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 is a condition when the water in the boiler is cool enough and the flue gas temperature going through the boiler is cooled to a point where the hot gasses start to condensate. The average return water temperature where this takes place is about 130ºf. This is what we call the flue gas dew point temperatures. This is similar to a cold drink in the summer and the glass sweats. The cold drink inside and hot humid air outside causes condensation on the glass. Another example is the ground gets hot during the day and the night air cools off. If the conditions are correct by morning, we have dew on the grass. We cannot change physics, but we can change the conditions and keep it from happening since it is all temperature and flow related. The cast iron or steel boiler always has the same flame temperature so the change must be in the gallon per minute (gpm) flow of water through the boiler. Slow the water down in a boiler and the water gets hotter quicker. Speed the water up and there is less thermal transfer, so the water does not get as hot and condenses longer.
The bigger problem with flue gas condensation is what is included in the byproducts of combustion. Among other chemicals there is hydrochloric acid, carbonic acid, and sulfuric acid. When the boiler is allowed to continually condense in the flue passes the acids are eating at the iron or steel. When the return water gets returns warm enough flue passes now dry. The acids become dormant. This is also a good reason to have the gas or oil boiler cleaned every year. There are boilers today designed to condense therefore made of materials that resist the effects of the acids. These boilers are made of cast aluminum and stainless steel and are called 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). If the boiler is installed in one of the above applications boiler protections should be considered here are two types of bypass piping. We have two different bypasses, a boiler bypass and a system bypass. There are two basic types of bypass piping. They are boiler bypass and system bypass. Boiler bypass keeps the system flow rate higher than a system bypass. The advantage to keeping the system flow rate higher is better heat output from the radiation at any given water temperature and better air elimination. In other word speed the water up get more heat and slow the water down get less heat. Baseboard Chart With that said and before all the hate mail starts coming in let me say this. This will produce a fuel savings although minimal in a high-water volume system, less savings in a low water volume system. There are a lot of subjects discussed on this site that will only make a minimal difference in fuel consumption. But add all or most of them into your application and we see a larger fuel savings. Boiler Bypass - This is a very basic way to somewhat protect the cast iron or steel boiler from flue gas condensation. The boiler bypass pipe will not be the best boiler protection but is an acceptable and inexpensive way of boiler protection. A boiler bypass will take cool return water from the system and pass it to the supply water and go back to the system. The water going through the bypass will not go through the boiler, thus, a boiler bypass. This will keep a higher flow rate in the system when compared to a system bypass. See system bypass below. There are benefits to a boiler bypass. When the water bypasses the boiler and goes into the supply, the supply water temperature drops slightly before going to the radiation. This will increase the comfort level of the home as cooler water is circulated through the system. The temperature change in the living area is slower thus more comfortable. Another advantage is you are not heating all the gallons of the water in the system through the boiler that saves fuel. See drawing
Boiler bypass piping is used on residential cast iron or steel boilers. The boiler bypass will vary the system water temperature as outside air temperature changes. I call it the poor man's outdoor reset. It works better with larger water volume systems but just as important with other systems. The type of bypass is determined by the direction of the water flow. On a boiler bypass the water should move from cold return pipe to hot supply pipe. Here are two examples of boiler bypasses, the first with the circulator mounted on the preferred supply piping location and the second with circulator mounted on the return piping. Circulator on Supply or Circulator on Return Although I show a diagram with the circulator mounted on the return piping, today's piping standard is having the circulator mounted on the supply pipe pumping away from the expansion tank connection. There is a misunderstanding in the industry that condensation in a cast iron boiler is related solely to return water temperature. While there is truth in this thought, low return water temperature is half the equation. I have heard techs discuss the different temperatures between 130f and 140f where condensation occurs and what the minimum return temperature should be. The concern is not only the return temperature but the average water temperature in the boiler due to flow through the boiler. Many years ago, I read an article from B&G I believe was written by Gill Carlson, stating flue gas condensation and thermal stress is a result of extremely cold water entering a boiler or cool water at a high flow rate. With this said, if we get cold return temperatures let's consider changing the flow in the boiler. Boiler bypass will allow us to get a higher delta-T in the boiler to get to the magic numbers needed in the boiler to avoid condensation. A boiler bypass will reduce the flow in the boiler and maintain a good flow rate in the system. It all comes down to water volume and flow rate.
For more information on bypass piping go here 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.
Cast iron or steel residential boiler piping using p/s piping, you may still need to install a bypass pipe between the boiler piping. If you use a p/s loop with boiler as secondary, I would use a system bypass or just a valve on the supply piping that can be partially closed to reduce boiler flow. Remember, reducing boiler flow the water gets hotter and can stop boiler condensation. I would then adjust the flow in the boiler until I get a 25ºf - 40ºf rise through the boiler dependent on type of system. If that causes the boiler to short cycle, open the valve slightly until the short cycling stops. Oversized boilers may not be as much of a concern for bypass piping. Boilers in a secondary loop, the boiler circulator will not be affected by the primary loop. In other word the boiler circulator will not reduce 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 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. This is measured centerline of the branch of the tee. 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 order of the secondary piping on the primary loop is the shortest hottest water temperature loop to the longest coolest water temperature loop. This type of system is a multi-temperature water system. I usually suggest a primary loop when you have multiple temperature zones. I prefer the single pipe with boiler as a secondary in systems where most of the zones are all the same water temperature, usually when retrofitting from a cast iron boiler. If you have all the same temperature secondary zones and a single zone with a different water temperature, just control the low temperature zone with a mixing valve or any industry standard of choice. 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/Tekmar 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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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.
