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Setting up the Air

Setting up the Air. North Seattle Community College HVAC Program Instructor – Mark T. Weber, M.Ed. Air - 1. Selecting Target CFM. Installation Example:. You have a 66,000 BTU input, 95% furnace with a 3 Ton A/c unit on top. Figure your cooling CFM first.

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Setting up the Air

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  1. Setting up the Air North Seattle Community College HVAC Program Instructor – Mark T. Weber, M.Ed. Air - 1

  2. Selecting Target CFM Installation Example: You have a 66,000 BTU input, 95% furnace with a 3 Ton A/c unit on top. Figure your cooling CFM first. Nominal CFM for a cooling unit is 400 to 450 CFM per Ton 400 X 3(tons) = 1200 Cooling CFM 425 X 3(tons) = 1275 Cooling CFM 450 X 3(tons) = 1350 Cooling CFM For this example we will use 400 CFM per ton because you get better cooling and more humidity removed from the air.

  3. 62,700 Heating CFM = 1.1 X Temperature Rise Selecting Target CFM Installation Example: For heating, the Gas Furnace is 66,000 input, 95% efficient. The formula is a little more complex. 66,000 X .95 (95% efficiency) = 62,700 Output Btu’s Heating CFM: Specific heat of air (1.08) The indoor split, or temperature rise is usually between 50 and 70 degrees. We’ll use 60 degrees.

  4. 62,700 Heating CFM = 1.1 X Temperature Rise Selecting Target CFM Installation Example: For heating, the Gas Furnace is 66,000 input, 95% efficient. The formula is a little more complex. 66,000 X .95 (95% efficiency) = 62,700 Output Btu’s Heating CFM: Specific heat of air (1.08) The indoor split, or temperature rise is usually between 40 and 70 degrees. We’ll use 50 degrees.

  5. 62,700 Heating CFM = 1.1 X Temperature Rise Selecting Target CFM Installation Example: For heating, the Gas Furnace is 66,000 input, 95% efficient. The formula is a little more complex. Heating CFM: 62,700 (1.1 X 50 degrees = 55) = Heating CFM 62,700 divided by 55 = 1140 CFM of hot air

  6. Selecting Target CFM Installation Example: Let’s compare. For heating, the Gas Furnace is 66,000 input, 95% efficient and puts out 1140 CFM of hot air. For cooling, the 3 Ton A/C puts out 1200 CFM of cold air. Certainly close enough for me.

  7. Selecting Target CFM Installation Example: 1200 Cooling CFM 3 Ton cooling unit 66,000 input, 95% efficient furnace 1140 Heating CFM On units with PSC motors - You choose a motor speed not a specific CFM - The blower will supply adequate CFM for the maximum allowable cooling and heating at up to a .5 external static pressure.

  8. Calculating Air Volume (External Pressure Method) 3 tons at 400 CFM per Ton = 1200 CFM We want to error on the high side rather than the low side because we were conservative in figuring our CFM Our target is 1140 – 1200 CFM

  9. Calculating Air Volume (External Pressure Method) 3 tons at 400 CFM per Ton = 1200 CFM If we go to the lowest speed, we get 945 CFM Our target is 1140 – 1200 CFM .5 static pressure This is too low!

  10. Calculating Air Volume (External Pressure Method) 3 tons at 400 CFM per Ton = 1200 CFM Our target is 1140 – 1200 CFM .5 static pressure If we use medium-low, we get 1080 CFM. This is right in our target range.

  11. Measuring Air Volume (External Pressure Method) Set up a manometer and set to zero Measure before and after the blower .32 The total is .43 .11 Most residential units are designed to run at an external static pressure of .5” or less.

  12. Calculating Air Volume (External Pressure Method) 3 tons at 400 CFM per Ton = 1200 CFM The previous unit had a static pressure of .43 Our target is 1140 – 1200 CFM This is still close enough for me.

  13. 62,700 Heating CFM = 1.1 X Temperature Rise Selecting Target CFM Installation Example: The effect the higher CFM will have is to lower the temperature split Heating CFM: 62,700 (1.1 X 50 degrees = 55) = Heating CFM 62,700 divided by 55 = 1140 CFM of hot air 62,700 divided by 1100CFM (from previous slide) = 57 57 divided by 1.1 = 52 degree temperature split or rise Is this enough? The only way to know for sure is the set the speed you want and after the unit has warmed up (10 minutes) measure the split. If the supply air is above 110 degrees, you’re fine.

  14. Knowing how to set up the air is important! If I know any two of the three, just like Ohm and Watt’s law, I can find the third. With heat. In the previous example, we had a furnace that had an output of 62,700 BTUs. The temperature rise was 52 degrees. What is the CFM? To find this, first multiply 52 by 1.1 (specific heat of air and is constant) = 57. Divide 62,700 BTUs by 57 = 1100 CFM

  15. Knowing how to set up the air is important If I know any two of the three, just like Ohm and Watt’s law, I can find the third. In the previous example, we had a furnace that had an output of 62,700 BTUs. The CFM is 1100 CFM. What is the temperature rise? Divide 62,700 BTUs by 1100 CFM = 57 But now, you have to divide 57 by 1.1 (specific heat of air) = 52 degrees

  16. Knowing how to set up the air is important. If I know any two of the three, just like Ohm and Watt’s law, I can find the third. In the previous example, we had a furnace that had an output of 1100 CFM. The temperature rise is 52 degrees. How many BTUs is this furnace producing? Multiply 52 degrees by 1.1 = 57 Multiply 57 by 1100 CFM = 62,700 BTUs You might want to study this as there will definitely be questions about this in the quiz.

  17. For more information please contact Mark T. Weber At North Seattle Community College WWW.NorthSeattle.edu Mark.weber@seattlecolleges.edu

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