Stainless Steel Used in Heating Systems

As global demographics change and living standards improve, indoor heating has surged by nearly 25% since 2010. To achieve our climate goals, it is imperative to minimize fuel consumption and thus reduce greenhouse gas emissions. Therefore, heating systems must continuously improve their efficiency.

© istockphoto.com/rglinsky

Traditional residential heating systems vary widely, depending on climate, fuel availability, and the prevailing technology at the time. But most systems work by burning some kind of fuel to heat water in a boiler or air in a furnace. The hot water is then circulated throughout the building through a system of pipes, heating rooms through radiators or radiant floors, while the hot air is circulated through ducts into the living space.

 Most fuels, whether fossil fuels such as oil, gas and coal, or renewable fuels such as wood or biogas, are emitted in the form of flue gases. Depending on the fuel, the emissions consist mainly of nitrogen, carbon dioxide and water vapour, with small amounts of nitrogen oxides (NOx), sulphur dioxide SO2 and particulate matter. These potentially harmful gases are discharged to the outdoors through a flue or chimney. When the flue gases cool below the dew point (approximately 56°C), the water vapour condenses into water, which combines with other compounds to produce corrosive acids. Each heating system has its own characteristics and requirements. The relatively cool flue gas temperatures of less than 60°C in a condensing gas boiler are very different from the flue gas temperatures of up to 600°C in a wood-burning heating stove. The former requires a flue resistant to acidic condensate, while the latter requires a flue resistant to high temperatures. Surprisingly, both can use the same molybdenum-containing stainless steel for guaranteed service life.

In addition to flue gases, circulating water and tap water also pose a major corrosion risk to residential heating systems and affect their efficiency. The corrosive substances contained in treated water, especially chlorides and free chlorine, will corrode the surfaces of the boiler water circulation system. Calcium and magnesium in hard water form scale deposits, which reduce thermal efficiency and lead to premature component failure. With its smooth surface and high corrosion resistance, molybdenum-containing stainless steels offer a complete solution for all equipment in combustion-based domestic heating and hot water systems: boilers, stoves, fireplaces, chimney flues and hot water tanks.

Boilers: stainless steel helps improve efficiency

The boiler is the core of the hot water heating system. The boiler generates heat by burning gas, oil, wood or even coal, depending on the technology. Most of the heat is then transferred to the water circulation system via a heat exchanger, while the unused waste heat in the flue gases is discharged outdoors. In order to increase the efficiency of the boiler and reduce the impact on the environment, the heat lost to the outside in this way must be minimized and used to heat the circulating water. The lower the temperature of the flue gases, the more energy is used to heat the house and hot water for the bathroom and kitchen. Therefore, the modern condensing boiler shown in the figure below is one of the most efficient heating devices on the market today. The durability and performance of a boiler also depends on the type of fuel, how it is operated and maintained, and the materials used.

Functional diagram of conventional boiler and condensing boiler

 Traditional boilers have a burner and a heat exchanger in the combustion chamber. The hot gases heat the water in the circulation system and are then discharged through the flue while still hot, losing a lot of valuable heat energy to the outside world. In contrast, a condensing boiler is much more energy efficient because it uses a condenser after the combustion chamber to extract more heat from the flue gases by preheating the incoming water in the heating circuit. This reduces the flue gas temperature to below the "dew point" of 60°C, and the flue gases condense and form acids. The combustion chamber and condenser are designed to resist corrosion from corrosive flue gases and acidic condensate respectively. The condenser can collect the condensate instead of venting it to the atmosphere, thus reducing air pollution.

 The materials used for the combustion chamber and heat exchanger largely determine the service life and performance of the boiler. Traditionally made of cast aluminum, ceramics and steel, these components are now increasingly made of 316L stainless steel. 316L, with a 2% molybdenum content, has very good pitting resistance and good high-temperature strength. In the latest high-efficiency condensing boilers, 316Ti stainless steel is used, and in some high-break models, 904L stainless steel is even used. 904L contains 4% molybdenum, which has better corrosion resistance and can ensure a long service life.

 Stainless steel for chimneys

The hot flue gases produced by boilers, furnaces and other combustion-type heating systems must be discharged through chimneys. The flues of traditional masonry chimneys are rarely leak-proof, so corrosive condensate and acidic flue gases will erode and damage the mortar joints, allowing moisture, dangerous gases and smoke to enter the room. Therefore, the flue needs to be lined. However, many old heating systems have severely aged linings or no lining at all. Modern linings are usually double-layered, generally with an insulation layer between the two linings. 316L stainless steel is now used as the inner lining of high-performance flues. This grade of stainless steel can resist acidic condensate, acidic flue gases and high temperatures. In mild environments, the outer lining can be made of the same material or 304 stainless steel. Stainless steel flue liners have a smooth surface, making it more difficult for smoke to adhere, allowing for faster exhaust, less smoke accumulation and reduced fire risk.

 Stainless steel is relatively light and easy to install, making it suitable for lining existing chimney flues as well as for newly installed chimney flues. 316L stainless steel can be made into sleeves and chimney outlets of various shapes and diameters. Stainless steel can withstand all fuels and is ideal for wood, oil and gas boilers, exhausting dry or moist flue gases at high and low temperatures. Double stainless steel linings are traditionally used to avoid dangerously high indoor pipe temperatures, but also to maintain flue gas temperature, ensure good ventilation and avoid condensation. For condensing boilers, insulation solves a different problem. Flue gas temperatures are very low and there is little natural ventilation, so maintaining flue gas temperature is extremely important for flue gas exhaust.

©Joachim Nicolaus

When replacing old heating systems in existing buildings, masonry chimneys must be lined due to the lower flue gas temperatures. The stainless steel lining, installed from above, is not only durable and watertight, but also avoids major construction work inside the building.

Double stainless steel pipes can also be used alone, without the need for a traditional masonry chimney as a support. This solution is low-cost, lightweight, easy to install, saves space and can add an aesthetic and eye-catching element to indoor and outdoor.

Hot water storage tanks store hot water for use in home bathrooms and kitchens. 316L stainless steel meets the highest demands in terms of hygiene and durability and withstands the corrosion risks of all types of drinking water. Unlike enameled steel tanks, which require regular replacement of magnesium anodes, 316L stainless steel requires minimal maintenance. Like the lining of chimney flues, the lining of stainless steel tanks is also smooth, which reduces the accumulation of scale and thus reduces maintenance and prevents premature aging of the tank. Heat exchange coils, also made of 316L stainless steel, have the same advantages in addition to good thermal conductivity. Stress corrosion cracking (SCC) is always a risk in hot water containing chlorides. To combat stress corrosion cracking, ferritic 444 with a 2% molybdenum content and duplex 2205 with a 3% molybdenum content are often used for linings and heating coils of hot water tanks. Duplex stainless steel hot water tanks are much lighter than conventional tanks due to their high strength.

 ©Viessmann 

The hot water from the boiler enters the hot water tank through the heat exchanger coil, transferring its heat to the water in the stainless steel storage tank

 At present, the most efficient heating system, its design and material development direction is to use molybdenum containing stainless steel materials. 316L stainless steel is used in a variety of heating applications, improving the efficiency and durability of boilers, chimney fittings, casings and outlets, flue pipes and linings, hot water tanks and water tank heat exchange coils. In addition, 904L austenitic stainless steel, 444 ferritic stainless steel and 2205 duplex stainless steel are also used in certain specific applications. Nevertheless, with emission-free electric heat pumps growing at a double-digit rate, home heating is rapidly moving away from burning fossil fuels - but not away from molybdenum-containing stainless steel. The excellent corrosion resistance and hygienic properties of molybdenum-containing stainless steel are essential for heat pumps.