Calling an ultraviolet unit a filter is a habit of the trade rather than a description, since it removes nothing at all. What it does is inactivate organisms in the water passing it, and where it is placed in the house decides whether that protection covers everything or almost nothing. Placement, flow rating and the stages ahead of it are the whole of the design.
Point of entry, and as late as possible
For a supply whose microbiological quality is in question, the lamp belongs at the point of entry so that every tap, shower and appliance receives treated water. Within that, it goes last in the treatment train and as close as practical to where water branches into the house, because ultraviolet leaves no residual and cannot protect anything that enters the plumbing downstream of it. A lamp placed early, with filtration after it, is treating water that then passes through untreated equipment before reaching anyone.
Sizing on peak simultaneous draw
Dose is intensity multiplied by exposure time, and exposure time falls as flow rises, so a unit rated for a given flow delivers its dose only up to that flow. Size on what the house draws when a shower, a washing machine and an outside tap run together rather than on average use, because that is exactly when the most water is passing and when the dose would otherwise be lowest. On a well, check the pump's delivery too, since the flow through the chamber is set by the pump rather than by anyone's choice.
What has to be upstream
Sediment filtration always, since particles shade organisms from the light. Iron and manganese treatment where the water carries them, because both film the quartz sleeve. Softening where hardness would scale it. And colour removal where the water is tannin stained, since dissolved organics absorb the working wavelength even when the water looks clear. Ultraviolet transmittance, not appearance, is what determines whether the light gets through, and marginal transmittance needs a larger chamber or better pretreatment.
Practical installation details
Fit isolation valves either side so the sleeve can be serviced without draining the house, and leave the clearance the chamber needs to withdraw the lamp, which on some units is nearly its own length. Mount it where a leak does no damage, since the sleeve o-ring is a wear part. Give it a dependable power supply and, where nobody would act on an alarm, a solenoid that closes the supply on lamp failure. Our whole house filter guide covers ordering the stages that feed it.
Questions people ask about ultraviolet light water filter
Where should the UV unit be installed?
At the point of entry, last in the treatment train, and as close as practical to where the water branches into the house.
Why does it go after the filters?
Because it leaves no residual. Anything the water passes through after the lamp is untreated, and filters ahead of it keep the water clear enough to treat.
What flow rate do I size for?
The peak simultaneous draw of the house. Above the rated flow, contact time drops and the delivered dose falls below what is needed.
Does clear looking water guarantee it will work?
No. Tannins and dissolved organics absorb the working wavelength without making water look cloudy, which is why transmittance is measured.