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mcvane 08-07-2007 09:35 AM

blow power of vents
Hi there.

We have an average sized 2 storey house with 4 bedrooms upstairs. It is a 10 year old house and I do realize that some vents are a lot more powerful than others. For example, some of the upstairs vents get a small trickling blow of air when the furnace is on (heating or air conditioning), and there are certain vents upstairs that do get it fine.

We visited a friend's place that have a very small place and where they had vents, it was so powerful, that they could blow your skirt up if you were wearing one!

I figure our HVAC system might not have the power to push air through all the crevices througout our house. But is there a way to install a 'mini-fan' that is placed inside the ducts to push either cold or warm air more powerfully?

We have oscillating fans upstairs in the hot weather (by the vents) to push what cool air that does come out into the living space. It kind of helps, but it's not very practical.


gshock 08-07-2007 10:36 AM

See if you can find the manual dampers that control the flow to each vent. Typically, these are near the unit, where the ducts branch off and serve each space. Chances are, you need to adjust the dampers slightly to get more air in some spaces, and less in others.

Start there and see what you come up with. Other than that, it's helpful to know more about your system before one could offer more specific advice.

mcvane 08-07-2007 11:02 AM

I have checked...(and I'm not good with terminology), but the dampers are open for each of the vents, so air should flow freely. I guess it's inevitable, but some of the dampers in the basement ducts I've closed, and air still flows through them very well (the basement ones are very strong).

The only thing is, is it possible there are dampers elsewhere (in inaccessible spots) that are blocking the air? Some of the duct work flows above our porch or above our garage into rooms above it.

If I totally block off dampers in the basement and main floor, could this push air more powerfully upstairs? I've tried playing around, but logically, I think a lot of air would be lost to the small gaps in between connecting duct work in the basement (they are not air tight I don't think).

Unfortunately, I don't have specifications on our HVAC system... (manual)

gshock 08-07-2007 11:32 AM

If there are gaps in the duct work, this could contribute to the losses you're describing. The dampers on the vents are not the same as the dampers in the basement. True, they both regulate air flow, but the dampers on the vents control how much air. The dampers in the basement can control volume also, but can also serve to shut down an entire branch, i.e. one whole room. If you've adjusted the dampers in the basement, and still get a large volume of air then I think you've either got the wrong damper or the damper blade is not turning when you turn the handle. As for the gaps, duct tape should allow you to seal most of the gaps. Excessive duct lengths (over your porch) can contributed to a loss of airflow.

Yes, if you close off dampers to the first floor, this should push more air to the second floor. It has no where else to go. The fan on your A/C unit puts out a specific amount of air, cubic feet per minute, CFM. Typically, it's fixed, but depending on the unit, you might have two speeds - high and low (obviously). If you can find the manual, or you're savvy enough to find on online, you can adjust this yourself (assuming your unit has this feature). Otherwise, a specific amount of air has to be distributed throughout your house. You can change where it goes, but you can't ever reduce the CFM to one space without increasing it to another.

There could be dampers elsewhere, but it's not likely. Typically, you'd never want to put a damper in a place you can't get to it. Is the duct above the garage truly inaccessible, or is it just inconvenient? It's worth a look, just to be sure.

mcvane 08-07-2007 12:06 PM

Thanks for your wealth of info Gshock.

Our basement isn't finished, so there is a duct that runs to the front of the house from the furnace. It runs to a dead space which is just below the wall that separates our garage entrance (from inside the house).

Then, the duct must go vertically up (it's all drywalled), and then runs in the drywalled ceiling of the garage and feeds the air to a vent at the extreme front of the house.

Beyond the lack of strength in the air flow to this vent (and another one that is above the porch which is adjacent to the room above the garage), in the winter cold, the ductwork below the porch is so cold that it takes a good 10 minutes for the heat to actually warm up and push 'hot' air into the room above the porch. This is an additional problem, which probably can only be solved by going underneath the room and doing a super insulation of the room floor above it...that room is always cold.

Anyways, I went a bit off topic there, but there you have it. I will try to see if there is a 2nd speed setting on the furnace and see if I can block off the basement from receiving any A/C air - it's not needed there!

Thanks again.

pjpjpjpj 08-11-2007 03:28 PM

Gshock covered most of it, but to respond to one of your first questions:

Sometimes, if the duct runout to the most distant grilles has lots of turns, was installed poorly, is leaky, or has some blockage in it, you may have a hard time getting enough air through it even if you attempt to block off other grilles. In other words, short of completely closing other grilles off (which you obviously don't want to do), in layman's terms, the air might find it easier to whistle out of a partially closed grille on the first floor than to traverse that long, windy, leaky duct to the second floor.

So, to answer your question, yes, you can buy small inline fans at your typical big-box home improvements stores. But I have only ever seen them for 6" and 8" round ducts, so if you have a rectangular main that goes up to your second floor, you may have a hard time finding a solution. I have one installed in my house, for a round duct that runs to my master bedroom. I also bought an accessory that senses the airflow and turns it on and off based on when the AC blower is on (and senses if the air is hot or cold, I can set it to only run in cooling mode). It works well, but it is noisy.... a high "whirring" noise that comes out of the bedroom grille and several other grilles in the house that are tapped off the main near the booster fan. I am used to the noise, but it might bother other people more.

Also, I would use aluminized tape rather than duct tape to fix those leaks - duct tape is not actually designed for AC at all (look it up).

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