Eductor Basics

You have had success using Class A foam during fire training exercises to combat automobile interior and other ordinary combustible fires, and you’ve now decided to implement nozzle-aspirated Class A foam. You already own a 95-gpm eductor that can be used during fire responses. The device has been virtually unused since its purchase and is located in the driver’s-side tool compartment of your first-out engine. While eductor technology is simple and requires only a few operational changes, increasing your knowledge of how these systems work, along with understanding system design features, capabilities and limitations will benefit you and your department in making more successful fire stops.

Eductors remain one of the easiest and least expensive ways to become involved in the application of nozzle-aspirated Class A foam. Based on simple design principles, eductors create an area of low pressure inside, which lets foam concentrate be forced into the eductor due to greater external pressure exerted by atmospheric air on the foam concentrate supply. Eductors have an inlet pressure rating, usually 200 psi, and a flow rating, such as 95 or 125 gpm, where the particular device is designed to operate. All eductors have a specific proportioning ratio range over which they can educt foam concentrate. (Foam proportioning ratio is defined as the percentage of foam concentrate that is metered into a plain water stream to produce foam solution). Older eductors will have metering valves with proportioning ratios of 1%, 3% and 6%. Newer eductors will also have a 0.5% setting specifically meant for use with Class A foam agents.

Older eductors can be used successfully with Class A foam. Even though newer eductors are more attractive, since they are equipped with a 0.5% proportioning ratio setting, an older eductor with a minimum proportioning ratio setting of 1% can still be used utilized for Class A foam needing to be proportioned at 0.5%. To do this, take a 10-gallon bucket and pour in five gallons of water. Take a five-gallon pail of Class A foam concentrate and slowly pour it into the water, mixing it gently with a small paddle. You can use the resultant foam concentrate/water mixture with the eductor set at a 1% ratio. The final proportioning ratio will be 0.5% since the foam concentrate was cut to half strength by adding an equal amount of water.

The two popular types of eductors found today are “inline†and “bypass.â€

Bypass eductors are typically plumbed into fire apparatus discharge piping inside the pump house and can be operated in either the “bypass†or “foam solution†position. When operating in the bypass position, water flows through the eductor with little restriction and foam concentrate is not siphoned into the water stream. This allows the use of higher plain water-flow rates than that possible when the eductor is set in the “foam†position. When placed into the “foam†position, water flow inside the bypass eductor is diverted into a narrow waterway (the eductor’s “throatâ€), which allows for foam concentrate pickup.

Inline eductors are externally mounted to a fire pump discharge and are typically kept as “loose†equipment on the apparatus. They are manually installed on an apparatus pump discharge outlet when the need arises during a fire response.

Operational Concerns

First among the issues encountered in proper operation of an inline eductor is hoseline diameter selection and hoseline length. All eductors should be purchased with information from the manufacturer regarding appropriate hoseline diameter and maximum hoseline length where the eductor can be used successfully. Due to design limitations inherent in all eductors, hoseline back pressure limits the total developed hoseline length and/or elevation of the nozzle. A hoseline creating too much back pressure will not let the eductor operate correctly and thus the eductor may not “pick up†the foam concentrate. Typically, 150 feet of 1½-inch hose or 200 feet of 1¾-inch hose is considered acceptable for eductor performance. Consult with your specific eductor manufacturer’s literature for hoseline size and maximum hoseline length limitations.

When using an eductor, nozzle selection is important. Choose a nozzle that is rated at the same flow as the eductor. For example, with a 95-gpm eductor, you must have a 95-gpm fixed-flow fog nozzle, a variable-gallon adjustable fog nozzle set at 95 gpm or an automatic fog nozzle capable of flowing at least 95 gpm.

After optimizing hose diameter, hose length and nozzle selections, choosing a brand of Class A foam concentrate required to get the job done is another consideration. Make sure a high-quality and environmentally friendly foam concentrate is purchased. There are wide selections of Class A foam makes and brands available. Make sure you get what you pay for. Low-bid Class A foams may not be the best choice for effective fire suppression or even, in field use, be as economical as higher-priced Class A foam concentrates.

Situation Sensitivity

You’ve made all the right choices, equipped the apparatus and told your crew to get to work with Class A foam. However, even after making all the right decisions to help ensure the team’s success, there are situations out in the field that may cause an inline eductor to fail to pick up concentrate. Here are a few items to look at in keeping eductors operational.

Inlet pressure. If the pump operator fails to provide 200 psi at the inlet side of the eductor, it may not operate effectively or at all. If your apparatus has high friction loss in the discharge plumbing, or if the eductor is located away from the apparatus (such as at the end of 300 feet of 2½-inch hoseline), you will need a higher pressure than 200 psi on the master pump discharge gauge to make the device work. In this situation, friction loss in the piping and/or hose is robbing the eductor of the inlet pressure needed to make it operate properly. Calculate the friction loss in the piping and/or hose and make sure you add it to the 200 psi required at the eductor inlet to arrive at the needed discharge pressure on the master pump pressure gauge.

Flushing. If eductor metering and check valves are not flushed after use, foam concentrate deposits may plug them. Dried solids stuck on the inside of the metering and check valves cause a blockage and a “no foam concentrate†flow condition at the eductor’s throat. Always follow the manufacturer’s maintenance instructions for your particular eductor after each use.

Nozzles. Is the hose team opening up the nozzle completely during fire attack? Is there an internal blockage at the nozzle, like rocks plugging the inlet screen, causing the nozzle to flow less than its rating? Using a nozzle rated less than the gpm rating of the eductor, opening a properly rated nozzle less than all the way or having a properly rated nozzle with an obstruction will cause less gpm flow to occur, potentially leading to no concentrate pickup. Also, using a fixed- or variable-gallonage nozzle rated significantly above the flow rating of the eductor may result in poor foam stream reach, even though foam concentrate pickup is occurring.

Hoselines. Is the hose team exceeding the maximum recommended hoselay length by the eductor manufacturer? Is the hoseline kinked? Is the hoseline operating at a high elevation? These conditions lead to excessive back pressure. If your maximum hoseline length is 200 feet, but the situation requires 400 feet of hose, you can run 200 feet of large-diameter hose to the inlet of the eductor and then run 200 feet of “attack†hose after that. As long as the operator calculates the friction loss in the supply hose prior to the eductor, and ensures that 200 psi is provided at its inlet, foam concentrate pickup will occur.

Delivery Rate

After making sure that your eductor system is operational, and ensuring that pump operators, nozzle teams and fire officers alike understand the capabilities and limitations of your foam system, keep in mind when using an inline eductor you have set your fire suppression capacity, your gpm flow rate, to whatever the inline eductor flow rate is (such as 95- or 125-gpm of Class A foam solution, for example).

Even if you are using an automatic fog nozzle, your hose team still can flow only whatever the rated gpm capacity of the in-line eductor is, whether using plain water or Class A foam solution. The gpm rating of the eductor becomes the limiting factor to liquid delivery rate out of the nozzle.


Even with these operation concerns, eductors remain one of the easiest ways to become involved in nozzle-aspirated Class A foam. Eductors have a long history of use in the fire service and they can be used effectively today.

Through careful planning and an education program on the particular system that your fire department has in service, you can see great results when using eductors to apply Class A foam for structure, grassland and other types of ordinary combustible-fueled fire challenges.

Dominic Colletti Jr. is a career firefighter with a large East Coast combination fire department and the founder of the CAFS Institute. He is a fire instructor specializing in training on the use of Class A foam and compressed air foam systems. Colletti can be reached at