Consider State-Based Sequences When Programming HVAC Controls
Opting for simple or complex sequences largely depends on the building operator’s skill and talent.
Last month, I started to explore when to use simple sequences and when to use complex sequences. Part of creating the correct sequence lies in the proper use of technology.
For example, let’s look at dehumidification sequences. These sequences are fairly common in most of the southern U.S., yet it has been my experience that dehumidification sequences are often improperly implemented, despite how common these sequences are.
This provides us with a perfect scenario to illustrate the approach of building a simple or complex sequence.
Let’s look at a simple sequence utilizing subcooling and reheat.
On a call for dehumidification, the refrigeration circuit will start, operate, and reject its recovered waste heat to the air hot gas reheat coil condenser.
This is a very straight-forward sequence that will operate effectively in most packaged rooftop units and can be executed via embedded nonprogrammable controls. The building automation technician would simply need to set a specified dehumidification set point, and the unit would control to the set point.
Now, contrast this to a complex dehumidification sequence.
22.214.171.124 Cooling and Dehumidification Coil Control: When this loop is enabled, the direct digital control (DDC) hardware shall modulate the cooling and dehumidification valve to maintain either the zone temperature (ZN-T) at set point (ZN-T-SP) or zone relative humidity (ZN-RH) at set point (ZN-RH-SP), whichever calls for more chilled water flow. The valve shall be modulated in sequence with the reheat valve and humidification valve as shown to avoid simultaneous cooling and reheating and simultaneous dehumidification and humidification. When this loop is disabled, the coil valve shall be closed.
It’s at this point that I’m going to navigate us into the weeds (just a little I promise).
In my experience, using a state-based sequence is one of the best ways to write HVAC control sequences. State-based sequences are those that have a series of states that are enabled/disabled and, based on these states the system(s), will do something.
A very common example of state-based sequencing is the control of a reheat VAV. You are either in cooling, satisfied, or heating states.
Now, at first glance, the complex sequence has the making of state-based sequencing. And that’s good, right?
The problem lies in how complex the state triggers are. While you will not always be able to have a single state trigger, you should strive to keep the amount of state triggers low. In this case, we have ZN-T-SP (zone temperature set point) and ZN-RH-SP (zone relative humidity set point) and the respective outputs of their proportional integral derivative (PID) loops.
I’ve used this exact sequence before and the problem is that the ZN-T and ZN-RH fight each other. While I understand the objective, you don’t want a low ZN-RH loop output to keep the ZN-T from getting too hot; this sequence is unnecessarily complex (especially in a VAV system).
Ideally, we would let the dehumidification sequence control to return and discharge air relative humidity (RA-RH or DA-RH). An even more ideal approach would be to have the air-handling unit control all of this using internal logic from the embedded controls.
Here’s the thing: A building automation programmer can effectively program this sequence. It’s not terribly difficult. But, after you and the BAS folks are done with the project, the building operator will have to run the building, and he or she now has a needlessly complex sequence with multiple variables.
Additionally, the programming is no longer part of the embedded controls within the equipment, it is now part of a custom program inside a BAS controller. This means, in most cases, troubleshooting the system will require an on-site visit by a BAS technician.
Switching back from technical to operational, one of the things you must consider as a specifier is the level of skill and talent possessed by the building operator. If you are creating a specification for a major university with an in-house controls team, then, by all means, feel free to specify complex sequences. But if you are writing a specification for a school district or commercial office building, then simpler is better in most cases.
In next month’s article, I’ll further examine the concept of state-based sequences and how you can structure your sequences so they are easier to program and troubleshoot. A side note, one thing I see a lot of design firms missing is collaboration between the owner and BAS contractor. Forward-thinking firms will work with two to three manufacturers of their choosing and will develop sequences the manufacturer will have a systems library for. This will reduce total design and install costs and will make for a compelling value proposition. More on this in future articles.