Possible condensate drain air locking?

[Schultzy]

Hi, I have been looking at an old Bradway ducted unit with the condensate drain on the negative air side of the fan. The drain was not flowing and appeared to have an airlock. With the fan off, it still did not flow until I lifted the cleaning flap ontop of the trap. Once opened, the drain flows freely. When closed, the drain flow stops. Can't leave it open during fan operation as it draws air back through the drain pipe. There appears to be a lot of suction in the drain pipe. The. Unit itself also appears to be not level as the water in the drain pan is significantly pooling in the opposing end to the drain. Do you think my problem could be an airlock between drain pan and trap??
Could I have multiple issues including to much static?

[Magoo]

You need a serious P trap after the drain pan, with vent after the P trap. The P trap holds water over and above the negitive pressure in tha fan casing,( as in inches water gauge ) so as water can flow away. The vent after the P trap is to allow the water/ condensate to drain and not add extra negitive pressure effects

[Schultzy]

Ok, so I sat back and thought about this a little. My problem originates in the fact that the drain wasn't flowing even when the fan wasn't running. Even when the drain pan was full.
I knew that the drain would only flow when the cleaning flap was opened on the trap. This was letting the air out. (with fan not operating)
The air handler was falling away from the drain. So the drain effectively needed to run uphill. To an extent, this would have been ok and would have drained if there was no trap. (given the head of water in the pan) When the trap is on there becomes an air lock as the air has no where to escape to.
I went to the length of mocking up a drain with a head of water and putting a trap on the other end. I drilled a hole in the top of the trap and simulated my problem with giving the drain a slight increase of angle. Put my finger over the hole, and no flow. Take finger off to release the air and flows a treat.

Solution, lower the airhandler on the drain end to level up the fall on the drain.

Mystery is, why has it done it now after being installed for 20 odd years?

[question mark]

hi mate
is it a flexible drain pipe or a fixed proper drain pipe , secondly has it been install with provision for air to to escape , may be it got dirty stuff in side the drain pipe which is enough to block the air passage of the drain.may be install a new drain identical to the old one and see if make a difference.
thanks buddy

[Schultzy]

The drain is internal to the air handler. It is a dual stage system. The coils are side by side. The drains run internally to the casing of the air handler. The air handler itself is approximately 3m wide.
The drain appear to be clear, I have flushed it several times and flows freely when there is no air lock.
Its sitting about 25mm off level across the entire air handler. Drains should flow downhill. Its almost like the internal drain itself is acting as a shallow trap.

[Mick13]

i had a similar problem not long ago, the unit was 5 years old and i couldnt work out why it was only happening now and had been fine for 5 years.
i traced the drain out and it turned out one of the hangers that held the drain up through the roof space had come off, so the drain sagged between the other 2 hangers on either side. once i re-secured it, it worked like a dream. that dip in the drain was as u said acting as a shallow trap, so it was trapping air between it and the normal p trap, and not allowing the water to flow as it could not displace the air.
thats my theory anyway :)