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The next part of our design for our example house will be to select a furnace that provides the right heating capacity and the right amount of air, 1200 cfm in this case or a blower that is rated for up to 3 tons of cooling and the right air conditioning coil. For our example house the computer tells us we need 57,568 btus of heat. We have natural gas available so we select the Goodman GMNTE080-4 furnace which has a heating output of 73,500 btus at full capacity and a blower that's capable of up to 4 tons of air conditioning. That's what the 4 stands for in the Model Number GMNTE080-4. The furnace has a capacity of 80,000 input. Good question! Why gas furnace manufacturers make us do a calculation to determine the real capacity of the furnace is beyond me.
Oil furnaces are rated by the actual amount of heat they produce but gas furnaces use a standard in their identification as the heat input. You're right. For example our 80,000 btu furnace will produce 73,500 btus and that's all we're concerned with. But if we took the same furnace in an 80% efficiency it would only have 64,000 btus output. Big difference!
So when a gas furnace is selected it should be always selected by the output btus only. We're not interested in the input unless we're looking at the at the energy efficiency only of the furnace. So if you're looking at that dinosaur sitting in your house be very careful to look for the output rating or bonnet rating btus.
We can't use the GMNTE060 furnace in our model house because it only has an output of 55,000 btus. And no we can't go to the next bigger size furnace such as the GMNTE100-5 because again bigger is not always better. We've used 8 degrees as our outside design temperature and we already have a 10% safety factor built in to our calculation. We also have plenty of excess capacity because our furnace has 15,932 btus capacity more than our required sizing of 57,768 btus.
In review we've determined that we need a 3 ton system. We multiply 400 cfm (cubic feet of air per minute) times the tonnage which in this case is 3 tons. Our answer is 3 tons times 400 cfm = 1200 cfm of air. So we need a total of 1200 cfm of air for 2000 square feet so we divide 1200 cfm divided by 2000 square feet and we need .60 cfm of air for every square foot of the house.
So as an example if we have a 12 x 14 foot room we have 168 square feet. We multiply 168 square feet times .60 cfm and we need to deliver 100 cfm of air into that room to properly air condition the space. Knowing the amount of air necessary for each room is important to design the duct system. For the rest of the house we'll calculate the amount of air required for each room so we again take the dimensions for each room, calculate the square footage of each room and then multiply the .60 cfm required for each square foot times the square footage of the house. Then on the drawing you'll see where we've listed the size of each room by square footage and then took that and calculated the cfm of air required and wrote this on our drawing.
Let's summarize our steps to this point:
A. We made a drawing layout of the house.
B. We determined the square footage of the house and for each room.
C. Then we did a load calculation of the house to determine what our heating and air conditioning loads were.
D. Next we selected the equipment for the furnace and air conditioning we need.
E. Determined the amount of air required for the total system.
F. We then determined the amount of air we needed for each room and plotted it on our drawing.
If you're still with me you're doing great and the rest should be fairly easy.
Now to do the duct sizing we need to locate the air handler or in this case the furnace and the indoor air conditioning coil. A central area of the house is the perfect location because then we have equal air distribution. If the air handler or furnace is located at an opposite end of the house, then the diffuser at the other end of the house will be the last and least amount of air. We've decided to locate the furnace in the center of the house because the more central to the house we're located the better we will be able to distribute the air properly. Plus it's a model house and we can do anything we want! No long duct runs and no room coming up short on air supply. We will need a plenum? What's a plenum? The plenum is the main part of the supply and return duct system that goes directly from the air handler or furnace to the MAIN TRUNK.
What is the Main Trunk? The Main Trunk(s) supply the air distribution to the Take Offs. The Main Trunk(s) are the part of the duct system that all the air from the system is going to travel in before we "take off" the main plenum to the "diffusers" or grilles. What is a Take Off? A "take off "is that part of the system where we take the air off the trunk to supply air to the living area of the house. Then from the "take off" we will go directly to the Diffusers where the air enters the rooms of the house. What is a diffuser?
A diffuser is called the register or the grill or that thing on the floor or wall where the air comes out of. A diffuser is that metal grille in the room that you see on all forced air systems where you can feel the air conditioned or heated air. Every room in the house needs a diffuser to supply the air conditioning and/or heating. Think of the Furnace or Air Handler like your heart. Then we have the area of the heart where the heart is located which is the heart cavity where all blood passes through which in our case is the Plenum. The arteries are the Main Trunk(s) and the capillaries are the Take Offs.
The diffusers are the lungs. And just like people when the arteries to the heart are clogged because there's not enough blood flow, HVAC systems are the same and will have premature failure if the Duct System is insufficient to allow proper air flow. The biggest problem in an HVAC system is a poorly designed or operating duct system. Restrictions and too small ducting for the required air flow cause HVAC Attacks!