Diaphragm expansion tanks are smaller than the old-school, plain-steel compression tanks they’ve replaced over time. This is a good thing because boiler rooms are stingier on space these days. Oh, and diaphragm tanks seem so carefree. Well, at least on the surface.

These tanks separate the air from the system water with a rubber membrane and pre-charge the air side of the membrane to 12 psi at the factory because that’s what you need in most American houses.

Now, about that pre-charge. I’ve watched them do this at the Amtrol factory and they do a fine job, but once those tanks get on truck, stuff can happen to them. It’s always a good idea to check that pressure with a tire gauge before you mount the tank on the system. Just like the tires on your car, the tank will lose some of its pressure over time. In fact, an engineer at Amtrol once told me that their Extrol tanks will lose air across the rubber diaphragm by diffusion at the rate of about 1 psi per year.

Did you know that? I didn’t know that.

Diffusion is what happens when there’s more of one sort of molecule — say, nitrogen or oxygen — on one side of a semi-permeable membrane than there is on the other side. Since the rubber diaphragm is a semi-permeable membrane, the air just says “Excuse me!” to the rubber molecules and sneaks through to the side that has the water. From there, an automatic air vent will spit the air from the system. Fresh water will enter to take the exiting air’s place, and that’s usually how diaphragm-type tanks fail. If you don’t replace the air that’s escaping into the water by diffusion, the expanding water will tear the diaphragm off its anchorage and the boiler relief valve will pop.

Now, about this semi-permeable-membrane business. You may not know what that is, but you sure have seen it in real life — probably a few days after your kid’s birthday party (if you blew up party balloons for the big bash, that is).

Did you buy the cheap latex balloons? Yeah, me too. They stay full for a time, but then slowly lose air. This happens because latex is a semi-permeable membrane. The air moves right though it as the balloon tries to contract.

But now consider those more expensive, shiny Mylar balloons. The ones the clerk in the store connects to a helium tank. Those things stay full a lot longer because Mylar is less permeable than latex. Sure, a Mylar balloon will eventually get saggy, but it takes longer to do so than a latex balloon.

Oh, and have you ever noticed how the material they use for potato chip bags looks a lot like Mylar? The less permeable the membrane, the fresher the chips will be. That’s why the expiration date of potato chips is probably further out on the calendar than your expiration date.

And by the way, this is the same issue that plagued the radiant-heating industry a few decades ago. It’s why the manufacturers of plastic- and rubber radiant tubing put oxygen-diffusion barriers on the outside, or sometimes the inside, of the tubing. Without the barrier, air will move through the tubing and attack the steel and iron parts of the hydronic system.

Diaphragm-tank manufacturers give you a Schrader fitting on their tanks so you can check and, if necessary, pump up the tank to the proper pressure for the job. That pressure needs to be whatever the system fill pressure is, plus a few pounds so you have positive pressure at the top of the system.

If you’re filling the system to 18 psi because the building is three stories tall, you’ll need to pump up the tank to 18 psi because the factory only puts 12 psi in there. And you need to do this while the tank is disconnected from the system. If there’s water pressure on the other side of the diaphragm when you’re pumping air into the tank, the air will compress too soon. You’ll think you have the proper setting, but you won’t. The relief valve will pop when the boiler heats the water.

Years ago, when I visited Amtrol in West Warwick, R.I., I asked about that 12-psi pre-charge. I was thinking about contractor friends who work in Breckenridge, Colo., and other Rocky Mountain high places where the air pressure is a lot lower than it is at the Amtrol factory. That Rhode Island 12 psi isn’t going to be 12 psi when it gets up in the Rockies.

They smiled and said that’s what the Schrader valve is for.

So use it.

The deep-drawn tanks that Amtrol makes nowadays showed up in the 1960s. Before that, they had a neat history. To prove that the concept worked, they built this square tank out of two streetlight blanks. They put a sheet of rubber between the two blanks and bolted it all together. The rubber separated the air from the water, and the Schrader valve was there on the air side.

It worked.

Their first commercially available version of the tank had a flying-saucer shape. They trapped a formed-rubber diaphragm between the two halves, and then crimped the edges to lock the tank together. This tank had welded legs so the contractor could screw it to the ceiling joists in place of the plain steel tank they were replacing. The tank didn’t have to hang from the ceiling, but old habits die hard, so Amtrol figured they’d accommodate what their customers expected.

During the 60s, they came up with the tank shape that you’re familiar with now. They use a hoop-ring method to attach the diaphragm inside this tank. I watched them make these, and the machine that does the deed is a true beast.

You feed a flat disc of steel into its maw and, using hundreds of tons of pressure, it turns the flat disc into a steel dome. The strength of the steel doubles as a result of cold working, and that results in a lighter, stronger tank. It scared the heck out of me when it punched. It’s the Mike Tyson of machines.

One of the Amtrol guys told me a fun story about their first cylindrical Extrol tanks. They were using an overstock of baby-carriage rims to manufacturer their tanks in those days. When the competition arrived and came up with their version of the product, Amtrol bought a few and took them apart to see what was what, as any good competitor would. They found small holes drilled into the competitor’s steel hoops. Those guys were mimicking what they thought was a necessary feature, but it was just the spoke holes for the baby carriage rims.

Engineers. You gotta love ‘em.

The early Extrol tanks didn’t have an air valve. To minimize potential leak paths and speed production, Amtrol charged those tanks with a measured chunk of dry ice prior to assembly. At room temperature, the dry ice evaporated and produced a 12-psi charge of carbon dioxide.

But then there are places such as Breckenridge, and that’s why you now see Schrader valves on these tanks. No more dry ice.

As for sizing, companies such as Amtrol make it easy. Just go to their site and use the online calculator. You’ll need to know the system volume in gallons, the difference in pressure between the fill valve and the relief valve, and the average water temperature. If you’re using a fluid other than water (say, antifreeze), you’ll have to correct for that, but it’s easy to do.

And don’t forget to check the tank’s pressure when you’re doing service. Isolate it from the system first, though. Otherwise, it will try to fool you.

Don’t let it.