If you’re considering converting an old, non-captive-type expansion tank to a captive-air expansion tank, an easy rule of thumb is to replace the existing tank with the model that is equal to the existing tank’s volume. For example, if a 15-gal. tank is hanging from the ceiling, replace it with a No. 15 bladder-type tank.
At the time that you do this, you must install an auto vent to get rid of the system air. To recap, on old non-captive compression tanks, automatic air vents are not allowed. If it’s a captive-bladder expansion tank, auto vents are a must. The new tanks come with a factory pre-charge of 12 psi. Why, you ask? Because the manufacturer assumes that in a residential setting there will never be more than two floor levels above the boiler. In order to “lift” the water to a height high enough to completely fill the system, it requires 0.5 psi of pressure to raise water 1 ft. of vertical height. Hence, the water in a two-story house would have to be pressurized to 10 psi in order to fill the system.
You typically want some residual pressure in order to “burp” any bubbles out of the top of the system. Hence, 20 ft. times 0.5 psi plus 2 lb. residual pressure equals 12 psi. That’s also the reason the factory ships the pressure regulators preset to 12 psi.
What if the system height is say, 30 ft. high? With 0 psi on the water side of the tank, the tank’s air charge should be pressurized to the system’s anticipated fill pressure. In this case, 30 ft. times 0.5 = 15 psi, plus 2 lb. residual = 17 psi. Then, you must also adjust the pressure regulator to compensate for the additional pressure necessary to lift the column of water to the top of the system. What happens if you leave the pressure of the tank at 12 psi but raise the fill pressure to 17 psi? You begin using the acceptance capacity of the tank up front, which means you’ll have that much less on the other end when the water gets hot and expands. You will see an increase in pressure once the system gets hot. It may not be enough to blow the relief valve, but it will increase.
If you do it right, increases in system pressure are held to a minimum. In some cases, overfill can be used to your advantage, but you want to make sure that you don’t overfill or you’ll ruin the expansion tank and the relief valve.
If you are dealing with a hydronic heating system that has non-oxygen barrier tube, you must use an expansion tank designed for potable water. This tank will have a phenolic lining on all metal parts to keep the oxygen that’s in suspension from eating the tank’s shell. Also, don’t forget to adjust the tank’s air charge downward. Potable water tanks typically come from the factory with a 45-psi pre-charge.
Although the tank manufacturers have told some contractors that it doesn’t matter what direction you install the tank, common sense should tell you that it does matter. Every drawing I’ve ever seen has shown the tank nipple on the top of the expansion tank. This keeps air away from where the water is supposed to be.
In a retrofit, where the old ceiling-hung expansion tank’s pipe goes up, install a tee in a normal configuration, bush down the end run and install an auto vent. Then on the branch, install a nipple long enough to give you clearance for the diameter of the tank and install an elbow facing down. Hang the tank right there, and don’t forget to properly support it.
If you’re using a floor-mounted tank, the piping will be similar. Install an auto vent at the top of the down comer to ensure no air gets to where there is only supposed to be water.
Isolation valves are cheap when it comes to expansion tanks. A tank is one of those items that is guaranteed to fail at some time. Why not make it easy on the next person who has to change it out? Besides, you might just be that next person.
Technically speaking, with captive-air bladder-type expansion tanks, over sizing is not a major concern. The cost differential between a No. 15 and a No. 30 is so negligible it’s not worth considering. Go with the bigger tank. If the tank is oversized, it allows you to slightly over fill the system. I can’t remember when I last used a No. 15 tank. If memory serves me correctly, it was back in the heyday of solar.
If we follow the instructions that come with certain fill valves and we shut off the make-up water, the oxygen comes out of suspension and is expelled from the system. Then the system volume and pressure are going to decrease, and you’ll be back there re-pressurizing the system. If you’ve calculated a system fill pressure of 12 psi and you manually fill the system to 20 psi, as the oxygen comes out of suspension the diaphragm will send a bit of water back into the system to compensate.
I know for a fact that the statement about shutting off the water to the heating system comes from the old days of “air in” systems and no low-water fuel cutoff.
With today’s “air out” systems, shutting off the make-up valve is a guaranteed way to get a callback. I guess that if you build that into your installation price it’s ok. It gives you the opportunity to see the boiler room again to see how things are going. I personally don’t have any spare time, so I leave the make-up valve on. I also install a low-water fuel cutoff device so I can sleep at night.
Tune in next month for the final installment of Expansion Tanks 101.