What Pressure To Set Expansion Tank?

12 psi is usually an appropriate setting for most home applications. Pressure relates to two important factors: 1) pressure lifts water to a high point of 2.34 feet for every pound of pressure; 2) sufficient pressure (NPSH) at the suction side of a circulator prevents cavitation and the water from boiling in the circulator. In all cases, it is important to pump away from the expansion tank so that maximum positive pressure exists at the circulator.

12 psi will lift water to 28 feet, normally sufficient for a two story house when the pressure tank is at a low point in the house. Positive pressure at the suction side of the circulator relates to the temperature of the water and the flow rate. The specs for the circulator may state the required NPSH. But again, in most home applications, 12 psi is sufficient to prevent boiling and cavitation.

In direct answer to your question, if the pressure tank psi is less than 12 psi in your case and the autofill valve is located at the same elevation as your pressure tank, your autofill valve will add water to the system until the pressure is 12 psi, thus partially filling expansion tank capacity and you will have less effective acceptance capacity. If the pressure tank psi is nominally greater than 12, the autofill valve will not add water (unless pressure drops for some reason) and your full acceptance volume will be available for expansion as the water heats.

The issue remains whether you have sufficient acceptance volume to absorb the expansion of water from the low temperature point to a high temperature point. Over a range of temperature from low of 60F to a high of 200F, thermal expansion is about 3%. My rule of thumb is to provide 5% of system volume for expansion tank acceptance volume.

Based on your info, I would likely do the same thing. I don't have an autofill valve for my Tarm -- never need to add water so far.

I have an autofill setup on my system. I keep it valved off.

If I had a leak develop that would drop my psi and require more water in the system, I would first want to stop the leak before letting more water in. If I had a system that didn't contain much water though, I might leave it turned on.

velvetfoot said:

From my initial fill yesterday, I could see the pressure gauge read 12 psi, which was nice rather than futzing around and worrying about the well pressure. Now to fix the leaks! There's 0 psi in there now!

We typically turn the water off when we leave the house (paranoid, yes), and don't worry about it. I wonder where the water would go, anyway, where you wouldn't notice it?

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Not paranoid...I do the same thing if I leave for any extended period. My brother returned from vacation to find a washing machine supply hose had failed, which flooded his basement. I try to err on the side of caution...easier to turn on well pump than to repair the damage.

JTWALL said:

Not paranoid...I do the same thing if I leave for any extended period. My brother returned from vacation to find a washing machine supply hose had failed, which flooded his basement. I try to err on the side of caution...easier to turn on well pump than to repair the damage.

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I also do this. I had a pinhole leak appear in a basement copper joint once, that I just happened to be sitting directly under at the same time. Also had the same type of leak appear in a fitting outside my DHW coil in my old boiler - that one made a very big puddle before I noticed it.

But there was that one time in a severe brain fart (my most severe to date I think) that I turned off the main breaker instead of the well pump in a rush on the way out the door. The fridge & freezer were in some mess when we got back almost a week later, ugggh...

Years ago I bought an automatic fill valve, thought about the issues mentioned, and then never installed it.

velvetfoot said:

The autofill valve is set at 12 psi. The expansion tank connection is near the autofill connection. Should I set the expansion tank to 12 psi, or a little higher?

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if your operation psi in boiler is running at 12 psi or a little higher the expantion tank is not taking on no water drop tank down to 8 psi then the tank will take on water.. anything below 12 psi on the boiler when hot the bladder in the tank will not move and whats the use of having it if your only running 12 psi in the boiler...

eco-one said:

if your operation psi in boiler is running at 12 psi or a little higher the expantion tank is not taking on no water drop tank down to 8 psi then the tank will take on water.. anything below 12 psi on the boiler when hot the bladder in the tank will not move and whats the use of having it if your only running 12 psi in the boiler...

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If you set the expansion tank to less than what the boiler is cold, you will waste expansion room from the tank taking on water before you even start to heat it & it is really needed.

An auto fill valve is a safety item, IMO. If your boiler overtemps (say a circulator went out), then the pressure safety valve blows. Your fire is still strong, and without more water, you could melt down your stove.

I completely agree with aeblank. Although the points made above against using a fill valve make sense, I still want the assurity of knowing that the valve is there. I think you'd certainly notice if there was any significant leak. You can hear when my fill valve is letting water in. It's actually louder than any circ's in my system.

aeblank said:

An auto fill valve is a safety item, IMO. If your boiler overtemps (say a circulator went out), then the pressure safety valve blows. Your fire is still strong, and without more water, you could melt down your stove.

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True.

But on systems with hundreds of gallons of water in storage on reserve the picture is changed a bit.

Mine is valved off, but if I was running a boiler without any storage, it wouldn't be.

The purpose of 12 psi is that this is an appropriate psi on a cold system in most home applications: the system always has positive pressure sufficient to prevent air admission through a vacuum or possible system failure through a vacuum (water contracts as it cools). With 12 psi on a cold system, when the boiler is fired and temperature rises, water will expand, psi will also rise and the bladder expansion tank will accept that expansion without system pressure rising to the point of system failure or blow-off of the pressure safety valve, typically at 30 psi. Sufficient expansion acceptance must be in place to allow system operation up to maximum temperature (about 200F) plus a safety factor. In my experience expansion acceptance to allow psi no greater than about 25 psi is good practice. Some 30 psi pressure valves start to weep of drip before 30 psi.

An auto fill valve is a safety item, IMO. If your boiler overtemps (say a circulator went out), then the pressure safety valve blows. Your fire is still strong, and without more water, you could melt down your stove.

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While this might be possible, a well-designed system will not let this happen. The boiler itself should shut down at the high limit point, and an overheat zone or loop, gravity fed, should be in place to bleed off the excess boiler heat production.

Remember to bleed the tank down (or fill the tank with air) to the desired pressure when there is 0 pressure in the system.

Should, sure. My dishwasher should not have started on fire a few weeks ago either. $hit happens, that's all. Back to the original point, more than 12 and less than 25psi, it would seem to me.

velvetfoot said:

Not sure why that would be?

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If you set your air pressure at 12psi while the wet side (system) is also at 12psi, you will in effect have half a tank of air and half a tank of water.

If you set the air pressure at 12psi while the wet side (system) is at zero, you will in effect have a full tank of air. And a lot more potential acceptance volume as opposed to the first case.

Proper install (I think) would have a shut off valve between the expansion & system, a drain between that and the expansion, and another valve on the air side to let air in and out depending what you were doing. Then you can valve it off from system, drain the water from the wet side, set your pressure on the dry side, close the air, then expose it to the system by opening the shut off back up. If I said that right.

The expansion tank precharge pressure should be set lower than the pressure of the autofill valve. The expansion tank is not part of the system until the system pressure exceeds the pressure in the expansion. The relationship is typically 80% of the fill valve setting is what the tank air pressure should be. A fill valve setting of 15 psi would result in a 12 psi expansion tank precharge pressure. As Maple1 mentioned always measure the expansion tank pressure with the tank isolated from the system and open to the atmosphere so that you don't measure the water pressure in your system.

Beagle Dad said:

The expansion tank precharge pressure should be set lower than the pressure of the autofill valve. The expansion tank is not part of the system until the system pressure exceeds the pressure in the expansion. The relationship is typically 80% of the fill valve setting is what the tank air pressure should be. A fill valve setting of 15 psi would result in a 12 psi expansion tank precharge pressure. As Maple1 mentioned always measure the expansion tank pressure with the tank isolated from the system and open to the atmosphere so that you don't measure the water pressure in your system.

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My take, reading the Amtrol and other manufacturers Engineering Guides is the tank should be "pre-charged to the minimum operating pressure at the location indicated on the drawing" Matching the pre-charge to the fill pressure will assure you have the full capacity of the tank.

Beagle Dad, I'm curious where you got that 80% guideline? If the pre-charge is lower than the fill pressure you are limiting the acceptance volume of the vessel. The expansion tank, and it's pre-charge is important for establishing the PONPC, it is part of the system, assuring you provide NPSH (net pump suction head) to the system.

Tank isolating should be with a lock shield type valve to prevent it from being easily isolated from the piping. The drain port is also a handy featue.

Most solar installers will slightly over-fill the system to provide a "safety seal" This assures some pressure when the collector and piping drop below the ambient fill fluid temperature. Tanks must be sized large enough to allow that, of course.

We get calls complaining of a leak in the solar piping or collector when temperatures drop and the pressure along with it. Same with chilled water or GEO loop fields, a bit of over-charge eliminates the gauge dropping, presenting as a leak down. Sometimes to zero

Fill in the blanks

Elevation from tank connection to highest point of the system________________________
Temperature of water when system is filled________________
Maximum temperature system will operate at______________
Total volume of system in gallons of fluid__________________
% of antifreeze (if used)_______________________________
Desired pressure at the highest point in the system_________

And I'll crunch it out for you.