The following piping information applies to cast iron or steel boilers.
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
Delta-T - This 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 cold enough and the flue gas temperature going through the boiler is cooled to a point that we encounter condensate. The average return water temperature where this takes place is below 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. It has cold drink inside and hot humid air outside or another example is the ground gets hot during the day and the night 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 related. The cast iron or steel boiler is always inputting the same flame temperature the change must be in gallon per minute (gpm) flow through the boiler. Slow the water down in a boiler and the water gets hotter.
The bigger problem with flue gas condensation is what is included in the byproducts of combustion. Among others 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 flue passes dry out, the acids become basically 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.
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).
1. When the boiler is smaller than the amount of radiation in the system.
2. When there is any cast iron radiation in the system.
3. If there is any radiant in floor heat in the system.
4. When the return water is cool enough to cause the boiler to condense and corrode.
5. If the average water temperature in the boiler cannot get to a boiler average temperature of 140ºf in a reasonable amount of time.
If the boiler is installed in one of the above applications boiler protection should be considered here are two types of bypass piping. We have two very 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 words speed the water up get more heat and slow the water down get less heat.
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. 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 temperatures change. 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 bypass the first with the circulator mounted on the preferred supply side of the boiler and the second with the circulator mounted on the return side of the boiler. Although I show a diagram with the circulator mounted on the return the standard today is to have the circulator mounted on the supply piping pumping away from the expansion tank connection.
There is a misunderstanding in the industry when conversation is related to return water temperature. 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 really the return temperature but the average water temperature in the boiler. If the average water temperature gets above the condensing state the boiler will stop condensing. Some of the newer boilers can now operate at lower boiler return temperatures than in the past so minimum return temperature must be determined by the manufacturer. If you cannot keep the return water temperature at spec's you can install a boiler bypass. A boiler bypass is more about flow and delta-T than return water temperature. Many years ago the boilers had more water volume. The amount of return water blended well in the boiler. Today the return water flow my exceed the water volume in the boiler. For this reason the boiler bypass makes sense. A boiler bypass will reduce the flow in the boiler and increase the flow around the boiler. As we slow down the flow through the boiler the average water temperature in the boiler gets above the condensing temp in the boiler as the delta-T is increased in the boiler.
I read an article I think may have been from B&G that stated flue gas condensation and thermal stress is a result of cold water entering a boiler or cool water at a high flow rate. A boiler bypass will reduce the flow in the boiler and maintain a flow in the system. It all comes down to water volume and flow rate.
System Bypass - Is another way of doing boiler protection. It is not the best way for residential products (see drawing). 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, a system bypass the circulator is dedicated to the boiler. The system flow decreases slightly, slow 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 - Is a piping idea that has gotten very popular. I personally feel everything should be piped primary/secondary (p/s). It does not offer full boiler protection but it helps. The way to determine if you have p/s piping is closely spaced tees. The tees create what we call hydraulic separation. This only means the flow in one pipe will not affect the flow in another pipe. One circulator will not affect another circulator flow.
There are a number of types of p/s. There is the boiler primary loop (see drawing), p/s loop with boiler as a secondary loop (see drawing) and straight pipe with boiler as a secondary loop (see drawing).
Cast iron or steel residential boilers and p/s piping you may still need to install a bypass pipe. If you use a p/s loop with boiler as a secondary loop I would use a system bypass or just a valve on the supply piping that can be partially closed to reduce boiler flow. Remember reduce boiler flow the water gets hotter. I would than adjust the flow in the boiler until I get a 40ºf rise through the boiler. 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. With the boiler being a secondary loop the boiler circulator will not be affected by the primary loop. In other words the boiler circulator will not reduce the system flow.
When p/s is applied with close spaced tees there are certain rules to follow precisely. The distance from an elbow to the closely spaced tees is a minimum of 8 times the diameter of the primary pipe (8 x dia. primary pipe). The distance between the closely spaced tees 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"). This is actually measured centerline of the branch of the tee. The distance from a closely spaced tee to an elbow is a minimum of 4 times the diameter of the primary pipe
(4 x the diameter of primary pipe).
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 that are all the same water temperature. If you would 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 anyway of your choice.
Variable Speed Circulators - are very useful when used for boiler protection. They will change the flow through the boiler as the water temperature changes. They can be set up as a set point or delta-T circulators. When setup as a set point circulator, the circulator would be installed with a single sensor on the return line to the boiler. This works best with p/s and the boiler as a secondary. The sensor is measuring the return water temperature and controlling the flow through the boiler dependent on that temperature. If the temperature of the returning water is below the set point the circulator runs slower. As the return temperature increases so does the circulator speed trying to get the temperature back to set point. As the return temperature gets warmer the speed continues to increase until the circulator gets to full rpm. If another zone opens it may slow down again if the return water temperature gets close to or below the set point. I would suggest the set point temperature of 130ºf.
A variable speed circulator setup as a delta-T circulator would use the same piping as above. This time 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.
3 & 4 way Valve Protection - 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 Tekmar 741 Actuator Motor)
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 Tekmar 720 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 teed together. (See Tekmar 710 3-way valve) (See 3 Way Valve 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 Primary/Secondary (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 a 3-way valve or ESBE Thermic Valve, Honeywell DPBV Literature