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Question DetailsAsked on 6/29/2016

Do I need to add cold air return to recently finished room?

I've recently finished an unused third stall garage space into a fun room/music studio which added 240 sq ft of living space to our 1800 sq ft townhouse. I tied into the HVAC supply line in the attic above with 6" flexible tubing for heating and cooling. I purposely took extra measures to make the room as air tight as possible to control sound. The airflow into the room is minimal at best. The 12x20 room has two large closets one of which is on the opposite side of the wall from the furnace/utility room. Would it help air flow and be acceptable to add a return vent to the bottom of the wall in the closet and tie a 6" duct directly into the return plenum at the furnace?

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3 Answers

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I hope you complied with firewall requirements between the garage and the new room otherwise tying into the attic duct would probably be a fire code violation (especiallywith flex ducting) - generally ducts are not supposed to enter or exit a garage in case of vehicle/gasoline fire - or be metal ducting and in some areas have to have fusible-link fire dampers at the garage penetrations too. And of course hopefully you got the conversion permitted (both building and possibly Planning and Zoning) so you are not forced to tear it out or have escrow closing delay issues when you go to sell.

You say this is a music studio - meaning basically airtight for all practical purposes, so the air you are putting into the room has nowhere to excape - so yes, you need a return vent.

If you did not maintain the standard gap under the doors, that might be a code violation, too - in probably most areas if you seal the bottom of the door there has to be a certain amount of open ventilation elsewhere in case the HVAC system becomes inoperational, to provide a minimum amount of air circulation. If the door passes into the garage (using garage as access), another fire code requirement would most likely kick in, in addition to the garage rated firewall requirement - required ventilation to the room from another "clean area", with fire-resistant door seals and bottom door seal between the garage and the new room. And in some areas, would have to have another egress bypassing the garage as well - a legal egress window or door.

One other thing if there is a door into the garage so the new room is not totally air-isolated from the garage - generally, all potential ignition sources (any electrical/ electronic devices or auxiliary fan or heater for instance) would have to be at least 18" above floor level, same as furnaces and water heaters and such. Basically anything that ignites or turns on and off falls in that category, including things like amplifiers and speakers and electric/ electronic foot pedals and such that might normally sit on the floor. Some areas exempt totally electronic devices, probably most do not but enforcement is variable. Technically, but again not always enforced, all the outlets are supposed to meet that height requirement too because they can arc, at least when items are plugged in and unplugged.

The return should be as much at the other end of the room from the intake as possible, and assuming the supply is in the ceiling yes the return would normally be low down to or in the floor.

The closet door would need to be supplied with vent louvers or be undercut to at least the free air square footage of the return register to avoid it being an airflow restriction when the closet door is closed. (And yes removing the door or blocking it so it cannot close all the way would technically avoid that problem but would NOT meet code unless all door hardware and doorstop and framing and such were removed from the closet so you are down to just a drywalled walk-through, so a door could not readily be put back on there - code assumes any place configured for a door will have one installed, for HVAC planning purposes).

As for tapping back into the plenum yes that would provide your needed return airflow - though has to be tapped in "upflow" or "before" the air filter. Whether 6" would be appropriate depends on the overall household duct and plenum sizing - it is possible that might be too big (assuming the 6" supply is right-sized), because the return would be right at the air handler so there would be essentially no friction losses in the airflow for that return, so it could end up taking more air than its share from the HVAC system. I would put on an adjustable louver return register so you can throttle the airflow if the room is getting too much cooling or heating or is stealing too much from the upstairs duct branch you tapped into. Though, if you tapped in at the end of the branch upstairs, it could be you will not get as much flow as you need with a 6 inch duct - would just have to see. Depends a lot on whether the garage (and the floor in that room) is insulated or not, and on whether the system was originally sized for heating the garage or not, because that 13% increase in conditioned space, if the garage was not "conditioned space" before, could end up being a 20-25% increase in total heating/cooling load and result in inadequate house-wide heating/cooling if it has bare (uninsulated) concrete floor and/or uninsulated walls, especially if outside walls rather than backed to another unit. Certainly keep a look on the furnace and A/C cycles during the peak demand periods of the first summer and winter to be sure they are not running excessively to try to keep up with the added demand, because townhouse HVAC systems tend to not be oversized - generally they skimp on their capacity sizing to cut cost.

For duct sizing you need to run through the ACA Manual D calculations for your case to determine the required amount of airflow and duct dizing for any particular condition.

Answered 4 years ago by LCD


Thank you for the reply and the expert observations and advise LCD. The builder insulated and finished the original three stall garage space in sheet rock. The HVAC system/plenums run through the attic parallel to the garage space with 6' flex ducting take off from the supply plenum to each individual room. After converting the third stall it was finished in like materials and I simply tied into the supply plenum with 6" insulated flexible ducting approximately15' worth into the ceiling of the new room. The closet doors are of course interior doors with about a 3/4 inch gap at the bottom to the carpet. One wall for each closet is the origially finished and insulated garage wall that houses the untility room on the opposite side. This wall also has an exterior quality door for entering the utility room (Furnace and water heater). The access door to the new room is a solid core exterior door that exits to the remaing two stall garage space. Electrical outlets are code height for interior dwelling installation which according to my electrical guru buddies is acceptable for the space I created. The construction and additions I performed were planned and discussed at length with several builder friends I confided in before each stage. Framing, electrical, drywall and HVAC. I have a friend that owns a heating and air conitioning company look over my system thinking that I would want/need to purchase a larger capacity unit but he was confident that the current unit would easily handle the addional space needs. I was prepared to buy larger but he said I didn't need it. All turned out as anticipated but the HVAC flow was always in question as a wait and see how it works and modify as needed. Luckily with this room addition all my resources (breaker box, HVAC) were all conveniently accessible through the existing interior garage wall or in the attic. The breaker box now resides in one of the closets. Now that I've confirmed through experts like you that I need a return it will be an easy Saturday morning project. Thank you for sharing your expert opinion and other excellent advise.

Answered 4 years ago by Skip


Ah - sounded like definitely a DIY job but was not clear you had expert advice - lucky man.

Seven last thoughts in response to your additional info:

1) Some code areas require a duct flare fitting where you go into the return plenum - basically a metal adapter from round flex duct to metal and sticks out about a foot from the metal ducting - like Z-flex and other make. Others allow just a sheet metal flex duct "coupling" - split-edge flanged and screwed to the plenum, round duct attached to the coupling just a few inches away. Also, generally (ask your HVAC guy) any inlet to the plenum is supposed to be two times the minimum plenum dimension (across the plenum from side to side) from the furnace inlet or air filter - 4 times the diameter if the "plenum" is flex ducting, as I recall. This is to avoid major turbulence at the air filter inlet which increases the airflow friction losses. And of course make sure it does not interfere with maintenance panels and is rated distance from exhaust flue.

2) The solid core door to the garage, because between living space and garage, should have metal firestop weatherstrip around the 3 sides and tight-fitting firestop sill strip (which with some type doors can also be the bronze spring strip), and should have self-closing mechanism or hinges. This would then act, with the firestop, as a firewall delimiter for the garage, meaning "garage" provisions about distance from floor to ignition sources would likely not apply, because that new room would no longer in within the "garage space" - it would be, being isolated by fire rated door and Type-X fire-rated drywall (presumably), be treated more like an adjacent addition with common wall with garage.

3) I think you said the garage wall(s) of the room were drywalled like the rest of the garage. Actually, the exterior walls can generally be 1/2 or in some areas 3/8" drywall, but the dividing wall between garage and living space is supposed to be 1 hour fire rated - typically 5/8 Type X - oitherwise this would be considered a partition wall only, and the new room would still be considered part of the garage - so probably a non-conforming use to build a new "room" like that within the garage. I have seen inspectors require that the ceiling or a wall be torn off interior rooms like that to open it back up because technically a garage cannot have a non-fire rated room within it. Alcoves and work bays and such with dividing walls fine, but not enclosed rooms unless fire isolated from the garage.

4) You did not say if the utility room has an exterior door, but in some or most code areas - as I think I mentioned before - this room would be required to have a direct-to-outdoors legal egress (door or legal egress window) - though a pass-through through the utility room to outside would presumably suffice for that. Keeps a person from getting trapped in the room in the event of a garage fire. Might become an issue come time to sell.

5) Taking up the third bay with a room, aside from needing a building permit in most areas (but not all, especially if DIY), could present an issue at sale time, if in your area your size townhouse is required to have 3 garage spaces - like in areas without any significant street parking and no authorized or adequate driveway parking space - i.e. zero yard and drive lots like in built-up parts of major cities.

6) 6" flex duct has open area of about 28 square inches - with a 29" wide doorway (30" door minus 2 stop strips) that would require a 1 inch airgap at the bottom of the door (or additional free air flow space through the door) - a 36" door (35 effective) about 3/4", 24" door (23" effective width) about 1-1/4" - so be sure you have enough airspace under the closet door or totally remove it or put a louvered vent of sufficient additional airflow rating to cover the required airflow.

7) One other thing if this job affected the utility room - make sure it has the required square footage of fresh air intake from outside, not from the garage UNLESS it has legal open area to garage AND the garage is large enough to provide the cubic footage so by code it does not need external makeup air openings. My preference - NEVER take air for an fuel-fired appliance from a garage or workshop if you don't have to - find another makeup air source to minimize the risk of explosion in event of gas or shop solvent/paint leakage. Course, if your appliances are in the garage, no easy way to avoid that short of enclosing them in a sealed utility room with outside connections. For in-garage furnace/hot water heater installations, if a tight-fitting garage door (the car entry door) I prefer to put the fresh air intake low so it mixes with and can partly (when appliances are not running) partly exhaust any higher-density solvents or fuel fumes - though you have to be careful in cold areas because a low makeup air opening allows cold air to pour in and can cause freezing of slab, pipes, cabinets, etc. That is why it is usually up near the ceiling - so any casual air exchange (other than required appliance makeup air) is warm air going out, not cold air coming in, even though that is not necessarily energy efficient.

Answered 4 years ago by LCD


LCD. I've discovered that local code does not allow the routing of supply ducts to the garage area. Although it's now a finished room, it is still considered garage space as originally designed. I will now be removing the duct and duct control I've installed and exploring the installation of a ductless mini split system. Thank you again for the replies and expert advise/considerations.

Answered 4 years ago by Skip

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