Today, there seems to be a lot of chatter about the new crop of HVAC units made with stainless steel. Although stainless steel can be used for the frame, fan plate, blowers and burners to increase their durability and strength (as compared to other metals), we’d like to focus on the heat exchanger, because that’s where the main “business” of heating forced air happens and where the materials and construction can most affect the efficiency and safety of the unit.
Today’s high-efficiency furnace units have heat exchangers made with aluminum, steel, iron alloys, or stainless steel. If a steel or iron alloy is used, it is typically galvanized, which means it has a zinc coating in order to make the metal resistant to rust and corrosion. Stainless steel has a chromium-rich film on a steel alloy containing 10.5% or more chromium, which makes it very resistant to rust and corrosion and is more durable (lasts longer) than galvanization. But, it’s not just the coatings on the metal, but the properties of the underlying metal that matter, too.
The most important properties for performance are:
- Thermal conductivity – a measure of how easily heat can be transferred (higher numbers are better).
- Tensile strength – how strong or durable the metal is when subjected to high pressures or stress (higher numbers are better).
Aluminum’s thermal conductivity is at least 4½ times greater than that of steel in any form, whether galvanized, stainless, or uncoated. For relative comparison, the thermal conductivity of aluminum is 1536, while steel is 314, and stainless steel is 108.
Thermal conductivity affects the speed and the amount of energy required to heat up the heat exchanger. Additionally, aluminum is much lighter weight and far cheaper than stainless steel. So, why spend the extra money on stainless steel? Aluminum will warp over time and can corrode.
Stainless steel, on the other hand, resists corrosion, but manufacturers still have to overcome the high costs and low thermal conductivity associated with stainless steel, as well as its considerable heavier weight. Thermal conductivity can be increased by making the walls of the heat exchanger thinner. A thinner-walled pipe is lighter as well. Because of stainless steel’s high tensile strength, it can continue to withstand high pressures even at thin wall thicknesses – something most of other metals cannot do! Next, manufacturers experimented with using “fins” – L-shaped protrusions that stick out from the stainless steel pipes – and found that the performance of stainless steel pipes with aluminum fins was overall 10% greater than even galvanized steel. The new crop of high-efficiency furnaces move air across the heat exchanger much faster than the older style furnaces ever did. Plus, the amount of temperature rise is less in high-efficiency units (35-70 degrees vs. 70-100 degrees), so thinner steel materials can be used.
Some high-efficiency units have two or more heat exchangers. The first may be made of something other than stainless steel – such as galvanized steel, aluminized steel or glass coating - because there is typically no condensation here (exhaust products are too hot).
With sealed combustion chambers, it is often difficult to get a look at the heat exchanger(s) – whether one big long one, or multiple shorter ones – to check their condition. Our technician, Larry Waterbarger, found a simple way and took this photo of an 11-year-old galvanized steel heat exchanger in good condition with no observable cracks.
The manufacturer’s warranty says it all: Those made of copper alloys or are tin-plated, typically have warranties of no more than 5-10 years; an aluminized heat exchanger may have a warranty of 15 years, and those made of stainless steel typically carry a lifetime warranty.
Safety is a big concern. If a heat exchanger (made of any material) develops a crack, deadly carbon monoxide gas is released into your homes air ducts. You should have a carbon monoxide detector, but the first step is to check your heat exchanger and look for possbile cracks.
It's Your Choice
So, is a stainless steel heat exchanger worth all the hoopla and extra cost? Now that you have this information, get quotes and you make the call.