Poor return air???


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Old 01-06-08, 12:02 PM
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Poor return air???

Hi Guys,

Im Usually on the electrical and plumbing forums and those guys are awesome.. So I'll take a shot here.

Airflow has been a problem since I bought the house.. Some vents are Poor, and others are just pitiful. There are 26- 5 inch flex ducts in the crawlspace, that have been beaten to death over the years, and sag, and were misrouted to the point of being folded in half. Ive gotten most of them straightened out, and replaced as needed. ( Iguess I should try rigid ducting...But thats my next question)

Now I realize that airflow, just like water, and electric, will seek the path of least resistance, so I dont expect all of the registers to be perfect at the same time. But I cant bear to spend the cash and time on a project that yields little to no results.

Here is what Ive done so far....
Ducts were cleaned ( I'm guessing only the main trunks,because I dont see anything passing thru ducts that are folded in half) In the last year. It made no improvement at all. Straightened and rerouted the ducts to eliminate the sagging ones( Theyre all picture perfect straight lines!) It got a bit better , but hardly worth the effort. Increased the fan speed, got a bit better, removed the filter(Temporarily of course) It got better, Ran the furnace with the fan compartment open, and that improved drastically. What can I do to increase return air into the system?
#2---can I use and "Insulation " Jacket on the rigid ducting that Im going to replace most of the flex with?
Should I keep the 5 inch diameter for everything, or downsize The "Takeoffs" toward the end of the run?-Is it fair to assume I want "Absolutely unrestricted return"(Of course in a perfect world , this isnt going to happen, But as close as I can Get?)
Sorry for dragging on , But My DIY skills never involved this stuff before. Thanks..
 
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Old 01-06-08, 03:40 PM
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Have a manuel D done! This is what we use to sixe duct work.
 
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Old 01-07-08, 04:09 AM
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Originally Posted by Unclediezel View Post
Ran the furnace with the fan compartment open, and that improved drastically. What can I do to increase return air into the system.
Gotcha! Your system seems to be starving for air, likely from insufficient (or undersized) returns. How many you have?
And BTW, you don't want to run your furnace for too long with the fan compartment door removed b/c it will cause the blower motor to pull an electrical current way over its maximum rating...the motor overloads and you know what happens when a motor is allowed to operated under such conditions ->-> ->->

Originally Posted by Unclediezel View Post
#2---can I use and "Insulation " Jacket on the rigid ducting that Im going to replace most of the flex with?
Insulation is always a blessing...but it will not improve your airflow, only the temperature of the air that will reach the various rooms in the house.

Originally Posted by Unclediezel View Post
Should I keep the 5 inch diameter for everything, or downsize The "Takeoffs" toward the end of the run?
Here is where comes into the picture the recommendation that airman has given you, which is, proper procedures should be followed to correctly size your ducts. This procedure is outlined (and explained) in a manual called "Manual-D" which teaches about "Residential Duct Design". The manual is sold by a company called ACCA
http://www.acca.org/store/product.php?pid=91
Yeah, 5-inch is a very common size for supply branches in residential applications, but their sizes should be double checked...and I am really suspicious that your returns are not up to snuff. You may have to add some.

And if you're going to do this yourself, arm yourself with a ductulator...you can get them for free at an HVAC supply store (pic below)


One last thing, you can't use Acca-D if you first have not had a cooling/heating load calculation done for your house. This is also called a load calc. A well done load calc will show you how much heating (winter) and cooling (summer) each room needs, but also how much air will get the job done. Airflows are measured in CFM (=cubic feet of air per minute). Acca...again...has a different manual for load calcs, it is called "Manual-J"

Did I scare you already?
Hope not...this is a great field...Indiana Jones is a boring dude compared to the excitement you get with HVAC
 
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Old 01-07-08, 08:48 AM
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Originally Posted by Unclediezel View Post

Ran the furnace with the fan compartment open, and that improved drastically. What can I do to increase return air into the system?

Thanks..
When I remove the main cover off my gas furnace, it can blow much more air as well. This is because the "wind drag" stress of cold air returns (even for normal sized ducts) is eliminated. Without feeling the improvement of wind flow one's self, its hard to tell if your unit's improved air blowing power is normal, or a blinking red sign (sort of speaking).

--------------

As a general rule, every live-able room in your house must have a cold air intake. Cold air intake vent in each bedroom, in the living room, in the rec room and in the basement (assuming you have a finished basement). Due to safey reasons, NO cold air intake in your bathroom (don't want to suck its DAMP air into the rest of the house), in your kitchen (due to kitchen fire spreading risk) and in your furnance room itself (fire spreading thing as well). As a general rule, the cold air intakes are between 20% to 50% larger volume size - compared to its total heat vents in the same area. The further away air intake vents should have larger openings. And the closer to furnace sizes have smaller openings. Thus, forcing more air intake from the further away from cold air intake vents. For cold air intake, use the largest inner pipe as possible. Some use 5" duct pipes and I've seen large 16" inner stud cavities - for even better air flow.

As a suggestion, make a floor plan of you home and mark down its cold air intake and heat vents. Also determine each vents "volume opening" size by taking its "grill opening" Legth x Width space. With all known numbers, then determine if your room's area cold air intake is between 20%- 50% larger then its heat vents. If cold air intake is less volume then its heat vents, then one need to install larger grill size "cold air intake" vents. Or, install more cold air intake vents to that area using new piping to the main cold air duct main run. Just like my basement has 2 cold air intakes off its basement floor that go up to its main duct run.


Another option is to ask a few HVAC Specialists to visit your house. Many of them perform free Inspections / Analysys of your existing heat system. Ask them to give you a recommendation of what needs to be improved. Thus, validating what can be further improved within your home.


Hope this helps as well...

.
 
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Old 01-07-08, 09:37 AM
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Thanks Guys.....You have helped immensely

This is because the "wind drag" stress of cold air returns (even for normal sized ducts) is eliminated.
Also...I get a "flexing" POP noise from the sheetmetal when the fan starts up, or shuts down, which I attributed to Poor workmanship in the sheetmetal up until now.( It doesnt pop with the door off)

Ive been thinking under the assumption, of "what comes out" had to "Go in". The fan can only supply as much as air as return will allow. However, Is it possible to go "TOO BIG" on the returns? Drilling a huge hole in the return trunk would solve my starving problem, But I dont think I should ...yet
Would I be wasting my time and money to dump the flex duct , in favor of rigid ducting of the same size , and make my adjustments from there?
Ive only been in the house for a year, and have no idea if its broken, or if it was just poorly designed from the begginning.
Due to safey reasons, NO cold air intake in your bathroom (don't want to suck its DAMP air into the rest of the house), in your kitchen (due to kitchen fire spreading risk) and in your furnance room itself (fire spreading thing as well).
Guess I'd better move them huh? That certainly answers part of my design integrity questions....
 
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Old 01-07-08, 10:38 AM
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Unfortunately, forced air "air flow" isn't an exact science. HVAC Specialists attend many different courses to determine what is "within range" and what it outside the norm. And even within range, some specialsts can fine tune a system better then others. Just like the electricity arena, exact science doesn't always apply. Its experience that over rides the basic rules of minimum design code. Yes, even in the HVAC field as well...

With this in mind, do call a minimum of 4 HVAC specialists in your local area. Many of them will provide a free estimate. Estimate on the things they would improve on your specific home. Could be main duct size, could be more hozizontal duct runs to each core room, and they could even recommend more (or better postioned) duct grills. As a minimum, they will recommend replacing easy accessable flex pipe with solid / smooth piping. Thus, less drag on its inner air flow. Or, go with a larger size smooth pipe in that area. With a visual inspection / Analysis and "their math numbers", they will advise what is best for your specifc home.

BTW: If popping sounds and it sounds like your furnace is struggling "too much" (like trying to suck frozen yogurt up a thin straw), then odds are, your cold air return duct system is "too small". If it were me, I'd let the Specialists measure pipe size against furnace fan CFMs and "wind drag" stress of the existing duct sizes - before buying large size main ducts.

Hope this helps as well...

.
 
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Old 01-07-08, 12:18 PM
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like trying to suck frozen yogurt up a thin straw
Thats a perfect description...

Yes , it helps as well, and is much appreciated.

Is a "Manual D" and a Duct calculation something I can do myself, or are there special tools involved.? Books and CD's and videos, And Knowledge I dont mind spending money on, as opposed to Not knowing what to do and "Screwing it up" to the point where replacement is my only option.

A neighbor of mine said he would look into it for me, But not to call the office for an appointment because they arent the most "TRUSTWORTHY " bunch. He asked if I had a Manual D and calculations done , and I told him No. He suggested for starters that the Flex duct dissapeared, replaced by rigid duct, Of the same size, To rule out Crushed or pinched Flex, and to do the calculations on a System that was functional , Instead of "UP" OR "Downsizing" pipes that are not working simply due to damage. Does this make sense?
 
 

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