JUST A LOT OF HOT AIR

Nowadays, when you’re feeling chilly, you just nudge your thermostat up a few degrees. Not too long ago, you’d have been in for a lot more effort: Until the 1880s, most American houses were still heated by an open fire. 

In those days, any room you wanted to keep tolerably warm had to have its own fireplace and chimney. This is one reason houses had such boxy, compact floor plans--the idea was to have as few of those expensive fireplaces as possible. Often, they were placed back to back so they could share a chimney. All this finally changed in the late nineteenth century, when the innovation of central heating made it possible to warm every room in the house with a single source of heat.

Of course, Americans were hardly the first to have central heating. As early as 100AD, the Romans used the hypocaust system, which conducted warm air from a fire into hollow spaces beneath a tiled floor. Ancient Korea may have used a similar system, called ondal, even earlier. By the 12th century, Muslim engineers had improved the hypocaust by using pipes--our modern heating ducts--which did away with the need for hollow floors.

The English had an early version of central steam heating as early as the 1830s, though it was of course limited to the fabulously wealthy. It took another fifty years for a proper central heating system to make it across the ocean and into ordinary Yankee homes.

Some central heating used steam or hot water piped to radiators, but most heated the air directly. Early systems burned wood or coal, which meant you were still liable to freeze unless you kept the furnace stoked. Later on oil and natural gas prevailed as fuel, since they could be fed automatically. 

All of these early “gravity” heating systems relied on the fact that hot air tends to rise (or more accurately, that gravity makes the denser cold air sink). Of course, since warm air just wafted its way into each room through big ducts, the furnace had to be located below the living space--one reason older houses had basements even on the West Coast.

Once individual rooms no longer needed a bulky and expensive fireplace for heat, houses could be laid out much more freely. The characteristic rambling floor plans of late Victorian houses such as Queen Annes--among the first adopters of central heating--were a direct outgrowth of their liberation from the fireplace.

After World War II, central heating systems began using a fan to actively push warm air through the ductwork. These so-called forced air units could use smaller ducts than the old gravity furnaces, and could be located anywhere in the house, even in the attic.

Thanks to our increasingly urgent quest for energy efficiency, today’s central heating systems make even those postwar units look antiquated. Gone forever is that consummate energy-waster, the standing pilot light, and many forced air units now boast efficiencies in the high nineties--about double that of old gravity furnaces. Electronic burner controls and programmable thermostats make it easy to forget that your furnace is even working. But don’t: next time you turn up the heat, think about how far we’ve come in just a hundred and twenty years.