VAV Reheat Terminal Unit Controls
Noncompliant programming can easily slip through until it is uncomfortably late in the process.
If anyone has commissioned anything, it is very likely they have commissioned at least one VAV hot water reheat terminal unit. They are ubiquitous, they are commodities, and they are anything but “simple.” Just like the other “simple” systems addressed in this column recently, VAV reheat units are often marginalized and poorly documented in the design and construction process.
There is a sense that everyone knows VAV boxes, so what could go wrong? It isn’t that things can go terribly wrong, but it is a challenge to get them to go exactly right. Mostly in the interest of energy conservation, some building owners and operators have put a lot of effort into standardizing their terminal unit controls and want things done a very specific way. In addition to energy-conscious operations, the value of standardization cannot be overstated when the quantity of VAV boxes can be in the scores, hundreds, or thousands for a single owner.
How can VAV reheat units be so challenging? Let’s count the ways. The following are a non-exhaustive list of the options one has for programming a VAV reheat box.
Heating airflow equal to minimum cooling airflow or allowed to modulate up to a set point heating maximum airflow?
If there are minimum and maximum heating airflows, how is the damper modulation sequenced with the reheat valve — in sequence or in parallel?
Integration with perimeter radiation in the same zone? If so, how is the VAV box control sequenced with the radiation — in sequence, in parallel, dependent on outside air temperature, etc.?
User setpoint adjustment? If so, is the adjustment limited at the central automation system?
Communication with the central AHU? If so, what data is shared — cooling command, damper command, reheat valve command, occupancy mode, etc.?
Time of day occupancy schedule? If so, same as its respective AHU or a separate terminal unit schedule?
Space occupancy sensor? If so, what happens in occupied versus unoccupied mode — temperature setpoints, minimum damper position, etc.
CO2 sensor? If so, what happens when CO2 concentration is above setpoint — modulate damper, central air handler outside air damper control, etc.?
Design engineers should be very clear and detailed about their intended VAV box controls. That is a good start, but the next step is for the controls contractor to take that design seriously and do what it takes to achieve the specified sequence of operation. This can happen, but controls contractors have also been known to provide whatever their canned or partially-programmable controllers are capable of, even if it is not exactly as specified. This is a classic case of being awarded the project and asking for forgiveness later when it is too late for most owners to make a big deal out of non-compliant sequences — especially if basic comfort control is achieved with the non-compliant programming.
The discovery of such non-compliant programming often falls on the commissioning professional’s shoulders, and it often isn’t proven to be a problem until a functional performance test fails. Controls submittals can be regurgitations of the design engineer’s exact sequences, but that does not mean the programmer implements what is documented in the submittals. Again, sometimes they do and sometimes they don’t; one never really knows how this will turn out until you see the programming or conduct the functional test.
Most projects do not have the budget to pay the commissioning professional to review programming code, especially for such “simple” things as VAV reheat units. Therefore, functional testing is where many anomalies show up first. Most of the time, functional testing is one of the final activities of the construction phase and enforcing major deviations from design sequences can be a substantial effort for the commissioning professional and owner. At a time when the owner is stressed about moving in and closing out the project on budget, the energy conservation and future operational priorities can slip.
What can be done earlier in the project? My recommendation is to be fully cognizant of the potential complexities of the terminal units starting with design review and extending through submittal reviews and functional test preparation. Verify that the design engineers clearly and comprehensively define all aspects of the VAV reheat control sequences and obtain written confirmation, perhaps multiple times, from the controls contractor that the programming will reflect the design. ES