Make-up air for gas furnace?
Our builder is finishing our new 2,000 sq ft house with a basement in the St. Louis Missouri area. Due to some crazy change order pricing I could not afford both air sealing and sealed combustion furnace and water heater. I chose air sealing the house in accordance with Energy Star 2018 guidelines. The insulator estimates we are at ACH3 @ 50 pascals, but no blower door tests have been performed. The gas furnace 80,000 Btu (80% efficient) and water heater are open combustion appliances.
The home has a 400-cfm range hood, a 125-cfm electric dryer, 120 cfm central vac, and a 100 cfm and 50 cfm bath fans. I’m not sure what the Btuh of the water heater is but my math for 80,000 Btuh gas furnace at 50 cu ft per 1,000 Btu is 67 cfm. Total cfm is roughly under 1,000 cfm., but that seems like a lot of leakage.
I’ve estimated the house volume at 35,000 cu ft. At ACH3 @ 50 that’s roughly 1,750 cfm. But does gas furnace back draft start at a lower differential, 30 Pa?
Would It be prudent to plan for a makeup air system? If so, my concept is for the make up air system to provide unconditioned filtered air to the basement buffer area.
I’m a mechanical engineer who has designed HVAC systems in the past and have no problem designing and testing a make-up air system.
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