Re: Add a Cold Air Return?
Yep ------ as I draw juice up a straw from my tetra pac. ---- I think I have a pretty good understanding how forced air systems work.
Originally Posted by rdesigns
Interesting --- as Sten mentioned comparing apples and oranges.
Having both the supplies and the return(s) in the ceiling is not necessarily a problem if the supplies have adequate throw and velocity. If the supplies are lacking these things, the re-location of returns will not fix the problem, but the size of the return could.
If you doubt this, look up at the duct system of any Wal-mart or other big-box store: you will see supply ducts and registers branching out all across the ceiling, but you will see only one return, and it will be located in the ceiling right under the rooftop unit.
The many commercial offices I do work in pretty much have the supplies and returns in the ceiling. This setup results in poor performance and less than desirable results.The complaints from the folks working in these offices are the same ---- in one area people are freeziing while people in another area are too warm.
Considering in many cases you will find returns 4 feet away from supplies --- some times closer. There's absolutely no way you can convince me this type of setup doesn't negatively impact the performance and without short cicuits in air flow.
I can tell you this ------- I've taken readings with an infrared thermometer and find huge stratification issues with those types of setups. The lower level ( the human level ) is far cooler with warmer layers upward toward the ceiling and beyond .
When I open a ceiling tile the temperature level is even warmer above the drop ceiling.
gulp ---- another sip from the juice pac.
There are 3 states ---- negative ---- equallibrium ---- positive.
Why does the air not simply get sucked from the ceiling into the return, or, in other words, simply short-circuit without actually heating or cooling the store? Because returns do not suck air;
they are no exception to the law of physics which states that air, water and other fluids always flow from an area of greater pressure to an area of lower pressure. The supplies drive the performance of any forced-air system, not the returns. Returns are simply a path for pressure relief.
Differentials exist until they reach equallibrium.
We have mechanisims that affect the different states ---- natural and mechanical. Of course we have displacement in which this is also one mechanism being dealt with here.
Introduce the mechanical blower --- air is drawn to the inlet ( you could say being sucked ) at one side --- with the design of the vanes ---- pushes air through the outlet side. If this was in sealed container the net would be zero air flow because there would neither be a negative or a positive pressure differential --- they would be at equilibrium.
Now cut an opening in the box only for the outlet --- you would end up with a negative pressure inside the box because the blower would remove the air inside and without anything to displace this would create a vacuum and the box would collapse.
.... he says while the juice in the the tera pac is emptied and drawing on the straw collapses the empty tetra pac ............
As for the ceiling supply / return scenerio ------- if the return was drawing more volume of air than being supplied then the airstream could be strong enough to draw the heat being introduced away before having a chance to benifit ---- this is more of the short circuit.
Hence a balanced system will provide optimum supply and return volumes maintaining proper air flow.
Personally I don't believe that's a good design.
If your system had been designed and installed properly, the supplies at the ceiling of the lower floor would easily heat and cool that area even with the one central return on the upper level.
Perhaps running the blower may help. The expeience with upper and lower levels is cooler air wants to settle on the lower level and running the blower creates more of an uncomfortable draft in those situations.
What to do now? Increasing the size of the return air path will help, because an undersized return increases the resistance that the furnace blower must overcome to move the air. If you can reduce that resistance on the return side, the supply side airflow (both velocity and volume) will increase, and you will notice improved performance in terms of better throw and mixing from the supply registers.
Running the blower on low speed constantly will also help mix the air better, thereby averaging out the temperatures of upper and lower floors.
The OP's setup is really not ideal and understandably has issues. This really should be evaluated properly by a HVAC professional.
"" an ounce of perception -- a pound of obscure "