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 Condensate won't flow to drain
Author: pLongos313 (MA)

I had a new draw through, upflow air handler installed with new duct work. Problem is the condensate won't flow to the drain, but stays trapped behind the evaporator coil. I believe this is due to the suction of the fan. The contractor increased the previous plenum size along with changing a few ducts from 6" to 7" even though he NEVER did any calculations. The previous set up worked perfectly and now all i can hear is water gurgling as the volume keeps increasing. He tells me nothing is wrong, but this unit is not operating efficiently or as intended. If I move the filter a little water starts to flow. It is pitched right. The p trap works as intended if i take off the filter. Its an open return so I am not sure how to decrease the static pressure behind the evaporator coil (0.5 wc). If the plenum was smaller won't that increase the air velocity? I attached the p trap pics. I am thinking the plenum is too big which is requiring more air volume which means the blower needs to work more, thus increasing the static pressure. Any insight will be much appreciated.

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 Re: Condensate won't flow to drain
Author: bernabeu (SC)

The trap needs to be a 'deep seal' type and LOWERED in relationship with the elevation of the pan.

The inlet pipe to the trap needs to be vertically long enough (H) for a 'head' of water to accumulate to 'force' flow.

The trap needs to be deep enough (H/2 minimum) so that the seal is not 'sucked away'.

One must ALSO allow for an increase in suction as the filter ages.

In your case I would allow 1-1/2 for H, and 2 for H/2+.

H = pressure of air handler instilled by the fan - ask your installer for a 'magnehelic' reading

Typical 'proper' installation:


The following is 'how not to':


"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

Edited 3 times.

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