How Many Btus?
When considering adding a fresh air inlet into a building, remember to consider the impact to the heating or cooling load will have on the building. You can evaluate the increase in load by calculating the BTU introduced in the building by using the following formula:
Fresh Air Sensible Btu = (Fresh air cfm x The Temperature Difference between the Indoor and Outdoor Air) x 1.08.
Let’s create a scenario and see how the formula works. First, we measure the amount of fresh air being pulled into the building, or use the design cfm, if the system isn't built yet. This is the Fresh Air cfm piece of the formula. For this example, let's assume 200 cfm.
Next, wel determine our temperature differences. For summer and winter seasons, use the outdoor design temperatures and subtract from the design indoor temperature for that season. Say our winter outdoor design temperature is 20F and our indoor temperature is 70F, the temperature difference would be 50F.
Now multiply the 200 cfm by the 50F temperature difference by the 1.08 constant, and we find that this winter the fresh air will add 10,800 BTU to the heating load of this home. The final question; Is the heating system able to handle the additional load? If we have a natural gas furnace, maybe so. How about if it’s a heat pump? Maybe not.
Dilution
The addition of fresh air into the building has several other benefits. As it pressurizes the building, some air is exhausted out of the building. This dilutes the existing air with fresh air, and forces a percentage of the indoor pollutants out of the building with each air change.
Combustion Air
If there are combustion appliances in the building and the combustion air is being pulled from inside the building, the outside air can be used to as combustion air for the appliances.
A supply register can also be cut into the supply plenum tom pressurize the mechanical room. This assures adequate combustion air for the burners and will increase the flue draft pressure to ensure the furnace or appliances will have adequate draft.
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