Down flow?

I have never seen this before. I started to take out the air filter and the dust was on the wrong side, I turned over the filter and sure enough it was backwards. Well, long story short… the air was flowing down through the unit. So instead of the fan blowing directly to the supply ducting (drawing air through the air handler coils) the fan was blowing air through the coils and then through the filter and then into the duct system. Has anyone ever seen this? My take is that the guys that installed the duct work installed the return to where the supply needed to be, and the supply to where the return needed to be and then the guys that set the air handler scratched their heads for a few and then reversed the fan, but failed to install the filter before the unit. I did check to be sure that the returns were drawing air and the supply air was blowing air. I’m stupid busy right now and didn’t have time to do any research on this, so hoping someone could help with any insight. Right now I’m thinking it just needs a filter on the return side of the air handler. Any comments are appreciated.


I believe that this is the cause of that issue. After reading this several time, that is the only conclusion that I can think of.

Thanks for our input Shawn.

It might be a downflow system and the filter is in the wrong spot, was there a place for the filter elsewhere, maybe in a return grill in the home. I know it is probably not a furnace but they do make just air handlers this way as well.

Your pics don’t give me much but it appears to be just a AHU possibly with electrical heat strips or not at all. The return air filter should never be on the supply side of the blower always on the return regardless of the type of AHU/furnace. AHU’s have a squirrel cage type blower and it can not be reversed it only turns one direction. If it rotates backwards it will cavitate in its own air and will move little to no air.

Thanks guys.

Would it be possible to physically turn the fan around?

This is the unit, I could of posted a pic of 100 of other units from other inspections that look just like this, but the air is flowing down through this one, with the filter at the bottom. I’ve seen down flow units before up north (and the filter location is at the top), but all I have seen down here are up flow or horizontal units. I have never seen a unit which forces air toward the intended location of the air filter with in the unit.
The air filter is at the bottom of this pic.

That pic indicates to me that the unit is a upflow indicated by the silver insulated duct along side of the AHU. Where were the supply air registers in the floor or the ceiling. You can draw air through a A-coil but not the electrical heat strips they will be in the direct air stream discharge from the blower if not they would trip on high temp sensor. I have never in 40 years of servicing A/C units seen a contractor or home owner install a filter on the discharge side of the blower. BTW a electric furnace can be installed in the upflow or down flow position by simply flipping the unit.

BTW again the writing on the Ahu is right side up which is normal for a upflow if the blower was installed to be a downflow the writing would be upside down on the Ahu not the coil box but the AHU

It is slab on grade everything goes to the attic.
I agree, hence the dilemma.
Once again thanks to all for your time.

Well I was not there but I would bet my RED hat against a fishing trip from you that the unit is an upflow;-) If in doubt one can always trace the duct out if there was an attic. Do you have any pics of the plenum on the AHU into the attic

It was all foam board, plenum and return, once in the attic it goes to insulated flex… except (what would normally be) the plenum was the return air.
No pic… but like I said everything looked normal as all other up flow units that I have seen, except that the air was moving through the unit down flow.

Not sure I want your hat… I have my own red hats:D. (Maybe I do want your hat:mrgreen:) But any excuse to go fishing is okay with me. :lol: In any case thanks for your time!
The house is vacant, if I get a chance I may ask the owner if I can re-enter to further evaluate.

I wish you would I would luv to see a pic of the plenum in the attic you have my curiosity up:D