Heat Exchangers: Copper vs. Stainless Steel

Many HVAC appliances have a component that is known as a &#;heat exchanger.&#; They come in many forms, but it is the metal that composes your heat exchanger that often makes one of the biggest impacts.

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What this piece of metal does is transfer heat from one fluid (e.g., hot water in your water heater) to another (e.g., domestic water running to your tap). There is a good amount of choice in the metal you can use for that heat exchanger, ranging from bronze and titanium to brass and carbon steel.

However, copper and stainless steel heat exchangers are the most commonly used because they are less expensive and still highly effective. One of the most common questions we are asked by our customers is some variation of: which is better a copper or stainless steel hot water cylinder, water heater, boiler, or other HVAC appliance?

The main concerns of a homeowner when choosing between copper and stainless steel should be thermal conductivity, durability, and price.

In this guide, we look at the pros and cons of copper and stainless steel heat exchangers.

Which Is Better: Copper vs. Stainless Steel Heat Exchangers?

Thermal Conductivity

The thermal conductivity of a heat exchanger determines how quickly it transfers the heat from the heating source to the distribution fluid. In this regard, a heat exchanger with copper is much faster at transferring heat than stainless steel.

Here are the basic thermal conductivity levels, measured in watts per meter pre-Kelvin, of the two different metals[1]:

• Copper: up to 401
• Stainless steel: lower than 20

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. So, if you need quick heating, copper will work to your advantage.

Why would you need to heat something quickly? That&#;s an important question to ask if you are choosing between, say, a copper vs. stainless steel tankless water heater.

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, you could find yourself waiting up to 72 hours before your pool is heated to 10 degrees Celsius.

Even if you don&#;t need to heat things quickly, the higher thermal conductivity offered by copper also leads to higher efficiency. As a result, using a heat exchanger with copper will lead to lower energy costs. After all, a heater or boiler that has to run for longer to heat your home, pool, or tap water is going to cost you more.

Durability

Durability is a big concern for heat exchangers when it comes to appliances like a boiler. This is because condensing boilers (the most popular type right now), release a corrosive condensate that can eat away at the metal in the heat exchanger.

A heat exchanger that cannot stand up to the condensate will quickly corrode, requiring a time-consuming and costly replacement. As a result, you will likely want to choose a heat exchanger that can resist corrosion over the long term.

In this case, the clear winner is stainless steel. Unlike standard steel, stainless steel has a property known as &#;passivation.&#; This refers to its ability to form a layer of oxide on itself in response to contacting air.[2]

This layer of oxide protects stainless steel from corrosion and rust, allowing for a longer lifespan than regular steel. It is essentially perfect to use in any heat exchanger that will be in contact with corrosive elements.

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On the other hand, copper is much more vulnerable to corrosion. The condensate turns copper atoms into copper ions, effectively dissolving the metal over time. This is a big problem for two reasons. First, because of the lower lifespan; then, because a corroded copper heat exchanger loses efficiency.

Considering that higher efficiency and thermal conductivity was the advantage for copper, having it reduced balances out the other way.

Price

Copper tends to be cheaper than stainless steel when purchased in the same quantity, and that holds true when used in heat exchangers. While that may tempt you into getting copper for your heat exchanger, remember that it is much less durable. You will have to buy more copper replacements to maintain its efficiency levels. As a result, copper can actually end up being more expensive in the long term.

Generally, you will find that heat exchanger manufacturers will offer copper as the default choice because they&#;re cheaper. These companies are aware of the trade-off between the cost and the lifespan, where the cost is a &#;pay now or pay later&#; issue. You either pay more upfront for a stainless steel heat exchanger that will last longer or pay later to replace the copper one sooner.

Overall

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 larger homes, then copper 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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Stainless steel resists corrosion in a wide range of pH levels, while aluminum will corrode if the proper fluids are not used to produce and maintain a narrow pH range. Aluminum heat exchangers require the use of special manufacturer recommended heat transfer fluids and inhibitors when starting up and maintaining the system. If the proper fluids are not used, there is a risk of damage to the heat exchanger, and manufacturers of aluminum heat exchangers may not honor their warranties if the proper fluids have not been used. Stainless steel heat exchangers, on the other hand, do not require the use of special fluids and are compatible with plain clean water and commonly available propylene glycol.

Aluminum heat exchangers are also much more likely to suffer damage if not maintained at regular intervals.

Annual maintenance is required both to monitor the fluids and to clean the heat exchanger to remove aluminum oxides that can build up and clog the condensate line. If not properly maintained, the glycol solution in aluminum heat exchangers will degrade, corroding the aluminum and creating &#;grey goo&#; in the condensate trap. Stainless steel heat exchangers are much cleaner internally, with none of the grey goo produced by aluminum.  Annual maintenance is recommended for stainless steel heat exchangers, too, but due to the strength and durability of stainless steel, you do not run the same risks of damage or failure if regular maintenance does not occur on schedule.

Depending on your heating system design, you may wish, or need, to run your boiler at a high flow rate. Aluminum can erode at high flow rates, while stainless steel heat exchangers operate very effectively at high flow rates.

Stainless steel heat exchangers are more expensive than aluminum, which is lightweight and has high thermal conductivity, but due to the longevity and corrosion resistance of stainless steel, they are likely to be a much better value in the long run. Aluminum will rust, corrode, warp, or break down long before stainless steel, so investing in a boiler with a quality stainless steel heat exchanger will increase the useful life of your boiler.

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