Reply14 flex for a 3.5 ton return is way too small.
The insulation you added will slow the condensation down in winter, but won't stop it.
New Guest
Condensate pipe suction
Living in the Boston, MA area. Back in March I discovered a lot of water in my flex ducts, both in the R-4 insulation and in the duct itself. Two of the flex ducts had mold sitting on top of the water. All the water was found in the take-off side of the flex ducts. Water was not in all the flex ducts – about half. I also found water in the R-4 insulation wrapped around main trunk mostly at the end of the trunk.
During the winter months water is often found inside the air handler, and last winter (08/09) created so much water it dripped into the pan below and set off an alarm. This system is located in an unconditioned attic.
We’ve been experiencing moisture problems since this system was installed when our second floor addition was built in 2004. To mitigate the moisture problem last summer (2009) I removed the existing ceiling mounted aluminum diffusers and installed solid surface (plastic) diffusers that seal the opening when closed. During the winter I always close off the return with insulation board.
When I removed the insulation and flex duct I found the metal duct work was not sealed, flex ducts were loose in some of the boot connections to the rooms below and not sealed to the main trunk. It should be mentioned the return ‘box’ (that connects the living space to a flex duct return) was never insulated.
What I did. I sealed all metal connections with mastic and/or tape. I cleaned as best I could the inside of the metal duct and fixed many of the connections where the interior metal flanges were not flush (screws pushed the flange). I rewrapped the main trunk twice with RFoil bubble wrap with .75” spacers (I used polyethylene pipe wrap with adhesive strips – worked great). Hypothetically this should give me a claimed R-8. I replaced all takeoffs installing them through the RFoil (not over) and sealed them with foil tape and mastic. I then completed the job by installing new R-8 foil faced flex ducts using zip-ties screws and foil tape. I also sealed the air handler with foil tape and for good measure I wrapped it once with bubble wrap – no spacers. I took my time and feel I did the best job I could to reinsulate the system.
New Problem. Everything seems to work well except I notice when the system is running the newly wrapped (twice) box that is connected to the air handler and the new return flex duct is being sucked in. After checking the return size requirements of my air handler, I believe my return is too small. My return grill is 14 x 24 (again – this is connected to a large box that protrudes into the attic and is connected to a 14” return flex duct). I now notice my condensate drip does not discharge until AFTER my air handler turns off – it is creating a suction in the drip pipe (p-trapped) and preventing water from discharging while running.
Stats: 3.5 ton air handler / condenser. This system is serving the second floor (3.0 ton handles first floor and is located in semi-finished basement.) Second floor has 3 bedrooms, 1 office, 1 walk-in closet, 2 bathrooms and a large, open to the first floor stair area; a total of 1050 square feet with 8 foot ceilings.
Other considerations: Roof is a hip design and has a short ridge at 14’, I don’t believe there is enough air movement through the ridge vent, which causes the attic to be very hot in the summer and warm (due to radiant heat) on sunny days in the winter. I also have bathroom soffit vents that must be moved to the roof, as I believe they are causing excessive moisture in the attic during winter months.
Suggestions… PLEASE: Any suggestions as to how I relieve the suction on the drip pipe would be greatly appreciated. I’m concerned that water will overflow the internal condensate pan, or worse – be pushed through my new duct work. I cringe to think I will need to increase the return size or introduce additional returns.
God Speed, Jim. We'll miss you - 2-26-23
14 flex for a 3.5 ton return is way too small.
The insulation you added will slow the condensation down in winter, but won't stop it.
Regular Guest
It also sounds like your vent is on the wrong side of the p-trap. It needs to be vented on the outlet side not the inlet. With your undersized return/ high static make sure its deep enough and filled with water. Contact someone to evaluate and correct your ducting.
New Guest
Hello BeenThere,
I was already thinking the size of my return was too small, but you got me thinking that the actual return flex duct was inadequate for the return grill size. I would like to throw this at you and see if you agree:
Presently (if my calculations are correct) I have a return in my 2nd floor hallway that measures 24X14 (336 square inch area), but the 14” flex duct return is only giving me only 153.8 square inches of throughput.
If I add another 14” flex duct to my existing return ‘box’ (you can see the box in my ‘progress’ picture attached) and then connect to the box attached to my air handler, on the other side from the existing flex duct – I would double my throughput to the air handler. Both ‘boxes’ I mention are big enough to accept the new duct.
Am I correct? Can I increase air flow by adding another flex duct to existing hardware?
Thank you for your time.
God Speed, Jim. We'll miss you - 2-26-23
Is it just a grille. Or is it a return air filter grille?
A 14X24 filter grille is good for about 540CFM. So adding another return duct would make it extremely loud.
A 14X24 grille, is good for about 900CFM without making noise/too much noise.
14" flex is around 600 to 700 CFM depending on length and turns/bends in it.
Would be better off having a contractor add another return to your house, insted of trying to move more through that grille.
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