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Want to exhaust vent the bathroom through the floor to the outside, requirements?

0 votes
Want to create an inverted P-trap for heat in the vent circuit, as hot air won't want to go down to escape as easily.

Would like to use ABS if allowable. Would like to mount the fan below the floor for reduced noise and easier replacement, with an intake above the shower and exhaust outside.
asked in Bathrooms by rm (130 points)
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1 Answer

+1 vote
Hi RM,
1. The requirements are the same as for the “throught the ceiling” installation:
2. “Inverted P trap for heat in the vent circuit, as hot air won't want to go down to escape as easily”. I’m not sure if I understand you correctly… would that be by going up from the register above the shower – horizontal section - and down to the floor level / under the floor where you hook it up to the exhaust fan? If so, you might have some issues with condensation running into the fan enclosure. You could install some kind of an in-line condensation trap to discharge it before reaching the fan box… I’m not sure because I don’t know how you are planning to position the fan and what kind of a fan design is it.
The final  horizontal section of the pipe should have a 1/8” per inch slope towards the outside allowing any condensation that may form in that line to flow outside the house. You should also support it every 4’ to prevent any sagging.
3. I don’t see any issue with using ABS or PVC pipe, unlike the “dryer vent” building code requirements bathroom vent discharge pipe materials are not listed. Just make sure you cement all the joints with proper adhesive. 
I don’t know the exhaust fan size you’re going to install but I believe at around 200cfm’s they require 6” diameter discharge pipe.
I hope this helps a little, please let me know if you have any other questions.
answered by darekrudy (21,730 points)
Thanks, this is quite helpful.

Re #2.  Venting or Exhausting air from ceiling level upwards seems to have a leaking problem due to hot air rising and inefficient sealing at the fan or exhaust outlet, as leaks allow the hotter air at the ceiling to vent adiabatically as quickly as allowed.  The intention of putting a exhaust inlet above the shower for warm vapor on the wall and vent to a lower height would inhibit the adiabatic flow of warm air through a leak.  The purpose of abs it avoid moisture in the wall cavity and to minimize noise and leaks.  In this way, air should only flow when the fan is activated.

The reference to inverted P-trap is rather than gravity using water's weight to seal a waste pipe, the bouyancy of lighter hot air would be gravity seal the 'top' of an inverted P-trap.  Clearly the differential pressure is not nearly so significant and winds alone could overcome the bouancy difference, however, it's and effort to minimize opportunistic leaks.

Will likely only use the min 50cfm and a 3" pipe.  Placing the fan below the floor in the crawlspace would allow reduced noise and for fan maintenance without interfering with the interior.  It also allows for un-interupted insulation in the ceiling.

You could consider this https://www.tamtech.com/store/bath-fan-range-hood-exhaust-dampers-indoor-air-quality,Product.asp to prevent air leaks and minimize or maybe even eliminate stack effect (watch the video). I don't have any experience with such type of a damper but it appears that it can be utilized in either type of installation (ceiling and floor).