UPDATED on March 2, 2017 with information on the Dettson furnace rated at 15,000 Btu/h.
Why are the smallest available American furnaces rated at about 40,000 Btuh? Back in the 1960s, a house in a cold climate may have needed such a powerful furnace — or even one rated at 60,000 or 80,000 Btuh. But these days, many new homes have design heating loads that are much smaller — as low as 10,000 to 20,000 Btuh. Over the past 30 years, building envelopes have become tighter and better insulated, but U.S. furnace manufacturers haven’t kept up with the times. For mysterious reasons, they don’t offer furnaces that are small enough for today’s energy-conscious builders.
I wrote about this frustrating problem in a 2009 article, “Heating a Tight, Well-Insulated House.” In the four years since the article was written, furnace manufacturers haven’t budged; their smallest models are still twice as big as most energy-efficient builders need.
John Straube outlines the problem
John Straube is a professor of building envelope science at the University of Waterloo in Canada, as well as a principal at the Building Science Corporation in Massachusetts. Straube has been wrestling for years with the question of how to heat small, energy-efficient homes. On February 7, 2013, Straube gave a presentation on the topic (“HVAC for Low-Load Buildings”) at the Better Buildings by Design conference in Burlington, Vermont.
Straube defines a low-load house as one with a design heat load of less than 25,000 Btuh or a design cooling load of less than 1.5 tons. Homes like this are increasingly common. “Peak demand for superinsulated houses of 2,000 square feet is often 20,000 btuh or less, and townhouses can be under 12,000 btuh,” Straube noted in Burlington.
Finding the right equipment is hard
If you install conventional (oversized) equipment in a low-load building, says Straube, “You get a choice of freezing or…