HVAC return air leaks

[elboco]

I am new here, although I have read some older threads over the past few weeks. I have a typical 2 story house with basement which has the typical problem of a hot upstairs in summer (east central illinois). I had a second HVAC unit installed for the upstairs several years ago and this helped even out the temperatures quite a bit. The HVAC unit for the downstairs is in the basement. However, there is a room on the SW side of the house which can get uncomfortably hot in the late afternoon; 78 when the rest of the upstairs is about 74. Last fall, I installed a duct booster fan in the duct supplying the room without much change in temperature. I now installed something that works well but it has led to other discoveries. I installed a separate 400 CFM fan that blows air from the finished part of basement (through its own filter) through the existing original supply duct for the room. I also installed a separate 200 CFM fan to the return air duct for the room and this warm air blows into the unfinished part of the basement (about 20' from the water heater). We have only had a few warm days so far but this setup seems to be able to keep the room at a comfortable 72.

I had reservations about doing this based on posts here and other sites warning of distributing mold, radon, etc. throughout the house along, with potentially depressurizing the basement. The most obvious symptom of basement depressurization would be the basement door closing and feeling a draft under the floor. I don't witness this with the setup running, however this situation has always occurred with the downstairs HVAC system running. I have no return vents in the basement, so it seems to me that this depressurization has to be due to leaks in the return duct system. So, the question is what are the most common sources for leaks in the return system? My house uses the plenum space between the floor joists for return air. A crude calculation assuming a 0.005" gap between the sheet metal forming the "bottom" of the duct and the floor joist and an average 10' duct run * 2 sides equals about 1.2 sq in of "leak" space per return; with 7 returns that would be about 8.4 sq in of "leak" area. Does this sound about right or is it more likely I have a gaping hole somewhere in the return in the finished side of my basement? Are leaks like this pretty typical?

[Todd S 2]

I'm curious, how large is your home that it requires two A/C units?
It is not uncommon for a home to be short on return air upstairs.
The basement doors slam because of this shortage.
This effects the performance and efficiency of the system.

[ampulman]

If you have a doorway leading to a garage, you can test for supply or return leakage. To do so, stand by the door and dangle a length of string by the door and just crack it open (with furnace blower running). (thank you Dash)

If the string pulls into the garage, there is return leakage. The opposite would indicate supply leakage.

Assuming you have return leakage, you need to 'sniff' your entire return system. I used a dowel with a 1" strip of (facial) tissue attached to the end and ran it by every joint, seam, and connection, especially hidden areas where the sheet metal/joist connections (panning) connects to the trunk, as well as the length of the panning. Don't forget the filter housing. You will pick up every leak, big or small.

If you don't have an attached garage, you can still do the above to evaluate your duct system.

[kimosobee]

Sorry to say but a lot of duct work is installed without sealing all the leaks because in most area's it is not a code violation that is enforced . You can fill those holes with a HVAC silicone. And everywhere the duct work is put together you can use a duct sealer that you apply with a brush. It hardens when it dries and will never fall apart like duct tape would after the adhesive breaks down. The duct sealer looks like a thick slurry. Run some masking tape about an inch or so away from each seam or joint where each section of duct comes together on both sides, and all the way around the duct where you can. Get a cheap 3"brush from the dollar store. Now paint the slurry on the duct seams then take the tape off. You don't have to use the tape unless you don't mind looking at your finished product. I use the tape to make it look neater.Good Luck!!

[Todd S 2]

Sorry to say but a lot of duct work is installed without sealing all the leaks because in most area's it is not a code violation that is enforced . You can fill those holes with a HVAC silicone. And everywhere the duct work is put together you can use a duct sealer that you apply with a brush. It hardens when it dries and will never fall apart like duct tape would after the adhesive breaks down. The duct sealer looks like a thick slurry. Run some masking tape about an inch or so away from each seam or joint where each section of duct comes together on both sides, and all the way around the duct where you can. Get a cheap 3"brush from the dollar store. Now paint the slurry on the duct seams then take the tape off. You don't have to use the tape unless you don't mind looking at your finished product. I use the tape to make it look neater.Good Luck!!

That's great if there is enough return air upstairs. If it wasn't for the leaks in the basement most systems wouldn't work.

[kimosobee]

I am new here, although I have read some older threads over the past few weeks. I have a typical 2 story house with basement which has the typical problem of a hot upstairs in summer (east central illinois). I had a second HVAC unit installed for the upstairs several years ago and this helped even out the temperatures quite a bit. The HVAC unit for the downstairs is in the basement. However, there is a room on the SW side of the house which can get uncomfortably hot in the late afternoon; 78 when the rest of the upstairs is about 74. Last fall, I installed a duct booster fan in the duct supplying the room without much change in temperature. I now installed something that works well but it has led to other discoveries. I installed a separate 400 CFM fan that blows air from the finished part of basement (through its own filter) through the existing original supply duct for the room. I also installed a separate 200 CFM fan to the return air duct for the room and this warm air blows into the unfinished part of the basement (about 20' from the water heater). We have only had a few warm days so far but this setup seems to be able to keep the room at a comfortable 72.

I had reservations about doing this based on posts here and other sites warning of distributing mold, radon, etc. throughout the house along, with potentially depressurizing the basement. The most obvious symptom of basement depressurization would be the basement door closing and feeling a draft under the floor. I don't witness this with the setup running, however this situation has always occurred with the downstairs HVAC system running. I have no return vents in the basement, so it seems to me that this depressurization has to be due to leaks in the return duct system. So, the question is what are the most common sources for leaks in the return system? My house uses the plenum space between the floor joists for return air. A crude calculation assuming a 0.005" gap between the sheet metal forming the "bottom" of the duct and the floor joist and an average 10' duct run * 2 sides equals about 1.2 sq in of "leak" space per return; with 7 returns that would be about 8.4 sq in of "leak" area. Does this sound about right or is it more likely I have a gaping hole somewhere in the return in the finished side of my basement? Are leaks like this pretty typical?

What type of A/C did you add to the second floor, mini-split, air handler with duct in attic? If you had duct in the attic you could easily add R/A any where

[elboco]

Thanks for all the responses.
The home is 3000 sf with tall ceilings in 2 rooms so the conditioned volume would be equivalent to about 3600 sf excluding the approx 1800 sf in the basement. The home is 15 years old and it was pretty common for homes of this age to have separate units for upstairs and downstairs, although both units are typically located in the basement. I think new construction now has zoning. Also, since one can't judge the A/C requirements based on size alone, the property has no trees which provide shade and the family room with a tall ceiling has approx 100 sf of windows that face NW, so it gets plenty of afternoon sun. The room that gets warm is a corner room with a SW facing wall with a window, while the lower half of the SE facing wall is exposed to the 120+ deg garage attic and the floor is exposed to the 85+ deg garage.

The basement door doesn't slam; I measure aprox 0.02 in water pressure differential between the first floor and basement. Ampulman, I did your tests and the string pulls ever so slightly into the garage. I tried the tissue test and found several places with small leaks. Kimosobee - thanks for the tip. I think I will try sealing the ducts later this summer using duct tape, but as Todd_S_2 suggests, sealing it all up may cause other problems! (hence the first pass using duct tape - I can remove it if it causes problems)

I tested the returns as best as I could by placing a sheet of paper over each return to see how much suction was at each return. The downstairs 4 downstairs returns can hold a pretty heavy piece of paper while the 4 upstairs returns are much weaker and can only hold tissue. Actually, one return has practically zero flow - it turns out that this return is "t'd" with a return from the family room and the branch that goes upstairs has 2 90 bends - the branch goes up a first floor wall, under a hallway floor, then up an upstairs hallway wall.

Unfortunately, when I had the second system put in, I simply asked for a second unit upstairs and didn't ask for an overall system evaluation and manual J printout (because I didn't know any better). Chances are the contractor would have found my leaky returns and could have made improvements to the existing system. Oh well.

The upstairs unit is an air handler with duct in the attic, so I could easily have another supply and return run to the "warm" room. How long have mini-split systems been around? I just did a google search on mini-split because I was unfamiliar with the term. I spent 6 months in Taiwan 1 year ago and our apartment had 3 mini split systems and I was thinking that they would have been an ideal solution for our house had I known about them 8 years ago! I assume that the primary advantages of mini split systems vs a window unit are that they are more efficient and better sealed since a mini split system only needs to have the coolant lines sealed.

At this point, I'll leave everything as is since I am certain that my dual fan system will not depressurize the basement any more than the existing HVAC system.

[kimosobee]

Elboco Check the model# on your air handler, and the MOD# & SER# On your outdoor condensor that you added to get your 2nd floor cooling under control.Then would you tell me how many supply openings you have and what size insulated flex they are, 6" ,7",etc. I also want to no how many return air openings you have, what size they are and where their at . In bedrooms , hallways or just 1 centrally located area. All of the dimensions are stamped on the flex line along with the "R" rating. I'm just curios to see if you are taking in enough return air?

[skippedover]

Elboco, you've gotten a lot of answer but the one pointed question you asked I did not see answered. Before you do anything else, you need to size the systems. If the original system was sized to do the whole house and then you added a new system upstairs without ever doing anything with the original unit, it stand to reason you've now got significant over sizing of the system. I imagine that when the second system was added, the first system supplies to that new zone were disconnected? If so, then the system for the first floor is killing itself when it runs and has no place to dump most of the air! :eek2:

Trying to remedy duct issues without knowing the load on the system is tantamount to building a road without knowing how much traffic will use it! Should it be one lane each way or 4 lanes each way? There's a world of difference. So repairing air leaks is noble but a waste of time IMO until you get things sized and know exactly what you're dealing with. From there you can design a plan to cure problems and deliver comfort. JMO.

[tipsrfine]

Get a blower door test "worst case depressurization" test done. There may be times when your house goes into really bad depressurization & that could could cause backdrafting & possibly flame roll-out, either of which could easily cause death.

[Shophound]

The room that gets warm is a corner room with a SW facing wall with a window, while the lower half of the SE facing wall is exposed to the 120+ deg garage attic and the floor is exposed to the 85+ deg garage.



Let's look at this segment of your post for a moment. If I read it correctly, you're saying at least one half of a wall in this SW room (which is warmer than other rooms on the same floor served by the same a/c system) is exposed to an attic where temperatures can soar.

How well is this wall insulated? Have you considered adding radiant barrier on the attic side of this wall (if it is accessible)?

How well is the floor of this room insulated where it is over the garage?

You have tried solving this problem by supplying more conditioned air to this room with your one fan, and extracting more air with the other fan. That is one way to address this problem...increase the volume and turnover rate of air to the problem room.

The other way is to reduce the heat the room gains each day during hot weather. That is why I asked about the wall shared by the attic, and the floor that is over the garage. It's possible (can't say for sure without actually seeing it) that by improving these two areas significantly, your existing upstairs system can handle this room just fine without your added two blowers.

Check it out.

[elboco]

skippedover, only one supply from the first system was physically disconneted (supplies a walk in closet); the rest of the supplies are not disconnectd although about half have the registers closed. Unfortunately, I should have had the HVAC contractor do all this analysis 8 years ago; now I'll know for next time.

shophound, I am not sure what kind of insulation is in the floor of this room. I can check on the wall as the garage attic is easily accessible and adding insulation was on my list. I am considering adding powered attic ventilation in both the garage attic and the house attic, although I haven't figured out if this will improve comfort or end up costing significantly more in running exhaust fans 10 hours per day.

kimosobee,
Air handler is a Lennox G50UH-36A-070-2
Compressor is a Lennox HS26-036-3P, ser 5802A65694

I apologize if I don't use the correct terminology here. All flex duct in the attic are R4.2 and I put some Miraflex R25 over as much of the supply and return ducts as I could after I realized how hot my attic gets. There are 2, 12" returns; one located in the hallway about 10' from the return plenum and the other in the MBR about 30' away. Both returns use 14x20 filters that are in duct board boxes.

On the supply side, there is is an 8" round flexible duct , 6" round flexible duct and 2 approx 12" square plenums fabricated from duct board. One of these duct board plnenums has 3, 7" flexible duct. The other duct board plenum has a 6" flexible duct near the air handler, then 2, 7" ducts and 3, 6" ducts about 15' away.

To summarize, qty 2 12" returns.
supplies: qty 5 7", qty 4 6" and qty 1 8".


Thanks!

[tipsrfine]

I am considering adding powered attic ventilation in both the garage attic and the house attic, although I haven't figured out if this will improve comfort or end up costing significantly more in running exhaust fans 10 hours per day.
It can cost more than just the cost of running the fans if you don't have an adequate thermal boundary/air seal between that attic & the house. It will further depressurize your house, increasing your heating cost along with increasing the liklihood of back-drafting & flame roll out.

[Shophound]

I am considering adding powered attic ventilation in both the garage attic and the house attic, although I haven't figured out if this will improve comfort or end up costing significantly more in running exhaust fans 10 hours per day.
It can cost more than just the cost of running the fans if you don't have an adequate thermal boundary/air seal between that attic & the house. It will further depressurize your house, increasing your heating cost along with increasing the liklihood of back-drafting & flame roll out.

Thanks, Tips, you beat me to it.

I agree with Tips. If you have an a/c system in the attic that serves the second floor of your house, it is difficult to assure the air seal between house and attic that Tips mentioned. Essentially every joint in the ducting must be sealed, every supply boot that penetrates the attic floor or a wall the attic shares with a room must be sealed, and any filter access doors in the return air stream that are in the attic must be sealed, along with the air handler cabinet itself.

I would seriously look at the walls of the warm room that are shared with attic space. I'm seriously serious. I'd also find out about the floor of the warm room over the garage. If you have to knock out a section of drywall in the garage ceiling and poke a flashlight up there to get a look around, do it. Drywall can be patched if you find there's insulation up there. If there's not, it will be tempting for you to hedge about ripping out the garage ceiling to add insulation batts...blown in insulation may be your best bet there. Worth the time and money to do it, IMO, especially if you plan to remain in the house for some time.

One thing I wish would thoroughly penetrate the American psyche...from home builders to home owners...we NEVER build, remodel, or live in a house just for ourselves. It is always for more than just us. We're very payback minded when it comes to making comfort and energy improvements to a house...if we were more broad minded we could add these into resale value of a home (in my book they're more valuable than granite countertops :D ).

[tipsrfine]

Shophound, I believe the op said an attic fan, not an ac system. Of course what you have stated is still true about the air-sealling. Powered ventilation is more & more being frowned upon as being needed. As long as there is enough net free natural ventilation in the attic & it's properly air sealed & insulated from the house with the air seal & insulation being together to form a proper thermal boundary. I'd also be concerned about an attic fan in the garage attic if it's attached to the house. Make sure they do not share any thermal by-passes.

[Shophound]

Shophound, I believe the op said an attic fan, not an ac system. Of course what you have stated is still true about the air-sealling. Powered ventilation is more & more being frowned upon as being needed. As long as there is enough net free natural ventilation in the attic & it's properly air sealed & insulated from the house with the air seal & insulation being together to form a proper thermal boundary. I'd also be concerned about an attic fan in the garage attic if it's attached to the house. Make sure they do not share any thermal by-passes.

I mentioned the a/c system because in the OP's case, it is in the attic, and being it is in the attic, it represents an extension of the thermal and pressure boundaries of the house. This is why I went on about air sealing the entire attic side of the HVAC system. Add a PAV and you could end up pulling indoor air through duct and plenum leaks when the a/c is not running.