3.3 Heaters & chemical feeders

Pool and Spa Heaters

Heaters raise water to a comfortable temperature and hold it there. Three technologies dominate, and the CPO exam expects you to know how each works, its efficiency, and how a bypass protects it.

Gas Heaters

A gas heater burns natural gas or propane in a combustion chamber and passes pool water through a copper heat exchanger above the flame, transferring heat rapidly. Fast heat-up makes gas heaters the natural choice for spas and for intermittent use where water must warm quickly on demand. Standard units run at roughly 80–84% thermal efficiency, and high-efficiency condensing models do better. Gas heaters work in any weather, but fuel cost rises with use. Proper water balance is critical: aggressive (low-pH, low-alkalinity) water corrodes the copper exchanger, while scaling water clogs it — so balanced water directly protects the heater.

Heat Pumps

An electric heat pump does not create heat by combustion; it moves heat from the surrounding air into the water using a refrigerant cycle, essentially an air conditioner run in reverse. This makes it very efficient: a coefficient of performance (COP) of 5–6 means it delivers five to six units of heat for every unit of electricity it consumes. The trade-offs are that heat-up is slow and performance falls as the air cools, because there is less ambient heat to harvest. Heat pumps are ideal for maintaining temperature over a long, warm season rather than for quickly heating a spa.

Solar Heaters

A solar system pumps pool water through dark collector panels — usually mounted on a roof — where the sun warms it before it returns to the pool. Operating cost is close to zero and it is the greenest option, but output depends on available sunshine and adequate panel area. Solar installations use a bypass / diverter valve, often with a controller, to send water through the panels only when the panels are hotter than the pool; otherwise the water would lose heat. Many heaters of every type include an internal bypass so that only part of the flow passes through the exchanger, which protects the heat exchanger from excessive flow and improves overall efficiency.

Sizing and Protecting the Heater

Heater capacity is rated in Btu per hour (British thermal units), and a larger Btu rating heats a given volume of water faster. Operators size a heater to the pool's surface area, desired temperature rise, and how quickly the water must warm — a spa that must reach temperature in an hour needs far more Btu per gallon than a large pool held at a steady temperature all season. Whatever the technology, keeping the water chemically balanced and maintaining adequate flow through the unit are the two habits that most extend a heater's life, because unbalanced water corrodes or scales the exchanger and low flow overheats it.

Chemical Feeders

Automatic feeders deliver sanitizer and other chemicals steadily so operators are not dosing by hand, improving consistency and safety. Know these four common systems:

• Erosion / tablet feeders — water flows past trichlor tablets (some units use cal-hypo) and slowly erodes them, releasing sanitizer; a flow-adjust dial sets the feed rate. They install on the return line, downstream of the heater, with a check valve to keep corrosive gas from backing up into equipment. Never mix trichlor and cal-hypo in the same feeder — the combination can react violently, even causing fire or explosion.
• Peristaltic (liquid) pumps — a small motor turns rollers that squeeze a flexible tube, metering liquid chemicals such as sodium hypochlorite (liquid chlorine) or muriatic acid without the fluid ever touching the pump mechanism. They dose precisely and are easily automated.
• Salt chlorine generators (SWG) — water carrying dissolved salt passes through an electrolytic cell that converts chloride into chlorine (hypochlorous acid) on site; the chlorine then reverts to salt, so sanitizer is generated continuously from the salt reservoir. They require the correct salt level and periodic cleaning of scale from the cell.
• Chemical automation (ORP/pH controllers) — sensors continuously measure ORP (oxidation-reduction potential, a proxy for sanitizer strength, read in millivolts) and pH, and the controller switches the feeders on and off to hold the setpoints. An ORP around 650–750 mV generally corresponds to adequate free chlorine. Automation improves consistency, but probes must be calibrated and the water still verified with manual tests.

Heater and Feeder Reference

Use the table to match equipment to the job: pick a heater by how fast and how cheaply it must run, and pick a feeder by the chemical form (tablet, liquid, or salt) and the level of automation desired.