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Small single story condo in southern California. Newer horizontal attic mounted gas FAU with new ductwork.

There is no heat in the bathrooms and adding electric heat is not an option.

Are there reasons why I wouldn't want to add a small register in each bathroom? If it's okay, then my main HVAC question is if each register needs to be a home run back to the plenum or can I feed both registers from a single run such as the following? (I know not to takeoff from the end of the plenum)

Plenum 6" flex > 10x4 to 6 (guest bath) > 6" flex > end boot 10"x4" to 6" (master bath)

With respect to flex duct should this be an 8" to guest, reduce to 6" to master or is a 6" duct probably okay for both? Total branch from plenum to end boot is under 10'.

The bathroom sizes
Guest: 7'6"x5'0" (37 SQFT) (Left on floor plan)
Master: 8'4"x5'6" (45 SQFT)

Floor plan

FAU

Attic

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    This is just a comment because it's just an opinion, not directly answering your question...but here goes: I'm generally not in favor of a forced air vent INTO a bathroom because it blows out humidity from a shower and stink from you know what into the house UNLESS you have a pretty powerful vent fan in the bathroom. An alternative might be an oil filled plug-in radiator that can provide gentle warmth in a bathroom very safely. Just an opinion.
    – George Anderson
    Commented Dec 21, 2022 at 19:48
  • A quick note from the OP (ME) since it has come up a few times, I’m in no way considering a return in either restroom, but I understand including it in your comments. Are there any other considerations other than register distance to bath fan etc that I might consider about a bathroom application - also, I appreciate the comments about maybe even dropping down to a 4” supply to reduce the chance of register dampers being closed. - All good comments so far.
    – Richard
    Commented Dec 21, 2022 at 20:33
  • Sorry I know you're getting a lot of advice you didn't ask for but ayway .. Don't put a register above a toilet especially if you have air conditioning. The cold air pouring down onto users of the toilet will be uncomfortable.
    – jay613
    Commented Dec 21, 2022 at 21:25
  • the above-toilet register can also cause condensation if you have kids or coffee drinkers; moving it to a corner lets the air warm before it hits the cold tank.
    – dandavis
    Commented Dec 21, 2022 at 23:27
  • No A/C, heat only. I was attempting to keep the register away from the showers and a reasonable distance from the exhaust fan. My next priority is to try to not put it directly over the vanity. Any air flow issues snugging it closer to a wall (still ceiling mounted)? Thank you for the comments about placement, I was hoping to get guidance if necessary.
    – Richard
    Commented Dec 22, 2022 at 0:48

2 Answers 2

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Those are small bathrooms and a 6 inch supply would be too much heating in my opinion. You'll find yourself adjusting the damper which defeats the efficiency of your system due to turbulence.

Rule-of-thumb is 1 CFM per 1 sq. ft. A 6" duct gives you 100 CFM. I'd go no larger than a 4" duct with a standard 4"x10" register. Attach a 6" to the plenum and then split to two 4".

You need to know the total CFM of your furnace blower because if it's already maxed out then you might be stealing comfort from other rooms which were previously comfortable.

Definitely don't put a return in the bathroom unless you wish to introduce excess moisture into your HVAC and funky smells into the rest of your house. Hint, do not introduce excess moisture into your HVAC unless it's a proper humidifier.

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  • I’m fine for volume on the blower, I agree and will take the advice to not oversize this supply to reduce the chance someone closes the register damper. I was considering putting a damper at the plenum on this branch to better control the volume to the bathrooms. These are interior walls so I just need to get “a little warmth” when it’s a freezing cold 65 degrees out here! ;)
    – Richard
    Commented Dec 21, 2022 at 20:58
  • @Richard A damper at the start of the trunk is definitely better than dampering at the register in terms of turbulence. You made me laugh with "when it’s a freezing cold 65 degrees out here!" I don't turn on the heat unless we have sub-50 overnight or low 50's several days in a row; in Central New York 65 is "Goldilocks" temperature! =)
    – MonkeyZeus
    Commented Dec 21, 2022 at 21:05
  • No one runs 4" ducts anymore, even for half baths. Turbulence at the output doesn't change efficiency in any meaningful way, and is generally considered a good thing due to mixing and distribution. Strong drafts are rarely desirable.
    – isherwood
    Commented Dec 22, 2022 at 13:49
  • Your statement about CFM being "maxed out" doesn't make much sense unless the fan is variable and not usually running at full speed. Airflow is always a balance, and adding flow to two small rooms won't change much in terms of run cycles. They'll be slightly longer, but comfort won't be significantly affected.
    – isherwood
    Commented Dec 22, 2022 at 13:58
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I'm not an HVAC tech and don't know the relevant code well, but here's some general wisdom:

  • Yes, any bathroom in homes with central heating should have a heat run. It's unthinkable to not have them in my climate. (It's -12°F here today.)
  • You wouldn't install returns, though. You don't want to drag bathroom odors and moisture through the home. (Modern high-efficiency homes exhaust through heat recovery units.)
  • Modern standards call for at least 6" supply ducting, and sometimes larger for very new systems. Oversize where long runs or many bends exist.
  • Avoid flex except where you really need it. Corrugation increases flow resistance. Use rigid flex if possible.

In your case an 8" line split into two 6" lines would probably be just fine.

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2
  • How would a return be any different on balance than a register? If the pressure and volume of the room are constant, doesn't it stand to reason that pushing air into the room also pushes smells out? If it's off a kitchen or living room, could it actually be better to have a return because it will be way more diluted?
    – dandavis
    Commented Dec 21, 2022 at 23:31
  • If smells or excess moisture are present an exhaust fan should be running.
    – isherwood
    Commented Dec 22, 2022 at 13:45

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