Got a burning question about your radiators? Whether you need to know how to bleed them or are wondering why they're silver, we've got all the answers in this comprehensive guide.
The pipe on this wall likely feeds a radiator sitting on the floor above this one-pipe steam unit. Photo by Alli Coate.
Hot-Water Radiators 101
In hot-water radiators, a pressure-reducing valve between the city water and your hot-water heating system keeps it filled all the time. Most two-story homes need 12-psi pressure, and that's the factory setting of the valve. If your old house has three stories and there are radiators on the top floor, you may need to adjust the valve to feed water at 18-psi pressure to make sure the radiators at the top are full.
Once filled, a circulating pump moves the heated water from your boiler to your radiators and back. In the old days, many hot-water heating systems didn't have circulating pumps; the water flowed by gravity, with hot water rising and cold water falling. Because of this, many freestanding, cast-iron radiators have their pipe connections at the bottom of the radiator. The heated water enters the radiator and rises by convection, while the cooler water inside the radiator falls back to the boiler.
Before circulating pumps arrived, the path of least resistance for the water was always the top-floor radiators. The old-timers slowed the flow to the uppermost radiators by inserting a metal orifice (a nickel-size piece of metal with a small hole) inside the radiator's supply valve. A contractor friend told me his grandfather would make these from Prince Albert tobacco tins. He would use tin snips to cut a circle and then punch a hole with a nail—worked like a charm.
The challenge, though, is that when you add a pump to the system, the path of least resistance shifts to the radiators on the first floor, and that often causes the radiators upstairs to be cold. Where there is no flow of hot water, there is no heat. If you vent and don't get any air, and the radiator still doesn't get hot, this is most likely the problem. The pros know this, and on a troubleshooting call, most will remove the orifices from the upper-floor radiators and install them on the lower-floor radiators to help balance the system.
Steam Radiators 101
If you have steam heat, each of your radiators will have either one or
two pipes. All steam radiators take advantage of gravity to get the condensed steam (called "condensate") back to the boiler. The key to making it all work is to keep the system pressure low. If you can't heat your old house with 2-psi pressure or less (that's the pressure the Empire State Building uses), something is wrong.
High-pressure steam can hold the air vents on a one-pipe steam system closed, and with the vents locked closed, the air can't leave the system. If the air can't get out, the steam can't get in. High pressure also can cause the condensate to stay up in the system, and this can lead to hammering sounds and high fuel bills. The device that controls the pressure is the "pressuretrol," and it's on the boiler. For house heating, it should always be on its lowest possible setting.
One-pipe steam radiator sections often are connected only across their bottoms. A section is like an individual slice in a loaf of bread. Steam is lighter than air, so when it enters a one-pipe steam radiator through the supply valve at the bottom of the radiator, it will rise, pushing the air ahead of itself. The air will leave the radiator through the air vent, which is on the last section, and about a third of the way down from the top. Why? If the vent were at the very top of that last section, the lighter-than-air steam would close it before most of the radiator got hot. Remember, if the air can't get out, the steam can't get in.
Two-pipe steam radiators have the steam-supply valve on either the top of the radiator or (more rarely) on the bottom. The return—the pipe the condensate uses to return by gravity to your boiler—is always at the bottom of the radiator. This might take the form of a steam trap, or it might be what we call a "vapor" device, which is found in dozens of shapes and sizes. Unlike one-pipe radiators, you can adjust the supply valve on a two-pipe radiator to let in more or less steam, which is the main advantage of this system. In a one-pipe radiator, the steam and condensate share that confined space within the one-pipe supply valve, and if you throttle that valve, you'll get a lot of banging noises and squirting air vents as the steam tosses the water around in the tight confines of the partially closed valve.