I remember when I was a kid my Dad told me that math was his favorite subject in school “because there’s no opinion to it, the right answer is the right answer”. How I wish that were true in engineering problems.

For this installment of “The Invisible Edge: Ingenuitive MEP Delivery” we’re going to go a bit more technical than last time. Let’s get into control theory… So, about the whole “2/3rds” thing used in pressure transducer locations for fan/pump control in variable air or variable pumping arrangements:

“Locate Pressure Transducer at 2/3rds of the duct/pipe”. Okay… is that 2/3rds of the overall length to the furthest valve? Is it after 2/3rds of the potential design flow has branched off? Is it after 2/3rds by count of control valves or VAVs?

Here’s my take: Forget the whole thing and put the pressure transducer where your duct or piping supply/return mains leave your mechanical room. (Make sure it’s downstream of the minimum flow control bypass valve if present). The trick is to dynamically reset the setpoint based on the most open valve position. If you do that, the pressure becomes just a reference instead of a critical value that’s always a compromise between energy use and possibly starving coils/zones. ASHRAE Guideline 36 even agrees with me… though I personally like just using the single most open device (reset DP to keep it between something like 75% and 95% open… depending on valve type) instead of the overly complicated voting system.

The benefits of this approach are numerous:
Energy Efficiency – System DP is now controlled to the minimum needed to avoid a starved zone or coil.

Reduced cost – no huge run of sensor wire if the remote DP is wired back to the central controller

Increased reliability – That long sensor wire I just mentioned is just begging to be damaged. I’ve seen sensor wires get caught in someone’s pro-press jaws too many times, during both new construction and remodels. I’ve seen other wires get chewed through by rodents. Some communicate the value instead: that works until that one VAV controller the DP sensor is wired to goes down and your entire system defaults to a static DP Setpoint.