When considering the fresh air requirements in a residence, we can go beyond the 15 CFM per person/1 person per bedroom plus 1 rule of thumb, and use the Total Heat Formula to calculate how much additional load is being introduced into the building. Of course, the reason this is important is that it results in proper sizing of the equipment, enabling it to handle the total heating load in the structure.
Here’s an example….
If a tight 2,000 square foot home with 8-ft ceilings needs to change 25% of the air per hour during the heating season, and there is a temperature difference of 53 degrees between the outdoor air temperature and the indoor air temperature that is being maintained, the following formula calculations will tell you how many BTU’s per hour need to be added to offset the colder air being introduced into the building to satisfy the fresh air requirement.
First, the CFM is calculated based on the cubic foot area of the building and the percentage of hourly air change that is necessary.
So, we can calculate the total CFM necessary in our specific example by plugging in the following numbers:
2,000 x 8 x 0.25 = 4000 ft3/h (cubic feet per hour)
And, then our next step is to refine our concept of time relative to the situation by employing another simple arithmetic step:
4,000 / (Divided By) 60 minutes = 67 CFM
And, with the temperature difference….TD…. (53, as mentioned above) and the CFM (67, as we calculated) known, the Total Heat formula will provide the BTU requirement information we are looking for.
By the way, the TD in situations such as this is calculated based on what is known as a “worst case scenario” and is derived from the outdoor design temperature. For example, if the outdoor design temperature in Tucson, Arizona was 20-degrees (“worst case scenario”) and the temperature to be maintained inside the building was 73 degrees, then the temperature difference would be listed as 53 degrees. You can plug your numbers into the formulae mentioned above and research the design temperature information for your particular situation to perform a calculation on any building.
On to our final calculation….
Our Total Heat Formula…..Qs (Total Heat) = 1.08 x CFM x TD…..when employed in our specific example…..shows:
Qs = 1.08 x 67 x 53 = 3,835 Btu/hr.
Which means that when the furnace size is determined for our example residence, we would have to consider an additional heating load of 3,835 BTU’s because our building is tightly constructed, and the HVAC system is designed to control the amount fresh air being introduced into the living space.
Until next time…
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