Cast Iron and Steel Boiler Protection from Condensation
Caution :All drawings are conceptual drawings for illustration purposes only and may be incomplete.
There are rules to follow when piping cast iron or steel boilers that are overlooked at times. Some important terms are Boiler Bypass, System Bypass, Delta-T, Flue Gas Condensation, Primary/Secondary piping. Variable flow circulators set up as minimum return water temperature or delta-T circulation, 3 or 4-Way valves, and ESBE thermic valves. All these are a form of boiler protection from condensation that will cause premature boiler leaking. Some of these options are better than others. The type of protection depends on how cool the returning water temperature and how long the cool water will be returning to the boiler. You decide what level you want or need.
Delta-T - is the difference in temperature between the hot supply and the cool return. As the water goes through the boiler it will pick up temperature as passes through the boiler. The temperature difference is Delta-T. A delta-T can be a temperature rise or drop. The rise would be through the boiler and a drop in delta-T would be the system. A normal residential system delta-T would be a drop of 20ºf as the system gives off heat. A normal boiler delta-T would be a rise of 20ºf - 40ºf rise.
Flue Gas Condensation - This condition occurs when the water in the boiler is sufficiently cool, and the temperature of the flue gas passing through the boiler is lowered to a level where the hot gases begin to condense. The typical return water temperature at which this happens is around 130°F. This phenomenon is referred to as the flue gas dew point temperature is reached. It can be likened to a cold beverage in summer, where the glass sweats due to the cold drink inside and the hot humid air outside causing condensation on the glass surface.
Another illustration is when the ground heats up during the day, and the night air cools down. If conditions are right by morning, dew forms on the grass.While we cannot alter the laws of physics, we can modify the conditions to prevent this occurrence, as it is all related to temperature and flow. The flame temperature in a cast iron or steel boiler remains constant, so the variation must occur in the gallons per minute (gpm) flow of water through the boiler. Slowing down the water flow in a boiler results in the water heating up more rapidly. Conversely, increasing the flow reduces thermal transfer, the water does not heat up as much and condenses for a longer period.
The more significant issue with flue gas condensation lies in the byproducts of combustion. These include various chemicals such as hydrochloric acid, carbonic acid, and sulfuric acid. When the boiler is allowed to continuously condense in the flue passages, these acids can corrode the iron or steel. However, when the return water is warm enough, the flue passages dry out, and the acids become inactive. It is important is a steel or cast iton boiler the flue passes must be dry when the boiler demand ends. This is also a compelling reason to have gas or oil boilers cleaned annually. Nowadays, there are boilers specifically designed to condense, constructed from materials that withstand the effects of these acids. These boilers are made of cast aluminum and stainless steel and are known as mod/con or high-efficiency boilers.
Bypass Piping - When installing a cast iron or steel boiler we need to protect the boiler if any of the following applications. If any of the following conditions do exist, the potential of premature boiler failure may occur due to thermal stress or corrosion from flue gas condensation (sweating). When a boiler is installed in one of the aforementioned applications, it is essential to consider boiler protections. There are two distinct types of bypass piping to take into account: the boiler bypass and the system bypass. The two fundamental types of bypass piping are the boiler bypass and the system bypass. The boiler bypass is designed to maintain a higher system flow rate compared to the system bypass. One significant advantage of maintaining a higher flow rate in the system is that it results in improved heat output from the radiation at any specific water temperature, as well as enhanced air elimination. In simpler terms, increasing the speed of the water leads to greater heat output, while reducing the water speed results in less heat being produced. Baseboard Chart Before any negative feedback begins, I would like to clarify the following. This approach will yield fuel savings, albeit minimal in systems with high water volume, of course with increases comfort levels and less so in systems with low water volume. Many topics covered on this site will result in only slight reductions in fuel consumption. However, by incorporating all or most of these strategies into your application, we can achieve more significant fuel savings. Boiler Bypass - This is a fundamental method to provide some protection for the cast iron or steel boilers against flue gas condensation. While a boiler bypass pipe may not offer the most effective protection, it serves as an acceptable and cost-effective solution. The boiler bypass directs cool return water from the system to the supply water, allowing it to re-enter the system without passing through the boiler. As a result, this maintains a higher flow rate compared to a system bypass. There are advantages to implementing a boiler bypass. When water bypasses the boiler and enters the supply, the temperature of the supply water decreases slightly before reaching the radiation. This contributes to increased comfort in the home, as cooler water circulates through the system. Additionally, the gradual temperature change in the living area enhances comfort. Another benefit is that not all gallons of water in the system are heated through the boiler, leading to fuel savings. See drawing
Boiler bypass piping is utilized in residential cast iron or steel boilers. This bypass helps to adjust the system water temperature in response to changes in outside air temperature. I refer to it as the budget-friendly outdoor reset. It performs more effectively with systems that have a larger water volume, but it is also significant for other types of systems. The kind of bypass used is based on the direction of the water flow. In a boiler bypass, water should flow from the cold return pipe to the hot supply pipe. Below are two examples of boiler bypasses: the first features the circulator installed at the preferred supply piping location, while the second has the circulator positioned on the return piping. Circulator on Supply or Circulator on Return While I present a diagram depicting the circulator installed on the return piping, the current piping standard recommends mounting the circulator on the supply pipe, directing the flow away from the connection to the expansion tank. There is a common misconception in the industry that condensation in a cast iron boiler is solely dependent on the return water temperature. While this notion has some validity, low return water temperature is only part of the overall picture. I've heard technicians debate the various temperatures, such as 130°F and 140°F, at which condensation occurs and what the minimum return temperature ought to be. However, itâs crucial to consider not just the return temperature but also the average water temperature within the boiler, which is influenced by the flow through it. System Bypass - Is another way of doing boiler protection. It is not the best protection for residential products Circulator on supply, or Circulator on return. When a system bypass is applied instead of a boiler bypass, we lose some of the system temperature control and flow as some of the water is diverted back into the boiler. We also lose the flow in the system. When a boiler bypass is used, the circulator is dedicated to the system, with a system bypass the circulator is dedicated to the boiler. The system flow decreases slightly, slowing water in the system get less heat from the radiation. Again, the system bypass is used mainly on commercial boilers. Check manufacturer’s specs. Primary/Secondary (p/s) Piping - Is a piping idea that has gained popularity over the last decade or so. I personally feel everything should be piped primary/secondary (p/s). The way to figure out if you have p/s piping is the use of closely spaced tees. Closely spaced tee's create hydraulic separation. This means the flow of one circulator will not affect the flow of another circulator.
When using cast iron or steel residential boiler piping with primary/secondary (p/s) piping, it may still be necessary to install a bypass pipe between the supply and return piping of the boiler. If you opt for a p/s loop with the boiler functioning as secondary, consider using a system bypass or simply a valve on the supply piping that can be partially closed to decrease boiler flow. Keep in mind that reducing the boiler flow will increase the water temperature and can prevent boiler condensation. Subsequently, adjust the flow in the boiler until you achieve a temperature rise of 25°F to 40°F through the boiler, depending on the system type. If this adjustment leads to short cycling of the boiler, slightly open the valve until the short cycling ceases. Oversized boilers might not pose as significant a concern regarding bypass piping. For boilers in a secondary loop, the circulator of the boiler will remain unaffected by the primary loop, meaning it will not diminish the system flow. When p/s is applied with close spaced tee's there are certain rules to follow precisely. The distance from an elbow on the supply to the closely spaced tee's is a minimum of 8 times the diameter of the primary pipe (8 x dia. primary pipe). The distance between the centerline of the branches of the closely spaced tee's is a maximum of 4 times the diameter of the primary pipe not to exceed 12" (4 x dia. primary pipe not to exceed 12") whichever is closer. A good rule here is the closer the better. The distance from a closely spaced tee to a return side elbow is a minimum of 4 times the diameter of the primary pipe 4 x the diameter of primary pipe, flow shown from right to left. The arrangement of the secondary zone piping on a boiler primary loop, which connects from the boiler and returns to it with all secondary zones branching off the loop using closely spaced tees. This is structured from the shortest, hottest water temperature loop to the longest coolest water temperature loop. This configuration represents a multi-temperature water system. I typically recommend a boiler primary loop when there are multiple temperature zones installed. In cases where most zones maintain the same water temperature, particularly when retrofitting from a cast iron boiler, I prefer using a single pipe with the boiler as a secondary. If all secondary zones share the same temperature but one zone has a different water temperature, simply manage the low-temperature zone with a mixing valve or any standard method commonly used in the industry. Setpoint or Delta-T Circulators - are very useful when used for cast iron boiler protection. They will change the flow through the boiler as the water temperature changes. Delta-T circulator - Shown here in a primary/secondary configuration. The circulator has two sensors one installed on the supply side of the boiler and one on the return side of the boiler. You set the delta-T you want to maintain and the speed changes to meet the set point. If you set it for a rise through the boiler of a 40ºf delta-T, the circulator will speed up or slow down to maintain that delta-T.
Both of these circulators could be used on cast iron boilers piped as a manifold system on a p/s piping system. I do not suggest these circulators be applied as boiler circulators for high efficiency boiler unless the boiler manufacturer condones it. Here we show the Delta-T circulator on a manifold system. The way it works is the circulator will determine the delta-T the boiler is producing. The speed of the circulator will increase or decrease as needed raising or lowering the flow through the boiler. If the flow increases the circulator will add more water to the return to increase the temperature entering the boiler. As the delta-t gets closer to the Delta-T set in the circulator the circulator slows down to maintain the proper delta-T. 3 or 4-Way Valve - These valves are used for cast iron and steel boiler protection by blending supply and return water from the boiler and system together. This is done by manually adjusting the valve or adding a motor for automatic mixing. The latter is by far a lot better application, due to conditions changing as the boiler cycles on and off and zones open and close See Watts/Tekmar Actuator The 4-way valve has 4 ports, two are piped to the system and two are piped to the boiler. The internal butterfly will move (when motorized) and protect the boiler against flue gas condensation and thermal shock. See Watts/Tekmarref" 4 way valve The 3-way valve has 3 ports. One is piped to the supply of the boiler, one to the supply of the system and one to the return of the system and return to boiler, which are connected together with tee's.
See Watts/Tekmar 3-way valve
When using a 4 way valve without an actuator, not what I would suggest due to system changes, here is a pictorial description of valve positions. See position drawing
ESBE Thermic Valve - Is very similar to a 3-way valve except it works on water temperature not electricity. Pipe the boiler primary/Secondary (P/S) and install the Thermic valve as a system bypass. When using (P/S) the use of a system bypass does not affect the flow in the system due to the hydraulic separation of p/s piping. See drawing for use of P/S and ESBE Thermic Valve in p/s piping and a cast iron boiler. The operation of the Thermic valve is simple. Shown in the next 3 drawings is the operation of the valve. First is the valve shown in a closed due to cool boiler water. Next drawing the water has warmed up enough to allow some water from the boiler to enter the valve and mix return water with boiler water to the system. At this point it is acting like a boiler bypass. The third drawing is when the boiler water is warm enough, and the return temperature is warm enough, the delta-T of the boiler keeps the boiler supply warm enough, there is no water flow through the bypass pipe. The Thermic valve is wide open.
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If the average water temperature exceeds the condensing temperature, the boiler will cease to condense. Many of the newer boilers are now capable of functioning at lower return temperatures than those of earlier models. The minimum return temperature should always be specified by the manufacturer. If maintaining the return water temperature within specifications proves challenging, a boiler bypass can be installed.
A boiler bypass primarily addresses flow and delta-T, rather than just the return water temperature. In the past, boilers had a greater water volume, allowing for better blending of return water within the boiler. Nowadays, the flow of return water can sometimes surpass the water volume in the boiler, making the boiler bypass a sensible solution. By implementing a boiler bypass, we can reduce the flow within the boiler while increasing the flow around it. Slowing the flow through the boiler raises the average water temperature above the condensing threshold. With reduced flow through the boiler, the delta-T increases, allowing us to reach a safe temperature that prevents condensation.
Years ago, I came across an article from B&G, likely authored by Gill Carlson, which stated that flue gas condensation and thermal stress result from extremely cold water entering a boiler or from cool water flowing at a high rate. Therefore, if we encounter cold return temperatures, we should consider adjusting the flow within the boiler. A boiler bypass will enable us to achieve a higher delta-T in the boiler, reaching the necessary conditions to avoid condensation. Ultimately, it all boils down to managing water volume and flow rate.br>
For more information on bypass piping go here
There are multiple ways of piping p/s piping, we will look at the most common. The first one, or the original type is the boiler Primary Loop. The other is closely spaced tee's on the system piping placing the boiler in a secondary piping arrangement.
Setpoint Circulator - can be set up to monitor the boiler supply temperature using one sensor mounted on the boiler supply pipe. boiler supply pipe. When set as a set point circulator the single sensor would be strapped to or installed in a well on the supply pipe. The sensor is measuring the supply water temperature and controlling the flow through the boiler dependent on that temperature. If the temperature of the supply water is below the set point temperature the circulator runs slower. As the supply temperature increases so does the circulator speed trying to get the temperature back to set point. As the temperature gets warmer the speed continues to increase until the circulator runs to its full rpm. If another zone opens it may slow down again if the supply water temperature gets close to or below the set point. I would suggest the set point temperature of 130ºf to 140ºf.
