HVAC System Rules of Thumb

Frequently, when I encourage operators to consider the loads their systems serve, as I did
in my previous post, I’m asked how someone who is not trained in design can go about doing that. I often suggest that they apply rules of thumb and there are a number of great resources out there on this topic. One of my favorites, appropriately titled HVAC
Equations and Rules of Thumb
is illustrated below.

If you take a minute to browse through the table of contents you will discover that the handbook contains nearly 800 pages of useful information if you are in the HVAC business, especially when a  used copy can be had for under $20.

Another favorite of mine is the ASHRAE Pocket Handbook. This little gem really will fit in
your back pocket and can be a valuable reference for countless useful pieces of information, like duct or piping friction rates, properties of materials, and in the context of our
current discussion, load check figures, as illustrated below. 

In fact the handbook contains the information needed to take your load estimate one step further by including all of the basic information you need to do a manual load calculations using the Cooling Load Temperature Differential (CLTD) method including an outline of the steps in the procedure, the necessary equations, and where the required information can be found in the guide, as illustrated below.

New copies of the handbook can be obtained for under $40 if you are an ASHRAE
. I have seen used copies for under $15 in used book stores and
, especially the earlier (pre-2005) editions.  Given that the rules behind physics
haven’t changed much lately, an older version will probably do just fine.

An important caution regarding using rules of thumb and techniques like the CLTD method; in contrast with modern load calculation programs used by experienced practitioners, they are at best approximations and should generally not be used as the basis for selecting equipment. But, as a technique to get a feel for what you think might be going on in your system or make a first pass assessment of what the load might be as you begin a design or acquisition process, they can serve you well, as they have in my career.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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