Heating, Air Conditioning, Fridge, HVAC/How many registers
I recently built my second new home. Unfortunately, this time I went completely independent and subbed out everything and consequently I apparently learned a lesson. I shopped for my HVAC and got quotes from about 5 people. Each one beating the previous guy's price by a couple hundred dollars.
Now I have a little problem. My west facing wall has lots of windows. One is about 6x8 and is on a roughly 12x12 bedroom. That room gets warm. There is a second bedroom next to it with a standard 32x60-ish window that also gets hot (the room) but not quite as bad. Again both rooms are on the west facing wall of the home and get lots of sunlight throughout the day. I am concerned that this guy I went with was a little lost in how he laid out the registers. I KNOW that he was a LITTLE lost because right off the bat he placed two registers on the immediate joist off the wall, placing it about 2 inches off the interior wall. I told him he needed to move it because I needed to put crown there and he acted like I was crazy for telling him that the register couldn't be 2 inches off a wall. Now for the point: almost ALL of the registers are mounted interior in the rooms. Consequently, no cold air ever gets to the outside of the room. It makes sense to me (now) that registers should be placed outside, particularly in rooms that are likely to be hot, and sucked interior toward the returns.
Where I'm at now: I want to add ducts (and registers) off the main trunk to supplement the flow to four bedrooms. The new ducts will be placed toward the exterior walls so that the cold air is then sucked toward the inside. Can I have too many registers for the unit and trunk? It's a 5 ton unit and I have two approximately 20x20 returns at opposite ends of the house. I know that it's rated for 2000 cfm and I don't want to exceed the rated output with the ducts right? Shouldn't a 6" be rated to X cfm and an 8" to Y cfm? Can I add them up that way and get it to 2000 cfm?
I recently read another post on here where one guys adds up 4 8", a 7" and a 6" and says those ducts equal 250-something square inches. Not using my math it doesn't but that makes me wonder if I should be sizing registers by cross section and matching to the return. But if so, that doesn't seem right for a house this size (2900 sf) with two 20x20 returns.
I strongly recommend you have a licensed HVAC.contractor come to your home and perform what is called a HEAT LOAD ANALYSIS
on the rooms in question. An individual heat load on each room will tell you exactly how many CFMs (Cubic Feet per Minute) of air flow each room requires in order to properly heat and cool....
At that point, the sizes of the registers in each room will tell a good contractor how many CFM you are currently getting vs how much more you may need. It is very possible the the size/diameter of the duct runs to those rooms is insufficient. The keys here are the load calculations (make sure your contractor is very familiar with these before he comes out!), then making the proper adjustments to your duct, registers and return air without compromising your system.