HVAC Rules of Thumb
If you’ve been in the HVAC industry for a while, you probably have some “Rules of Thumb” that you use throughout your day. Here are a few that you may not have used before and could prove to be useful!
The 1.7X rule
This rule of thumb states that the MAX heating capacity required for your comfort cooling application is 1.7 x the cooling load. For example, if the cooling required is 30 tons (360,000 btus), then the MAX heating capacity required would be 612,000 btus (1.7 x 360,000). This assumes no lighting, no people, no internal heat gain, and a -10 degF ambient design temperature. The heating requirement could be more, though it is extremely rare.
300 sq ft per ton
If you need a ROUGH estimate on how much comfort cooling a commercial building needs, this is a good place to start. The cooling requirements depend on many factors (insulation, population density, window type, internal equipment, lighting, etc), so use this rule of thumb carefully.
150 cfm per ton for makeup air (DOAS)
If a portion of your building needs makeup air (100% outdoor air), the amount of cfm PER ton is about 150. For example, a 15,000 cfm makeup air unit with cooling (or a DOAS unit), would require roughly 100 tons of cooling.
0.1 tons/person in densely populated zones
This is a handy rule of thumb for estimating the cooling load for a densely populated area, like an auditorium, gymnasium or sanctuary. For example, if an auditorium will have up to 400 people at maximum capacity, the cooling load would be close to 40 tons (400 x 0.1). If the space is insulated well, this rule of thumb reduces a bit to 0.08 tons per person.
50 heating btuh per square ft
Just like all rules of thumb, this is just a place to start, and will heavily depend on insulation factors and heat generating equipment. In reality, most commercial buildings (offices, retail, schools) are probably closer to 25-35 btuh per square foot, but warehouses or areas with more ventilation could require 50-75 btuh per square ft (or sometimes even more!). This also depends where in the country or world you are. In the midwest this is a good rule of thumb. If you are in southern California, or in the arctic circle, this rule of thumb is not so helpful.
1.3 cfm per sq ft
If you need to quickly determine how much cfm you need to deliver to a space, this is a great place to start. If the cooling load is relatively small, the system may require closer to 1 cfm/sq ft. In spaces that are more density populated, like conference rooms, gymnasiums or auditoriums, the space may require closer to 2 cfm per sq ft (or sometimes more!)
Are these useful?
What are some rules of thumb that you use every day?