Whole-house air cleaners effecitve with Central Returns?
Does a whole-house cleaner provide any benefit if the house is fed off of a central return?
My thought would be that without returns in each room all I am doing is pulling in a small portion of the houses air for cleaning and the whole-house cleaner becomes a "room closest to the return" cleaner
A central return, still has all the air in the house go through it.
If it didn't. Then you would get no heat or cooling to the bedrooms.
You can't put air in a room that the air can't escape from.
Think of it the opposite way. With a good media filter or EAC, the air comming out of the supply ducts is clean. Its' not a central vacuum system. Most dust is going to settle ot the floor. That's why we have vacuum cleaners. Short of making the inside of you home a wind vortex (like the inside of a bagless vacuum), you'll only collect a fraction of the dust that's present in your home... but at least with good filtration what dust is sucked in doesn't collect on your coils, inside you suply ductwork and isn't redistributed from the supplies.
Where I work, we have room with as much as 2000 CFM of dust collection with duct velocities around 2000 fpm and static pressures of several inches in a 500 square foot room.... and 90% of the the dust in the air still falls to the floor and on equipment surfaces. The aspiration points only collect from 1-2 feet away at most.
That darn gravity thing causes so many problems.
Good to know; so a contractor telling me to put return ducts in every room to get any benefit from the whole-house air cleaner is just up selling the work then?
Two contractors actually -
First said put a return duct in each room by drilling through the bottom plate and installing a duct between the studs low to the ground and mastic the interior space for air tightness.
Second suggested installing "bypass" ducts in high in the walls of the bedrooms, then a corresponding duct on the opposite side of the wall to create a passage through it for the central return to draw air through rather than just under the door (got to admit this one sounded crazy to me, but the first one made sense to an uneducated homeowner).
If your bedrooms have enough under cut on the doors that they are maintaining same temp as the rest of the house.
You don't need individual returns.
If those rooms are not maintaining temp, then you may need returns ran, or more under cut.
Or, more supply.
But, just adding a media air filter does not require adding returns to all the rooms.
I personally still prefer multiple returns to having undercuts under the doors... but I wouldn't retrofit multiple returns to improve filtration. Again, you're still going to supply clearner air with a media filter and running a continous or intermittent fan has other advantages.
I was only pointing out that you shouldn't expect the level of dust accumulation to dramatically reduce. It will only remove dust particles that are unlucky enough to stay suspended in the air long enough to make it a few feet from the return and get sucked in.
Look at it this way. Your typcial vacuum cleaner is probably about 100 CFM. The same as 1 supply register in a room. Would you turn on the vacuum and point the hose in the middle of the room and expect it to collect all the dust in the air? No, but it will get some of it, and more improtantly particles that are sucked in won't get blown back out the supplies. A system without a EAC or media filter is like running a big fan... it just stirs the dust around.
I don't think the contractor is crazy or trying to rip you off. My returns are installd in a similar manner. It's not the best desing, but inexpensive and works pretty well if the joists are seled off completely to the attic.
Originally Posted by sarco
The joists in my basement are panned where needed and the space between wall joists on interior walls between rooms are used for hte return air duct. Then a retrun grill is installed in both rooms on opposite sides of the wall... so the return is shared by both rooms. Works well, simple, and 5 rooms and hallway can be handles by just 3 ducts. I did however, add a sheetmetal partition to seperate the rooms to light and sounds aren't so easily transmitted between rooms. I adjusted the position of the partition in the wall to bias the return airflow depending on the size of the room it served.
There's probably less than 25' total of metal ductwork including the return plenum.
multiple returns for improved filtration? um, no.
multiple returns for improved climate control, sometimes.
individual returns do improve room to room comfort, BUT the leave tremendous unfiltered DUCTWORK! dust and microbes WILL collect in this duct, requiring regular duct cleaning. a properly installed, and balanced central return system will maintain clean healthy ductwork. preventing the need for regular cleaning of the ducts. or at the worst, only the LARGE central ductwork will require cleaning.