Weighing the Options
What mechanical system does the owner really want?
Are you talking with owners about what types of systems they use in their facilities? If so, how often? They talk about the challenges with maintaining these systems and perhaps vent their frustrations about shortcomings. In this month’s article, I would like to examine the pros and cons of various types of HVAC systems, mainly from a performance aspect and why an owner might choose one over another. The other topic to discuss with these systems is how easy they are to integrate into a central control system.
DX systems (split or packaged)
In a light commercial application, this is an easy solution for an HVAC system. Outside air quantities are relatively light, not a large sensible load, etc. In small spaces like a retail store or small office, residential equipment (5 tons or less) works well, whether it is a heat pump, straight cooling with electric heat, or gas furnaces with cooling coils.
Sometimes we see this type of equipment used in larger commercial projects like individual rooftop units for school classrooms for zoning purposes. If an owner pursues this option, make sure he/she knows to use a separate unit that will treat the outside air loads required for a facility with a lot of people in it.
If you can get the owner to consider all the factors associated with multiple units, such as power consumption, flashing in multiple roof curbs, and additional structural support at each unit location, then a central DX unit with VAV units is an option that may be more cost effective. This is one unit for an owner to deal with versus multiple units. And with VAV boxes, you can limit the zone sizes to internal versus external spaces, and in some cases have both within the same space if it is large enough and it maintains high occupant comfort between zones. A word of caution: if at all possible, use DX packaged equipment with factory-mounted internal controls specifically made to be used on a VAV system. While DX VAV technology has made great strides in my last 30+ years in the industry, it is still best left to the manufacturer to wire and test these units as they come off the assembly line instead of trying to figure it out in the field.
For large commercial facilities, campus wide cooling systems or multi-story buildings chill water may be a better option. Chillers these days can run as low as 10 percent of total capacity, and the ability to control the leaving air temperature on an air unit is much easier. Cost per ton, this type of installation is higher than DX but performance-wise and power consumption-wise, it is more cost-efficient. Air units can be placed strategically in the building to serve various areas (e.g. each floor); and with a little maintenance like regular filter changes, checked belts and greased motors, the units will last 20 years.
One of the biggest fears we hear from owners is the ability to maintain these systems. Especially in rural areas, the level of HVAC technician to work on these types of systems is much higher than a technician who works on residential and light commercial systems. However, this is an opportunity for the mechanical contractor to capture a service client for life.
So we have talked a lot about cooling options, but what about heat? With residential equipment, gas heat or heat pumps are easy. Heat pumps heat much better with today’s refrigerants such as 410A vs. R-22 that are in most units, which are 10 years or older. In milder winter climates I would definitely have no issue with using these for heat. Gas heat is also terrific, and if you have a large facility with multiple units, have the gas company increase the gas pressure at the meter to keep the pipe sizes for the gas smaller.
On larger units, your best options are a hydronic coil (water or steam from a boiler) or an electric coil.
The same fears go along with a boiler system that I mentioned for chill water — having a qualified maintenance staff to service them. Typically boilers are inspected annually by state agencies, so proper maintenance is critical for the owner to keep an operating permit. However, the ability to control the leaving air temperature with close tolerances is easy, and some say “wet heat” provides a more even air temperature.
Electric heat is an option when front-end cost is the driving consideration. There are limit switches that burn out over a relatively short amount of time and heating elements (wire coils) that can burn out as well. In markets where owners can get a really good rate from the electric company for their power, electric heat is attractive. The owner just needs to be prepared for regular repairs on various portions of the heating coils, and they likely will have a limited life compared to the air unit that it is served by.
If your owner is really concerned with what it costs to operate a system, then suggest to them a Variable Refrigerant Flow (VFR) system from manufacturers such as Mitsubishi, Daikin and LG (to name a few). These systems work extremely well at providing heating or cooling for any given zone at the same time. With digital scroll compressors, the condensing unit only works as hard as necessary to meet the building load. And the SEER rating for these systems far exceeds any traditional DX equipment. The front-end cost can be a little pricey, but when you take into account the unusable space that has to be provided for mechanical rooms, the electrical installation savings, or steel for rooftop equipment, and operating costs can be attractive to an owner.
Given all these options, how should the mechanical contractor guide the client through a decision regarding how to best control these systems? For a small office, an owner can put in a web-based digital thermostat and tell through a smartphone app whether the system is running, adjust schedules, get reminders about regular maintenance items, etc.
For multiple residential units on a commercial project, the first thing to get out of the client’s thought process is to control any system with time clocks. Besides being a labor-intensive option, the owner has no way to see what the system is doing or how it is performing against design. Guide the owner to a small DDC central control system. While there is some additional cost as opposed to just using thermostats, a building manager can see what is going on with the facility from a central location. And, many of these units now come standard with controls that can integrate with a central control system, so there is a lot of “plug-and-play” capability where making a small investment in some software can allow you to maintain and troubleshoot the system, many times without making a service call to an HVAC contractor. Also, if the problem does lead to a service call, having a control system allows the technician to do some problem-solving by seeing different statuses of the unit on a computer monitor before actually going in a ceiling or on the roof.
VRF systems typically have control systems that are designed specifically for these systems. That being said, they have the capability to have “non-VRF” equipment, such as outside air units to be tied into them. The installation is fairly straightforward.
Large chill water and heating water systems with central plants lend themselves to a more robust DDC software that you might see from Johnson Controls or Siemens (just two examples). There are control valves, dampers, space sensors, etc. that all have to work together for the system; there is software that has to be written for the individual parts as well as integrated into a system. Intimidating as it may be to an owner, trying to do it any other way would work a facility manager to death and cause the system to malfunction.
We as contractors must understand the needs of our clients first and foremost. If front-end cost is the driving force, then the contractor does not need to try to convince an owner to use a chill and heating water system. If they are interested in energy efficiency or longevity and ease of use, then a central chill/heating water or VRF system may fit the bill.
That being said, the contractor needs to lead the owner to incorporate certain features or equipment into the facility that will allow the system to perform better, and at the end of the day, have a more satisfactory experience with their HVAC system.
Charles “Chip” Greene is president of Greene & Associates Inc., mechanical contracting firm based in Macon, Georgia. Greene is a graduate of Mercer University with a BBA degree in management and has over 30 years of experience working on commercial HVAC and plumbing projects in the educational, medical, and institutional markets. Contact him at firstname.lastname@example.org.