Cast Iron and Steel Boiler Protection from Condensation
Caution :All drawings are conceptual drawings for illustration purposes only and may be incomplete.
At the time of the original writing, the residential heating industry was significantly different. The majority of boilers were manufactured from cast iron or steel. In the early 2000s, the industry introduced high-efficiency aluminum and stainless-steel boilers, known as modulating-condensing (mod-con) boilers. These materials provided greater resistance to flue gas condensation compared to their cast iron or steel counterparts, thereby mitigating previous concerns. The adoption of these boilers increased substantially, and by 2020, over half of the residential boilers installed in the United States were high-efficiency models. The primary distinction between these two boiler designs was that flue gas condensation could adversely affect cast-iron boilers over time, depending on installation practices, radiation type, system water volume, and operating water temperature. The construction of mod-con boilers significantly alleviated issues related to corrosion failure. There are specific guidelines to follow when installing cast iron or steel boilers that may sometimes be overlooked. Key terms include Boiler Bypass, System Bypass, Delta-T, buffer tanks, and Primary/Secondary piping. Variable flow circulators can be configured for minimum return water temperature or delta-T circulation, using 3 or 4-Way valves, and ESBE thermic valves. These measures help protect boilers from condensation that can lead to premature failure. The best option depends on the temperature of the returning water and the duration for which cool water is returning to the boiler. The appropriate level of protection should be determined based on these factors.
Delta-T - denotes the temperature difference between the hot supply and the cool return. It can indicate either a temperature rise (through the boiler) or a temperature drop (through the system). In North America, a typical residential system's Delta-T usually shows a temperature drop of 20ºF as heat is distributed. A boiler's Delta-T typically ranges from a 20ºF to 40ºF increase.
Flue Gas Condensation - occurs when the boiler's water is sufficiently cooler than the hot flue gas passing through the boiler, causing the flue gas temperature to drop to a level where condensation begins. This typically occurs when the return water temperature is below 130°F. An analogous phenomenon can be observed when a cold beverage undergoes condensation on a hot day. Similarly, dew forms on grass when the ground warms during the day and the air cools at night.
Although the laws of physics cannot be changed, the conditions can be adjusted to prevent flue gas condensation by regulating temperature and flow. The flame temperature in a cast iron or steel boiler remains constant, so adjusting the gallon per minute (gpm) flow changes the heating rate. Slowing down the water flow increases the heating rate, while increasing the flow decreases heating rate and prolongs condensation.
Bypass Piping - Protective measures are essential for cast iron or steel boilers to prevent thermal stress or corrosion from flue gas condensation, which can cause premature failure. 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. 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 technique helps protect cast iron or steel boilers from light to medium flue gas condensation. A boiler bypass pipe provides a cost-effective solution, although it may not offer the most comprehensive protection. 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. This maintains a higher system temperature and a lower boiler flow rate compared to a system bypass.
Incorporating a boiler bypass offers several advantages. When system return water bypasses the boiler and mixes with the hot supply, the temperature of the supply water slightly decreases before reaching the radiation, enhancing comfort in the home by circulating cooler water through the system. Furthermore, the gradual temperature change within the living area contributes to increased comfort. Additionally, not all the water in the system is heated through the boiler, resulting in fuel savings. Boiler bypass piping is commonly used in residential cast iron or steel boilers. This bypass helps to adjust the system water temperature in response to changes in outside air temperature. It can be referred to as a budget-friendly outdoor reset. It performs more effectively with systems that have a larger water volume but is also significant for other types of systems. In a boiler bypass, water should flow from the cold return pipe to the hot supply pipe. Below are two examples of boiler bypass configurations: one features the circulator installed at the preferred supply piping location, while the other has the circulator positioned on the return piping. While the provided diagram depicts 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.
It is often believed in the industry that condensation in a cast iron boiler depends solely on the return water temperature. While this idea has some basis, low return water temperature is only part of the overall picture. Technicians frequently discuss appropriate return water temperatures, such as 130F and 140F, where condensation can occur. System Bypass - The system bypass approach to boiler protection is less effective for residential applications. Utilizing a system bypass rather than a boiler bypass diminishes system temperature control and flow, as it results in some water recirculating back into the boiler. In a system bypass, the circulator is dedicated exclusively to the boiler, as opposed to a boiler bypass where it directly serves the system. This method is predominantly employed in commercial boiler operations. Please refer to the manufacturer's specifications for further details. 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 - IThis method has gained popularity in residential applications over the last decade. It employs closely spaced tees to achieve hydraulic separation, ensuring that the flow from one circulator does not interfere with another's operation. Typical configurations include situating the boiler within a primary loop or incorporating it into secondary piping with closely spaced tees.
For residential cast iron or steel boilers using p/s piping, a bypass pipe may be necessary between the supply and return piping dependent on your attached system. Adjusting the boiler flow can help regulate water temperature and prevent condensation. The target temperature rise through the boiler should be between 25F and 40F. If short cycling occurs, slightly opening the valve can mitigate this issue.
When implementing p/s piping with closely spaced tees, adhere to specific guidelines: maintain an 8x pipe diameter distance from an elbow on the supply side, ensure a maximum 4x pipe diameter distance between the centers of closely spaced tees, and keep a 4x pipe diameter distance from a closely spaced tee to a return side elbow. See the two main types of Primary/econdary piping drawings. 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 Drawing of 4x's Rule , 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. Setpoint Circulator - can be set up to monitor the boiler supply temperature using one sensor mounted on the 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 130f to 140f. Delta-T circulator - Shown here in a Primary/Secondary Equipped with two sensors, one on the supply side and one on the return side of the boiler. It regulates a set temperature differential (delta-T) by adjusting its speed. For example, if set for a 40F rise, it will increase or decrease speed to maintain that delta-T. These circulators are suitable for use in manifold systems and primary/secondary applications but should only be used with high-efficiency boilers if approved by the manufacturer.. 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 setting in the circulator the circulator slows down to maintain the proper delta-T. Buffer Tank - The use of a buffer tank has become more common over the past 12+ years for two main reasons. 1) Newer homes typically have more heating zones than older houses, which helps in reducing boiler short cycling. 2) The tank can be heated to a higher temperature to reduce flue gas condensation. 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 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.
Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, installations should be reviewed in a per-job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc and its personnel accept no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website.
Conditions that may necessitate boiler protection include:
Baseboard Chart
See drawing
Circulator on Supply or Circulator on Return
However, it is important to consider not just the return temperature but also the velocity of the return water temperature and boiler Delta-T. Modern cast-iron boilers are now designed to operate at lower return temperatures than earlier models and have reduced water volume. The minimum return temperature should always be specified by the manufacturer, and if not specified, assume between 20F to 40F.
Years ago, an article from B&G, likely authored by Gill Carlson, stated that flue gas condensation and thermal stress resulted 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.
For more Boiler Bypass Piping Information click here
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.
Boiler Primary Loop. or Boiler as a Secondary Loop.
More Information on Buffer Tanks and Tank Piping.
