Chemical Pot Feeders: How Bypass Feeders Work and How to Size One for Closed-Loop Systems

A chemical pot feeder (also called a bypass feeder) is a pressure vessel piped across a pressure difference in a closed loop, so that a batch of treatment chemical is drawn into the circulating water without shutting the system down. This guide covers how a pot feeder works, how to size the pot to your loop volume, and what to check before you buy. It is about industrial bypass pot feeders like ERE's Pot Feeder, not the tablet chlorinators sold for swimming pools.

What Is a Chemical Pot Feeder?

A chemical pot feeder is a sealed, pressure-rated tank with a threaded fill cap and four pipe connections that lets you add water-treatment chemicals to a closed recirculating loop while the loop stays in service. Typical uses are corrosion inhibitors, biocides, cleaners and passivation chemicals in hot-water heating (hydronic) loops and chilled-water loops.

Manufacturer literature describes bypass feeders as a rugged and dependable way to introduce chemicals into closed-loop liquid systems, widely used in hot and chilled water systems. ERE's unit is built with a carbon steel head and body, a Buna-N O-ring, a blue epoxy finish, a domed bottom and a 3.5" threaded closure cap. It has four 3/4" NPT connections, two upper and two lower.

How Does a Bypass Pot Feeder Work?

A bypass pot feeder works by diverting a small stream of loop water through the pot, where it mixes with the chemical charge and carries it back into the main circuit. The driving force is a pressure difference between the two piping connections, not a pump on the feeder itself.

1. Isolate the feeder. Close the inlet and outlet valves. The label on ERE's 2-gallon unit says not to open under pressure, to close the isolation valves and to bleed off pressure before opening.
2. Relieve pressure. Open the drain or vent on the pot until it is at atmospheric pressure. The label also warns that contents may be hot.
3. Charge the pot. Remove the cap and pour in the chemical your treatment supplier specifies, leaving headspace for the cap and gasket.
4. Close the cap. Check the gasket sits in its groove. The label states the cap should seal with hand-tightening pressure only and must not be struck with a tool.
5. Return to service slowly. Open the valves slowly, as the label directs. Loop water now flows through the pot and carries the chemical into the circuit.
6. Close off when done. Follow the chemical supplier's contact time, then close the feeder valves until the next charge.

Where Does the Flow Through the Pot Come From?

The flow comes from a pressure difference in the main loop, most often across the circulating pump, or across a partially closed balancing valve or a purpose-fitted restriction. The higher-pressure tap feeds the pot and the lower-pressure tap returns to the loop, so water moves through the vessel whenever the loop is circulating.

Which ports act as inlet and outlet depends on the installation drawing for your piping layout, so follow the manufacturer's guidance for your unit. The two valves you add on the feeder connections are what make it serviceable: they let you isolate and refill the pot without touching the main circuit.

Which Chemicals Can Go in a Pot Feeder?

Only chemicals that your water-treatment supplier approves for this use, and that are compatible with carbon steel and Buna-N, should go in the pot. That normally means closed-loop corrosion inhibitors, glycol-loop treatments, biocides and system cleaners, dosed per the supplier's program.

• One chemical at a time. The label on ERE's unit warns against combining chemicals in the dispensing device without specific instructions.
• Hot or cold water only. The same label states hot or cold water only and do not use for steam.
• Read the SDS. Under WHMIS, suppliers must provide hazard labels and safety data sheets. The CCOHS WHMIS overview explains what those documents contain and what workers handling the chemical need to know.

How Do You Size a Pot Feeder?

Size the pot by working out the volume of concentrate one dose requires, then checking that it fits with headspace. The calculation is: system volume × dose rate = volume of chemical per dose. Your treatment supplier sets the dose rate, so use theirs, not a rule of thumb.

The table below uses an illustrative dose of 1% by volume of concentrate to show the logic. It is not a recommendation for any specific chemical.

If you are regularly making multiple charges, the larger pot is the better fit. If your loop is small and you top up only occasionally, the 2-gallon unit is easier to handle. Estimate the loop volume from the pipe runs, heat exchangers and expansion tank rather than guessing; an under-estimate under-doses the system.

What Are the Specifications of ERE's Pot Feeder?

ERE's Pot Feeder is a carbon steel bypass feeder rated for 300 psi at 200°F (93°C), supplied in two capacities. The table shows the published part data.

Both models share the same head and body material (carbon steel), Buna-N O-ring, blue epoxy finish, domed bottom, 3.5" threaded closure cap and four 3/4" NPT connections. A specification sheet is linked on the Pot Feeder product page.

A pot feeder is a pressure vessel, and pressure-equipment registration rules vary by province. For background on how pressure vessel design is codified, see the ASME Boiler and Pressure Vessel Code, Section VIII Division 1. For an installation in Quebec, confirm requirements with the Régie du bâtiment du Québec; elsewhere, check with the provincial pressure-equipment authority. Ask ERE for the current spec sheet if your inspector needs it.

Should You Use a Pot Feeder or a Metering Pump?

Use a pot feeder for occasional batch dosing of a closed loop that loses little water, and a metering pump when you need continuous or proportional dosing, for example on an open cooling tower with ongoing make-up and blowdown.

In a tight closed loop the chemical stays in the system, so a periodic shot is often all the program needs. That is why the pot feeder is a common, low-maintenance choice in hot and chilled water loops.

What Mistakes Cause Pot Feeder Problems?

Most problems trace to opening the pot while it is pressurized, overfilling it, or putting the wrong chemical in it.

• Opening under pressure. Isolate and relieve pressure first, every time.
• Overfilling. Leave headspace; a full pot can make the cap hard to seat.
• Ignoring the gasket. A misplaced or worn gasket is the usual reason a cap will not seal.
• Under-estimating loop volume. This under-doses the whole system.
• Using it on steam. The unit is for hot or cold water only.

For filtration and treatment equipment that sits alongside a treated loop, browse the water treatment systems collection, or see the Chemicals collection that lists the Pot Feeder.

Frequently Asked Questions

What is a chemical pot feeder used for?

A chemical pot feeder is used to add treatment chemicals, such as corrosion inhibitors or biocides, to a closed hot-water or chilled-water loop while the loop stays in service.

Can a pot feeder be used on a steam system?

No. The label on ERE's Pot Feeder states hot or cold water only and do not use for steam.

Is a chemical pot feeder the same as a pool chemical feeder?

No. Pool feeders dispense chlorine or bromine tablets into a pool return line. An industrial bypass pot feeder is a pressure vessel for batch-dosing liquid treatment chemicals into a closed process or building loop.

How much chemical does my pot feeder need to hold?

Multiply your system volume by the dose rate from your treatment supplier. If the result does not fit in the pot with headspace, split it into multiple charges or choose the larger capacity.

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