What’s a Design Envelope?

The Armstrong Design Envelope (Video)

At its core, the Armstrong Design Envelope is a new way of approaching equipment selections for HVAC systems. Most consulting engineers approach the problem of sizing equipment using the following process:

1. Determine the peak requirements for this system based on the most extreme day (heating or cooling) in the calendar.
2. Add a percentage to cover an unforeseen variability
3. Using these "peak design day"requirements, select the HVAC components that operate most efficiently at that single point

If the components of an HVAC system are selected using the "design point" selection process, any or all of the above changes might affect the efficiency of the HVAC system, and they might need to be upgraded.  For example, during the design and construction phases of a project, it's very common that the components of an HVAC system would be re-selected a number of times to adjust for design changes, or different selections of building materials. The Design Envelope approach minimizes equipment changes during the design and construction phases of a project, and throughout the life of a building, and provides maximum efficiency through control and component integration.

The Design Envelope approach to equipment selection involves an entirely different mindset, whereby consulting engineers would size equipment as follows:

1. Determine the peak requirements for the system based on the most extreme day (heating or cooling) in the calendar.
2. Using performance charts showing performance characteristics for the product family (figure A above) and individual units (figure B below), select the size that offers the best performance for the design day, and provides sufficient capacity around that initial point to accommodate the possible variations.