Choosing an ultraviolet unit comes down to matching the dose it can deliver to the flow rate the house actually draws, then making sure the failure of a lamp cannot go unnoticed. Everything else is packaging. The units look similar and their price differences usually reflect chamber size, lamp quality and monitoring rather than any difference in the underlying method.
Size on peak flow, not average use
The dose an organism receives depends on how long it spends in front of the lamp, so a unit rated for a given flow delivers its stated dose only up to that flow. Exceed it, as happens when a shower, a washing machine and an outside tap run together, and the dose falls at exactly the moment the most water is being used. Size on the plausible simultaneous draw of the house rather than on the average, and remember that a unit installed on a well may also have to pass the flow the pump delivers rather than the flow anyone chooses.
Class ratings and what they are for
Units are commonly offered in two grades. The higher grade is intended for water that is known or suspected to carry contamination, and delivers a substantially larger dose with monitoring to prove it. The lower grade is intended as supplementary treatment on water already believed to be safe. Where a well has tested positive for coliform, or where surface water can reach the casing after heavy rain, the higher grade is the appropriate specification. The EPA notes that private wells are not federally regulated under the Safe Drinking Water Act, so this choice is the owner's alone to make.
Monitoring, alarms and the shut off valve
A lamp continues to glow long after its germicidal output has fallen below what the unit needs, so appearance proves nothing. A sensor that measures ultraviolet intensity through the water is the only honest indicator, and it should drive an alarm the household will actually notice rather than a small light in a basement. Where the water supplies people who would not act on an alarm, specify a solenoid valve that closes the supply when the lamp fails or intensity drops. That converts a silent failure into an obvious one, which is the whole point.
Serviceability decides whether it stays working
Annual lamp replacement and sleeve cleaning are not optional, so buy a unit you can actually service in the space you have. Check the clearance needed to withdraw the lamp and sleeve, since some chambers need most of their own length in free space at one end and basements rarely offer it. Check that lamps and sleeve o-rings are readily available for the model rather than proprietary and slow to source. Our uv bulb replacement guide covers the annual job and the pretreatment that keeps the sleeve clean between visits.
Questions people ask about uv light water purifier
What flow rate should the unit be rated for?
The peak simultaneous draw of the house, not the average. Above its rated flow the contact time drops and the delivered dose falls below what is needed.
Do I need a unit with an intensity sensor?
It is the only way to know the lamp is still delivering a useful dose, since a lamp keeps glowing long after its germicidal output has faded.
Should the supply shut off when the lamp fails?
On any system treating water known to carry contamination, yes. A solenoid turns a silent failure into an obvious one that gets fixed.
Does a UV purifier need filters in front of it?
Always. Turbidity shades organisms from the lamp and iron or hardness films the quartz sleeve, so sediment and any needed treatment go upstream.