Eliminating Air & Dirt in Modern Hydronic Systems
The air and dirt found in today’s piping systems have not changed all that much over the years. What has changed, or evolved, are the components within a piping system. This would be true for chilled as well as hot water systems. On the hydronic side, perhaps the biggest change is the heat exchanger. Many of the efficient condensing boilers on the market have small diameter heat exchanger tubes. Tankless water heaters are also becoming boilers or heat sources for hydronics. For the best heat exchange, it is imperative that these small diameter passages stay wide open.
Failing to provide a wide-open passage for the fluid leads to some interesting problems. The first indicator may be a different sound to the system. Inadequate flow can cause areas of steam formation. This can lead to percolation or cavitations. This presents as a moan, hammer or high pitch resonance: the “elk bugle” phenomenon.
Let’s talk dirt
A prudent first step is to provide an appropriate dirt elimination device. The device chosen will need to be sized for the application and placed in the best spot to allow it to do its job successfully. For years the “go-to” fitting has been the y-strainer, a simple and efficient fitting that places a screen in the fluid flow. Depending on the application, y-strainers are available with different mesh sizes, and they can be equipped with a drain-off valve to allow flushing of the strainer. A pressure gauge may be installed to indicate the reduction in flow. This is not unlike the gauge seen on swimming pool filters.
A strainer or screen also is seen on the inlet side of various components such as boiler feed valves, backflow preventers, mixing valves, etc. Remember, these are maintenance sensitive methods of dirt removal. Failure to clean and check them periodically will result in reduction of flow and, worst case, in blockage of flow. Ideally, any device with a serviceable screen should be installed with isolation valves to speed the clean-and-check maintenance.
In recent years, the industry has started embracing dirt separators. The main difference between these and the y-strainer, seen in most styles, is the lack of the fine mesh screen. The separator works by first presenting a “wide spot” in the piping. This allows for a velocity decrease, slowing of the flow. A “low velocity zone” is another name for this concept. The slowing of the velocity allows some of the particles to fall to the bottom chamber. Additionally, a media is placed in the fluid stream to divert the particles downward. A large bottom chamber collects the debris until it can be drained off. The separator does not impede the fluid flow as it does its work; the y-strainer collects debris and restricts the flow as it traps the particles in the fluid stream.
The separators will remove much smaller particles, down to 5 microns .0002", according to lab testing. It will take multiple passes to provide this type of performance. Automatic blowdown valves can be added for periodic blowdown. Flow velocity through a separator affects performance. Expect to see four feet per second (fps) as a minimum and up to 10 fps as the maximum. Higher flow rates will affect the performance.
Dirt separators are a wise choice when retrofitting boilers to old iron pipe systems, which can “shed” rust and scale particles on a continuous basic. Think of a dirt removal device as you would the oil or air filter on your vehicle; it needs to be sized and maintained properly.
Place the dirt separator where it will see the most flow. One suggestion is to add a separator on both sides of a primary secondary piping. Consider the inlet side to the boiler, mounted low in order to capture the dirt where it tends to migrate based on density. Another common and recommended location is on plate-type heat exchangers.
Now for some hot air
As with dirt elimination, air elimination has seen similar improvements in technology over the years. The same concepts apply in air removal. Basically, slow the flow and allow the air to be trapped and released via a high point float style vent. The escape hatch is the high point, with dirt elimination it is the low point.
Hydronic systems can never be 100% airtight. Air is added any time there is water makeup. Air can enter the system at gaskets, valve packing, etc. Oxygen can migrate into systems through the walls of PEX and other polymer tubing. Air can also be pulled into systems via improperly placed air elimination devices.
Heating water will also drive air out of a solution. The amount of dissolved air and gases a fluid can hold depends on both pressure and temperature. Air in a system can lead to noisy operation. This is often the first indicator to the owner or operator of a hydronic system problem. A mixture of air and water reduces flow and can cavitate circulator pumps. Air trapped in heat exchangers or against the walls of boiler castings can degrade heat transfer and lead to accelerated corrosion. Large air pockets or bubbles can stop flow completely.
The ability to capture and eliminate microbubbles is one improvement seen in recent years. Microbubbles are often too small to be seen by human eyes. Filling a glass at your kitchen faucet is one place to see microbubbles in action. The water appears milky at first until the bubbles rise to the top of the glass. In a closed loop system these microbubbles need to be gathered and removed to prevent the problems mentioned above.
The older ramp-style air eliminators are not designed to handle microbubble elimination to the level that microbubble separators are. The concept behind microbubble separators is similar to the concept of dirt separation. Provide a spot in the piping to slow the flow and allow the air bubbles to be combined and vented out. The same media seen in some dirt separators may be used in air elimination. The design of the media is to provide a place for the small microbubbles to cling to. As the size of the bubbles increase, they rise to the top of the separator to be vented out of the system. A media with many small “vanes” and sharp surfaces provides a spot in the fluid stream to intercept air molecules.
As with dirt separation, air eliminators need to be sized correctly and placed in the system where they can do the best job. Multiple air elimination devices are often combined. An example would be small manual vents on individual radiators, fin tube baseboard or hydronic coils in air handlers. These are intended to provide “high point” air removal.
Small float type air vents can also be used on high points. Thermal solar arrays are a common place to locate float style vents. Both solar air separators and air vents need to be manufactured to much higher operating temperatures. Look for brands built for solar application when working on a thermal solar system.
A good fill and purge is the best step to starting a hydronic system. Flushing a system with a full port hose fitting will eliminate much of the work required by both the air and dirt separators. Some brands of air separators have a hose cock feature built in to aid in initial air removal.
Recently, the market is seeing combination air and dirt separator devices. These are intended for larger systems, generally 2" and larger. These combo devices place both functions in one housing, which can be a space, time and money saver.
Also, there are some triple duty devices that provide air, dirt and hydraulic separation in one device. These have four connection ports to provide the primary/secondary function. Extended upper and lower housings with air and dirt scrubbing medias are built into one packaged device. Many manufacturers offer the single or multi-task devices, often up to 12" pipe size, and custom sizes are available if a system requires them.
So, keep it clean and get rid of the hot air!
Bob “hot rod” Rohr has been a plumbing, radiant heat and solar contractor and installer for more than 30 years. A long-time RPA member and columnist for Phc News, Rohr has joined Caleffi North America as manager of Traning and Education.