Replies

1. 
   cussnu2 | Dec 09, 2010 06:04pm | #1

   Doug Rye, the radio guy, says that a furnace can only blow as much as it can suck.  In other words, if it can't get the return air, it can't blow it out.  You might want to check your return air and make sure it is up to delivering enough volume.  I know my own furnace wasn't getting enough return air  out of the main living space because it would pull the basement door closed when it ran.  The basement was under negative pressure which isn't such a great thing considering the easiest way for it to get air was to suck air down the flue!

2. 
   rdesigns | Dec 13, 2010 12:41pm | #2

   You've done the first two steps (load calculation and equipment sizing/selection)--but the the next step is proper duct design. As cussnu2 points out, insufficient return air is a very likely suspect. It's the most common problem I see in my job as a mechanical inspector.

   Although there may be a nice big return air plenum at the furnace itself, there can be restricted return air paths connected to it, meaning that certain rooms/spaces in the house may not have adequate return paths to relieve the pressure created by the supply ducts that serve that same room/space.

   You can test this:  With the furnace blower running, and with all windows and doors closed, stand on the outside of the suspected room and feel along the bottom of the door. If you feel air rushing under the door to the outside of the room, it means the room is being pressuriized due to not having an unrestricted return air path from that room.

   You can open up the path by 1) undercutting the door even more; 2) installing  hi/lo transfer grilles in the partition; 3) installing a jumper duct ceilitg-to-ceiling; 4) a new or larger return air duct connecting the pressurized space to the main trunk duct on the return.

   If there's a basement or 2nd floor with a door that separates the levels, do the same bottom-of-the-door check on each side of the door to see if there's any imbalance. One whole level can be pressurized in comparison to the other. Basements very commonly are de-pressurized in relation to upper floors. If this is the case, it means that the upper floors have inadequate returns. It does NOT mean the the basement has too much return, as some installers might mistakenly contend.

3. 
   Clewless1 | Dec 16, 2010 09:34pm | #3

   As they do in commercial installations ... change the pulley(s) on the fan and motor. HOWEVER ... the fan must be compatible with the increased speed, so you need to evaluate that (or have someone do it for you). Most fans (i.e. the squirrel cage, not the motor) have a range of capability. Changing the belt sheave can move that fan into the upper range if it will allow it. There are fan curves that would tell you how far you could push it.

1. 
   DanH | Dec 16, 2010 09:52pm | #4

   Except that most residential blowers now are direct-drive.

   (I was gonna suggest increasing the voltage to 240V, but I'd guess there's probably a problem with that idea too.)