Many HVAC devices use what is known as a “heat exchanger” to transfer heat from one fluid (e.g., hot water in your water heater) to another (e.g., domestic water running to your tap). The heat exchanger – in the most basic terms – is a piece of metal that allows the heat from one fluid to pass onto another fluid. Heat exchangers come in many forms, but it is the metal that composes your heat exchanger that often makes one of the biggest impacts.
While you have a large array of choices for the metal used in your heat exchanger – metals ranging from bronze and titanium to brass and carbon steel – copper and stainless steel tend to be more common as a result of their low manufacturing costs and high effectiveness. The main concerns of a homeowner should be thermal conductivity, durability, and price. In this guide, we look at the differences between copper and stainless steel heat exchangers.
The thermal conductivity of a heat exchanger determines how quickly the heat exchanger passes the heat from the heating source to the distribution fluid. In this regard, a quick look at chemistry books should show copper as the clear winner. Thermal conductivity, measured in watts per meter pre Kelvin, various across metals:
- Copper: Up to 401
- Stainless steel: high teens
On average, the thermal conductivity of copper is 20 times that of stainless steel. In practical terms, this means that copper can transfer heat 20 times faster than can stainless steel. So, if you have a need for quick heating, copper will work to your advantage.
Why would you need to heat something quickly? For example, if you own a swimming pool and plan on going swimming on an autumn day, a water heater with a copper heat exchanger can get your pool ready for you much faster. With a stainless steel heat exchanger, in contrast, you could find yourself waiting up to 72 hours before your pool is heated to 10 degrees Celcius.
But even if you don’t have a need to heat things quickly, the higher thermal conductivity offered by copper equates to higher efficiency. This can lead to lower energy costs. After all, a heater or boiler that’s running longer to heat your home, pool, or tap water is going to cost you more.
When we speak of durability, our main concern is whether the heat exchanger can handle the corrosive condensate that is released by a boiler. The main type of boiler used in modern homes in the condensing boiler. In this boiler, condensate makes contact with the heat exchanger.
This is a problem because condensate is highly corrosive. A heat exchanger that cannot stand up to the condensate will quickly corrode, requiring a time-consuming and costly replacement. Thus, choosing a heat exchanger that can resist corrosion can benefit you in the long term.
Here, the winner is stainless steel. Unlike standard steel, which is susceptible to corrosion, stainless steel has a property known as “passivation,” which refers to its ability to form a layer of oxide on itself in response to contacting air. This layer of oxide protects the metal from corrosion and rust, allowing for a longer lifespan.
Copper, in contrast, is much more vulnerable to corrosion. Condensate turns copper atoms into copper ions, effectively dissolving the metal. This is a big problem not just because of the lower lifespan but also because a corroded copper heat exchanger loses some of its efficiency, reducing the thermal conductivity, its main advantage.
Copper heat exchangers tend to be cheaper than stainless steel heat exchangers. Because of this, some homeowners opt for copper. But copper can end up as the more costly choice, with its lower lifespan due to its susceptibility to corrosion.
Generally, boiler, water-heater, and heat exchanger manufacturers offer copper as a choice primarily for its cost-effectiveness. These companies are aware of the trade-off between the cost and the lifespan. Much like electric vs. gas boilers, that trade-off is a “pay now or pay later” issue: either pay more upfront for a device with a longer lifespan or pay more later to replace and repair a device with a shorter lifespan.
Much like the gas vs. electric issue, the ultimate choice comes down to whether you are thinking long-term or short-term. If you plan on adding value to your home by installing high-quality HVAC equipment, go with the long-term option (e.g., gas boilers and stainless steel heat exchangers). The long-term option will save you money and reduce the need for HVAC services and replacements.
So it should be obvious that stainless steel, the more costly of the two metals, is better for long-term thinkers. However, should you really need a heat exchanger with the highest conductivity to quickly heat large bodies of water (e.g., a pool) or huge homes (e.g., a mansion), copper – the higher maintenance metal – might be the better choice. Of course, stainless steel can do everything copper can, just at a slower pace and slightly higher price.
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