Why Some Compression Tanks Fail

Packaged boilers have been popular for a very long time and they sure do make a contractor’s life easier. These days, most packaged boilers come with a circulator in the carton, and not mounted on the boiler. The one exception would be on those boilers where the manufacturer builds in primary-secondary pumping to protect the boiler from low-temperature return water.

I love the idea of putting the circulator in the carton instead on the boiler because it gives the installer a choice. He may want to install the circulator on the supply side of the boiler, pumping away from the compression tank, which is always smart. Or, he may want to install it on the return side of the system because of habit. Life is filled with choices.

The boiler manufacturers who sell the packaged boilers don’t care where contractors install the circulator. That’s because manufacturers don’t have to go upstairs to vent air from the radiators, which is usually required when the circulator goes on the return side of the system, pumping toward the compression tank.

A circulator has to create a difference in pressure when it starts, but it doesn’t care whether it does that by increasing its discharge pressure, or lowering its suction pressure. The compression tank, being the point of no pressure change in the system, makes that decision for the circulator.

Pump away from the tank and the pressure goes up. Pump toward the tank and the pressure at the circulator’s suction goes down. And when pressure goes down, air comes out of solution. We call that Henry’s Law.

And that’s why they have to go bleed radiators.

But let's say we have a contractor who hasn’t heard about this. He’s installing a packaged boiler and he decides to pipe the circulator on the return side. He also decides to pipe the automatic feed valve just above the circulator because it's easier for him to do it that way. This is his habit.

The compression tank, which is the point of no pressure change, is on the outlet side of the boiler, and that’s the point of no pressure change. For the circulator, it’s like a hydronic bookmark. The circulator can’t change the pressure at that point because, when it runs, the circulator can’t add water to the tank, or take water out of the tank. And since it can’t do that, it can’t change the pressure inside the tank. We call that Boyle’s Law.

Okay, so let’s say you’re the circulator. What will you do? You have to create a difference in pressure from one side of yourself to the other, but you don’t have to necessarily increase the pressure at your discharge to do that. You could just as easily drop the pressure on your suction. It’s six of one, half-dozen of the other as far as you’re concerned.

You’re pumping into the boiler and you have to flow through the boiler before you reach the point of no pressure change (the compression tank), so let’s say you’re able to raise your discharge pressure by about one pound to reflect the pressure drop though the boiler. But that’s as high as you can go because you have to stop raising pressure once you hit the point of no pressure change. You’re going to show the rest of your differential pressure as a drop in suction pressure.

It goes like this: There’s a 12-psi feed valve on the system, and there’s a 12-psi air charge in the compression tank. The circulator, one of those small ones you’ll find in most houses, starts. It’s discharge pressure goes up to 13 psi. It’s suction pressure drops to seven psi. Taken together, that’s a six-psi differential across the circulator, which is typical for a small, single-speed, water-lubricated circulator.

Here’s the problem: The feed valve, which is set to feed whenever the pressure in the system drops below 12 psi, is right there at the circulator’s suction. The circulator just came on and dropped the pressure by five psi right at that point. We’re at seven psi now, so the feed valve is going to do its job because high pressure goes to low pressure. Always.

Now how often do you think this will happen?

Do you think it will happen every time the circulator starts?

It won’t.

It happens only once, the very first time the circulator starts. Think it though. On the first cycle, the pressure at the circulator drops to seven psi. The feed valve feeds while the circulator is running, bringing the total system pressure up by about five psi. That extra water goes into the compression tank because that’s the only place it can go. The rest of the system is already filled. The extra water in the tank squeezes the air on the other side of the tank’s diaphragm, causing the overall system pressure to increase.

This happens all the time. A contractor will fill a system to 12 psi, run it once, shut it off and now notice that the pressure gauge reads about 17 psi instead of 12 psi.

He scratches his head and wonders how that happened. The next time the circulator starts, it again lowers its suction pressure, but this time, it lowers it to 12 psi because it’s now starting at about 17 psi. The feed valve doesn’t feed again because the pressure never dropped below 12 psi.

The contractor is standing on the outside, watching this happen, and he figures it’s no biggie because the relief valve on the boiler is good for 30 psi and the system pressure is just up to 17 psi or so. He shrugs and scratches his head. Stuff happens, right?

Now the burner comes on and heats the water, causing it to expand. The “extra” water from expansion moves into the compression tank because that’s the only place it can go (other than out the relief valve – which it will do in a moment).

The contractor watches the pressure gauge climb and suddenly the relief valve starts to drip. He figures there’s something wrong with that compression tank. Maybe it’s too small. Maybe it doesn’t have enough air in it. Maybe he should go get a larger one. Or maybe he drains water from the system and starts all over again. Or he leaves the job and hopes for the best.

Okay, here’s a hydronic fact of life for you. When you increase the system’s fill pressure by 50 percent, the required compression tank size for that system doubles. So if you think the tank is now too small, you’re right. It wasn’t too small when the system fill pressure was at 12 psi, but it’s too small now because the system fill pressure went up to about 17 psi.

Contractors will either oversize tanks, or bring the busted ones back to you as return goods. And all because of where the circulator is in relation to the fill valve and the tank.

And that contractor chose to pipe it that way.

Sometimes, habit can hurt.