Condensing Gas Furnaces: the Vulcanaire Supertherm of HVAC

Our government moves to mandate them:

The new proposed rule, announced by the U.S. Department of Energy (DOE) yesterday, would require that gas-burning residential furnaces achieve 95 percent energy efficiency by 2029. The stated goal is to lower consumers’ energy bills in the long run and limit harmful emissions…

The DOE press release says that homeowners would be able to meet their proposed energy efficiency standards by installing condensing gas furnaces, which reuse gas and water vapor that normal noncondensing furnaces vent into the atmosphere.

The technology may be a novelty to residential heating, ventilation and air conditioning (HVAC) units, but it’s not new at all. It’s basically the same as the Vulcanaire Supertherm, which Vulcan produced during the 1960’s and 1970’s.

Schematic of the Vulcanaire Supertherm, from Vulcan’s Bulletin 600B, 1976.

Both HVAC and air compressors attempt to do the same thing: change the properties of air, whether that change is temperature, pressure or both. The main difference is that HVAC units are primarily built to change temperature while air compressors are built to change pressure. The Supertherm acted on the air’s temperature to increase its volume and thus its ability to do useful work, while condensing furnaces act on the air’s temperature to directly increase that temperature and by doing so its ability to heat the building.

One of the first tests of the Vulcanaire Supertherm at the yard of Horn Construction, Long Island, November 1964

The Supertherm lasted long enough in Vulcan’s product line to enter the energy cost (and inflation) dominated 1970’s, but its post mortem was as follows:

Although the unit worked as intended and performed well, the simple concept didn’t translate into a simple design. With numerous parts and complex fabrication and assembly, the unit was uneconomical to produce and difficult to install. Air compressor manufacturers learned how to use hot exhaust gases in other ways to improve the energy efficiency of their products.

By the late 1970’s, even with the elevated energy prices of the era, production of the Vulcanaire Supertherm had gone cold.

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