Weighing the Options

﷯What mechanical system does the owner really want?

﷯A﷯re 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 s﷯ystems (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. I﷯n 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.

﷯Chill w﷯ater

F﷯or 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.  

Heating

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, Dai﷯kin 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.

﷯Controls

﷯G﷯iven 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 smart﷯phone 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 cgreene@greeneandassociatesinc.com.

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