When to Replace a UV Bulb and How to Tell It’s Worn Out

The UV lamp used to treat pool or hot tub water works differently from what you’d expect — it doesn’t gradually lose the ability to light up, it loses disinfection effectiveness. The lamp can still visibly glow long after it has stopped producing UV radiation at the needed wavelength with sufficient intensity.

Why it “glows but doesn’t work”

Effective UV disinfection needs radiation at a specific wavelength and intensity. As the lamp ages, intensity declines gradually and smoothly — unlike a classic light bulb, which is either on or off. Visible light (often with a faint purple tint) can persist long after the disinfection effect has dropped below a useful threshold.

When to replace the bulb

• Per the manufacturer’s recommended operating hours — typically one season for year-round use, longer for seasonal use. This is the most reliable guide, not a visual check.
• Rising chlorine consumption despite a functioning UV lamp — a signal that the UV component of disinfection isn’t contributing as it should anymore.
• After a power outage or surge, which could have damaged the bulb even without visible symptoms.

Step-by-step bulb replacement

1. Disconnect the UV lamp from power and wait until it’s fully cooled — bulbs operate at high temperatures.
2. Open the housing’s service cover per the instructions for that specific model.
3. Remove the old bulb carefully, usually from a fluorescent-tube-like socket, without excessive force on the fragile glass.
4. Check the quartz sleeve the bulb sits inside — deposits on it block radiation from reaching the water, so it’s worth cleaning or replacing it during the swap.
5. Fit the new bulb of exactly matching type and power, with no fingerprints on the glass (use clean gloves or a cloth).

Why the bulb type must match exactly

Different UV lamp manufacturers use bulbs with different bases, power, and wavelengths — a visually similar bulb from another manufacturer might not fit the housing at all or won’t deliver the correct radiation intensity. Always order by the specific UV lamp’s type designation, not by the part’s general appearance.

FAQ

How long does a UV bulb typically last?

The most commonly cited operating life is around one season of intensive use, but the exact figure always comes from the manufacturer of that specific model.

Can I tell a worn bulb by the colour of its light?

Not reliably — a drop in disinfection intensity isn’t always accompanied by a visible change in the colour or brightness of the visible light.

Does the quartz sleeve need replacing along with the bulb?

Not always, but it’s worth checking and cleaning it of deposits during the swap — a fouled sleeve reduces effectiveness even with a new bulb.

Replacement UV bulbs and water-treatment accessories are in the water treatment category. More articles on disinfection and water treatment are in the Guides section.