Wheeler On HVACR: The Problem With 80+ AFUE Gas Furnaces
It was back in the early 1980s that we first started to hear of high-efficiency furnaces. That was when Lennox came out with, and started a successful advertising campaign to sell, its new and innovative 90+ AFUE gas furnace. It created a public stir that eventually led to federal legislation that now requires all residential gas furnaces to be more than 80% efficient.
At the time, I was the training manager for a Carrier distributorship, and to counter interest in its rival's product, Carrier Corp. produced a training program (which I taught) that outlined all the methods that companies could use to create higher-efficiency gas furnaces, and the problems each method would create. It's a shame that more furnace manufacturers didn't see this program, because a few of them used the methods that Carrier talked about and had the problems that Carrier foresaw, which resulted in product recalls, customer dissatisfaction, and huge losses - and there are still problems that people are experiencing and may yet be unaware of. What are these problems?
All furnace manufacturers have known of methods to increase furnace efficiency since the early 1920s, because Carrier Corp. (for one) produced a line of very efficient gas furnaces (90+ efficiency) way back then. However, the idea was soon abandoned because they were just too costly to produce and the savings to customers didn't justify the extra expense. Gas furnaces needed some wasted energy to get chimneys and vents to draw properly (to heat them up), and anything that was much more than 80% efficient would have problems with acidic moisture condensation in the heat exchangers, which would cause them to rust out if the moisture wasn't dealt with properly.
Well, the new federal efficiency legislation forced all residential furnace manufacturers (kicking and screaming) into the high-efficiency furnace business, and before long, almost all offered a line of mid-efficiency (80+ AFUE) and a line of high-efficiency (90+ AFUE) gas furnaces. But the question is; was anything really gained?
Before the legislation was passed, all gas furnaces produced between 60% and 70% efficiency, so the jump to 80% efficiency only yielded a 15% to 25% improvement in savings on a gas bill. However, to cause the furnaces to draw in chimneys and vents, a vent fan had to be added to the furnace. This added a significantly higher price tag to the product, and all the savings on the gas bill were just added to the electric bill. In addition, now that vents and furnaces are forced to draw by being slightly pressurized, common venting with gas water heaters isn't a good idea, because the furnace vapors can exhaust into the residence through the water heater flue.
Then there is the problem of acidic moisture that 80+ AFUE furnaces create. Oh, furnace manufacturers are careful to design their equipment so it doesn't eat through their heat exchangers today. However, water does tend to condense in long runs of, or in single-wall sections of, or at the discharge of, aluminum vent pipes, causing them to corrode and leak. And with brick chimneys, the acidic moisture can destroy the cement that holds the bricks in place.
Where does the acid in the moisture come from? Some of it is naturally produced from the odorant in the gas and by the combustion process, and the rest comes from chemicals that are used inside a home, which can combine to create moisture that is as acidic as vinegar. You may have already heard of the results of this problem. And where does the moisture come from? The combustion output of each cubic foot of natural gas is one-third carbon dioxide and two-thirds water vapor. So, burning the gas creates the moisture. In fact, most of the extra energy savings obtained from 90+ AFUE furnaces comes from condensing the combustion byproducts into moisture, which is why these furnaces require a drain to be connected to them.
So, why have I written this article? Because I want you to be aware of problems that your customers may well be having due to some more poorly thought-out federal legislation. And since we now have to live with the laws, perhaps the better choice for customers is 90+ AFUE furnaces that are properly installed with new plastic venting and a drain for the acidic condensate that is formed. <<