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How indirect heating works


Dan Holohan
July 16, 2009
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If you're working with folks who are rich, cold, and miserable there's a good chance they'll be living in big 19th Century homes that have heating systems of the "indirect" variety. These can be as confusing as adolescence so I thought I'd tell you what I've learned of them so far.

The Dead Men chose this system because it combined heating with ventilation, both of which were very important folks who could afford central heating in those days. The name "indirect" comes from these meteorite-sized hunks of iron that hung in the basement and directed warm air upstairs to the first, second, and sometimes even the third floor of a big house. The air moved through tin or sheet-iron ductwork that is a joy to behold. Such workmanship! And more often than not, the Dead Men brought in fresh air from the outside to mingle with the basement air and provide ventilation to the rich folks upstairs. At the time, this central heating system was the absolute best that money could buy.

Most of the indirect radiators were put together just like small cast-iron sectional boilers. Some, however, were made of rows of steel fin-tube. In every case, though, it's tough to figure out what size they are because you can't see them and very little information from their manufacturers remains. And you're not about to start trashing that wonderful ductwork, are you?

Here are some things I know for sure, though:

If a steam system served the house, the indirect radiators had to be at least 14 inches higher than the boiler water line. That was to allow for the gravity return of condensate back to the boiler. Most of these systems worked on low-pressure steam vapor. Several ounces did the trick. If you're replacing an old boiler you have to be very careful where you position the new boiler's water line 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 heat.

Within the duct, the indirect radiator has to be a good 10 inches below the top, and eight inches above the bottom. The radiators had to be tight against the sides of the duct. These dimensions are crucial to the proper flow of air across the indirect radiator. Sometimes, a cast-iron unit will fail and you might want to replace it with a homemade nest of fin-tube radiation. Watch what you're doing in a case like this because the flow of air is so subtle here - and so important.

When the Dead Men used the indirect radiators for ventilation as well as heating (which was most of the time), they always tried to get the outside air to come in from the bottom of the radiator. If this wasn't possible, they took the next best choice and brought the fresh air in from the side opposite the warm air outlet. And keep in mind that the way they allowed for the incoming cold air was to have the heated air leave the house through cracks. When you weatherize the house you lose the leaks. That which stops what goes out, also stops that which comes in. Interesting conundrum, eh?

They sized the hot air flue based on the square foot of connected indirect radiation. They allowed two square inches per square foot of radiation if they were heating with hot water. They allowed 1-1/2 square inches per square foot of radiation if they were using steam. 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 it's a hot water job, or by 1.5 if it's a steam job. That will give you a good estimate of the square feet of radiation inside that duct.

Generally, the registers in the rooms are 25% greater in area than the flues that serve them. There aren't any 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. 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 was also known as the 2-20-200 Rule). Simply put, the Mills Rule allowed for one square foot of radiation (steam or hot water) for each 2 square foot of glass, each 20 square foot of exposed wall, ceiling or floor, and each 200 cubic feet of room volume. They'd total these three things and come up with a radiation load, to which they'd add their standard pick-up factors for the pipe load. Once they had this figure, they'd add 25% more if the system was heated indirectly by steam, and 35% more if the room was heated indirectly by hot water. This allowed for the fresh air and for the limited convection currents in the rooms themselves.

Nowadays, even wealthy homeowners often decide to abandon the ventilation side of their indirect systems so they can save some bucks on fuel. If they ask you to help with this, know that you can seal up the fresh air inlet and work only with the air in the basement. But you will have to find a way to get the upstairs air to the basement. Often, a louvered basement door is all it takes to make it work.

You can also estimate the size of the indirect radiator by checking the size of the steam or hot water tapping feeding it. Check this against the size of the hot air flue (as I mentioned before) and make sure the two more or less agree. If they don't, figure the higher of the two for purposes of sizing your replacement steam boiler (base the hot water replacement boiler on the heat loss, not the radiation load).


1-1/4" tapping - up to 80 square feet of indirect radiation.

1-1/2" tapping - up to 100 square feet of radiation.

Hot Water:

1-1/4" tapping - up to 60 square feet of radiation

1-1/2" tapping - up to 90 square feet of radiation

2" tapping - up to 100 square feet of radiation.

And just in case you forgot, one square foot of radiation in steam is equal to 240 BTUH. In hot water work, it's equal to 150 BTUH (assuming the average water temperature is 170 degrees).