Choosing an ultraviolet unit is a sizing exercise, not a shopping preference. The unit has to deliver enough light to every drop passing through at your highest flow rate, in water clear enough for that light to travel through. Get the flow rating and the plumbing position right and the unit runs for years on a lamp change; get either wrong and it passes untreated water while looking perfectly healthy.
Size on peak flow, not average use
Dose is light intensity multiplied by exposure time, so the faster water moves through the chamber, the less each drop receives. Manufacturers publish a rated flow at a stated dose, and that rating has to cover the worst case your plumbing can produce: two showers plus a washing machine, or a hose bib wide open. Work out that peak from your fixture count and your pump capacity, then choose a chamber rated above it. Undersizing shows up as nothing at all, which is exactly why it is dangerous.
Where it goes in the line
The chamber belongs at the end of the treatment train, after sediment filtration, after any iron, manganese or hardness treatment, and downstream of the pressure tank so it is not cycling with the pump. Everything after it is unprotected, so keep the run to the house short and sanitize that pipework when the unit is commissioned. Mount the chamber where you can withdraw the lamp and sleeve along their full length, which usually means allowing clear space equal to the chamber at one end.
Transmittance is the hidden specification
Rated flow assumes water that transmits ultraviolet light well. Tannins, dissolved iron and manganese absorb it, and turbidity scatters it, so colored or cloudy water cuts the delivered dose sharply even with a new lamp. Reputable suppliers state the transmittance their rating assumes and will derate the unit for poorer water. If your well test shows color, iron or manganese, the fix is treatment upstream rather than a bigger lamp, because a bigger lamp cannot see through what it cannot see through.
The controller, the alarm and the shutoff
A lamp that has failed leaves no evidence in the water, so the unit's electronics are part of the safety case. Look for a controller with a lamp failure alarm and, better, an intensity sensor that watches actual output through the sleeve rather than just counting hours. A solenoid valve that closes on lamp failure or power loss turns a silent failure into an obvious one: no water instead of unsafe water. Households that lose power regularly should plan for that behavior deliberately.
The routine that keeps it honest
Lamps lose output long before they stop glowing, so they are replaced on schedule rather than on failure, typically once a year. The quartz sleeve is cleaned at the same visit, since a film of mineral scale on the sleeve blocks light as effectively as cloudy water. Change the sediment prefilter on its own schedule, and sample the treated water for bacteria once a year to confirm the whole arrangement is doing its job. Our whole house UV filter comparison covers which units publish sensor and alarm options.
Questions people ask about water uv filter
Where should the UV unit be installed?
Last in the treatment train, after filtration and softening and downstream of the pressure tank, with the shortest practical run of pipe to the house.
How do I know what flow rate to buy for?
Size for the peak simultaneous demand your plumbing can create, not average daily use, then pick a chamber rated above that figure at the published dose.
How often does the lamp get changed?
Typically once a year. Output decays steadily while the lamp still lights, so replacement is scheduled by hours rather than waiting for failure.
Do I need a sediment filter in front of it?
Yes. Particles shield organisms from the light, and a prefilter also keeps the quartz sleeve cleaner between services.