Winterize your pond UV lamp: when to turn it off, how to drain it and store it without breaking it

Every spring, we see UV units arrive with cracked quartz. Almost always for the same reason: the unit was left full of water outdoors during a freezing night. Winterizing a UV unit takes fifteen minutes and avoids a repair costing several dozen euros.

Why frost is the UV's number one enemy

Water increases in volume by around 9% when it freezes. Inside a UV unit, it has nowhere to go: it pushes against the thin, rigid quartz tube, which cracks. Sometimes the crack is invisible until the pond is refilled in spring, when water starts reaching the bulb and the electrical housing.

A UV unit that is disconnected but left full of water outdoors faces exactly the same risk as a UV unit that is running.

When to stop it

• Water remaining below 10 to 12 °C: suspended green algae no longer proliferate.
• In Belgium and northern France, this generally means late October or early November.
• Filtration pump: it can keep running if you wish, with the UV simply switched off and drained if it is on a separate circuit.

Leaving a floating thermometer in the pond means you don't have to guess: when it reads below 10 °C for several days in a row, that's the signal.

The 6-step procedure

1. Disconnect the UV.
2. Isolate it by closing the valves, or switch off the pump.
3. Drain it completely by disconnecting it and tilting it, with the end caps open.
4. Remove the quartz sleeve if possible, clean it with white vinegar, and let it dry.
5. Store the device in a garage or cellar, protected from frost. If you cannot dismantle it, leave it empty with the caps loosened.
6. Note the date of the last bulb: in spring, you will know whether it needs replacing.

And what about the rest of the pond in the meantime?

Turning off the UV is part of a broader preparation. The fish slow their metabolism, feeding should be reduced and then stopped below 8 to 10 °C depending on the food, and dead leaves should be removed before they sink. Under the ice, organic matter continues to decompose and releases gases: this is why a permanent opening, created by a heating element or a small air pump placed near the surface, remains important.

However, avoid circulating deep water in winter: the fish take refuge at the bottom, in the warmest layer, around 4 °C. An air stone placed too low would cool it.

The case of heated koi ponds

Some koi owners keep their pond between 10 and 15 °C throughout the winter, sometimes under a shelter. In this setup, the logic changes: the water remains warm enough for algae to grow, the fish continue to feed lightly, and the filtration remains fully active. The UV can then continue running, provided the entire circuit is protected from frost, including the outdoor pipes.

In that case, remember the bulb: a UV lamp that operates twelve months a year uses up its lifespan twice as fast. An amalgam lamp, which lasts longer and is more effective in cool water, then becomes a sensible choice if your device accepts it.

Spring restart

When the water rises above 10 to 12 °C, generally in late March or April:

• install a new bulb if the old one has already completed a season;
• Reinstall the clean quartz sleeve and check the seals;
• Refill with water, check for leaks, then plug it in.

If you also start the filter with bacteria, wait 24 to 48 hours before turning on the UV to give them time to settle.

You may also like

For the rest of the pond: How to prepare your pond before autumn to avoid 6 months of sludge.

READ THE ARTICLE

Frequently asked questions

— Can the UV be left on all winter?

Technically yes, if it remains in circulation and protected from freezing, but this is unnecessary in an unheated pond: algae do not grow in cold water, and the bulb wears out.

— The UV is buried in a filter. What should I do?

Turn it off and, if the filter is emptied for winter, empty the UV housing as well. If the filter remains filled with water and frost-free indoors, the UV can remain in place, unplugged.

— Does a running UV prevent the quartz from freezing?

It limits the risk as long as the water circulates, but a power outage during a freezing night is enough to compromise it.

— Can the water turn green in winter without UV?

Rarely. During a sunny thaw in February, a slight green tint may appear, but it generally disappears when the UV is restarted in spring.

In summary

• Stop when the water drops below 10 to 12 °C.
• The unit is emptied and stored in a frost-free place: the quartz is at risk.
• An opening in the ice for gas exchange.
• Heated koi pond: UV can remain in operation, with the circuit protected from freezing.
• New bulb and seals checked upon restart.

Last updated: September 28, 2026.