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What specifics will I need to collect to figure out if our furnace and A/C are sucking too much at the source/intake?

I don't know the proper terminology for these things, do forgive me for any errors here. We are in a 1,000 sq ft home. The air intake for the furnace and A/C is in our hallway near our bathroom and two bedroom doors. Any time the thermostat turns on, any doors that are within a foot or so of being closed are sucked closed, hard.

Is there a way to determine if our furnace and A/C are sucking more in than it should? Is there even a way to turn the volume of air down somehow?

Also, most likely contributing to quite a bit of our home energy cost is a whole house attic fantasy we no longer use mounted in the hallway right next to the furnace and A/C intake. Is it a problem having this whole house fan still installed?

The louvers don't actually open and close when the thermostat turns on, but I'm sure it sucks some air from the attic.

  • central return LOL; this is one reason why I love my radiators – Tyler Durden Feb 28 '15 at 4:15
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    Radiators are great for heating, not so much for cooling. – iLikeDirt Feb 28 '15 at 4:33
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This is common in HVAC systems with a single central return. My system does this too. It's simply the nature of a central-return system. The only practical way around it is to have a balanced system with per-room returns sized the same as the supply registers on those rooms.

There are a couple of things you can do about it. One is to spend hundreds to thousands of dollars necessary to re-do your house's return ducting to put a dedicated return in every room. Another thing that might help is to replace your doors with heavier ones on less-well-lubricated hinges. Or you could just keep the doors wide open.

As for the whole-house fan, remove it, seal up the hole, drywall the ceiling, and pile insulation over the top to the level of the rest of the insulation on your attic floor.

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  • Your explanation makes perfect sense, thanks a bunch! – RON8O Mar 4 '15 at 4:40
  • Louvred doors might be an interesting approach since they could flow through. Somehow I doubt this will be enough. Good luck! – BrownRedHawk Mar 4 '15 at 14:39
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    Undercut doors, too. They'll still slam, though. These solutions are for improving return air paths when the doors are closed, not preventing them from slamming shut. – iLikeDirt Mar 4 '15 at 18:22
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How high are the bottom of the doors off the floor? you may be able to solve your problem by cutting bottom off of doors 1 1/2" max.

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Check the CFM rating of your blower unit against the square footage of your home, and the capacity of the furnace/AC units. It could be too high and in need of adjustment or replacement with a lower CFM unit (contact an HVAC pro if you aren't confident in making this assessment yourself.) Also, check that you have the right air filter in place ahead of the blower unit in the return duct, since a missing or too small filter could cause the blower to run faster than designed.

To keep a comfortable temperature in a living space, it is rarely necessary to move so much air that doors slam shut. And what happens after the doors slam? The airflow is restricted so the blower unit is either dumping a lot more heated/cooled air in other parts of the home, or it's working it's butt off (i.e. wasting energy) trying to move air against a significant restriction.

If that all checks out, consider partly closing the dampers (either on the supply duct or on the vent in the room) for the affected rooms, since air can only be "sucked" from rooms where it is being significantly supplied.

Lastly, I second the comment on checking/changing the door gaps. If the home was built with hardwood and is now decked out in plush carpet, the door spacing is probably too small or nonexistent. You need a 1" to 1 1/2" door gap to allow rooms to properly heat/cool if the doors are closed.

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  • Thanks for the tips everyone. Two of the doors are at about ¾" gap at the bottom and the third is only about ½". The filter is the correct size and has been replaced recently. I'll climb up in the attic and look at the CFM rating and see what I can find there. Nothing changes after all of the doors slam shut, except that the blower has to work harder because of the restricted flow. Not very efficient! – RON8O Mar 5 '15 at 21:10

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