We’ve come a long way since the days of gravity-hot-water heating. The physics haven’t changed a bit, but the systems sure have. Trapped air was a challenge back in the day before we had effective air separators and circulators. The Dead Men would slope their big horizontal pipes slightly upward to encourage the air to rise out of solution when the water got hot. It usually floated up into an open expansion tank in the attic. Have you ever seen one of those? They were often riveted at the top and bottom; most had a gauge glass. Some of those tanks were made of copper. I’ve known a few contractors who licked their lips at that sort of scrap history.

On these jobs, the Dead Men would usually pipe their cast-iron radiators with the inlet and the outlet both at the bottom and on opposite ends because they didn’t have circulators. The water would rise slowly to the top of each radiator as the colder water fell out of the radiator and into the return pipe. This becomes a challenge when you add a circulator to the system because the hot water now finds it easier to just scoot across the bottom of the radiator. It looks like an air problem from the outside, but it’s not. You solve it by throttling the flow into the radiator. 

Oh, and the only way to get the air out of those big cast-iron radiators when they’re bottom-to-bottom connected is to manually vent them, starting with the lower floor and working your way upward. 

Just add water

They filled those earliest systems through a funnel that was at the very top of the piping. Imagine having to carry that many buckets of water up all those stairs. Gosh. 

When city water pressure showed up, the Dead Men filled their systems from the basement, and they used an overflow pipe that ran from that open expansion tank in the attic down to the basement. They’d fill until they got overflow downstairs, and then they’d have to dump some water from the boiler to lower the level in the tank a bit to allow for expansion when the water got hot.

Altitude gauges showed up next, which did away with the need for an overflow pipe from the open tank. These were simply pressure gauges with a modified face that also read feet of altitude above the position of the gauge. A column of water 2.31 feet high (that’s 28 inches) will exert 1-psi static pressure at its base, and that’s what the altitude gauge showed. 

The Dead Men would measure the altitude from the manual fill valve to the center of the expansion tank in the attic. They’d set the movable red needle on the altitude gauge to that height, and then they’d let the water enter. As it rose in the system, the water put static pressure on the gauge. That needle, connected to the gauge’s bourdon tube, moved and when it met the stationary red needle the system was filled. Simple!

Of course, if they added too much water to a system, the water would just overflow from the tank and out onto the roof. That was embarrassing, but it did no more harm than rain on the roof would do. 

A concern, though, was always the air temperature up there in the attic. During the dead of winter, the water in the open tank was liable to freeze, and that presented a danger since the frozen water would block the expansion space. That’s why they moved the tanks to the basement around 1915, giving us what we came to call the compression tank. Compression tanks compress air because they’re closed. Expansion tanks allow water to expand. They’re always open. That’s the difference and why we use two different names for the tanks. Few jobs these days have expansion tanks. 

Automatic water feeders arrived not long after the altitude gauge and that gave us a way to manually vent radiators without having to monitor the flow of water into the system. Factories set the feed valves at 12-psi because that’s what we need in a typical two-story building. If the building is taller, you have to increase the pressure to get the water to the top. Allow a few extra pounds of pressure once the water reaches the top to vent the highest radiators. And when you’re done with that, make sure you pump up the compression tank’s air pressure to match the water-fill pressure. 

It took a while for us to figure out the best place to install the feed valve was wherever the compression tank was because that tank is the one point in the system where the circulator can’t change the pressure. This is because when the circulator runs it can neither add nor remove water from the compression tank or the line connecting the compression tank to the system. We call this the “Point of No Pressure Change” (sometimes abbreviated as PONPC). Fill at that point and the circulator won’t be able to trick the fill valve into feeding when the circulator starts and stops. 

We also learned that if we pump away from the compression tank, the circulator will apply its full differential pressure to the system. That helps to bust up trapped air bubbles out there in the system and bring them back to the air separator, where we can get rid of them once and for all. Less air in the water means better heat in the radiators — and that’s what we all want. 

We also found the air separator works best if it’s in a spot where the water is hottest, or where the system pressure is lowest. This is because of Henry’s Law, which tells us that gases dissolve in liquids in proportion to pressure and temperature. So the air separator can be near the boiler (the hottest place), or up at the top of the system (the lowest-pressure place). But since all the water has to pass through that big supply pipe that leaves the boiler, this is the probably the best place to put an air separator.

It’s just physics

Boilers sometimes wound up on the roof of a building, and not because the builders were afraid of floods, but because the buildings were tall. Having the boiler at the top of the system makes sense because the pressure up there needs to be only about 3 psi, the same pressure you would need at the top of any hydronic system. That’s the pressure that allows you to vent air from the high point, and to keep the water from flashing to steam, should it be hotter than 212° F. 

Put the boiler in the basement of that tall building, and the pressure is going to be much greater. You may have to invest in a high-pressure boiler and all that goes with that. 

So, sometimes what seems strange at first (the boiler’s where?) makes sense when you think about the physics going on here. 

And that boiler, no matter where it finds itself, is going to have an aquastat, which reminds me of another historical tidbit. The first aquastat had a bellows that moved when the water got hot. It was sensing pressure, not temperature. Attached to the top of the bellows was a fulcrum which attached to a short steel bar, weighted on both ends. Attached to the weights were chains that ran through ceiling-mounted pulleys. These chains controlled the draft doors above and below the firebox. Keep in mind they were burning coal in those days. 

As boiler pressure rose, the bellow expanded, tipping the weights, which moved the chains that closed the dampers. When the fire died down, the pressure receded. The bellows sensed this and contracted, shifting the weights once again, which moved the chains and opened the dampers to give the fire more oxygen. Back came the flames. 

You may get to see the remnants of one of those in some old building. When someone asks you what the heck those old pulleys and chains up there in the rafters are all about, tell them this story.

History has always reminded me of driving. You have to look at what’s behind you as well as what’s ahead. It’s all part of the big picture, and seeing that big picture is what makes you a great troubleshooter.