Descending from the sink drains, the pipes should run in environments where the canalization passes and therefore be protected from frost , and generally have a slope and position that ensures the flow of water towards the gray tank. However, there may be flat points, elbows or curves which, in certain situations, create small pools of water .
An old trick consists in blowing into the drain (like a blowgun), using a piece of pipe, to send away any water that may be stuck in the pipes. The most modern version of the blow pipe is the pump-operated sink plunger: lift the piston, place the small suction cup on the drain and "blow" (without exerting force, just remove the stagnant water) by pushing the piston.