Thread: Return Air requirements

If a jump duct system and or louvers are provided over bedroom doors what effect doe under cutting the doors do to operation and balance?Should under cut be eliminated ?

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No, the undercut of the door will have no effect if a proper return, jumper, or transfer grill is installed. If however there is an issue with privacy or just aesthetics, the door can be replaced or corrected with no ill effects. It is a non-issue in relation to the HVAC system when the need for the undercut door was eliminated by the installation of the other return air path.

The undercuts in the door help the air from the supply registers get back to the return when the doors are closed. The laundry rooms and other non sleeping rooms with often have just a grill since your not worried about people looking in. Many times when the house is built a manometer is used to make sure there is not to much pressure being built up in the room when the FAU is on. The door is the easiest way to lower pressure if the ducts aren't able to do it.

Originally Posted by Larry Hood

If a jump duct system and or louvers are provided over bedroom doors what effect doe under cutting the doors do to operation and balance?

Originally Posted by Robert Ernst

The undercuts in the door help the air from the supply registers get back to the return when the doors are closed.

Robert,

I suspect you missed this part of his post: "If a jump duct system and or louvers are provided over bedroom doors"

As Jim said "No, the undercut of the door will have no effect if a proper return, jumper, or transfer grill is installed."

In Florida, and most areas which use the ICC codes, the undercut door is a dinosaur from the past, never worked well even back then, mostly it just allowed noises outside the room to get inside the room, and noises inside the room to get outside the room - to really help, a door would have to be undercut quite a bit, then it would look like someone walking around in pants with the legs too short ... yuck!

The transfer duct or grille allows sufficient return air exchange (when properly sized and installed). With a transfer duct/grille installed, either the contractor does not understand the purpose of the transfer duct/grille or they are just set in their old ways and 'that is how I've always done it'.

Yeah, it would be better to replace the door with a door which was not undercut, but if the door is not undercut a lot, then why spend the time or the money to replace the door? It's not a "defect" in the sense that it is wrong to have the undercut door to the bedroom (unless undercut quite high); however, it is a "defect" in the sense that anyone still thinks it is needed and necessary.

Originally Posted by Jerry Peck

Robert,

It's not a "defect" in the sense that it is wrong to have the undercut door to the bedroom (unless undercut quite high); however, it is a "defect" in the sense that anyone still thinks it is needed and necessary.

The door just needs to be cut enough to keep it clear of the carpet.

Originally Posted by Robert Sole

The door just needs to be cut enough to keep it clear of the carpet.

Correct, but that would not be addressed as "undercutting" a door for return air, that would simply be a part of the door installation and adjustments, the higher the pile of the carpet the more the door will need to be "trimmed" off at the bottom.

Having installed many doors in old buildings and having to trim, shim, and adjust the doors as needed to match out-of-square door openings and sloping floors ... I can tell that I would refer to that as "having trimmed" a lot of doors; to the extent that I have had to cut the bottom rail out of doors (and the top rails - have to keep the door panels looking 'centered' top to bottom), remove some of the core inside the doors, and then glue the bottom rail back in to the door looks like it was manufactured that way.

"Undercutting" is to allow air "under" the door, thus the "under"cutting term. At least that is my story and I am sticking to it.

Undercutting the door 3/4" will allow for return air from most normal size bedrooms. I would rather not have undercut doors but most of the homes in my area do not have individual returns in each bedroom.

Originally Posted by Rod Butler

Undercutting the door 3/4" will allow for return air from most normal size bedrooms. I would rather not have undercut doors but most of the homes in my area do not have individual returns in each bedroom.

Let's see, a 2'8" door with a 3/4 undercut looks like about 24 cubic inches. Don't see many rooms with only 24 cubic inches of air coming into them. So no, a door to a room with a 3/4" undercut will work with almost no room. Remember, if that return doesn't get back all the air it puts out, we can have some real problems. Review your Building Science Rod.

Originally Posted by John Ring

Let's see, a 2'8" door with a 3/4 undercut looks like about 24 cubic inches. Don't see many rooms with only 24 cubic inches of air coming into them. So no, a door to a room with a 3/4" undercut will work with almost no room. Remember, if that return doesn't get back all the air it puts out, we can have some real problems. Review your Building Science Rod.

sigh,

Oh Grasshopper. I mean John. No, I mean Grasshopper . . . At first I wondered if your reply was a joke from a buddy of mine. (Scott, is this you posting as John?)

A 32” wide door with a ¾” undercut does not give you 24 cubic inches, it gives you 24 SQUARE inches. But then I understand I need to go slow. Since airflow is measured in cubic FEET per minute we must first convert to square feet. Twenty four square inches divided by 144 equals .16667 square feet. Now we can talk cubic feet per minute or CFM.

A typical bedroom might be 14 feet by 14 feet, a random size but let’s roll with that. At 500 square feet per ton of cooling that would come out to about .39 tons of cooling and at 400 CFM per ton that gives us about 157 CFM. I’ll pause for a minute while you do the math.

Ahem.

A typical residential duct system might be sized at a velocity of between 800 to 1000 feet per minute, it will vary based on the designer but that’s close. For a return system you can rely some on cracks around the door but we’ll leave that out for this discussion. Using the 157 CFM divided by the .1667 square feet (remember the 24 SQUARE inches divided by 144 to get square feet?) will give us a velocity of 942 feet per minute. (Take the CFM and divide it by the square footage to get velocity) Like I said, undercutting the door would not be my preferred method for return air but it happens. Bottom line is that 3/4" undercut will work just fine.

I’m still not sure your reply wasn’t a plant just to get a response. Actually I think it’s fromDoug. It is you isn’t it Doug? I knew it. I've been punked.

Originally Posted by Rod Butler

sigh,

Double sigh ...

Rod, did you notice that the person asking the question was from Central Florida? (I will pause briefly for that to soak in ... )

Ahem ...

The FLORIDA building code REQUIRES either a transfer duct or a transfer grille ... undercutting doors for this purpose IS NOT AN OPTION.

Originally Posted by Jerry Peck

Double sigh ...

Rod, did you notice that the person asking the question was from Central Florida? (I will pause briefly for that to soak in ... )

Ahem ...

The FLORIDA building code REQUIRES either a transfer duct or a transfer grille ... undercutting doors for this purpose IS NOT AN OPTION.

Jerry, thanks for imitating my style, shows you have class. Or at least can recognize it. And I appreciate your pointing out the Florida Building code requirements, I did not know that. I didn’t know Florida had a building code and that wasn’t my point.

I did not notice that the OP was from Florida and if you read my post you'll see I had no reference to code, it was in reference to how a system works. My (admittedly sarcastic) reply was to the person that knows very little about the functionality of HVAC systems and even less about math and physics.

I have read your posts and I think you are very knowledgeable if not opinionated about home inspections. However you do have a problem with reading what the poster's intent is. Truth is I am not a HI, but I do have a great respect for the profession. I am a HVAC engineer and have designed building mechanical systems for the last 35 years and I do know HVAC. My intent here is to first; learn from other professionals and second; to share my knowledge. When someone challenges me in such an immature and demeaning manner I pull out my sixshooter. The guy I responded to attacked me regarding my knowledge of building systems and I let him have it.

I have no intent in advancing a bulletin board war of words, it is demeaning and much too time consuming for both of us. If we meet again on another topic, we’ll deal with it from there.

The OP asked whether the undercut was sufficient, and I don't see a demonstration here that 157 CFM can negotiate a 24 square inch opening without pressurizing the room.

Originally Posted by John Brown

The OP asked whether the undercut was sufficient, and I don't see a demonstration here that 157 CFM can negotiate a 24 square inch opening without pressurizing the room.

All I said was that a 3/4" undercut was sufficient. I did not endorse it,like I said it is not my preferred method.

And yes the 157 CFM will pressurize the room, to the tune of about .0008"WC when the door is closed. A typical furnace will have about 0.50" WC forexternal static pressure.

Peace?

Originally Posted by Rod Butler

Peace?

Absolutely, peace is much better than war, also much less time consuming and much less costly.