Return Ducts,Too Big?
This question is about a forced air HVAC unit. (Central Air with the heater portion being natural gas). It is in a new 2400sq' house, installed in an insulated 4' high crawl space. It has three return ducts coming off a central return line in the middle of the house. The first floor is 12”x 24” located in the living room just below the thermostat. The second floor has one 12”x12” in the master bedroom mounted low on the wall as you come in. The third vent is 16”x16” just outside the bedroom in the upstairs hallway mounted at eye level.
They all had those cheap blue filters and seemed to work fine (noise, airflow, etc). I was told they should be replaced with the better corrugated paper filters with more surface area that filter more. I did that and soon I noticed that it was pretty noisy, but tolerable when operating. After doing some research I read that it would be better to have more and bigger return ducts. When it was time to replace the filters, I noticed that the cutout in the actual ductwork where the filter box mounts to the main return duct was about half the size of the filters. Bigger is better right? I then took a sawsall and enlarged the actual opening to almost double what it was, to almost the size of the corresponding filters. I thought by doing so everything would breath better and the fan motor would be less stressed. Big mistake? The noise is now intolerable, and the better filters are being distorted and sucked inward. I tried running without any filters and things are whisper quiet although very drafty near the return ducts due to the air moving so fast. I then tried putting the cheap blue filters on to slow the air flow and that seemed to help a lot as far as noise and airflow.
My question is , are there better air filters with very low resistance out there? Or should I be satisfied with the practically useless ones that I have back on. Can I have one of those statically charged filters installed down in the crawl space and do away with filters up in the main house? Should I close up the openings back to the original size? I assume the airflow rates were checked and balanced when the HVAC was installed in the first place , but I have very little faith in the builder at this point because of other major problems.
Thanks for any insights in advance,
I'd stay away from those statically charged filters,too restrictive unless very large,then they don't chare very well.
However,a properly sized 4-5" thick pleated media,would do a good job.Installer/salesman,shold test the static to see if your system can deliver the required air flow,with a filter like that first.Duct remediation can be done,if needed ,with the filter install.
Here's a photo of a system designed with two filters to minimize static(think resistance to air flow).
Not all may agree,here's what one Pro said,"Actually the supply air static dictates the return air static"
Last edited by dash; 03-14-2009 at 03:38 PM.
If the pleated air filters you were using were 3M filtretes.
Originally Posted by Yanni
Try a general pleated air filter. They are less restrictive, but filter better then the see through fiberglass ones.
Thanks Dash and Beenthere,
The pleated filters that I originally upgraded to were made by Naturalaire with an ASHRAE rating of 8 and a Merv rating of 8. They seemed to work ok until I opened up the ductwork cutout to match the filter size. I will look for pleated filters with a lower merv rating to try out. I put the see through filters back on until I can get an HVAC guy that deals with Goodman next week.I saw a number of choices on Goodmans website for larger filters and humidifiers installed inline on their units. For right now the air flow and noise seem to be very good although I know it is not filtering very well. Seems like it is a trade off between clean air and blower efficiency. I am hoping the Tech will be able to measure the airflow and recommend the appropriate filter unit.
Thanks again, Yanni
Have them check the static pressure of the return at the furnace.
And also, have then check it at the return air filter grilles.
The duct work may be ok. So that all you really need, is large return air filter grilles installed.
He should be able to tell you which you really need.
Wrote a note to myself to check those items.
Watch the pleated filters you are buying! If they look like they are made from the same filter material as your coffee filter, do not buy them!!! They will plug very quickly, and cause excessive air flow resistance. A LOT of the 1"pleated filters (1" thick) will cause you problems with early plugging and excessive air resistance. A good idea is to buy the less expensive "see thru" 1" filter and spray both sides with Endust before installing. This will increase the dust catching ability without adding to the flow resistance!!
".....A good idea is to buy the less expensive "see thru" 1" filter and spray both sides with Endust before installing."
-never heard that one before. good idea though a mid-range pleated usually gets me a static drop of .1 max unless its been neglected. Even at .2 the filter is only a participant to duct issue.
Someone recommended precut media filters as a compromise. They seem like they would do the trick with low resistance and high efficiency. The only downside is they cost twice as much as a good pleated filter.Any Comments on that? A years worth with two month servicing intervals for three grills would cost $. I am new to these forced air systems so is this a reasonable cost?
Last edited by beenthere; 03-17-2009 at 10:05 AM.
Reason: Removed price