I’m trying to address a problem with return air in my house’s HVAC system. I believe I have a solution but since I’m a beginner with HVAC I would appreciate feedback both on the general plan and the design details.
Here’s the equipment:
Furnace: Trane XV80, model TUD100R9V5J (2 stage, 52k BTU/80k BTU)
AC condenser: Trane XE 1200 model TTP042-SF-1B (3.5 ton).
The air handler / furnace is mounted vertically in a closet – it draws air from the crawl space and sends it up into ducts in the attic. The problem is there’s no duct work in the crawl space, nor is the crawl space sealed – it has a dirt floor. (There are return air grills in most rooms but they go directly to the crawl space, not duct work.)
To complicate matters, a concrete beam separates the closet’s crawl space from the rest of the house, so any return air must pass via the 5” high gap between the top of this beam and the bottom of the floor.
My first thought was to pan over the beam between the closet’s crawl space and the rest of the house, then add ducts. However, there just isn’t room. It’s a very-low ceiling crawl space and there’s already plenty of plumbing and electrical conduit down there - adding ductwork would prevent access to entire parts of the crawl space.
Fortunately, the house has a very open design. All rooms connect to a central hallway, and the closet shares a wall with the central part of this hallway. So if I can duct to the hallway I can fix the return air issue.
To do this I would cut one hole in the closet’s floor, another in the wall with the hallway and duct between the two by building a box that covers both holes. The return air would then pass thru a grill in the wall of the hallway, the box would duct it down to the closet’s crawl space where it would then travel a short distance to the bottom opening to the air handler. I would also seal the floor of the closet’s crawl space as well as openings over the concrete beam to the rest of the house’s crawl space.
Assuming this basic plan is correct I put some thought into the return air grill design:
Since there’s already a filter at the base of the air handler I don’t plan on putting another one in the return air grill. According to Hart & Cooley’s website the ACCA recommends a maximum air velocity of 500 fpm for unfiltered grills. For a 3.5 ton condenser (1400 CFM) that gives me a grill area of no less than 2.8 sf (403 square inches).
To help with the cosmetics in the hallway I would like to use a wood grill. The most open design I could find (Pattern Z from Patterncut, Inc.) covers approximately 30% of the opening - that increases the grill area requirement to 2.8 sf/ 0.7 = 4 sf (576 sq inches), or an opening approximately 42 inches high and 14 inches wide.
If I relaxed the ACCA requirement a bit I could use a smaller grill. Hart & Cooley suggest a higher maximum velocity of 600 fpm – that would allow an opening of 3.3 sf (480 sq in), or an opening of about 40 in high and 12 in wide.
While I realize that bigger is better when it comes to return air grill size, it would be great (for cosmetic reasons) if the smaller one (based on H&C's spec) worked. But I don't have a feel for how much I'm pushing the envelope here. It would be great to hear from some folks with experience.
That’s all I have – thank you for reading this far and even more so for any feedback.