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The back room of my house is quite warm in the summer. The room is on the north side of the house (Alabama, USA), so the outside walls don't get direct sun exposure. The attic insulation is a little on the low side, but comparable to the rest of the house. The HVAC air handler is on the opposite end of the house. There are two air returns, one near the air handler and the other just a few feet from the room in question. The nearby return has a 40 foot run to the air handler. The air handler feeds a trunk line that runs about 35 feet. Near the end of the trunk are two 6 inch flexible ducts that go the remaining distance to the room (25-30 feet).

I have considered a few alternatives to resolving the problem.

1) replace the 6 inch ducts with larger ducts.

2) add another duct line and duct.

3) install a duct fan to boost air flow.

While I'm completely capable of doing the electrical part to code (and safely), I'd prefer either of the other two options. Which of these options is best? Or is there another better option?

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Can you supply some sketch of the situation + alternatives considered? –  Peter Ivan Jun 7 '13 at 10:39
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up vote 2 down vote accepted

I have a couple of long runs too and I used a fan until I upgraded the HVAC unit. Before you get a booster you need to measure the air flow out of the two vents and compare them to other vents in the house.

How to measure air flow? Well pick up an Air Flow Meter. However I have a suggestion before you spend $$$ on this. Get a large trash bag, configure it to open about the size of your duct opening (take off cover), hold it tight on the duct opening, measure how long it takes to fill... compare several vents. Note the air should be running when starting this - since how long it takes air to come out factors very little into the cooling equation. If you are confident with the bag results and they are obvious then skip the pricy tool. ##Side Note## (A friend of mine uses one of those garden wind spinning things and measures the rotations over a given time to compare air flow. You have to have the angles/distance the same from the vent and he has to have his iphone slow wdown the frame rate to count. But damn that mcgyver-bastard, pretty sure its more accurate than my empty trash bag! But it does involve slow-motion so my solution is more practical)

I would also take temperature readings inside each duct while on after 5-10 minutes. How cold is the air you are getting?

If you are getting comparable air flow and temp you can add another duct or bigger. Really up to you.

If you aren't and I am guessing you aren't then you need to first get rid of the flex lines. They squash air flow, they are affected by outside temperatures more, and they leak. Put rigid metal ducts in. If the ducts are in the attic then you need to insulate the crap out of them.

Then if you still have issues you need to measure air flow and temp again. If air flow is poor you are looking at a booster. If it is relatively the same, add another duct.

Then you are talking about adding a return to the room and over-insulating the attic above the room.

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The garbage bag air-flow meter showed that I had similar output air volume/time both close to the air handler and in the back room. An IR thermometer showed 66 degrees at a duct near the air handler and 74 degrees at the ducts in the back room. Looks like I need to add insulation to the flexible duct line. It has some insulation on it already. –  Les Bartel Jun 7 '13 at 22:09
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That is your issue. 74 degrees will take forever to cool down a room. My issue was opposite of yours - in the winter I had hot air coming out at 76 degrees... It was 35 foot duct under concrete slab... I know great design. It didn't get better until I got a unit about twice the size I needed. –  DMoore Jun 8 '13 at 4:09
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I think DMoore nailed it already, but as I have explored this topic due to a similar problem in my own house, I'll list some other options to consider for future visitors:

  1. Evaluate sources of heat in the room. Computers, lamps, etc. Replacing an inefficient device with one that generates less heat can be a huge benefit.

  2. Insulate or replace windows. Windows can be the biggest source of heat leakage. If the window gets heavy sun, also consider window awnings as a classy looking (and green!) option.

  3. If you think you need a vent fan but can't install one for some reason, another alternative is a powered vent booster which replaces the vent cover. Not cheap, but easier to install.

  4. Add a portable air conditioner to the room. Not the most elegant option but if the main AC is just not up to snuff it may do the trick.

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