will cause the pressure to hold steady at the mains 60 psi.

The volume of expansion of the water is determined by the incoming water temperature, and how hot the WH is set for, and the amount of water used. An undersized expansion tank will not absorb extra water as well as a larger expansion tank. In the worst case, having the precharge set too low on an undersized tank will cause you to not use the full expansion space on that tank. I expect your incoming water is not so cold as it would be in a cold area.

I expect that when you repeat your experiment, you will see a rise, even with your reduced precharge. Also note that the mains pressure often changes with time of day reflecting water usage. This change might be larger in non-water-tower places, but even in water tower (or hilltop water tanks), the water department will let the levels drop and to use cheaper electricity in the middle of the night to top off the tanks.

 

[go_hercules]

I follow your reasoning, and will check my pressure again several times after the heater runs after showers. I am always willing to experiment to learn something. I do think, however, that it will be repeatable and here is why. Let's say you set the precharge at 60 and the water pressure is at 60, there would be no water in the tank when in service at equilibrium. Now let's say you set the precharge at 55 - at equilibrium there would now be some amount of water with less air since it's squeezed down the point that it's at 60 psi like the water. Now, here's the rub between our thinking. Is there ENOUGH air volume left to compensate for any expansion it might see? If there is, then I still say you will hold 60. If there is not enough volume, then there will be a rise with a lot of expansion. And that is why I snuck up on it by reducing the pressure slowly just enough to get me there. And I did this in the worst case scenario that my system sees, which is after a long shower. As for fluctuations in the mains, I don't think that would have any effect since I have a functioning pressure reducing valve.

 

[James Henry]

https://delcowater.org/thermal-expansion-tanks/

 

[go_hercules]

That Delcowater link is pretty good, but I see an error that is repeated over and over in articles and explanations. In their diagram called figure 2, they show an expansion tank with 60 psi of air and water. Notice that there is a volume of air, maybe about a quarter of the total volume. Well, the only way to get to that situation would be to start with an air precharge much lower than 60. If you started with a precharge of 60, then an equal water pressure of 60 would not displace any air at all. Now what I am doing, starting with a lower precharge, will get you to the situation in that diagram.

Now for figure 3 in their article. COMPLETELY WRONG. The water obviously expanded causing a decrease in air volume and corresponding increase in air pressure. BUT, they show a water pressure of 105 as well. The water pressure would be 60 or damn close to it, since the water was free to expand. They missed the whole point of the expansion tank.

 

[Reach4]

Longish text, so I added some numbers.
1. For simple math, let's assume that atmospheric pressure is 15 psi. When you read the pressure with your pressure gauge, we can say that is measured in PSIG... (gauge pressure above atmospheric pressure. In the non-academic world, when we say psi, we usually mean psig. But for the ideal gas law, we need to use the absolute pressure for calculations.

So if we presume that at time 1, we measure 60PSIG. So lets call that P1 = 75 psi. With me so far?

2. Let's also assume that the air temperature is constant for simplicity. It would not be, but for simple math, let's pretend. Also pretend the diaphragm is perfectly limp, so the water pressure is the same as the air pressure as long as the water pressure is equal to or greater than the air pressure.

3. Let's assume the pressure tank volume is 642 cubic inches (2 gallons), and since when P1=75, the tank holds 642 cubic inches of air (and 0 water). Let's call that air volume V1 (volume 1). Let's presume that we finish the shower, and the water heats up, expanding by 321 cubic inches. The water volume is 321 cubic inches. So now the air volume becomes 321 also. We can call that V2. What is the pressure now?

4.
(P1 * V1) = (P2 * V2). That is the simplified Boyle's gas law.

5. Solving for P2, we get P2= ((P1 * V1)/V2).
So P2= (75*642)/321 = 150 absolute psi. But on the pressure gauge, we subtract off the 15 psi, so the pressure we would see on the gauge is 135 psig.

The full version of this is called Boyle's law. It does not ignore temperature.
https://en.wikipedia.org/wiki/Boyle's_law

 

[go_hercules]

I checked the water pressure twice tonight after 2 different long showers were taken and the water heater had cycled completely. Each time, the pressure never rose above 60 psi, which is what the PRV is set at. I would have to say that this method works great. It is very simple. Pre-charge to the same pressure as the water pressure. Let the water heater cycle. If the pressure goes up beyond the target pressure, let out just a little air at a time until the gauge reads the target pressure. You're now at the perfect pre-charge pressure. Remember, this must be done without cracking a faucet, letting the ice maker run, etc. while the water is heating. Like I mentioned, even if you get close, you will limit the pressure rise, I simply wanted to be at the optimal setting. Take it or leave it. If in doubt, just try it.

 

[hj]

You are assuming that air has ZERO volume. When the water expands is will ALWAYS create an increase in pressure. The expansion tank does NOT "eliminate" the expansion, it moderates it so the pressure increase is less than if it were caused by "incompressible" water. It makes no difference HOW MUCH air is in the tank, because when you turn the water on the tank is at system pressure. IF the tank's charge is less than the system pressure there will be less space for expansion to occur. If it is above system pressure there will be more air, but the tank will NOT start absorbing pressure until the expansion increase the system pressure to match the tank. I am not sure what you are measuring, but it CANNOT happen.

 

[wwhitney]

Now for figure 3 in their article. COMPLETELY WRONG. The water obviously expanded causing a decrease in air volume and corresponding increase in air pressure. BUT, they show a water pressure of 105 as well. The water pressure would be 60 or damn close to it, since the water was free to expand.

This is where you are mistaken, Figure 3 in the reference is correct.

If you have a closed system with a flexible membrane separating two fluids, the pressure in each fluid will be equal. If not, the higher pressure side would push on the membrane more than the lower pressure side, moving the membrane towards the lower pressure side. This movement would provide more volume for the higher pressure side, reducing its pressure, and reduce the volume for the lower pressure side, increasing its pressure. The movement will only stop once the two pressures are equal.

In other words, the pressures on the two sides of the membrane will be in equilibrium.

Cheers, Wayne

 

[go_hercules]

I give up. I will try one more time.

First, let me explain again where my system is at. I can repeatedly take long hot showers, let the water heater reheat with all other water off, and the system pressure never rises above the regulated 60 psi which I have set. I have done this now about a dozen times at different times of day and results are 100 percent consistent. So hopefully we at least agree that my system works entirely as expected. It is doing EXACTLY what it is supposed to do.

I initially, like most everyone else, set the tank air pressure to equal the regulated water pressure, in my case 60 psi. Then I turned off all other water, took a long hot shower, let the water heater complete it's reheat, then checked the water pressure. It had risen to 72 psi. So I let a very small amount of air out of the tank to bring the indicated water pressure down to 60 psi. This was done immediately after the water heater shut off and the maximum amount of expanded water was trapped in the system. Later, I bled the system and checked the air pressure in the tank. It was right at 55 psi.

Someone above had mentioned that maybe my water gauge and air gauge didn't agree with each other. With this method, it doesn't really matter. The end result is the end result. HOWEVER, just for curiosity sake, I let the water heater cycle, opened some taps to relieve any built up pressure, then checked the air pressure with the expansion tank installed. It showed exactly 60 psi, same as the water gauge. That is what you would expect after opening a tap, the air pressure would equal the water pressure. It did, so I confirmed that my gauges were in agreement with one another.

So, by setting the pressure slightly below 60, the bladder is compressed at least slightly, allowing a small amount of water into the tank. When thermal expansion takes place, the bladder begins to compress the air immediately, and there is more than enough residual air volume to completely handle the expanded water.

No matter what else we may disagree on, it's beyond theory at this point. The system works perfectly. Like I said before, if you are anywhere close you should be fine. In my case, even if the pressure rose to 72, that was acceptable, it would never trip the T/P valve, etc. I just wanted it to be optimal.

 

[go_hercules]

Wayne, your explanation ONLY applies if the compressibility of the two fluids are equal. In this case, the water, not being compressible will expand when heated, since it's allowed to against the bladder. The air, being compressible, will absorb this expansion. The water pressure will remain constant. The air volume will decrease, thus raising it's pressure. If both fluids were air, or both water, then yes, the pressures would equalize, but they're not.

To prove it to myself, I let the water heater cycle. Confirmed system pressure remained at 60 psi, then measured the air pressure in the expansion tank. It was just below 80 psi. Then I cracked a water tap, and checked it again, and the air pressure dropped back to 60.

 

[wwhitney]

The problem we have is that the behavior you are describing is incompatible with physics. If the pressure never rises above 6o psi in the system as the water tank goes from cold temp (~60F) to hot temp (~130F), the system is not closed. Some fluid is flowing out of it somewhere. E.g. a tap that is dripping ever so slowly, or an air precharge valve on the expansion tank that starts leaking at 60 psi, or something.

So, by setting the pressure slightly below 60, the bladder is compressed at least slightly, allowing a small amount of water into the tank. When thermal expansion takes place, the bladder begins to compress the air immediately, and there is more than enough residual air volume to completely handle the expanded water.

Sure, that is how an expansion tank works. But what you are missing is that if the system is closed, as the air volume in the tank decreases because of expanding water, the air pressure goes up, which means that the water pressure goes up.

Cheers, Wayne

 

[wwhitney]

Wayne, your explanation ONLY applies if the compressibility of the two fluids are equal.

That is not correct. Just think of the membrane itself for the moment, it has some fixed area A , exposed to air on one side, and exposed to water on the other side. The force on the water side of the membrane is A * P_water, and the force on the air side of the membrane is A * P_air. If there is a difference between P_water and P_air, there is a net force on the membrane, which means it will move.

As to water being incompressible, that is only an approximation, it has a very low but non zero compressibility.

To prove it to myself, I let the water heater cycle. Confirmed system pressure remained at 60 psi, then measured the air pressure in the expansion tank. It was just below 80 psi. Then I cracked a water tap, and checked it again, and the air pressure dropped back to 60.

Where are you measuring "system pressure"? If your water pressure gauge were on the water side of the bladder in the expansion tank, it would read 80 psi when the air side of the tank reads 80 psi.

Cheers, Wayne

 

[go_hercules]

Then Wayne, what would you expect to see on a water pressure gauge after the heater cycled with an expansion tank in the system, and the PRV set to 60 psi? I would expect 60, I mean that's what it's intended to do. Mine does exactly what it is supposed to, and everyone is in shock.