Germicidal lamps look interchangeable and are not. Four characteristics have to match the system they go into, and a fifth, the one people shop on, tells you almost nothing about how much disinfection you will get. Understanding the difference makes ordering a replacement straightforward and makes comparing systems possible.
The four things that must match
Physical length, since the lamp has to sit correctly within the quartz sleeve and the chamber. Base and pin configuration, which determines whether it connects at all. Electrical characteristics, because the lamp and the ballast are a matched pair and a lamp designed for a different drive will not strike or will run wrongly. And lamp type, since low pressure and amalgam lamps behave differently across temperature ranges. Order against the system model number or the part number printed on the old lamp.
Why wattage misleads
Wattage describes electrical input, not germicidal output, and certainly not the dose delivered to the water. Two lamps of the same wattage can differ in the proportion of their output at the germicidal wavelength and in how that output holds up over the lamp's life. What matters for treatment is the dose the whole system delivers at a stated flow and water clarity, ideally from independent validation, which is a system specification rather than a lamp one.
Output decline and the annual rule
Germicidal output falls steadily through a lamp's life, and manufacturers rate lamps for a service period on that basis, typically a year of continuous operation. The visible glow persists long after the useful output has gone, which is why appearance proves nothing and why the annual replacement is a rule rather than a suggestion. Amalgam lamps generally hold output better over a wider temperature range and cost more, which is why they turn up in higher duty systems.
Handling and disposal
Do not touch the glass: skin oils etch quartz when it runs hot and create hot spots that shorten life, so use gloves or a clean cloth. Isolate the power before opening a chamber and never look at a lit germicidal lamp, since the wavelength damages eyes and skin with no sensation at the time. Let a lamp cool before handling. Lamps contain a small amount of mercury and go to an appropriate collection point. Our uv bulb guide covers sourcing lamps and sleeve o-rings by model.
Questions people ask about uv lamps for water treatment
Can I choose a lamp by wattage?
No. Wattage is electrical input, not germicidal output. Length, base, electrical characteristics and lamp type all have to match.
How long does a germicidal lamp last?
Typically rated for about a year of continuous operation, after which output has fallen even though the lamp still glows.
What is an amalgam lamp?
A type that holds output better across a wider temperature range than a standard low pressure lamp, at a higher cost.
Why must I not touch the glass?
Skin oils etch the quartz when it runs hot, creating hot spots that shorten the lamp's life.