A UV lamp does not fail the way a household bulb fails. It keeps producing visible light long after its germicidal output has dropped below what disinfection needs, so a glowing lamp tells you nothing about whether the water leaving the chamber is still being treated. That is the single most important thing to understand about these lamps.
Solarization: why a lit lamp can be a dead lamp
The germicidal output sits in a narrow ultraviolet band, and the quartz envelope gradually darkens with use, a process the trade calls solarization. Output falls steadily while the faint blue glow you can see continues. Manufacturers publish a service life for exactly this reason, and the correct practice is to change the lamp on that schedule whether or not it still lights. Units with a UV intensity sensor give you a real measurement instead of a calendar.
The quartz sleeve matters as much as the lamp
The lamp never touches the water. It sits inside a quartz sleeve, and the ultraviolet has to pass through that sleeve to reach anything. Hardness scale, iron and silt film on the sleeve absorb the light and quietly reduce the dose. Clean the sleeve at every lamp change with a soft cloth and a mild acid descaler if there is a film, and inspect it for hairline cracks. Handle the lamp itself by the ceramic ends only, because skin oil bakes onto the glass.
Doing the change safely
Cut power to the ballast first, then shut the inlet valve and relieve pressure. Withdraw the lamp, disconnect the lamp connector, then remove the sleeve nut and gland seal if you are cleaning the sleeve. Replace the O-rings rather than reusing them; a weeping sleeve seal is the usual cause of a chamber that drips after service. Refit, restore water, check for leaks, then restore power and let the lamp warm up before you draw water.
Sanitize the plumbing after opening the system
Opening a UV chamber puts the treated side of the system in contact with room air and your hands. On a private well, where the owner carries full responsibility for water quality because private wells are not federally regulated under the Safe Drinking Water Act, that matters. Disinfect the downstream plumbing after service, and if you are on a well that has ever tested positive for bacteria, retest after the work. Our well water testing guide sets out the sampling routine.
Questions people ask about uv light bulbs for water treatment
How do I know when a UV lamp needs replacing?
Follow the manufacturer service interval or the intensity sensor reading. Visible glow is not evidence of germicidal output, because the lamp keeps lighting after its useful output has faded.
Can I clean a UV lamp instead of replacing it?
Cleaning helps only if the quartz sleeve is dirty. It cannot restore a lamp whose envelope has solarized, since that loss of output is a change in the glass itself.
Is it safe to look at a UV water treatment lamp?
No. Germicidal ultraviolet damages eyes and skin. Never energize the lamp outside its chamber, and cut power at the ballast before opening the unit for service.