HVAC duct run for bathroom remodel

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MTy

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I need some help on getting the new HVAC duct run right.

A bit of background on the old bathroom layout. The old vanity area was part of the master bedroom which is apprx. 14 x 20. The toilet and shower was on one side and the closet on the other. There is a register in the master in front of the window and was a register in the furdown above the vanity. There was also a small register in the WC area. There was no supply register in the closet. The return is in the master on the other side of the window near where the new bathroom door is.

Pictures of current situation and a numbered picture of where I think the new registers should go.

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"1" and "2" would put the new registers on either side of the door. One for the bathroom and the other back into the master. "3" would add a register to the closet. It appears the current line is 10 which reduces to eight.

I plan to use flex duct, what is the best way to do these runs? Will my chosen locations work? I know ideally the bathroom register is on an exterior wall or window but the window will be in the shower and I don't really want air coming down on the vanity, although maybe that doesn't matter.
 

Dana

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Can't really do much here without knowing the room by room heating/cooling loads and cubic feet per minute (cfm) for the room loads are. Start by running a room by room LoadCalc or CoolCalc load calculation on the house, and look up the cfm of the furnace/heat-pump/AC you're using.

Any room with a supply duct also needs adequate returns, ideally all of the supplies and returns (and air handlers) would be inside both the thermal (insulation) & pressure boundary of the house. Ducts above the insulation in an attic requires punching difficult to reliably air seal holes in the ceiling, and any duct imbalances between supplies and returns use "the great outdoors" for part of the return path through whatever air leaks it can find.

Since it's looking like a full-gut type of rehab, it could be an opportunity moment for air sealing the wall sheathing as the primary air barrier, and a decision tree moment for whether the roof deck gets insulated (expensive, but it puts the ducts inside the thermal & pressure boundary), or perhaps the ducts get run in soffits under an air-tight ceiling with a vented attic, vs leaving it all in the attic and doing your damnedest to seal both the house & ducts enough, then sizing the equipment correctly.

Getting this stuff right makes a great deal of difference on both comfort & system efficiency, but it's a lot bigger thing to analyze than sizing the ducts for a bathroom-only rehab.
 

MTy

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Can't really do much here without knowing the room by room heating/cooling loads and cubic feet per minute (cfm) for the room loads are. Start by running a room by room LoadCalc or CoolCalc load calculation on the house, and look up the cfm of the furnace/heat-pump/AC you're using.

Any room with a supply duct also needs adequate returns, ideally all of the supplies and returns (and air handlers) would be inside both the thermal (insulation) & pressure boundary of the house. Ducts above the insulation in an attic requires punching difficult to reliably air seal holes in the ceiling, and any duct imbalances between supplies and returns use "the great outdoors" for part of the return path through whatever air leaks it can find.

Since it's looking like a full-gut type of rehab, it could be an opportunity moment for air sealing the wall sheathing as the primary air barrier, and a decision tree moment for whether the roof deck gets insulated (expensive, but it puts the ducts inside the thermal & pressure boundary), or perhaps the ducts get run in soffits under an air-tight ceiling with a vented attic, vs leaving it all in the attic and doing your damnedest to seal both the house & ducts enough, then sizing the equipment correctly.

Getting this stuff right makes a great deal of difference on both comfort & system efficiency, but it's a lot bigger thing to analyze than sizing the ducts for a bathroom-only rehab.

While those things are ideal, the ducts have to stay in the attic.

Maybe I should ask a more specific question. Assuming airflow is proper otherwise, can I get at least the right amount of airflow to the 85 sq. ft. bathroom and 45 sq. ft. closet with a 6 inch flex duct trunk line (reduced from existing 8 straight pipe line) with 6 inch branches to each room or do i need an 8 inch trunk line the whole way with 6 inch branches to each room?

Each branch would be the appropriate 24 inches apart with 24 inches on the trunk line after the last register.
 

Dana

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Without knowing the cfm requirement for the room you really can't say. It's unlikely that a correctly sized system with correctly sized ducts would need anything as big as 6" flex to serve the cfm needs of 130 square feet of insulated space with limited window area (like most bathrooms). But most systems are oversized, making it impossible to find true comfort for the rooms at the ends of long runs.

Since it looks like you're planning to just wing-it, install tweakable balancing vanes right at the take-off(s) from the trunk to be able to dial back the flow as-needed. Adjusting the flow at the register or register boot is too crude and doesn't work as well.

Be sure to mastic-seal all seams and joints on the balancing vanes, take-offs, register boots etc. With flex it's usually more reliable to apply the mastic to the interior of the flex, slipping it over the take-offs and boots, then zip-tie it to keep it from slipping off. Cutting back the flex inside the insulation layer leaving enough insulation to fully cover the take off or balancing vane is important to limit condensation issues. Sealing the vapor barrier outer cover of the flex to block humid attic air from being able to reach cold metal during the cooling season saves a lot of aggravation chasing down condensation drips.

Be absolutely sure to provide a return path to a return register from any room that has a supply register. Using well-sealed partition studwall bays (glue the drywall to the stud bay) with a grille near the top on one side, near the bottom is a reasonable "jump-duct" to use for return paths on low to moderate cfm rooms, even though current IRC code would require hard metal return duct inside the stud bay. An inverted U of insulated duct into the attic to get by an obstructive wall can also work as a jump duct.

When using flex duct it's important to keep the flex stretched straight to it's maximum, and be supported at least every 4' with 1.5-2" straps. If it's just flopped into place, has sags or crimps, or looks like ball of mating snakes the internal duct resistance becomes very substantial, and doesn't come anywhere near meeting it's rated flow. When making turns in flex duct keep the radius long, or use hard-piped slip jointed ells (then mastic-sealing the slip joints) to make the transition.
 
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