A: two causes lead to standing water at the bottom of AHU: one cause is poor insulation, drop of external condensed water, such case can be improved simply by adding 1＂thick PE insulation from outside.
Another cause: U-tube is improperly installed or blocked (See 4-3-2), or insufficient gradient of horizontal tube behind U-tube, or when the height of riser behind U-tube is ≥3m, the water pipe is full, no drainage may occur under vacuum siphon effect (Add 1×Φ1/2＂open-end tube just above the riser); if water tray is fully loaded with AHU in operation, while drain pipe discharges small amount of water, then such case will occur.
※Before first use of AHU, it’s preferred to run AHU for a short time (≈2h) to generate condensed water, or charge drain pipe with water, to fill and seal the bottom of U-tube.