Andrew Yeckel > Stories > Furnace replacement

Retrofitting our house for a modern furnace

When we bought our house in 1995 it had a Round Oak Moistair Heating System. These furnaces were manufactured in Michigan by the Round Oak Stove Company, later renamed The Beckwith Company. In 2020, we decided to replace it with a modern HVAC system.

The earliest mention I've found of the Moistair Heating System is 1900. This advertisement appeared in The Saturday Evening Post in 1919. In a curious bit of history, the name of the furnace was initially denied trademark protection on the grounds that the words "moistair heating system" were merely descriptive, but in 1920 the U.S. Supreme Court ruled in favor of the company and granted trademark protection.

This type of furnace has no fans. It relies on hot air rising by buoyancy and is often referred to as a gravity furnace. This method works best for small multistory buildings. These furnaces are often called octopuses because of the many large ducts required for effective air circulation.

The furnace is located near the center of our small basement, where it takes up quite a lot of space.

Originally built to burn coal, the furnace was retrofitted to burn natural gas, probably by the 1950s. This box attached to the bottom of the furnace houses an electronic switch that controls a large gas burner inside the furnace.

This type of furnace has poor fuel efficiency, around 50% compared to 80% or more for modern forced air furnaces. Natural gas prices have been low for awhile, but we expect prices to increase in the future, possibly a lot. The replacement we've chosen is a 96% efficient Lennox. After demolishing the old furnace and ducts, the installers have plenty of space to install the new furnace.

Here the installer is extending gas piping to the new furnace. This is definitely not a do-it-yourself job.

The duct previously attached to this register was much larger than the replacement duct. Custom fittings are manufactured onsite by the installers. This one is mounted on the basement ceiling underneath the living room.

Laying out the new ductwork is a creative enterprise. The new system has a total of eight hot air ducts. This duct snakes over the gas and water pipes and under the joist to reach its outlet in the kitchen floor above.

All four of these ducts return cold air from the main floor of the house to the furnace, to replace hot air that is pumped upstairs.

Here is a closer view showing where the cold air return ducts join together in big box called the plenum. From there the cold air falls downward and flows through a high-quality filter into the bottom of the furnace for reheating.

There are two more cold air return ducts on the other side of the basement that also connect to the plenum. The installers somehow found usable pathways for all these ducts through the mess of existing wiring and piping.

The new furnace is tucked behind the chimney. The round footprint of the old furnace is still evident on the cement floor. Much usable space has been recovered, both on the floor and overhead. The new ductwork is hardly visible in this view across the basement.

Here are some closer views of the furnace from front and back.

           

The white PVC pipes are the combustion air intake and exhaust vent which extend outside the house.

           

Although the old gravity furnace might seem clumsy, it was a very well designed system for its type. The installers were able to reuse all of the existing registers in the house, needing only to add three registers to the two-story addition we had built in 2001.  The vertical duct tucked next to the cupboard in these pictures carries hot air to the second story of the addition. It is the only visible intrusion of the new system into the living area.

            

We added central air conditioning as well. We've never had it before. But we're heading into winter, so it will be six months before we get a chance to use it.

The new furnace, on the other hand, is already getting a lot of use. It can be run from an intuitive WiFi application, which allows us to easily experiment with the heating schedule. It will also allow us to monitor the system and alert us in case of failure when we are away from home. The air quality is much better, too, which matters a lot when we can't open the windows for six months.

We paid a lot of money to the HVAC company, but the work and equipment is of excellent quality. This kind of retrofit is challenging. It requires skill and experience to build a good system, especially in a climate as demanding as Minnesota. Our tests have found that the new system delivers well balanced heating everywhere in the house. No space is too cool or too warm. I think we will hardly need to touch the louvers anywhere.

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Several observations indicate that the Round Oak furnace was not the original heating system for our house. The furnace was installed on top of the cement floor, but other evidence suggests that this floor was originally dirt, which would have been typical for a house of its age. For this reason gravity furnaces are often found embedded in cement rather than on top of it. The presence of the Round Oak on top of the cement suggests a timeline in which a different furnace originally installed onto the dirt floor was replaced after the floor had been covered in cement.

Evidence found in the chimney also suggests that a different furnace preceded the Round Oak. There is repair work around the flue opening where it was apparently modified to fit the Round Oak flue during its installation. This contributes to a general appearance that the furnace installation was a retrofit rather than original to the house.

Flue openings in the kitchen and the master bedroom show that supplemental heat was available on the ground and upper floors. The kitchen flue opening, no doubt used by a cooking stove, remains unfilled, covered only by a decorative metal plate. The bedroom flue opening, presumably attached to a small heating stove, has been filled with cement.

A picture emerges of a simpler, smaller furnace installed on the dirt floor of a fieldstone basement, supplemented by coal or wood burning stoves on the ground and upper floors. All these appliances would have needed frequent tending to maintain output. I can imagine that the bedroom stove was principally used at night when the basement furnace was allowed to burn low. Stirring and fueling the bedroom stove in the middle of the night would have been so much easier than tending the furnace in the basement.

As time moved on, more things happened. The Round Oak furnace was converted to burn gas instead of coal, and automated with a thermostat. The kitchen stove and flue were removed and a gas stove installed. The upstairs stove was removed. These things need not have happened at the same time, but I suspect they did. The Round Oak furnace running on gas was easily powerful enough to eliminate the need for supplemental heating, and automating operation of the furnace meant that it would no longer be necessary to tend a fire during the night.