Old Air Handlers Behaving Badly and How Analytics Can Help

Old Air Handlers Behaving Badly and How Analytics Can Help

FacilityConneX is all about bad behavior. By that, we mean equipment not doing what it is supposed to do – equipment doing stupid things.  We flag the behavior with analytics and discovery, bring it to your attention, tell you how to fix it, and how much you’ll save when you fix it.

There are so many reasons that equipment behaves badly – we’ve lost count – but here is a particular scenario that impacts many thousands of air handlers across the country.

Are these Air Handlers Behaving Badly?

One of our customers has a multi-building campus. Like so many other facilities, this campus has a wide variety of air handlers, some quite old, and some much newer. The oldest ones went into service with purely pneumatic controls.  In the absence of direct digital controls (DDC), pneumatics were a great solution but they had limitations; the logic that they performed had to be simple and wasn’t necessarily efficient.

For example, the mixed-air temperature (the temperature of the airstream after mixing of the outside-air and the return-air but before heating or cooling coils) was typically controlled by modulating the mixed-air damper in order to meet a mixed-air temperature setpoint.

The discharge or supply air temperature was controlled by modulating chilled water flow in cooling mode or hot water (or steam) in heating mode.

Controlling the operation of an air handler through these two parameters, the mixed-air temperature (MAT) and discharge-air temperature (DAT) setpoints, works but it can be highly inefficient with regard to energy consumption. The essence of the problem is that there is no inherent reason to control the mixed air temperature (other than avoiding freezing a coil).

But why bother talking about how old pneumatic air handlers were controlled? Because, many of those old air handlers are still in service, and though they have been upgraded with digital controls (many are still using the pneumatic actuator) they often are still using the original pneumatic logic.

Why? Well, when it comes time to upgrade to digital controls the most obvious approach is to duplicate the existing pneumatic logic in the DDC sequences. To do otherwise would require a complete review of the air handler and the downstream componentts that it serves. A new sequence of operations would have to be written and that’s not usually part of the scope-of-work for the controls upgrade.

So, we end up with an old air handler with new DDC controls, running old pneumatic logic. So what, what’s the harm? The answer is that unless the relationship between the MAT and the DAT setpoints is just right (and it rarely is), the air handler will use excessive cooling or heating energy – not just some of time but consistently. Here’s why:

What if the MAT and DAT setpoints are the same, say 65°F? If the return-air is 75°F and the outside-air is 55°F, the mixed-air damper will adjust for 50% outside air. The 65°F mixed-air then runs through the inactive heating and cooling coils and discharges at 65°F, then “it’s all good”, right? Wrong!

Typically, the indicated DAT will be higher than the indicated MAT even when the cooling and heating coils are inactive.  For starters there’s roughly a 1.5°F temperature rise due to energy added by the fan and motor. Steam or hot-water pipes within the airstream add yet more heat. The discrepancy between the two temperatures is further accentuated by the tendency for the MAT sensor to read lower than expected. This is due to stratification within the air handler and the nature of the averaging-type sensor.

Consequently, if the MAT sensor indicates 65°F, the air will arrive at the DAT sensor at 69°F and the unit will respond by calling for chilled water to bring the DAT back down to the 65°F setpoint. At that point, FCX intelligence comes into play. FCX advanced analytics understand that the unit could avoid mechanical cooling by bringing in more outside-air (economizing) and declare a “fault” situation. Not only do FCX analytics identify inefficient operation but they calculate how much money is wasted by the unnecessary mechanical cooling.

So, as a general statement, if the MAT setpoint is “too high” relative to the DAT setpoint then the unit chronically runs mechanical cooling – wasting energy.

Alternatively, if the MAT setpoint is “too low” relative to the DAT setpoint, then the unit chronically calls for supplemental heat. FCX analytics  identify that fault as well. They understand that a reduction in outside-air would negate the need for supplemental heat, and then calculate how much heat energy is wasted.

And all this chronic heating and cooling is due to an artifact of the control logic – not a response to real conditions – that’s the stupid part.

Campus personnel try to adjust the MAT and DAT setpoints seasonally to minimize unnecessary use of chilled and hot water, but it’s an impossible task because conditions are constantly changing.  Worse yet, some of the air handlers have a reset schedule on the discharge air temperature. Normally this is a good strategy that can reduce reheat at the VAV’s. However, in this case the changing DAT setpoint changes the relationship with the static MAT setpoint causing the air handler to waste heating or cooling energy. Even slightly miss-calibrated sensors will further increase waste.

What’s the solution? FacilityConnex has recommended that the sequence of operations for these old air handlers be changed so that the mixed-air temperature is monitored but not directly controlled. Only the discharge-air temperature is directly controlled. The outside-air damper and the chilled-water coil become stage one and two for cooling. This has several energy-saving consequences:

  1. Chilled water will never be used when economizing could provide the necessary cooling.
  2. Hot water or steam heat is never used except when the outside-air damper is closed to its minimum setting (reducing the volume of outside-air that must be conditioned).
  3. Finally, the recommended sequence allows for the use of a discharge-air temperature reset schedule, which can be extremely useful in reducing reheat downstream at the VAV’s.

FacilityConneX – like a really smart person watching your equipment in real time, 24 hours a day.

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