Gas Furnace Efficiencies (80% vs 90%+) - What One is Right for You?

So, you're looking into a new furnace, but where do you start? There's different brands, efficiency ratings, number of stages, and sizes. With so many factors to choose from it can be difficult to know if you are making the right choice for your home. This article will focus on the different efficiency ratings of furnaces, and a brief description of how they work.

I'll start out with AFUE (annual fuel utilization efficiency) ratings, or simply just efficiency. Old furnaces back in the 70s or before were commonly 70% or less efficient. Luckily over time manufactures have gotten better and better at bringing those numbers up into the 80-90% range. Today there is two basic categories as far as efficiency for gas furnaces. There are 80%, also known as standard efficiency, and 90%+, or high efficiency.

The 80% furnace is a simple open combustion design, meaning the burner compartment is open to the room and draws air from the room for combustion. This furnace has only one basic heat exchanger, usually tubes, which carry the combustion gases from the burners through the air stream and then out the flue. As the blower in the furnace passes air across the heat exchanger, the heat from the combustion gases is transferred to the air being circulated through the ductwork. These furnaces do not produce condensate like 90%+ units. Many homes built in the recent past were fitted with 80% furnaces because they are cheaper and simpler to install. These furnaces work well, but use more energy to provide the heat for your home.

The 90%+ furnace is a bit more complex than the standard units, and of course more expensive. Currently there are a few options to choose from ranging from 95% all the way to 98% AFUE ratings. 95% being a single stage and 98% a fully modulating furnace, more on that in another article. High efficiency units are more costly upfront, as well as to install. These furnaces have a primary and secondary heat exchanger (which is built from expensive Stainless Steel). Unlike the 80% which runs the combustion gases through one simple heat exchanger to transfer the heat into the air of your home, the 90%+ runs them through a second very efficient heat exchanger to get every bit of heat possible out of the combustion gas. This process cools the gases so much that the water vapor begins to condense to liquid in the furnace. This condensation would (and if allowed to happen does) destroy a standard 80% furnace. Thankfully this doesn't hurt the 90%+ furnace, because the secondary heat exchanger is built from stainless steel, which will not corrode from the condensation. The condensate forms only in the secondary heat exchanger, and harmlessly flows through a built in water trap, then out to an appropriate drain.

Pros

-Cheaper upfront cost

-Simpler installation

-Simple design = less maintenance

Cons

-Uses inside air for combustion

-Uses more energy = higher cost long term

-Usually doesn't qualify for energy rebates

Pros

-Uses less energy = Less cost long term

-Quieter operation

-Uses outside air for combustion

-Often qualifies for energy rebates

Cons

-More complex system

-Higher upfront cost

So there is the basic difference between the 80% and 90%+ gas furnaces. Everybody is interested in saving money on their energy bills, and helping out the environment a bit by using less propane/natural gas, so 90%+ furnaces have become very popular. Unless upgrading to a 90%+ unit isn't possible, we don't install very many 80% furnaces these days. If you are looking into a new furnace, 90%+ is the new standard for comfort and efficiency. Hopefully this article taught you something new! Check out our other blog posts for information on other topics, we try to add new articles regularly.