What is the rule of thumb for sizing the cold air returns? I am suspicious my gas furnance is starved for air as it can take several hours to raise the temp from 64 to 68 in the morning (Outside temps in the low 40s). I have 4,500 sq ft of conditioned space with three 32" openings and four 16" openings in the cold air return ducting. The furnace is a Trane XR90.
There are many more factors involved in dealing with the question of raising the temperature than simply return air opening sizes.
To properly evaluate your return air sizing, more information is required, such as total air delivery quantity in cubic feet per minute, the amount of outside air introduced into the system in cubic feet per minute, the layout of all rooms, the air quantities delivered to each room, the size of the return air duct, if any, the size in square feet of each return air opening, the placement of the return air openings, and the ability of the furnace to deliver heat into the system, and also, basically the adequacy of the syetem selected for the size of your home. And that is not to mention the heat loss in your structure, through glass areas, general leakage, etc., or because of possibly inadequate insulation or improper vapor retarders. A VERY complicated issue.
Architect (NY) and Home Designer (PA)
Posts: 2487 | Location: Tobyhanna, PA | Registered: 24 October 2005
As mentioned above there's a lot that goes into designing ductwork. Trouble is your system is installed and not on a drawing board...so what you need to start is a duct-o-lator (an hvac mechanic should have one) and a static pressure test. The duct-o-lator is a wheel slide chart that will give you the different duct sizes and the cfm(cubic feet per minute) that can be moved at a design static pressure. The static pressure test is similar to having your blood pressure taken. No manufacturer recommends higher than .5 inches total external static pressure. Static pressure is a measurement of the resistance of air flow...the higher the number the less the flow. Your best bet is to locate a mechanical contractor that is CERTIFIED in air balancing... less than 3,000 technicians in the nation carry this certification. Try National Comfort Institute at 1.800.633.7058 for a list of those nearest you. Good luck.
Posts: 1 | Location: Chicago | Registered: 09 May 2006
Adding to the previous two postings: I am NOT an HVAC contractor, certified, or otherwise. I did, however, take a few proactive steps, at fairly minimal cost, that paid big dividends in my own house. The first was to wipe the exterior of the ducts clean, at each joint or seam, and then seal those joints with 4" aluminized peel-and-stick tape (NOT "duct tape"). This stopped the air leakage, and delivered all of the feed air to the registers, and all of the return air to the furnace. The second was to ensure that all the balancing dampers were open at the main trunk of the feed duct, wherever there is a branch take-off. The third thing I did was to wrap the newly-sealed ducts with aluminized duct insulation, essentially a fiberglass- reinforced Mylar over fiberglass insulation, having an R-4 insulation rating, with a minimum of 4" overlap at each joint, which served to arrest conductive losses, and ensure that more of the conditioned air was getting to its intended location.
These three steps can be taken by anyone with minimal mechanical skills, and have a generous payback for the time and money invested. Once you have ensured that the conditioned air is getting to where it needs to go, the next step is to hire a competant HVAC technician to take you to the next level.
Posts: 105 | Location: West Haven, Conn. | Registered: 15 November 2005
You stated your heating unit was gas. Are you having the same problem cooling? If not you may not have the gas pressure set proper on the main control. If you are using propane the unit came for natural gas and will have the incorrect orfice and pressure settings. This will cause the unit not to develop the amount of heat needed in the exchanger resulting in poor preformance and high gas consumption. You should always ask for a professional when it comes to installing a gas furnace.
Try opening the blower door with the fan running,only pull it open about 1/2 in.there is a safety switch behind the door that will shut down the system if you go to far.If you have a hard time pulling the door open and it slams closed when released its probably starved.Next check the air filter for its thickness.Some of the 3 and 4 inch paper filters cause a lot of static pressure also if there is an a/c unit the cooling coil could be dirty.M
