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Old 02-09-2008, 10:14 AM   #1
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Lousy HVAC setup


We just moved into a house and I found out that on all 12 places where we have ducts going into the air distributor, there is no seal. I imagine this could be the cause of low pressure, lousy heat and wastefully heating the attic (where the furnace/blower is located).

Should I use "mastic" or duct tape. I don't know exactly what mastic is, but bought a tube of stuff called UL 181 listed water based duct sealant (in a caulk type gun) as well as the fancy foil duct tape.

All of the ducts are flex ducts and follow very long winding paths despite taking up all of the floor space in the attic. I was hoping to shorten them as much as possible by creating direct routes (again, the floor space in the attic is already completely useless). When they sealed the duct to the elbow they used duct tape. I have read here they should be using sealant and zip ties:

http://www.eere.energy.gov/buildings..._seal-0783.pdf


Finally can we insulate the aluminum air boxes on the supply and return side using flame spread 25 insulation?? What other things should I look out for ???


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Old 02-09-2008, 12:05 PM   #2
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Mastic is the way to go unless your joints are manufactured seams that fit snug then you should use the foil UL181 tape. [expensive stuff. not regular gray duct tape]

On any joint like in the pic, it needs to be mechanicaly connected first so always check that. The mastic wont "weld" things together, just seal the seams.

Fiberglass drywall mesh tape [it will be called "scrim" on your tube of mastic] is good for the larger joints. Put on a scratch coat of mastic using a cheap paint brush.. Inbed the fiberglass tape and put another coat over the tape.
Be generous in the application. You can also get the mastic in small buckets.

Check back in a few weeks/months to see how the joints are holding up.

Insulating the ducts is a plus if they run thru unheated areas...

Check the filter on the handler and change on a regular basis. Quick check of the coils to see if they need to be vacumed is good since its a new house.

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Old 02-09-2008, 06:53 PM   #3
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your pix X 12 take-off is a lot of heated air leaking if you take off the flex reach up with a small hammer and knock those lips tighter on the inside of the duct to pull up that take-off tighter to the trunk.go to $$$ store and get some cheap 1" paint brushes and a bucket of mastic duct sealer and paint 360 degrees around all the take-offs after you bang them tighter to the trunks(use mastic only metal to metal).NOW THE FLEX shorten up the runs but watch the elbow'd flex as it hits the registers don't pinch it.remove the flex off the take-off,and run the foil around the take-off so that when you slip the flex back on you catch some of it when you tape around the flex to the take-off(do these flex take-off hang down)from the truck supply if they do try to HOME RUN them as short as possible?after you 360 the flex onto the metal collar (catching that foiled tape on the collar)as you go around then pull the insulated outer part of the flex over that and 360 it again(hope that is insulated flex) there?with that mastic go all the way back to the unit and start at the duct off the unit and seal all clips and bracket screws(with the unit fan off)that stuff is like TAR and paint it on very rich all the way around.if the discharge duct is screwed to the top of the furnace tap the flanges down around the top of the unit and paint away... if it is connected cheaply give if a couple of coats when that stuff harden"when it seals and cures it is permanant"
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Old 02-10-2008, 04:12 PM   #4
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What is percentage energy loss should I expect with insulated ducts through unconditioned attic space? In a best case scenario?

None of the ducts are properly sealed and all of them use very long runs and often compressed. What a freaking mess......
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Old 02-10-2008, 04:44 PM   #5
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i won't even guess on the % of lost heat and the time put on the heating cycle to make your setpoint.you need to get the heated air from out of the furnace to the rooms with minimal air loss and temp loss along the run..is the metal work insulated on the insides,and does the flex have insulation as the outer wrapping.i would think about getting a laser thermometer with a laser pointer on it so you can a least see what the unit is sending out and see what your sealing work will accomplish on the discharge ducts

Last edited by biggles; 02-10-2008 at 04:47 PM.
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Old 02-10-2008, 05:35 PM   #6
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The metal boxes are insulated on the insides and all of the flex is insulated with R6.

So I guess what I am going to start off with is putting mastic all over every connection, shortening the flex runs and making sure they are not compressed. Hopefully this will help. I just can't believe anyone would go to the trouble of installing a central heat/air, almost do it correctly, but then leave out a few major points. Thanks for the tips. I am sure it will help..

Last edited by pcampbell; 02-10-2008 at 05:37 PM.
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Old 02-10-2008, 05:49 PM   #7
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When a flex run is more than 12' the static in it kills the flow of air dramatically, you may want to change some to round pipe if they are excessivly long.
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Old 02-11-2008, 03:07 PM   #8
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Thanks so much. This is very interesting. I would say NONE of the runs are less than 12 feet. The only thing that look ok are the returns. They are all 8" and very short, very direct.

What I have decided to do is

Seal seal seal

I am shortening several runs. Some of the runs literally ran 25 feet in circles in order to go 5 feet. It's nuts.

Get rid of awful 135+ degree angles. Put in some hard elbows at the ADU (they all currently go out straight and then turn in the flex.)

I am going to remove and cap 2 of the runs at the ADU. These are redundant runs pushing air to places we don't want them in rooms that have multiple ducts. This should increase pressure to the places we want air I think.

Insulating around the registers. They ripped out the insulation to install the registers in the attic but didn't put insulation back when they were done.

Finally, I have a duct fan that I may put to use. It is Fantech's most powerful 6" fan. I may hook this up on a dial to the living room. This will allow us to really pump in some air when we have guests over, or when my wife is freezing hopefully. This is the room we are in most of the time.

I am really hoping I will see some good improvements in pressure, temperature and of course lowered heating bill. As far as I can see we were are currently heating an extra 500 sq ft right now.

BTW, it was 6 F out this morning and we are heating the entire house with a Rinnai 21500 BTU direct vent heater. We see about 63 in the room furthest from the unit. This is not warm enough, but I almost got the 38k BTU model, and really wonder if that thing couldn't heat the entire house from one location with no duct work.
That is in a room with huge window and door exposure, 3 walls to the outside. I see condensation on the inside of our door to the outside ... thinking this means we need to get a good storm door.

Last edited by pcampbell; 02-11-2008 at 03:09 PM.
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Old 02-11-2008, 04:42 PM   #9
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if you have to turn a piece of plex into a 90 degree to catch the supply duct,you might want to add metal elbows from the supply duct to go in straight with the flex.the shorter runs and minmal of turns off the main supply trunk the better the air will be to the space.
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Old 02-14-2008, 02:38 PM   #10
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I am back. I have sealed all joints with mastic and removed literally about 100 feet of duct. The pressure is definitely better... BUT:

I had knocked down some insulation so I had to crawl behind the furnace (which is wedge up against where the floor/ceiling meet!!!) When I was back there I noticed there was hot air just RUSHING at the point where the heater meets the A/C. We have this lying on the floor. From right to left is supply air box, A/C, heater, return air box. There is air blowing out of the joint between the heater and A/C like there is no tomorrow. Can I assume this should be sealed?????????? Unfortunately the unit is on the ground only about 2" off the ground on some 2x4's and it is going to be extremely difficult to get at the crack on the underside.Someone mentioned something about possibly the wrong size shroud/collar being used?

Last edited by pcampbell; 02-14-2008 at 02:45 PM.
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Old 02-14-2008, 05:21 PM   #11
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that is a big leaking point when the put that on the horizontal like your describing,and needs to be sealed 360 degrees around return to blower...blower/furnace... discharge duct.suggestion here....shut the gas off break the union away from the unit,,and TILT the unit UP to get a good sealing job under there (don't use duct tape it will dry out with HOT air blasting it),the refer lines should be liberal enought to even add some more 2x4s under it to just lift it straight up also as an option.once you get some working room to reach the entire bottom.if you remove some of the flex discharges can you see into the AC coil and reach the metal connection from the supply duct to seal?if you paint on duct sealer all the way around inside the leaking and then all around the outside connection let it dry....then run the fan to see the progress made.that area in the metal supply section is the highest pressure the air will be before it diffuses out into the flex runs.i would recommend screwing in some more ZIP SCREWS before sealing it ,but your near the evap coil with the freon...if it is safe to screw down the flange that won't hurt.if they didn't do the supply duct you can be sure they never even consisdered the return after dragging that unit up into the attic and mounted on 2x4s just zip it on the sides and top and were done.you might of want to consider hanging(maybe 6" off the attic floor) it from your roof beams with threaded rod and 2 lenghts of KINDORF(replaces the 2x4s) in your spare time probably have to move the gas line up or down but the refer lines can move up or down...i could give you a suggestion also on the AC side of the system ......but lets not go crazy here.

Last edited by biggles; 02-14-2008 at 08:56 PM.
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Old 02-20-2008, 12:15 PM   #12
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I was able to get my hand under and seal 360 degrees around the entire box. I let it dry, turned on fan and it has all but stopped it entirely. I put another coat on for good measure. Now air is coming out of the PVC pipe that is used to drain water from the condensor. There is an opening in the PVC pipe which I assume is to displace the air to allow the water to flow (like the opening on a gas can?) I would say the amount of air coming out is much less than that which was coming out around the junction between furnace and condensor, but still...

Is all of this normal?
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Old 02-21-2008, 05:30 AM   #13
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take a cup of water and pour it down the vent the air is coming out,it dried out since the last AC run that will prime it no big deal there.you'll have so much air soon you won't know what to do with it.
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Old 02-22-2008, 08:01 PM   #14
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So far, we see a noticeable improvement in that it is much easier to warm up the living area.

What I've done is :

-Re-route all flex ducts to make them shorter and taut
-Remove 2 runs to rooms that had multiple registers
-Change any straight connections to the aluminum box that turned immediately with elbows
-Put insulation everywhere, around boots, elbows, etc. There was already insulation on the ducts, but they did not have insulation around any of the boots!
-Seal all joints, elbows, connections with mastic. Seal the holes shown in the picture. Seal everything everywhere else I could.

Before when it was 68 in the house it would be 69 in the attic. Now it is 72 in the house and 62 in the attic. I believe regardless there is going ot be some heat up there because the furnace itself is warm and hot air rising through fairly poor insulation on floor of attic.

Anyway ,thanks for all of the help.
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Old 02-23-2008, 01:08 PM   #15
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the flue pipe run... off the furnace is banging 180F(hit that with your laser) easy,so thats attracting some of that attic cold,won that battle and kept a contractors billing you away...enjoy it!

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