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fcxdfm (Mechanical) (OP)
20 Sep 06 23:07
I had always thought that P-trap at the air-handling unit condensate drain connection is needed all the time, no matter if the cooling coil is blow-through or draw-through configuration. However, in the project I am working on, I select a wall-mounted BARD unit with blow through configuration. I was asked why the P-trap is needed. My point is that P trap in the blow through configuration can prevent the cold air at the coil from escaping through the drain pipe. Am I correct here? Anyone can share your experience with BARD unit? Or could you advise anything on this P-trap topic?

Thank you so much!
quark (Mechanical)
20 Sep 06 23:13
I don't know about BARD but you are right.

fcxdfm (Mechanical) (OP)
21 Sep 06 9:15
Thanks, quark.

Another engineer just told me that as long as no negative pressure at the coil connection, no need for the P-trap. I am really confused, so blow through AC unit does not need P-trap???...By the way, in the project, the condensate drain will discharge into the underground drain pipe through the air gap and hub above ground...

Thanks again.
ChrisConley (Mechanical)
21 Sep 06 13:48
Blowthrough units do need P-traps. The without a P-trap the condensate line becomes a 3/4" leak path from the unit.
KiwiMace (Mechanical)
21 Sep 06 14:33
I think on blow through units, you need to be more aware of the depth of the trap.  A high static unit will blow out a shallow trap.
Helpful Member!  imok2 (Mechanical)
21 Sep 06 18:33
fcxdfm (Mechanical) (OP)
21 Sep 06 18:44
Thank you all for help!
cme (Mechanical)
22 Sep 06 11:45
trane has details for draw thru and blow thru on their website
trashcanman (Mechanical)
26 Sep 06 16:01
Always have the depth of water in the P-trap 1" more than the max S.P. of the fan (positive or negative.  Thus, with a blow-through unit producing 6" s.p., the p-trap should have a water seal 7" deep.

For a draw-through unit producing say,-1.5" s.p., the p-trap water seal should be a min. of 2.5" deep.

Hope this helps.

fcxdfm (Mechanical) (OP)
26 Sep 06 22:08
Thanks again to you all.
Engineer6512 (Mechanical)
28 Sep 06 14:38
I would always install a P-trap regardless of whether it is blow through or draw through. You have to consider all conditions including when the unit may not be in operation. When the unit it not in operation there would be nothing to prevent drain gases from migrating up into the unit / ductwork and potentially into the occupied spaces.
HVACctrl (Mechanical)
28 Sep 06 17:38
P-traps are not expensive. Why not just put a deep one at every condensate drain and be done with it?

Not meaning to sound smart-alecky, but it seems to me that its such a minimal expense.... might as well do it.


ChrisConley (Mechanical)
28 Sep 06 18:52
P-traps aren't expensive, but sometimes the installation of them can be.

We reviewed a project where the housekeeping pad had been deleted and the contractor attempted to use 3" of drop to trap against almost 6" of static.

The resulting 'fix' involved coring through the concrete floor and then installing a P-trap in the ceiling space of the floor below.... on 8 air handling units.

The issue was due to 'equal' product being supplied. The equal product that arrived on site was 100mm taller than the design equipment. The solution was to delete the housekeeping pad. Live and learn.
hmm97 (Mechanical)
28 Sep 06 20:05
It depends to the size of the unit and the working SP.
A high working Sp of a AHU will blw out lots of air and creates high noise. Better to have a trap.
HVACctrl (Mechanical)
28 Sep 06 20:56
Good point. You probably pointed out (hopefully, anyway) an extreme example of this, but something I hadn't considered.


lilliput1 (Mechanical)
29 Sep 06 23:08
Note also that if you don't have enough drop for the trap, the condensate would back up in the pan and the pan will remain flooded during summer.
AbbyNormal (Mechanical)
7 Oct 06 11:44
The Bard wall mounts are blow through and will blow cold air out the drain line.

On draw through a good point is to have a high 'stem' on the trap, refer to 'book page' 116 of my favourite link.

Take the "V" out of HVAC and you are left with a HAC(k) job.

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