Now let’s see how hydronic heating tanks work
The Compression Tank
• The Compression Tank’s Vital Role in Hydronic Systems
Now let’s see how compression tanks work. When you fill a closed hydronic system with cold water and then heat it to a high limit, you wind up with about five percent more water than you started with. That’s because water, like most everything else, will expand when heated.
Since you already have the system completely filled with water you’re going to be in trouble if you don’t have a compression tank to accept that “extra” water. Without a compression tank, the relief valve will probably pop every time the burner comes on.
You can’t compress water, but you can compress air. And once you trap a pocket of air in a tank, you can use it to give that “extra” water something to squeeze, The air in the tank becomes a pneumatic spring. It takes up the slack whenever the water starts to bulge.
There are two kinds of compression tanks: the closed steel compression tank and the diaphragm tank. Let’s take a look at them.
Plain steel tanks
A closed steel compression tank has no moving parts. Normally, it starts out with a cold fill of about 2/3 water and 1/3 air. As the system water expands, that “extra” water moves into the tank and squeezes the air cushion.
The now-compressed air creates an increase in system pressure. You can see this on the boiler’s pressure gauge. Keep in mind, though, that this pressure increase has nothing to do with the pressure the circulator develops or the static pressure the column of system water exerts. This pressure is created solely by the expanding water. It’s a pressure created by a rise in temperature. The higher you raise the water’s temperature, the more it will expand. The more the water expands, the greater the increase in pressure will be.
Of course, you have to take this into consideration when you select the tank, but if you do a good job of sizing, the pressure will usually rise only a pound or so by the time the system reaches its high-limit temperature.
As the system cools down on its off cycle, the water shrinks and allows the air in the tank to expand back to its original volume. You’ll see this as a drop in system pressure on the boiler’s pressure gauge.
So what you’re really seeing on the pressure gauge as the system heats and cools is the expansion and contraction of air inside the compression tank. How much of a change in pressure you’ll get depends on how big an air cushion the water has to push against. In other words, the size of the tank.
This also explains why the relief valve pops when you lose your air cushion in a steel tank. Without an air cushion, there’s no longer anything for the expanding water to squeeze.
A fitting that prevents waterlogging
If the steel compression tank is directly connected into the system, air will eventually leave the tank and work its way up into the radiators. That’s because the system water can reabsorb the tank’s air cushion and move it (by gravity circulation) out of the tank and back into the system.
B&G’s Airtrol Tank Fittings prevent the air from leaving steel compression tanks by creating a gravity-flow “check valve” between the tank and the system. Airtrol Tank Fittings solve the escaping air problem once and for all because they stop gravity circulation.
Every steel tank can be made better with the addition of an Airtrol Tank Fitting. We know of systems with steel tanks that were installed decades ago. They’ve never lost their original air cushion! Thanks to the Airtrol Tank Fittings, you’ll never hear of an air-related complaint on these jobs.
Now let’s take a look at diaphragm-type tanks. Diaphragm tanks separate the air from the water with a flexible rubber diaphragm. These tanks serve the same purpose as steel compression tanks, but they’re generally smaller because one side of the diaphragm is pre-charged with compressed air.
When you start out, the air side of the diaphragm is fully expanded and flush against the inside of the tank. But when you connect the tank to the system and feed water into the other side of the diaphragm, the water’s pressure pushes back against the compressed air and squeezes it like a balloon. As long as the tank is in good working order, the water and air never touch each other. Should the diaphragm fail, however, the tank will lose its air cushion and the relief valve will almost surely pop on the next firing cycle.
Most residential diaphragm tanks are pre-charged to 12 psi to match the fill-pressure needs of a typical two-story house. When the feed-water pressure reaches 12 psi, the system will be filled to the top floor and will be under several pounds of pressures on both sides of the diaphragm will be equal. It pays to check the air pressure in a diaphragm tank before you install it, because some of the air may have escaped during shipment and storage.
Pump it up!
Naturally, if you have a building taller than two stories, you’ll have to pump up the air side of the diaphragm to match the feed-water pressure you’ll be using to get water up to the top and pressurized. This is very important. If you don’t pump up the tank to match the fill pressure, the relief valve will probably pop. This happens because the water pressure, being greater than the air pressure, will have pushed the diaphragm all the way back before the water begins to expand. When the burner fires, the expanding water has nowhere to go.
You can’t check the air pressure in a diaphragm tank when it’s connected to the system because the water pressure on the other side of the diaphragm will compress the air and give you a false reading, you first have to disconnect the tank from the system.
• The Compression Tank’s Vital Role in Hydronic Systems