i guess i could just take the door off and leave it open

[fledglingme]

recently i was asked a question that i didn't know how to answer. it was how big of a cut should be made to a bed room door bottom to allow the air to escape and reenter the heating cycle of the furnance? (return side of the system) when i started thinking about this, it occured to me that now that i completed a lot of remodeling, i'm asking for mysef as well. looking around my home the supply outlets are all 4x9" most rooms have one, but the master has two. at 36square inches. each room's door to the upstairs landing would be cut up aprox 1.25" (doors are 30" wide) the master a whooping 2.5"-both numbers seems like a lot to me. add to this the fact that the landing is under60 square feet and i have 5 doors opening onto it. if i cut 1.25" off of 3 doors and 2.5" off of another, and none from the last door( closet)- things are going to look awfully peculiar.so how do you handle this? i'd be fine with an average cut for all, but how to arrive at a number is the hard part. is there a reasonable approch to this problem, or is it just a case of following what finish carpenters do and arbitrarily cutting off .75 " and going to break?

 

[KLF]

Forced air systems should have ducted returns from each room. If not, consider louvered doors. You can get louver inserts to add to existing doors.

 

[bogus one]

Most FAU systems actually have one centralized return inlet, typically in a hallway at the "core" of the home, or, as in your two story at the top landing. A typical undercut is 1". I think this would be more than sufficient.

 

[snobdds]

Most FAU systems actually have one centralized return inlet, typically in a hallway at the "core" of the home, or, as in your two story at the top landing. A typical undercut is 1". I think this would be more than sufficient.

This is how mine is set up, one big return on each floor. They happen to be directly above the furnace with one straight shot down. I think it's for less ducking running all over the place.

We took all the carpet out for hardwood and there happens to be about a 1" gap on all the doors. I think it plenty.

 

[-Spike-]

Newer homes around here have vents in the ceiling for return air, they simply route to the hall just outside each room. Otherwise, 1" is pretty standard.

 

[Jetboy]

Newer homes around here have vents in the ceiling for return air, they simply route to the hall just outside each room. Otherwise, 1" is pretty standard.

This seems like the cleanest solution. Or just put a vent above the door through the wall.

My house was built in 1951 and has both a heat and a return in every room except the upstairs (main floor) bathroom, which only has heat.

 

[LK1]

Don't forget to change your air filter at least every couple of months. Get a HEPA one if you can but flow is most important.

 

[Redgrrr]

3/4 to an inch on bedroom doors and inch plus on bathroom for the fart fan to actually work

 

[D'Animal]

That is the way our house is built.

Most FAU systems actually have one centralized return inlet, typically in a hallway at the "core" of the home, or, as in your two story at the top landing. A typical undercut is 1". I think this would be more than sufficient.

 

[AJP]

I have forced air heating and cooling. All of my rooms have heating vents and - except for the bathrooms - they all have two air return vents - one at the floor level that is opened only during the winter to draw cooler air back to the furnace and one near the ceiling (opened only during summer) to draw warm air back to the a/c unit.

It appears that many homes use the space between studs (closed off with galvanized sheet metal) to provide the return ... not sure exactly how they transition between the floors though - would imagine fire code would have something to say about that? Also always a little uncomfortable about the possibility of mold getting into that type of arrangement ... just idle thoughts

 

[fledglingme]

not doubt, having returns built into each room would be best. but after a home is built an occupied- it's a little late to dream about the ideal return. it just seemed a bit odd that an outlet would be soo big, and the under cut a "choke point" and low! i'll assume here, that the returns built into a room are sized the same as the outlet supplying. i'm not willing to do anything right now, but think the vents mounted above the door has a lot of merit. in this area it's sorta rare to see airconditioning in hvac systems

 

[-Spike-]

If you use primarily heating, using the undercut of the door is ideal as a return, as it pulls cool air from the floor. If you can't hear the air rushing under the door, you're probably fine. Just because the supply vent is a certain size doesn't mean the fan is pushing all the air that size vent can handle.

 

[splitshot]

Newer homes around here have vents in the ceiling for return air, they simply route to the hall just outside each room. Otherwise, 1" is pretty standard.

This is the way we ran our returns, but the woodstove makes it a moot point.

 

[AJP]

in this area it's sorta rare to see airconditioning in hvac systems

its pretty much "SOP" here in Toronto on anything with forced air systems. A typical installation also has return air run through an electrostatic filter box at the furnace intake and in winter the heated air is humidified through a variety of rube goldberg devices on its way back to the rooms. Actually Toronto is usually damp enough in winter (the other major contributing factor to our rusty vehicles) that humidifying is not necessary very often. Farther north it is more of a necessity. Most folks are also spec'ing heat exchangers as well to allow greater flow of fresh air in the winter without loosing too much energy... I have been trying to figure out how to retrofit this effectively ....

 

[MOfj40]

Having worked as a mechanical contractor, here's a little insight as to why hvac systems are built the way they are in new homes (at least in the eastern united states). The general contractor subs out the hvac install to a mechanical contractor. On spec homes they generally are only willing to pay for one or two centralized returns which are less efficient regarding air flow through the structure but are much cheaper. Your average spec home buyer won't pay extra for individualized returns.

So someone like myself comes in and sizes/installs the supply duct, based on the volume of the room and the thermal coefficient of the building materials. The centralized returns are sized based on the needs of the unit and installed. All this is done just after rough in - no sheetrock, interior doors, etc. General contractor or trim subcontractor then installs doors later in the building process with (generally) no input from mechanical contractor on undercut needed for airflow. I've dealt with only a couple contractors that asked what they should undercut the doors to, but they also typically built custom homes and were willing to pay the extra for individual room returns.

Joe-new-homeowner moves into his new residence assuming all is good. Maybe it is, maybe not. Perhaps everything is perfectly balanced, but more likely they have hot and cold spots throughout their house. Too many variables to know for sure, unless the home buyer has done their homework. Unfortunately, the "average American" is looking for the right price in the right area rather than informing themselves on best practices for building and being willing to pay a little more for quality construction. Instead they either deal with it hot/cold rooms, or pay a mechanical contractor like me to come back through and balance their system by installing extra returns, louvers or supply side dampers and using a manometer to balance airflow in each room of the house.


For your application, through wall vents or louvered doors are the quickest/easiest fix. However they will increase the amount of noise transmitted from room to room significantly if they are even on both sides of the wall. If you can verify that the stud cavity is not blocked between floor and ceiling, the best option would be install a vent on the bottom of the "inside" wall of the room and install a vent on the top of the same stud cavity on the "outside" wall in the hall. That will greatly reduce transmitted sound while still providing good airflow