Ultraviolet components look generic and are not. Lamps of the same nominal wattage differ in length, base and electrical characteristics, sleeves differ in length and diameter, and o-rings differ in section. Ordering by description rather than by model is how a household ends up with a week of untreated water and a part that does not fit.
Identify the system, not the part
Start from the system's model number, which is on the controller or a label on the chamber, because manufacturers publish parts lists against it. If that has worn off, the old lamp usually carries a part number printed along the glass, and the sleeve may be marked at the closed end. Photograph both before removing anything. Measuring is a last resort and needs care: lamp length, pin configuration, sleeve length and the sleeve's outside diameter all have to match, and near enough does not work.
Lamps and ballasts go together
A germicidal lamp needs a ballast supplying the right electrical conditions, so the two are specified as a pair against the system. A lamp that will not strike in a system with a known good lamp usually means the ballast has failed rather than the lamp, and swapping in a lamp of a different specification will not work even if it physically fits. When a system is old and the original lamp is discontinued, the manufacturer will normally name a current equivalent rather than leaving you to guess.
What to keep on the shelf
A spare lamp, because a failure at an inconvenient moment otherwise means weeks of untreated water while a part is shipped. A spare sleeve o-ring, since it is replaced at each service and perishes with age. And the sleeve itself is worth having only if yours is old or has been scratched, since a scratched or etched sleeve scatters light and cannot be cleaned back to condition. Keep the system's model number written somewhere that is not the lamp you are about to throw away.
Handling the parts you fit
Do not touch the lamp glass: skin oils etch quartz when it runs hot and create hot spots that shorten life, so use gloves or a clean cloth. Isolate power before opening the chamber and do not look at a lit germicidal lamp, since the wavelength damages eyes with no warning sensation. Depressurise the water side before removing a sleeve. Old lamps contain a small amount of mercury and go to an appropriate collection point. Our uv bulb guide covers the annual change in full.
Questions people ask about uv light parts
Can I order a lamp by wattage?
No. Length, base, pin configuration and electrical characteristics all have to match, so order against the system model or the lamp's part number.
The lamp will not light. Is it the lamp or the ballast?
A known good lamp that will not strike usually means the ballast has failed, since the two are specified as a matched pair.
What spares are worth keeping?
A lamp and a sleeve o-ring. A spare sleeve is worth it only if yours is old or scratched, since scratches cannot be cleaned out.
Why should I not touch the lamp glass?
Skin oils etch the quartz when it runs hot, creating hot spots that shorten the lamp's life. Use gloves or a clean cloth.