A chlorine injection pump is the small metering pump that doses disinfectant into a well line. It is the component that decides whether a chlorination system works, and it is the component that fails silently. Everything else in the train is passive; the pump is the only part with moving parts, a prime to lose and a setting to drift.
How the pump is triggered
Dosing must happen only while water is moving, or the line receives a slug of chlorine when nothing is flowing and none when a shower is running. Two arrangements handle this. The pump can be wired to the well pump circuit, so it runs whenever the pump runs, which is simple and common. Or it can be driven by a flow switch or a water meter contact in the line, which responds to actual flow rather than to pump operation and doses more proportionately when demand varies a lot. Either way the injection point sits ahead of the contact tank, since the chlorine needs its time in the water before anything else happens to it.
Diaphragm pumps, tubing and materials
The residential standard is a small electronic diaphragm metering pump: a flexible diaphragm pulses, drawing solution from the tank on one stroke and pushing it into the line on the next, with check valves at each end holding the direction. Stroke length and stroke rate set the dose. Everything wetted has to be chemically compatible with hypochlorite, which attacks many materials over time, so use the tubing, foot valve, injection fitting and seals the manufacturer specifies rather than whatever is in the drawer. The injection fitting itself needs a check valve so line pressure cannot push water back into the solution tank, and it needs to be reachable, because it is the part that scales up.
Priming, dialing in and proving it
A new or refilled pump usually needs priming, since a diaphragm pump struggles to pull air out of a suction line. Most have a prime or bleed function for exactly this. Dialing in the dose means adjusting stroke and rate, then testing for a chlorine residual after the contact tank and adjusting again, which takes a few passes. It is not a set once and forget job, because the correct dose depends on flow, on how much iron and sulfide the chlorine has to oxidize before it can do anything else, and on pH. Test for a residual after the contact tank, and separately test for bacteria after the whole system, since the residual proves dosing and the bacteria test proves the outcome.
The failure modes that hide
Every common failure looks identical from the tap: water that appears entirely normal and is not being disinfected. An empty solution tank. A lost prime after an air lock. A foot valve fouled by sediment in the bottom of the tank. Hypochlorite that has degraded in the tank over weeks so the dose is far weaker than the setting implies. An injection fitting scaled almost shut, so the pump strokes against a blockage. A cracked suction tube drawing air. Build a routine around it: check the solution level, watch the pump actually stroke, verify solution is moving, and test a residual regularly. Keep a spare tube set and check valve cartridge, and handle concentrated hypochlorite with gloves and eye protection.
Questions people ask about chlorine injection pump
Should the injection pump run all the time?
No. It should dose only while water flows, either wired to the well pump circuit or triggered by a flow switch, so the dose matches actual water use.
Why has my chlorine pump lost its prime?
Usually an empty solution tank, an air leak in the suction tube, or a fouled foot valve. Most metering pumps have a prime function to clear the suction line.
How do I know the pump is dosing correctly?
Test for a chlorine residual after the contact tank, and test for bacteria after the whole system. The first proves dosing, the second proves the result.
Does the chlorine solution go off in the tank?
Yes. Hypochlorite loses strength over time, faster when warm or in light, so a tank mixed weeks ago can dose far less than the pump setting suggests.