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Open Roads Forum  >  Truck Campers

 > A ducting/HVAC air-flow question

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btggraphix

Golden, CO

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Posted: 01/21/09 12:47am Link  |  Print  |  Notify Moderator

Hi all, I decided to pull this question into a seperate thread. For additional reference on what I've been doing, check out my "running some wires while I have my belly open..." thread.

The short story is I have now switched out the old 2" duct to the basement/service bay with a 4" (and sent the old 2" into the cabin.) I had a dealer do a portion and I am now considering changing a little of what they did.

The 4" comes down near the front driver side:
[image]

goes across to the driver side by the converter and gets squeezed a little. Not too bad but I could move the converter over a little to help:
[image]

Between the side and the fresh tank:
[image]

And then back towards the middle. You can see how it drops well lower than the height where the belly will have to squeeze up. Note, it also gets squeezed to get through a cross member and the grey tank. It continues down the middle, and then is dropped down to a 2" hose. THEN, another 2" is split off, upwards, which comes back OVER the grey tank, and is dumping back towards the front. The other continues off to the right, squeezes below a cross member, and heads to the service bay:
[image]

Here's how far the 4 incher drops at that one spot:
[image]
I also have 1" of insulation on the belly pan, so this will really squish the 4 incher. I realize the circle to an oval may not reduce it's cross-sectional area much, but I woul dhave to cut out the insulation. Also note that just past this low spot, it squeezes through this:
[image]

That's pretty tight though I can't really tell how much.

Now, just past these tight spots, it is cut down to a 2". then, the second 2" T's off, upwards, and goes back ABOVE the grey tank to dumps forwards:
[image]

What I am contemplating, is if I wouldn't be better off, dropping from the 4", to 3, 2" ducts (BEFORE that squeeze down under the tank. Area-wise, unless I am forgetting something, the cross sectional area of a circle is pie-r-sqared....so a 4" duct would be something like 3.14*4 = 12.5ish. Each 2" duct is 3.14*1 = 3.14 (duh) so 3, 2" ducts would still be a reduction in flow (down to about 9.5 sq.in.) BUT it's less drop than down to just two 2" ducts, and I could split them BEFORE the tight spot, and run 2 of the ducts to the service bay (where I am having the freezing issues and need to most heat) and leave one to the front. This way I think the 2 inchers would fit through that tight spot better. On the other hand, flow rate is a whole different beast. Perhaps going from the 4" to the 2" where is does, the rate of flow would kick up and the same volume of air would potentially still get through.

I don't like that major smash down (it'll make it harder to get the insulation to stay, the belly pan to raise/bolt is and/or I'd have to remove insulation) but on the other hand the service bay is the primary need, the furnace does seem to be blowing in there pretty well as-is and the bed of the truck has 3.5" of plywood sitting directly below the belly so removing insulation isn't likely to cause issues. Plus, it is already this way......so the easything is to try it like this, and if I feel like changing later I can. But I know once I button it up, I won't likely open it up again for awhile. I take that back, I am going to leave a tempertaure gauge down there so I will have to open it up at some point to retrieve it.

Anyway, thoughts? Theories? HVAC daydreams? Nightmares?


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bb_94401

Columbia Falls. MT

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Posted: 01/21/09 02:02am Link  |  Print  |  Notify Moderator

When I was contemplating putting in ducting for a diesel heater in my TC basement, I came across this guide on the Espar site on ducting design. It starts on page 59 of their Espar Product Catalog.

It gives rating points to each component in your duct design (elbows, tee, grates, reductions, length of hose, hose dia., hose type, etc.). Then they sum the rating points and determine if the ducting is effective or too restrictive for the fan on the heater.

You might adapt it to your design alternatives and see what the tradeoffs are. They use it to prevent the heater from overheating, which is just another way of saying that the air is not flowing, which is what you are interested in avoiding. Choose the heater model which is closest to you heater fan capacity and use its point maximum as a guide.

You also might consider rigid, smooth ID PVC piping for long runs, as that is what they used in my TC basement, as it has less resistance to flow.

* This post was edited 01/21/09 10:22am by bb_94401 *


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PR Connection

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Posted: 01/21/09 08:23am Link  |  Print  |  Notify Moderator

BT

One thing I did to improve air flow although the change was minimal it did help was to foil tape all connections to the furnace plenum. You will notice that all connections are done with a metal flange that just spins on the furnace. There is a fair amount of air leakage from all these connection. Tape them all tight (all that you can gat at ) and you will notice a difference. Even the plenum box has an upper lid to it that I taped to increase air flow to the ducts where it belongs.

Dan


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pjay9

Tacoma, WA

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Posted: 01/22/09 03:49pm Link  |  Print  |  Notify Moderator

You certainly have some restriction points. It would seem to me that getting a duct company to make you a wide flat insulated duct with proper ports would be the answer. The 2" 2" duct "T" is absolutely useless unless there is a diverter inside which I doubt. It wouold be better to come directly off the 4" side by side and the flow would split close to equally depending on the back pressure of each 2" run. The fabed duct would solve many issues of duct restrictions, getting thru tight spots and decreasing resistance in the duct. I am sorry to say that you may not get this done by Sunday...if you don't do anything else change that 2x2 90* junction to side by side at the 4". Maybe bottom it up for now after you take some measurements and get an estimate for a proper duct and install it in the spring. Looks like you have the room under there to do it. Capt PJ


2005 Lance 1161, 2004 Dodge CTD 3500 Dually 19.5's Stabiloads Roadmaster Sway, 2009 20' Raider 185 Pro Fish 90hp & 9.9 Yamaha vintage Penn elec.downriggers EZLoad roller trailer


btggraphix

Golden, CO

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Posted: 01/22/09 09:31pm Link  |  Print  |  Notify Moderator

The theory the service guy was going by, was that he knew I needed the most air in the service bay, and the old "T" was etup the other way....the straight going to the front, and the side one going to the back. You can tell by hand that there is definitely some air coming towards the front, and more air going to the service bay now....so in his eyes, mission accomplished.

However, I have to figure by going first from 4" to 2", you lose air overall, and then you split so you are splitting up a smaller total to begin with.

I decided to try to put three 2" ducts from the 4" duct, but now I hit a snag: I can't find anywhere in Denver to buy 2" flex ducting!

I have checked two dealers, Camping World and home depot. Any ideas? Actually, I might change the thread title.....

pjay9

Tacoma, WA

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Posted: 01/22/09 09:40pm Link  |  Print  |  Notify Moderator

can you use some 2" pipe for the straight runs and scavinge some of your flex for other places?

btggraphix

Golden, CO

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Posted: 01/22/09 09:54pm Link  |  Print  |  Notify Moderator

You've read my mind....I was thinking about that exact thing, but it would be really tough. Partly because there are several tight spots where I would have to use the flex plus the junctions, and then the couple of hard to reach spots. There will be 2 lines all the way to the service bay meaning I need to basically DOUBLE that length. I don't think it's very feasible. 5 Star RV in Henderson is going to look in their salvage yard for me....that's my only "lead" other than cold calling HVAC and/or other RV places.

sleepy

Oak Ridge,Tennessee

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Posted: 01/22/09 10:12pm Link  |  Print  |  Notify Moderator

The sides of the ducts have friction... in the the middle of the ducts there is a lot of air moving that isn't touching the walls (and worse still the "corrigations".

The more ducts that you have the more drag you are creating.... hurting your efficiency.

A good rule of thumb...

A 4" pipe is equal to 4 ea 2" ducts

4x4 = 16 sq in

2x2 = 4 sq in X 4ea = 16 sq in

Look at how much surface area inside the pipes and the amount of space taken up by the ducting it self.

Remember you lose 15% efficiency with just one 90 degree elbo... 15% of the remainder with the next 90 degree elbo, and so on.

A smooth walled "tin can" transition through tight areas makes more sense... the walls would be very thin... very little restriction. Clamp your duct on each side.


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pjay9

Tacoma, WA

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Posted: 01/22/09 10:43pm Link  |  Print  |  Notify Moderator

Why don't you see if an HVAC place can make you a wide flat duct! it is worth a try...far more effective than the flex as Sleepy has pointed out! it looks like you could run the whole lenght the way the 4" is done...those tin bender fellows are really pretty good at what they do. they may even have a good idea for you if you call them up and show them the pics maybe take the TC over to them and let them figure your runs...there is stuff out there now for heating system I don't even know about since I built my house. All my ducts run in the ceiling buried under r40 insulation and vent out in the ceiling instead of the floor like in most homes, my home ducts are big flexible and insulated with a plastic sheath over that then buried. So see those tin benders...they should be able to solve your probelems. AHh! what's another week to get it right?

btggraphix

Golden, CO

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Posted: 01/22/09 11:45pm Link  |  Print  |  Notify Moderator

pjay9 wrote:

AHh! what's another week to get it right?


Very true, but if you don't set the goals, you don't make progress! The wiring and the ducts to the service bay and the insulation are all so intertwined I REALLY don't want to get hung up one little piece of the puzzle....

Tired of talking, I want to make some more progress!

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