Various materials are used for tankless water heater heat exchangers, both primary and secondary when applicable. This guide to tankless heat exchangers explores:
Let’s compare tankless water heater heat exchanger options.
What is a Heat Exchanger?
As the name implies, a heat exchanger is the medium, the part, through which combustion heat is transferred into water. Heat exchangers are constantly being heated with up to 199,000 BTUs of heat, more than the capacity of most gas furnaces. They’re also cooled by water as cold as the mid-30 F flowing through the pipe surrounding them. As a result, these parts must be extremely durable and resistant to cracks caused by heating and cooling. They should also conduct heat very well. The heat exchanger is termed the primary heat exchanger when the unit also has a secondary heat exchanger.
What is a Secondary Heat Exchanger?
Secondary heat exchangers are found in gas tankless water heaters, gas furnaces, boilers and other gas-fired systems, both natural gas and propane, with condensing technology. Their purpose is to raise the efficiency of heat transfer, often from the low or mid-80s to the mid to upper 90s. In gas tankless water heaters, they do this by using hot combustion gases to heat incoming water before the gases are vented. This is the distinction between condensing and non-condensing tankless water heaters.
There’s more information in our Tankless Water Heater Buying Guide. You might find the entire guide useful, or you can use the Content Navigation box at the top to jump to the section Gas Water Heaters: Condensing vs Non-Condensing.
- Pros: Secondary heat exchangers are so effective that units featuring them can be vented through a wall using PVC, rather than vented through the roof using metal, because the exhaust has little heat left in it.
- Cons: Exhaust gas is very acidic, so it will destroy some metals.
Heat Exchanger Materials with Pros and Cons
Tankless heat exchangers are made from several metals and alloys. This list shows the metals, durability, resistance to corrosion and other advantages and disadvantages.
This is the most common metal used in heat exchangers by all brands. The metal is affordable compared to other options, but its benefits go beyond cost. It has the highest (best) heat conductivity of any metal at 401, about 20 times higher than stainless steel, so heat is rapidly transferred. Copper heat exchangers handle expansion and contraction very well too. The single drawback to copper is that it is slowly dissolved by the acidity of combustion gases while also reducing its conductivity. For these reasons, it cannot be used in secondary heat exchangers.
Stainless steel contains some copper, so it is a copper alloy, but the copper level isn’t as high as in metal labeled copper alloy. The benefit of copper alloy is that it offers excellent thermal conductivity due to the copper, but it resists corrosion much better than pure copper. Takagi condensing water heaters are among those that have copper alloy primary heat exchangers.
This metal offers many good qualities, but it isn’t the best in any category. It’s lighter than copper and stainless steel. Its heat conductivity rating is 237, so about 40% less than copper but 1000% more than stainless steel. It resists corrosion better than copper because it forms its own oxide layer that buffers it from acidity. Several Bosch units have aluminum secondary heat exchangers including the Bosch Therm C 1210ES and Greentherm C950ES Series.
Almost all secondary heat exchangers are made from stainless steel because they must be highly resistant to the acidity of combustion exhaust gases. An exception is the Noritz NRC98 Series units with commercial-grade copper heat exchangers.
There are various grades of stainless steel, though not all manufacturers reveal what type they use.
Takagi uses 316L stainless steel, a marine-grade stainless suited for salt environments. The “16” refers to 16% chromium in the blend.
Noritz uses STS 304 stainless steel heat exchangers. STS304 is the European name for A2 or 18/10 stainless steel with about 18% chromium and about 10% nickel.
Navien uses stainless steel for both heat exchangers, not just secondary exchangers. This ensures the best corrosion resistance while still offering efficiency ratings up to .97 UEF, among the highest in the industry. That’s one of the reasons Navien is comfortable offering an industry-best 15-year warranty on the heat exchanger, a length matched by Takagi and Bosch.
Noritz EZ111 and EZ98 models have dual stainless steel heat exchangers too, though its warranty is just 12 years.
What Material is Best for Heat Exchangers?
As we’ve noted, each has its pluses and minuses. For low cost and heat conductivity, copper is tops. For fighting corrosion, stainless steel can’t be beaten.
We have two recommendations on the matter, and both apply to condensing tankless heaters:
1). If you buy a condensing gas tankless water heater, avoid models without stainless steel secondary heat exchangers.
2). If lower cost is your goal, copper primary heat exchangers or alloys are fine. When you’re willing to pay more to get better durability, dual stainless steel heat exchangers are the best option.
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