Making old furnaces new again, with innovative retrofits that improve efficiency

Research Insights share findings from CEE's energy efficiency research, drawing real-world links to help Minnesotans save energy and money, while lowering carbon emissions.

Many of the furnaces in Minnesota’s single-family homes are 15 years or older, and usually aren’t replaced until they either stop working or become less effective. Generally, the older the furnace, the less efficient it is — most standard furnaces operate at efficiency levels 15% lower than current technologies on the market, resulting in wasted energy and higher heating bills.
But just because you have an older furnace, doesn’t mean you have to replace it. You can modify — or “retrofit” — your existing furnace to help the unit operate more efficiently, which will save you energy and bring down those electric bills. New transport membrane humidifier (TMH) technology offers a great approach to retrofit an old furnace; it can even make your unit more efficient than newer models on the market. Transport membrane humidifiers work by transferring waste heat and water vapor from flue gases into the building air supply, an action that not only supports efficiency, but adds humidity to dry winter air.
RI-furnace.pngCEE’s Field Study of a Moisture and Heat Transfer Furnace Retrofit Device, led by senior research engineer Josh Quinnell, took a closer look at TMH devices to determine how much energy and money they can save consumers; establish implementation protocols for their use; and examine the technology’s non-energy benefits. As part of the study, TMH units were installed in five Minnesota single-family homes, each with standard efficiency (78% to 83%) furnaces. The team monitored and evaluated these units during two specific heating seasons (2015 and 2016), and used the data they recorded to estimate the expected annual and lifetime energy savings across all five sites.
Quinnell says of the study, “Research projects usually seem stale to everyone except researchers, so I was really surprised by the interest expressed by the homeowners, inspectors, and contractors in this project. Not only were they interested in learning about how the technology worked, but they shared my excitement about the savings potential.”
Along with observations about design, sizing, and installation, the data gathered through the study was used to estimate payback periods and market adoption rates. The project’s findings also helped determine how TMH technology can contribute to the achievement of strategic goals set by Minnesota’s Conservation Improvement Program.
Are you thinking about buying a high-efficiency furnace, or adding a retrofit to make your current one more efficient? Here are a few reasons why both are a good idea:

  • They eventually pay for themselves. If you’re purchasing a brand new high-efficiency furnace, the upfront costs can be significant, but so are the savings. Energy savings can add up to about 30% depending on changes in AFUE (annual fuel utilization efficiency), or the ratio of annual heat output of the furnace compared to the annual fuel energy it consumes.

  • Your furnace could last for decades. High-efficiency furnaces include built-in systems that remove moisture inside of the heat exchanger and have the ability to withstand the buildup of chemicals. They also include an efficient on-and-off cycling program that helps them last longer with proper maintenance.

  • They support better air flow, which means better indoor air quality. High-efficiency furnaces emit heat more uniformly and include filtration systems. These systems remove dust and mold from the air, well-known triggers for asthma and other respiratory problems.

Want to dig in further? Learn more about the field study of moisture and heat transfer furnace retrofits:

To learn more about our other CEE research projects, check out our project pages and Field Notes at

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Comment by Franco Oyuela on August 30, 2018 at 11:13am

Even if your furnace has a few years left in it, it may not be cost efficient to keep it if your energy bills during the heating season are high due to the furnace’s horrible efficiency. With that in mind, you need to assess how much it’s costing you just to keep the current furnace you have versus how much you’ll save with a new furnace.

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