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Author Topic: PVC pipe for A/C  (Read 2322 times)
TomC
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« on: October 20, 2009, 08:07:41 AM »

To try to get the maximum flow out of A/C ducting, I was toying with the idea of using large diameter PVC pipe (4-6") for my A/C ducting.  Mainly because it is smooth inside-as compared to the insulated flexible ducting sold at Home Depot.  I'm wondering though-would it produce a smell, or a even a toxic gasing output? If the flex insulated ducting from Home Depot doesn't restrict flow, then I'll use that-but it just seems to me with the accordian interior, it would slow the flow.  Good Luck, TomC
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« Reply #1 on: October 20, 2009, 08:31:15 AM »

I couldn't say with certainty that the setup isn't toxic in some way but I did see a bus at one point that used it, as an extra measure they had ran the pvc pipe inside an insulated sleeve of the expanding corrigated ducting.
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ruthi
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« Reply #2 on: October 20, 2009, 08:51:33 AM »

Tom, let us know what you figure out. We are about to be putting in the ducting for our air. We have had lots of ideas, but havent decided. We even considered using the pink 2 inch foam at HD for the duct work. We put a long one together as an experiment, it seems to work good. It wouldnt sweat, and you can make it the size needed, and it is lightweight.
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John316
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« Reply #3 on: October 20, 2009, 08:54:04 AM »

We used 2x7" regular sheet metal ducting. We had it custom made, and then installed it ourselves. We then covered it in insulation, sofar so good Grin.

God bless,

John
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« Reply #4 on: October 20, 2009, 09:18:58 AM »

I am no expert but I think you would need to be careful about static electric build up. I know PVC pipe builds up a strong static electrical charge if used in a dust collection system for a wood shop but I don't know if it is the wood chips, the air flow, or both. Most folks use a bare copper wire run inside the pipe to a ground to dissipate the charge.

Just something to think about.

Paul
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« Reply #5 on: October 20, 2009, 09:38:16 AM »

We also gave some thought to the plastic downspout rain gutters, and then insulate them. I think the measurements on them is 3"by 7.
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« Reply #6 on: October 20, 2009, 09:49:41 AM »

PVC is used for drinking water, so . . . . . .
If you're concerned about the possible odors, go to the store & stick your nose in a piece & see if the smell is objectionable. I haven't noticed any objectionable odors from PVC pipe, but the glue is another story. That stink will take a few days of good ventilation to go away tho . . .

The downspout idea will allow almost as much flow as a 6" pipe with only half the height.  Cool

Install a bare, grounded wire inside the ducts to take care of the static buildup.

Use the 'foam core' PVC & you'll already have some insulation built in!  Grin

The rule of thumb I remember about smooth vs flex ducting - for similar flow, the flex needs to be the next size larger.

Have fun with your project.
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« Reply #7 on: October 20, 2009, 09:51:38 AM »

Hi Tom,

BTDT years ago.. It will work but, you better think about insulateing the PVC. Even SCH40..

PVC will grow mold real quick if left damp..

Good Luck
Nick-
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« Reply #8 on: October 20, 2009, 05:34:05 PM »

PVC will work for ducting A/C. The problem with PVC is that if a fire occurs the burning PVC will produce toxic fumes.
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« Reply #9 on: October 21, 2009, 05:46:17 AM »

If you go to a commercial gutter and downspout supplier they have different size downspouts available. The small for homes and larger ones for commercial application and they are aluminum or plastic.
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« Reply #10 on: October 21, 2009, 08:12:31 AM »

Thanks-I'll check out the cutter theory.  Good Luck, TomC
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« Reply #11 on: October 21, 2009, 08:57:26 AM »

My only thoughts are: Does the interior diameter meet the AC specs for cu. ft. and there may be a possibility of condensation forming on the PVC.  Jack
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« Reply #12 on: October 21, 2009, 10:20:49 AM »

The reason PVC is not used for ductwork in normal construction is the toxic fumes if the PVC burns.  The HVAC will spread the toxic fumes throughout the ventilated space.

One has to decide for themselves if they think this is an issue in a bus.  If your bus is on fire you want out now anyhow.  I don't know if the toxic fumes would overcome a human before they could get out or not.
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« Reply #13 on: October 21, 2009, 02:19:50 PM »

 After reasearching this with some of the same thoughts, I built my ducts from wood. I could fabricate them to the size and shape I needed. I primed and painted the wood then cut it and put the painted side to the interior of the duct. Sounds odd, but when is the last time you saw condensation on wood?Huh    JIm
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« Reply #14 on: October 21, 2009, 03:36:09 PM »

...The reason PVC is not used for ductwork in normal construction is the toxic fumes if the PVC burns...
...I don't know if the toxic fumes would overcome a human before they could get out or not...

My uncle is a retired Fire Capt., from what I understand, one breath of certain plastic fumes is enough to incapacitate a person.  It's a shame then, that the natural reaction of humans when suprised or startled, it to take a big, deep breath to facitiltate "fight or flight"...

I for one would never use PVC to circulate HVAC air - only drain water from the evaporator coil...  The static build-up issue, and mold suceptability is another matter - most of the made-for-HVAC material is low-toxicity if burned (and usually doesn't sustain a flame) and has anti-fungal/anti-static treatments.  I wouldn't worry too much about the drag caused by the duct material itself so long as the duct is properly sized for the air handler and the entire system is correctly balanced - if the duct ridges are actually impacting your air delivery notciably in a <50 foot length, then your probably have other, larger, things to worry about.

-Tim

[Edit 1]  It seems to me that my post, though it's intended to be cautionary, might come across a bit negative and not very helpfull...  To maximize your air flow, you need to select the correct size duct for the volume/pressure you are dealing with, and in the event of the flexible ducting - you need to ensure that it is supported and kept in a straight line for long runs and bent gradually for turns. Nick B. can probably comment on the static pressure volumetric rating of your fan - but basically, your AC's fan will be most effective at a certain downstream pressure (less usually equates to more CFM).  Typically a system is designed by taking a BTU load, the controlled space's volume, and an air-change/Hr number and building a system around those numbers...  Also remember smaller diameter ducts with a fixed CFM will cause air to move faster throught the duct - and faster air generally makes more noise.  Diffusers and vent covers are usually backed by a larger-than-the-duct box which causes a slow-down in the air speed (the large surface area of a diffuser relative to the feed-duct's cross-section usually also slows down the air). -T [/Edit 1]

[Edit 2] If you really want to maximise your space, there is a duct-board material which is aluminum lined insulation on the outside, and exposed insulation in the inside.  Much like the above post where a custom duct was made out of wood, this material allows you to build plenums to whatever shape you need.  The inside is covered in an acoustic damping material which is fire-proof/resistant and anti-fungal.  It's very light, and rather flexible (probably more so than a bus), but still very rigid (so it will hold its shape). -T [/Edit 2]
« Last Edit: October 21, 2009, 03:53:32 PM by Tim Strommen » Logged

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« Reply #15 on: October 21, 2009, 05:19:05 PM »

Too large a duct will provide too slow air flow.  Bigger is not necessarily better.   You want to shoot for 800-1100 ft/min air flow velocity.   Cold, air conditioned air, needs velocity.   Air volume in cu. ft/min. divided by duct cross sectional area in sq.ft. gives velocity in ft/min.   

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« Reply #16 on: October 22, 2009, 04:03:28 AM »


 When I did my roof air AC install there was an installation manual/booklet that came with the AC. It was very specific about the size of duct to use. Not 6X4 inch or 6 inch dia. but the size was given in square inches  min and max. also vent placement and size and dia/radius of turns.  I belive this is done to provide the proper back pressure for the unit to function properly.  So maximum airflow may not be in your best interest??   I am not a HVAC guy, so the information from the manufacturer may be wrong.

                                                                    Jim
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« Reply #17 on: October 22, 2009, 04:49:35 AM »

Call me silly! But I'd use duct work for my system.......but I'm just silly! Roll Eyes
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« Reply #18 on: October 22, 2009, 05:42:45 AM »

We used UltraDuct. Great stuff to work with. I cut it with a straight edge and utility knife, creased with a screen spline installation tool and straight edge, folded and taped the seam with aluminum duct tape.    UltraDuct is a layer of high density closed cell foam with a layer of aluminum foild bonded to each side of the foam.  Total thighness is approx. 3/16". This enabled us to make our ducts to the exact specs of the AC manufacturer and is much lighter than PVC. Also easier to make turns when neccessary. 
    We purchased ours from Innovative Energy, Inc, Lowell, IN 800-776-3645.  We picked ours up at there place (4x8 sheets), but they will cut it into 2x4 sheets so it can be shipped via UPS/FedEx.  I dealt with Ron Rado (VP & Gen. Manager)   Jack
« Last Edit: October 22, 2009, 05:45:38 AM by JackConrad » Logged

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« Reply #19 on: October 22, 2009, 05:51:35 AM »

The bottom line is that there is no advantage to PVC over other materials, and there are several DISadvantages.

The greatest of these is the addition of a few cubic inches of toxic flammables, distributed from one end of the coach to the other, in a way which ensures the greatest toxic effect if a fire starts, and which will help the fumes and fire spread more rapidly.  One top goal when you design your conversion is to minimize or even remove things which can burn (that's why I used steel studs instead of wood 2x4s).

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