Steam heating arrived first, and since steam is lighter than air, it will always rise to the top of a radiator when it enters from the bottom (as it does in every one-pipe-steam radiator). It also tends to stay at the top of the radiator when it enters from the top, as it usually will when it’s in a two-pipe radiator.
Most one-pipe radiators had nipples across only the lower part of the radiator sections. We call these “column” radiators. They’re the ones with the wide sections. The steam entered from the bottom and rose, displacing the air as it did so. The air moved from section to section and then left the radiator by way of the air vent, which was always in the last section. You’ll have a tough time trying to use column radiators on a hot-water system because you can’t vent the air from the individual sections. You’d have to drill and tap each section for an air vent, which would look dopey.
So that’s a good place to look first if you’re thinking about converting an old steam system to hot-water. Do the radiator sections have nipples across both the top and bottom? If they do, they’re probably hot-water radiators, which the Dead Men started using on steam systems around 1900. That’s when vapor heating became popular.
Vapor heating is a system that runs on very low pressure – from zero to eight ounces, and never higher than that. This system became popular at the turn of the century because, before that, when central heating was new, the contractors were putting in systems that blew up an American building every 36 hours. They thought it was normal to have 60-psi pressure at the radiators back in those days. Blowing up buildings became sort of a national sport.
In 1898, The Carbon Club, which later became I=B=R, passed some rules that had to do with pipe sizing and pressure drop, and they made the bold statement that from that day forward, there wasn’t going to be a steam-heated building that couldn’t heat on two-psi or less. Things got better right away.
The hot-water radiators on those vapor systems had two pipes, and the supply pipe usually entered the radiator at the top. The steam worked its way across the top of the radiator, giving up its latent heat energy to the cool cast iron, and its sensible heat as well because the condensate dribbled from top to bottom. Compared to a one-pipe-steam radiator, two-pipe, vapor heating felt so much nicer. Those radiators, running at low pressure and heating from top to bottom, gave a more even radiant glow, and it all had to do with those upper push-nipples.
Many of the vapor radiators had no steam traps. Instead, they used an orifice device, either on the outlet side of the radiator, or within the supply valve. An orifice is a small hole through with only a certain amount of steam will pass at a given pressure. The Dead Men used charts to tell them the proper pressure for any given day, depending on the outdoor air temperature. This is why it’s so important to operate those old vapor steam systems with a vaporstat, rather than a standard pressuretrol. You need ounces of pressure, not pounds of pressure. Too much pressure will push through the radiator and pressurize the return main, and that will keep nearby radiators from heating.
Some of the old radiators had supply valves with a lever-type of handle. The lever had a metal pointer that you could position over these raised gradations on the supply valve’s bonnet. When first setting up a system such as this, the Dead Man, would move a peg on the valve’s bonnet that allowed the supply valve to open only to a certain point, and no further. He did this because this type of radiator valve contains a variable orifice, and each of those markings on the bonnet represents 10 square-feet of radiation. The Dead Man would install a radiator, and let’s say this particular radiator could put out 100 square-feet of Equivalent Direct Radiation. He would then set the valve at the 80-percent-of-maximum marking (80 square-feet EDR, in this case). When an occupant opened the valve fully, no more than 80 square-feet of steam could enter the radiator when the boiler was at a certain pressure. And since the radiator is capable of condensing 100 square-feet EDR, the only thing leaving that radiator would be condensate. That does away with the need for a radiator steam trap.
This brings us to today, when a contractor might remove that old supply valve because it’s broken, and replace it with a modern supply valve, which contains no orifice. It’s wide open. The steam will now blast through the radiator and enter the return lines, where it will block air from leaving other radiators, keeping them from heating. The usual reaction to this is to raise the pressure, which just makes things worse. The real solution is to install an orifice plate in the union connection of the new supply valve. The Dead Men often cut their own orifice plates from their
On those old hot-water systems, the Dead Men would use radiators that had nipples across the top and bottom, of course, and since those old systems worked by gravity (circulators hadn’t been invented yet), they would connect the radiator bottom-to-bottom. The hot water would enter the radiator at the bottom and rise to the top because it was lighter than the colder water that was already inside the radiator. The colder water would fall out of the radiator through the pipe on the bottom of the radiator at the opposite side and return to the boiler. That set up a nice convective current.
But when a contractor adds a circulator to one of those old systems, the hot water will zip across the bottom of the old radiator and leave through the other side. When you go on the job, the radiator feels hot at the bottom and cold at the top. This is because the water is moving too quickly to set up a convective current within the radiator. It appears to be an air problem, and you’re temped to bleed the radiator, so go ahead and try, but if you don’t get any air when you bleed, it’s not an air problem, so stop bleeding right away. What you have to do now is slow the water as it enters the radiator so that it will set up a convective current within the radiator. Want more heat? Throttle down on the inlet valve; don’t open it up. It sounds crazy, but it works.
Here’s something else I had to learn the hard way. Some of those older hot-water jobs had no compression tanks. Wander around the building as much as you want; you’re not going to find the darn thing. So how did the system operate without popping the relief valve? The Dead Men left air in the top of each of those bottom-to-bottom-connected radiators. The trapped air combined to act as the air cushion to take up the expansion of the hot water. It was ingenious, and it saved them money, but then along comes the modern installer. He works on the system and bleeds the air from all the radiators. He thinks he’s doing something good. What he’s doing is removing the “compression tank.” And that’s when the relief valve starts to pop on every call for heat.
I had to learn that one the hard way.
Another thing I had to get through my thick skull is the fact that old hot-water radiators contain a lot more water than fintube baseboard. Nowadays, the boiler manufacturers rate their boilers using a piping-and-pick-up factor of 1.15 for hot-water systems. That’s fine for baseboard and even for radiant, but If the system you’re working with has old radiators, and the big pipes that usually accompany those old radiators, that 1.15 pick up factor isn’t going to be enough. This is why all the boiler manufacturers show a boiler bypass in their installation instructions. Without that bypass (or primary-secondary piping), the new boiler will never reach its high-limit temperature. It will use more fuel than the old boiler used, and it will make everyone involved with the job miserable.
But it won’t make you miserable because you’re the one with the answers. Go get ‘em.