What is the difference in efficiency between upflow and downflow furnaces?

Question

I have been doing a bit of research on HVAC systems, and recently learned about upflow vs downflow configurations, and understand that upflow is supposed to be more efficient for heating and downflow is more efficient for cooling; however, I can't seem to find any quantitative information about how much more/less efficient the configurations are.

I'm curious because we recently bought a house in a southern/warmer climate, and the HVAC units are in the attic. According to everything I've read, that would be best suited to downflow, yet both of the units in the house were upflow (and with 95% efficiency furnaces).

So does it make any meaningful difference? Are we talking large numbers of dollars on my gas/electric bill, or just a few cents? (I'm inclined to believe it's the latter, but I haven't been able to find anything to confirm or invalidate that)

Answer 1

The benefit of air rising (not heat*) is negligible in contrast to the powerful fan and the fact that the path of the duct likely has much more elevation change than the duct trunk at the furnace cabinet. It would be more convincing to argue that a furnace should be in a basement rather than an attic to benefit from convection, but ultimately the weight of the air (what we're really talking about when we say "rising") isn't that important. Many modern units can be installed in either orientation--upflow in a basement and downflow in an attic, for example, or even on their side in some cases. I've always taken it to be a matter of installation space considerations and not efficiency.

In any orientation it's air over a heat exchanger. I don't think the air cares which way it's blowing. In all orientations the flame from the jet passes into an opening in the heat exchanger horizontally. Flipping the unit along its front panel plane doesn't change that.

Most of the articles a person comes across in searches seem to have been written by marketing departments or other people without much scientific expertise just to flesh out their websites for SEO (and which probably contain regurgitated false wisdom).

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* Conduction, convection, radiation--the three primary modes of heat transfer. Only in convection is gravity involved, and the heat must be carried by its medium to move in that way (radiation and conduction are always happening, but are less important here). It doesn't go anywhere by itself. Therefore, warm air rises, not heat, and only when in a parcel surrounded by cooler air.

Answer 2

A person might assert that it's most efficient if the equipment works with natural convection rather than opposing it: blow cold air downward; blow warm air upward. I believe this doesn't really matter. Ultimately the job of the blower is to overcome not only convection forces but also duct friction to push the conditioned air to where it is needed.

There are whole-system advantages when a space is cooled by blowing in air through registers in the ceiling, or when a space is heated by blowing in air through registers in the floor. Mainly this is due to the fact that good mixing naturally occurs between existing room air and newly-conditioned room air. Cold air delivered at the ceiling is going to naturally settle downward toward the floor, mixing with and cooling existing room air on its way, and similar happens with warm air delivered at the floor. We generally prefer our heads to be in cooler air and our toes to be in warmer air. We might be able to set the thermostat a degree or two warmer or cooler than normal as a result, and that's an efficiency gain.

Furnaces exist for upflow, downflow, and horizontal flow mostly for practical reasons having to do with the duct system. A system in an attic would often be positioned horizontally partly so the ductwork can lay on the attic floor, but also because many attics are not tall enough to fit HVAC equipment vertically. Whether the furnace is upflow or downflow doesn't matter when it's on its side!

In some buildings it makes more sense to put the ductwork in the floor; we use downflow furnaces to make that possible.

There is one big drawback to downflow furnaces: the burner is at the "bottom" of the furnace but exhaust gas has to go up. The exhaust of a non-condensing furnace needs to rise; the liquid condensate of a condensing furnace needs to drain/fall back toward the furnace. In either case the flue has to pass in front of the blower, which makes future maintenance of the blower a little more difficult.