Years after an HVAC engineer starts to design closed-loop water systems and then progressively moves up the ladder of engineering success, he can forget and/or overlook many of the issues, concerns, details, opportunities, etc. associated with engineering these systems. After all, it is just pipe sizing to allow water to flow to terminal units, transfer its energy, and return to the source of the energy to begin again. Pretty straightforward, you’d think, but as I look back at my experience learning to design a closed-loop heating system, I wrote down many things that need to be done regarding water systems beginning with the design phase.
Always start with a simple one-line flow diagram of the design to get an overview and to determine if the system will be primary, primary-secondary, or primary-secondary-tertiary based on application. The designer, using the building system heat loss or heat gain calculations, then decides on the water temperature difference: 10 or 20 degree delta-T, or maybe a larger delta-T to reduce the individual gpms and in turn reduce the overall pipe sizes to reduce first cost and possibly operating cost. Next, the designer has the option of using the lazy person’s pipe sizing spin-wheel that is based on gpm, velocity, and pressure drop, or take the time to set up a time-tested, value-added gpm range pipe chart noting pipe size and minimum-to-maximum gpm (i.e., 1.25 in 7 gpm to 16 gpm) per pipe size.