No, not reliably. Boiling water may kill drain flies or larvae it directly contacts, but it rarely removes all the organic film where they breed, so they can return. It also creates an avoidable scald and splash risk.
“Drain gnats” usually means drain flies or moth flies: small, fuzzy flies often seen resting near drains. Fruit flies usually breed around fermenting food or drink, while fungus gnats commonly come from damp potting mix.
Why boiling water is not enough
Drain fly larvae live in moist organic biofilm attached to drain surfaces, including beneath strainers and around drain openings. A quick flush may loosen debris but cannot reliably clean every breeding area. Colorado State University Extension identifies removal of this organic material as the key control measure.
What to do instead
Clean only surfaces you can safely access:
- Wear gloves and eye protection.
- Remove a lift-out strainer and clear visible debris.
- Scrub the strainer and accessible drain opening with detergent and a dedicated long-handled brush.
- Rinse carefully with hot tap water.
Do not dismantle plumbing, reach into the trap or insert drain-clearing tools. If drain cleaner or another chemical may be present, do not add hot water, detergent or another product; follow the label. For suspected chemical exposure, call the Australian Poisons Information Centre on 13 11 26.
Contact our Sydney plumbing team if the flies return after cleaning or the drain is slow, gurgling, foul-smelling or backing up. Avoid contact with backed-up wastewater.