Fat Bottomed Signals Make the Control World Go Round
The Signal Debate
What’s your favorite type of analog signal? Are you a 0–10VDC kind of tech, or more of a 4–20mA person? Maybe you’re one of those folks my mother warned me about, the kind who uses a 0–20mA signal.
The truth is, choosing your signal type isn’t just about personal preference. It’s also a smart way to spot hidden problems in a system. That’s why I’m a fan of a fat-bottomed signal, and why you should be too.
The Power of a Fat Bottom
A fat-bottomed signal is simply a signal with a false bottom built in. Ever been on a job where something felt off with an air handler? The trends show no economizing for days, or the supply air temperature is running high while feedback from the heating valve says it’s closed.
There's a good chance the actuator lost power. With a standard 0–10VDC feedback, 0V looks normal since a valve can sit at 0% open. That’s how the failure stays hidden.
But with a 2–10VDC setup, that same failed actuator would drop below range, instantly tipping you off that something’s wrong.
That’s the value of adding a false bottom.
Why It Matters
A standard signal can make you think everything’s fine while a damper or valve hasn’t moved in days. Meanwhile, efficiency tanks, comfort suffers, and eventually, you get THAT call from the customer. Honestly, this was always one of the first things I checked when I was out on service calls; it was low-hanging fruit every time.
Using a fat-bottomed signal acts like an early warning system built right into your control loop. It’s a simple tweak that catches silent issues before they turn into pounding headaches. One failure mode I frequently use is to hold the output at the average of the last 10 minutes and trigger an alarm — giving you time to fix things before the whole roof caves in.
Turning Signals into Failsafes
A false-bottomed signal isn’t just there to throw alarms—you can make it handle problems for you. Here’s a quick three-step way to turn any control loop into a built-in failsafe:
1. Leave room below normal
Give yourself a little buffer under the normal range. On a 2–10 VDC signal, treat 2 V as your normal low and anything under that as “something’s busted.”
2. Add detection logic and define the safe reaction
Tell the controller what to do when it sees that dip. If feedback drops below 2 V, flag it as a failure and react safely. Freeze the output where it was, average the last 10 minutes, or shut off a coil so you don’t cook the space.
3. Send out an alarm
Don’t stop there. Fire off an alarm or push notification so you know it happened and can fix things before they become a mess. Follow these steps and you’ll spot failures early and keep the system running while you sort things out.
Wrapping It Up
Picking the right analog signal isn’t just a “whatever works” choice. A false-bottomed setup lets you catch failures early and keep things running so you're not late to your 3:30 appointment at the bar.
Next time you’re wiring up a system, think about how that signal can double as a built-in diagnostic. Does it fail high? Does it fail low? What behavior will keep the system safe if the signal drops out? And depending on the area served, what behavior will protect the conditioned space? A little smart programming now saves you a lot of headaches later.
If this hit home, stick around. I plan on sharing more quick, practical lessons from the field to help you sharpen your skills and build systems that work with you, not against you.
