When hot water heating was new (and this goes back to the turn of the century) the Dead Men installed gravity systems because Homer Thrush had not yet invented the circulator. Up in the attic of these homes you’ll usually find an expansion tank that was open to the atmosphere. To fill these systems, you’d have to carry buckets of water from the well to the attic and empty them into the system. Imagine that! After a while, though, (and after city water became available) someone came up with the idea of using a ball cock in the open tank to keep the tank filled. Then, if the tanks waterlogged, the excess water just ran out onto the roof through an overflow pipe. No problem.
Some of those early gravity systems didn’t have attic tanks. The Dead Men would, instead, connect their cast-iron radiators across the bottom push nipples and leave a portion of each radiator filled with air. The collected air in the tops of all the radiators became their "expansion tank." This is sure to have you scratching your head if you’ve never been on one of these jobs. You’ll spend a lot of time wandering through that house, looking for a tank that’s not there.
After the circulator came along, the pipes got smaller and there was less expansion going on because there was less water in the system. The Dead Men abandoned those open expansion tanks in the attic in favor of closed steel compression tanks, which they usually hooked up to the butt end of a flow-control valve down in the basement. And for a time, there was peace in the valley.
But before long, the Dead Men leaned that, almost without exception, the compression tanks would lose their air cushions and fill with water. No one was quite sure why this was happening. They’d "solve" the problem by making it their habit to drain several gallons of water from every tank on every visit. This led to the common practice of draining all tanks, whether or not they needed draining. Generations were raised on this belief. You see a steel tank you drain it. Case closed.
And so it was for years. But then came the diaphragm tank. Diaphragm tanks don’t need to be drained, but they do lose their air pressure over time. That’s because the diaphragm is made of rubber, and rubber is a semi-permeable membrane. Gasses will pass through the rubber and into the water at the rate of about 1-psi per year, which is why you should always check the air pressure before you throw away one of these tanks.
But let’s get back to those steel compression tanks. The air sits on the water like cheese on a pizza. When the circulator runs, it can’t add any water to the tank because the circulator is operating within a closed system. For the circulator to add water to the tank, the circulator would first have to remove some water from the pipes. And if it did that, there would be an empty space in the pipes where the water used to be, and that’s simply not possible. In a similar way, when the circulator runs, it can’t take any water out of the tank and put it into the pipes because the pipes are already filled with water. Since the circulator can neither add nor remove water from that compression tank there won’t be any pumped flow in the line going up to that steel compression tank. With no flow, the water in the tank will be cooler than the water in the pipes because the water in the pipes is passing through the boiler. If you’d like a more detailed explanation of how circulators work in a closed heating system, get a copy of my book Pumping Away (and other really cool piping options for hydronic systems).
Okay, now for a bit of science. Gasses will dissolve in liquids in proportion to the pressure and temperature. As water cools, it absorbs air. When water gets hot, it releases air. Now, since the water in the compression tank is relatively cool (compared to the water in the pipes), it will absorb some of the air that’s inside the compression tank. That’s only natural.
Here’s what happens next. The hot water in the pipes rises by buoyancy into the tank as the cold water in the tank sinks down into the pipes. These flows pass each other in the single line that connects the compression tank to the system piping. We call this "gravity circulation" and it has nothing at all to do with the circulator. It’s just a natural phenomenon. Now watch this. The air that got absorbed up there in the tank eventually winds up in the piping because of this gravity circulation, and once that air-laden water comes up to temperature, the air gets released as bubbles. Those bubbles get pumped out to some radiator where they settle out of the flow because the water’s velocity out there in the system is usually less than it is closer to the circulator’s discharge. Someone winds up venting the radiator. That makes the system pressure drop. The fill valve senses this and opens, allowing fresh water to enter the system. This water goes up into the tank because that’s the only place that can accommodate it. Remember that the pipes are already filled with water.
So every time this happens, there’s a little less air in the compression tank, and a little more water. That’s how steel compression tanks get waterlogged. The way to prevent it is to use a special fitting that’s designed to stop gravity circulation between the tank and the system piping. Bell & Gossett’s Airtrol Tank Fitting is a good example of what I’m talking about. These things have been around for more than 50 years and they work well. Sometimes, those "old-fashioned" devices deserve a second look, especially if you’re spending a lot of time draining steel compression tanks.