An inline UV unit is a chamber the water passes through with a lamp along its axis, and it is simple to plumb and simple to compromise. Three conditions have to hold for it to disinfect anything, and none of them is visible from the outside, which is why a unit showing a healthy indicator can be doing very little.
Condition one: the water must be clear
UV works only where the light reaches the organism. Suspended particles physically shadow bacteria and cysts from the lamp, and dissolved iron, manganese, tannins and hardness absorb or scatter the wavelength, all of which cut the delivered dose while the unit continues to run normally. So the filtration upstream is what makes the disinfection real: sediment removal to a fine rating, plus iron, manganese and hardness treatment where present. A UV unit fronting inadequate filtration provides reassurance rather than safety.
Condition two: flow within the rating
The dose delivered depends on how long the water spends in the chamber, so a unit is rated for a flow and under doses above it. Size to the household's peak simultaneous demand rather than its average, because an undersized unit disinfects perfectly well at one tap and fails during a shower with a washing machine running, which is exactly when most water is being used. Manufacturers sometimes publish ratings at more than one dose level, so compare at the same dose.
Condition three: continuous power and a live lamp
The unit needs a permanent supply that will not be switched off casually, because a lamp that was off for an evening protected nothing when the taps were used. Lamp output falls steadily through a year of operation while the lamp still glows visibly, so lamps are replaced annually on the calendar rather than on failure. A UV intensity monitor reports the light actually reaching the water and catches both an ageing lamp and a fouled quartz sleeve.
Position and commissioning
Fit it last in the train, after all filtration and close to distribution, since nothing downstream of the lamp is protected: storage tanks, dead legs and any later contamination defeat it entirely. Leave straight clearance at one end to withdraw the lamp and the sleeve, which are as long as the chamber. Sanitize the plumbing downstream at commissioning, then test, and test again a week later. Our inline filter guide covers what belongs upstream.
Questions people ask about inline uv water filter
Why does the water need to be clear for UV?
Particles shadow organisms from the lamp, and iron, manganese, tannins and hardness absorb the wavelength, cutting the dose.
How should a UV unit be sized?
To the household's peak simultaneous flow, since the dose delivered falls as flow through the chamber rises.
Can the power be on a switch?
No. It needs a permanent supply, since a lamp that was off for an evening protected nothing when water was used.
Where should it be fitted?
Last, after all filtration and close to distribution, with straight clearance at one end to withdraw the lamp and sleeve.