December 2013

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Multizone AHU Retrofit Strategies

Breathing new life into an old beast

Steven R Calabrese

Steven R. Calabrese
Control Engineering Corp.

Contributing Editor

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Ive been running across a lot of the old multizone systems in my recent rounds of business. You know these systems, air handlers with a hot and cold deck, and a bunch of zone dampers local to the unit. You see a lot of these in older stand-alone single-purpose buildings. Like banks, which explains why I’ve been seeing a lot of these lately, as I’ve recently been involved in an ongoing rash of bank jobs (that didn’t come out right, did it?). Seriously speaking, I’ve been teaming up with a mechanical contractor that has been doing a lot of mechanical retrofit work with these types of stand-alone facilities, with my firm providing the design and installation of the associated controls retrofits that inevitably go along with these mechanical system retrofits.

So first an explanation on what a multizone is and how it was “traditionally” designed to operate. The unit has a supply fan that blows air through a set of stacked coils. On the bottom is the chilled water coil, and on the top is the hot water coil. On the leaving side of the coils are ductwork take-offs, one per zone. There can be as many as 15 of these on any given multizone unit! Prior to each take-off and after the coils, for each zone, is a zone damper, configured to be able to modulate and mix varying amounts of air blowing through the chilled water coil and the hot water coil. It is in this manner that each zone can get a totally unique temperature of air delivered to it. The chilled water side of the unit is referred to as the “cold deck”, and the hot water side of the unit is referred to as the “hot deck”.

The unit itself has the ability to maintain an active cold and hot deck at all times. Whether or not that is an economical thing to do is beside the point. Of course the unit equipped with economizer (outside and return air) dampers can maintain a cold deck temperature without the use of chilled water, for outside air temperatures suited for free cooling. Again, beside the point. The point is, conservation of energy notwithstanding, the multizone unit with an active cold deck (discharge air off the coil around 55 degrees) and active hot deck (discharge air off the coil around 95 degrees) can satisfy each and every zone connected to it, regardless of what any other zone is calling for. From a comfort standpoint, it doesn’t get much better than this. From an energy usage standpoint, maybe not so much, which is why you don’t see many of these being put in as new construction.

In a retrofit situation, typically the unit is overhauled, meaning that it may get new coils, a new supply fan, and maybe a new filter section. Control-wise, the damper and valve actuators are often pneumatic, so the existing dampers and valve bodies can remain and be reused, and the pneumatic actuators will be replaced with electronic modulating actuators. For each zone, a dedicated unit level digital controller will be employed to perform zone temperature control. The controller will have the input/output count required for the task. Inputs include a new space temperature sensor, as well as a new zone discharge air temperature sensor. An analog output will control the new electronic zone damper actuator. The method of zone control will be much the same as it was originally. The space temperature setpoint (per zone) is established, generally via a setpoint lever at the zone sensor, and the controller modulates the zone damper to mix the appropriate amount of cold deck and hot deck air to achieve the zone discharge air temperature required to satisfy the heating/cooling needs of the zone.

As for the unit itself, an equipment level digital controller will perform all facets of monitoring and control. Inputs include (but not limited to) cold deck temperature, hot deck temperature, return and mixed air temperature, fan status, and filter status. Outputs include a digital (binary) output to start and stop the fan, and analog outputs to modulate the economizer dampers and the chilled and hot water coil control valves.

Now to control the unit! We won’t control it like it was controlled in the old days. Meaning we won’t maintain the cold and hot decks at constant temperatures. First off, if outdoor air conditions allow for economizer operation, then we’ll attempt to maintain cold deck temperature using that alone. If that’s not possible, then we’ll modulate the chilled water valve to get us the rest of the way there. In the summer months, the hot water system is likely not even up and running, so the hot deck is what it is: return air at a temperature slightly greater than the general space temperature. We may even let the cold deck temperature drift a little, based on the individual zone demand, as we have that information available seeing that this is now a networked control system. Can’t let the cold deck temperature drift too much though, as we need to be conscious of the humidity levels as well.

Reliable Controls In the winter months, there will be no chilled water available, so economizer operation will sustain the cold deck. Again, we’ll let the cold deck temperature drift up as the outside air temperature drops, all the while keeping an eye on the zone temperatures and the demands of the individual zones. The hot deck will be active and we’ll maintain a hot deck temperature that will be reset based on zone demand, noting that the hot water itself will likely be on a reset schedule as well, dropping in temperature as the outside air temperature goes up.

There’s much more that we can do here, but I’m running out of space to discuss. The point is, the efficiencies that we can achieve over the old pneumatic system, with networked direct digital control, are much greater. By knowing what the individual zones are doing and knowing their needs, the equipment level controller can be programmed to make the right decisions for overall comfort control, and still operate the multizone unit to achieve maximum efficiency. Sure, they may no longer put these old beasts in on new projects, but for those buildings that still have these types of systems, overhauling the unit and retrofitting the unit with digital controls can make a lot of sense, especially when the alternative is ripping it out and putting in a whole new mechanical system!

Tip of the Month: Set up a trend on the hot and cold deck temperatures. If you’re letting zone conditions influence the setpoints of these temperatures (which you should), then trending these values gives some good insight into the needs of the zones as a whole, which could help you even further adjust your programming to establish the perfect balance of energy usage and comfort control.


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