7.3 Heaters, Flow Meters, and Chemical Feeders

Domain 2C does not end at the pump volute. The same water that left the filter usually passes a heater, a flow meter, and one or more chemical feeders. Chapter 6 already drew the eight-node loop and defined a flow interlock in systems language. This section owns the hardware: what the heater is, where the meter must sit, which feeder holds which chemical, and which switch is allowed to keep feeding when the pump is dead (none).

Independent teaching for AFO candidates uses the public 2025 handbook heading mechanical devices, plus 2024 MAHC feeder and heater interlock language as a voluntary model. Follow the AHJ and the equipment listing. Do not treat this as a quotation of the copyrighted AFO Manual.

Heaters

Commercial plants heat with one or more of four conceptual families:

• Gas (natural gas or propane) — a burner fires across a heat exchanger. Fast recovery, flue and combustion-air rules, high-limit switch, and a pressure or flow switch that must see water before the gas valve opens. Soot, poor venting, and a dry exchanger are fire-side and water-side disasters.
• Electric — immersion elements or an electric exchanger. Simple, often expensive to run at municipal scale, still needs flow so the elements do not bake in a dry tube.
• Heat pump — a refrigeration loop that steals heat from site air and dumps it into the pool water. Efficient in warm weather, slower recovery, and unhappy when the air is too cold. It is still a water-side exchanger: no flow, no heat, and scale still coats the tubes.
• Solar — collectors on a roof or rack, a diverting valve, and freeze protection. Supplemental in many climates. Night bypass and a draining or glycol strategy are operations issues; the exam idea is that solar is another exchanger in the same circulating stream.

Whatever the fuel, the heater is a restriction in the system curve (more TDH) and a victim of water chemistry. Chapter 5 taught that a positive (scaling) CSI — high pH, high alkalinity, high calcium hardness, high temperature — drops calcium carbonate on hot surfaces. The heat exchanger is the hottest surface in the loop. Scale narrows tubes, raises head, cuts heat transfer, and trips high-limits. Aggressive (negative CSI) water eats metal exchangers from the other direction. You do not need to rerun slide-rule math here. You need the hardware moral: balanced water is heater maintenance.

Flow / pressure switch interlock

The heater must prove flow (or a proxy pressure) before it adds energy. That switch is automatic in the Chapter 6 sense: on or off, not a probe brain. Bypass piping is common so you can isolate the heater without killing the whole loop — but a closed heater isolation valve with the switch jumped out is how exchangers melt. If the pump is off, the heater stays off. If the filter is so dirty that GPM collapsed, the heater should refuse to fire rather than cook a trickle.

High-limit, stack switch (gas), and low-water cutoffs are additional safeties. They do not replace the flow switch. They also do not replace a water test. A scaled exchanger can show "flow" on a clogged tube while the burner short-cycles.

Flow meters

The flow meter is how you know whether the pump and filter are actually delivering the design GPM Chapter 6 calculated. Nameplate horsepower is not a flow meter.

Location. Meters need a run of full, straight pipe — typically several diameters upstream and a shorter run downstream, per the manufacturer. Elbows, valves, and tees right against the paddle scramble the reading. Install on a flooded, full section, not on a downhill pipe that runs partly empty. Indoor rooms often put the meter on the return after filtration so you are reading water that actually went through the filter.

Reading. Face the dial or the digital GPM and write it down. Compare it to design turnover flow (333 GPM in the 120,000-gallon / 6-hour example). Low meter plus high filter differential is a dirty-filter story. Low meter plus a hissing strainer lid is air. Low meter plus a quiet, hot pump and a closed valve is dead-heading. Extra word problems wait for Chapter 8; the hardware skill is believe the meter only if it is installed so it can tell the truth.

Analog (variable-area) meters, paddlewheels, and magnetic meters all fail when fouled or when the pipe is the wrong size. If the needle never moves, do not assume design flow. Fail the meter, not the bather.

Chemical feeders

Feeders put sanitizer and pH adjuster into moving water after filtration (typical layout: filter → heater → meter → injection). Injection upstream of the heater can attack the exchanger with raw hypo or raw acid. Injection downstream of probes that the controller uses will poison the sample. Chapter 6 covered probe placement; here, the feeder itself is the machine.

Erosion feeders pass a slipstream of pool water over trichlor or bromine tablets (or sticks). Output depends on water flow through the pot, tablet surface area, and sometimes a control valve. Trichlor is acidic and adds CYA. Never put calcium hypochlorite tablets in a trichlor erosion pot, and never mix bromine and trichlor in one vessel. Incompatible solids in one feeder are a fire, explosion, or chlorine-gas event, not a convenience.

Peristaltic (tube) pumps squeeze a tube to meter liquid — sodium hypochlorite bleach or, on a separate pump, muriatic acid or another pH liquid. Diaphragm pumps do the same job with a pulsing diaphragm and check valves. Both need a flooded suction from a day tank, a check valve at the injection point so pool water does not siphon back, and a tube or diaphragm that is rated for the chemical. Bleach and acid each get their own pump, own tubing, own injection fitting.

Gas chlorinators meter elemental chlorine from a cylinder through a vacuum regulator. The safety idea you must know: the machine is designed so that a broken vacuum line stops gas flow rather than spraying Cl₂ into the room. That vacuum fail-safe is not a substitute for a dedicated gas room, ventilation, scales, PPE, and the Chlorine Institute / OSHA habits later chapters cover. Never treat a vacuum regulator as optional decoration.

Cal-hypo feeders may be erosion-style units built for calcium hypochlorite or slurry/dissolving systems. They raise calcium hardness as they sanitize — a Chapter 5 balance factor showing up as hardware. Keep cal-hypo dry and away from organics and acids. Do not use a trichlor pot as a "temporary" cal-hypo feeder.

Flow interlock — feed stops if circulation stops

If the recirculation pump dies, a strainer air-locks, or a valve is closed, a running bleach or acid pump can stack a slug in a dead pipe. When flow returns, the slug can dump into the basin — or hypo and acid that met in a shared line can release chlorine gas. 2024 MAHC §5.7.3.5.1.2 requires chemical feed components installed and interlocked so the feeder cannot operate in low or no-flow. MAHC §5.7.3.5.1.4.1 wants monthly challenge testing (unless the manufacturer specifies otherwise): stop recirculation flow to the feeder and confirm feed stops, then restore flow. MAHC also tells operators to evacuate bathers until the cause of an unexpected recirculation stop is evaluated — a health-and-safety rule with a hardware trigger.

A timer that runs the bleach pump all night while the recirculation pump is off is automatic and wrong. The interlock is the flow switch, paddle wheel, meter cutoff, or controller permission that sees water moving.

Never mix incompatible chemicals in one feeder. Dedicated vessel, dedicated pump, dedicated label (MAHC §5.7.3.5 language: feeders dedicated to a single chemical and clearly labeled). Trichlor + cal-hypo is a classic disaster. Acid + hypo in one day tank or one injection line is how mechanical rooms make chlorine gas without owning a gas cylinder.

CO2 versus acid for pH

Both carbon dioxide and mineral acid lower pH. They are not interchangeable hardware.

• Muriatic acid (HCl) or dry acid (sodium bisulfate) fed by a liquid or erosion-style pH feeder drops pH and consumes total alkalinity. That is useful when TA is high. It is harsh on day tanks, injectors, and anything that leaks. Overfeed etches plaster and crashes pH.
• CO2 dissolves to carbonic acid. It lowers pH with less TA destruction than a mineral-acid dose aiming at the same pH. Plants that already run TA on the low side often prefer CO2. Hardware: cylinders or bulk liquid CO2, a regulator, a diffuser or injection point, and the same flow interlock idea — do not dump CO2 into a dead line as a habit. CO2 is not a sanitizer. It does not replace chlorine.

Choose the pH chemical for the alkalinity story Chapter 3 taught, then pick the feeder that can be interlocked and dedicated. Do not feed acid through the bleach pump "because both are liquids."

Walk the stream in order: hot exchanger only if flow is proven, meter in straight pipe reading design GPM, feeders dedicated and silent when the pump is silent. Scale is a CSI problem with a melted-tube ending. That is heater, meter, and feeder hardware for an aquatic facility operator.