There once was a type of radiation that hung heavily inside metal ductwork in the basements of rich people’s homes. Much of it is still there. The Dead Men called this “indirect radiation” and they chose it for the wealthy because it combined heating with ventilation. The name "indirect" comes from the idea that the radiators aren’t in the same room as the rich people. They preferred their radiators to be discrete.
Indirects heat the incoming air, which then rises through the ductwork to the first, second, and sometimes even the third floor of a big house. They put together most of the indirect radiators just as you would small cast-iron, sectional boilers, and then they muscled them into place. Some of the indirects (and these came later) were made from rows of tightly spaced, finned-steel pipe. But whether they were steel or cast iron, it's often difficult to figure out what size these indirect radiators are because you can't see them. Unless you want to tear open the ductwork. And you don’t want to do that. So here are some tips that should help.
First, watch out when you’re replacing a steam boiler. Any indirect radiator has to be at least 14 inches higher than the steam boiler’s waterline. This is to allow for the gravity return of the condensate back to the boiler. Most of these systems worked on very-low-pressure steam, usually just a few ounces. If you're replacing an old boiler you have to be very careful where you position the new boiler's waterline because indirect radiators often hang very low. If you set your replacement boiler too high, it may partially fill the indirect radiator with water, and that will seriously cut down on the radiator’s output.
Look closely around any indirect steam radiator and make sure the air can get out. Ask yourself that key steam-heating question: “If I were air, could I get out?” Look for an air vent on the outlet side of the indirect. The air has to be able to make it completely through the unit if the steam is to arrive on time inside the unit. I mention this because many of those vents are gone now. They leaked and some knucklehead replaced them with pipe plugs. Plugs don’t vent well. That’s why we call them plugs.
Within the duct, the indirect radiator has to be about 10 inches below the top, and eight inches above the bottom. The radiators has to be tight against both sides of the duct. These dimensions are crucial to the proper flow of air across any indirect radiator. Sometimes, a cast-iron unit will fail and you might want to replace it with a homemade nest of fin-tube radiation because no one makes cast-iron indirect radiation these days. Watch what you're doing with the fin-tubes, though, because the flow of air is so subtle here, and so important to the unit’s Btuh output. Respect those dimensions I just gave you.
When the Dead Men used the indirect radiators for ventilation as well as for heating (which was most of the time), they always tried to get the outside air to enter from the bottom of the indirect radiator. If this wasn't possible, they took the next best option, which was to bring the fresh air in from the side opposite the warm air outlet. There are no return-air ducts in this system. The ventilation air enters the house without benefit of a fan. The only way for it to do that is for warm air that’s already in the house to leave though the cracks around poorly fitted windows and doors. If you weatherize the house, you’ll lose those leaks, and if the warm air can’t escape, the cold air can’t enter. All ventilation, and a good portion of the indirect radiator’s output, vanishes when you weatherize.
Interesting conundrum, isn’t it?
The Dead Men based the size of the hot-air flue on the square feet of connected indirect radiation. They allowed 1-1/2 square inches per Square Foot of E.D.R. when they were heating with steam, and two square inches per Square Foot E.D.R. when they were using hot water. They sized the cold air flue to be somewhere between two-thirds and three-quarters the size of the hot air flue.
If you have absolutely nothing else to go by, you can measure the length and width of the hot air flue to get an idea of what's happening. Multiply one by the other to get square inches. Then divide the total by 2 if you’re heating with hot water and 1.5 if you’re heating with steam. That will give you a good estimate of the square feet of radiation inside that duct. Another way to guesstimate is to look at the pipe size feeding the indirect radiator. For steam, the Dead Men would generally use a 1-1/4" pipe to feed up to 80 Square Feet E.D.R. of indirect radiation, and a 1-1/2" pipe to feed up to 100 Square Feet E.D. R. of indirect radiation. If it was a hot-water job, they would use 1-1/4” for up to 60 Square Feet, 1-1/2” for up to 90 Square Feet, and 2” for up to 100 Square Feet E.D.R... And keep in mind that they weren’t using pumps on those hot-water systems; this was based on gravity flow.
Generally, the registers in the rooms are 25% greater in area than the flues that serve them. Again, there are no fans to move the air in this type of system. Everything works by natural convection. That means the air moves more quickly to the upper floors than it does to the lower floors because of the chimney effect of the taller, second- and third-floor flues. Typical air velocities are 1-1/2 feet per second to the first floor, 2-1/2 feet per second to the second floor, and 5 feet per second to the third floor. Notice how the air speeds up as it moves higher. Because of these differences in velocity, each flue served only one floor. And since the air moved more quickly to the upper floors, the Dead Men usually made these flues about 25% smaller than those serving the lower floors. They also used smaller registers on the upper floors. This can get tricky if all you're looking at is the register. And please don't try to equate any of this to a modern forced-air system. It's very different.
Because they used this system for ventilation as well as for heating, they had to allow for more radiation. Their general rule of thumb in the old days was to take a heat loss of the space using the Mills Rule, which the legendary John Mills of H.B. Smith fame came up with. The Mills Rule allowed for one Square Foot E.D.R for each 2 square foot of glass, each 20 square foot of cold wall, ceiling or floor, and each 200 cubic feet of room volume. The Dead Men would total these three things and come up with a radiation load for the building, to which they'd add their standard pick-up factor for the pipe load, which in those days, was 1.56 for both steam and hot-water work. Once they had this figure, they'd add 25% more if the system was heated indirectly by steam and a whopping 35% more if they were using gravity hot water. This allowed for enough output to heat the cold incoming air. And, by the way, if you use the Mills Rule today you’ll come up with a boiler that could probably heat the house with the roof removed.
Consider how all of this can affect a replacement boiler size if you’re not going to be bringing in fresh ventilation air. Nowadays, even wealthy homeowners often decide to abandon the ventilation side of their indirect systems so they can save on fuel. You can seal the fresh air inlet and work only with the air in the house, but you will have to find a way to get the upstairs air back down to the basement. Often, a louvered basement door is all it takes to make that work.
If your indirect radiation is on a steam system, you’ll have to know its size in Square Feet E.D.R. to come up with the proper size for the replacement boiler. Take all of what I’ve told you here into consideration when you do. I hope it helps.
If the indirects are serving a hot-water system, begin with a heat-load calculation on the building as it is today, and base the size of your new boiler on that. These systems, with their high water content, get along beautifully with outdoor-air reset controls and modulating-condensing boilers. Use that approach and the indirects will find the proper output for any given day. Just make sure the air from upstairs can find its way back down into the basement. And don’t forget to open the ductwork panels so that return air has a way back into the cool side of the heater.