Avoiding Costly Mistakes in Light Commercial HVAC System Design

Avoiding Costly Mistakes in Light Commercial HVAC System Design

While residential and commercial HVAC system requirements are similar, there are defined differences that residential builders who are venturing into light commercial construction need to know about so they can avoid making potentially costly mistakes.

Basically, unlike residential construction where windows help ventilate a home, commercial construction does not rely on windows for ventilation. Instead, the outside air, general exhaust, toilet exhaust and process exhaust must be provided by — and specifically calculated for — commercial HVAC mechanical systems.

So, in addition to the heating and cooling calculations needed in residential construction, builders must also add newly introduced outside air to their commercial calculations.

This additional requirement necessitates that residential builders select and install commercial-grade mechanical components rather than install what they typically use in new homes.

Elements Common to Light Commercial Systems

To help familiarize residential builders with the differences in commercial systems, we have compiled a list of elements that are common to light construction systems that are not found in residential systems.

These elements apply specifically to buildings up to two stories high. While the basic principles are the same for systems used in larger commercial buildings, the configurations for systems in large and small buildings differ completely.

Roof Top Units – Heat, Cool, and Provide Outside Air


HVAC rooftop mechanical units provide the heating, cooling and most ventilation requirements for light commercial spaces. Essentially, they are the workhorses of light commercial construction. 

A rooftop unit is generally a factory “catalog made to order” assembly that includes a condenser and an expansion coil for cooling, a heat source and a single fan for forced air heating and an intake opening for outside air. 

The fuel heat flue discharges on the side of the unit.


Power Wiring – Complex and Expensive

Power wiring varies depending upon the size of the HVAC unit. 

 Units up to three tons can be powered by 120 volts, 208 volts or 240 volts. The most common units — those seven tons to 15 tons and larger — can be powered by 208 volts, 240 volts or 480 volts. 

Power is supplied through a disconnect mounted on the side of the unit. Frequently, a 120-volt convenience receptacle for servicing the unit is either included with the unit or installed in the field. 

Larger units may require a custom-ordered disconnect, which can be expensive. 

Also, keep in mind that, for the largest units, breakers in the panel are not sufficient circuit protection. For greater protection, you will be required to alter the electrical service gear for the entire building.


Outside Air Control – Economizers or Power Exhausts


Outside air control can be improved with an “economizer” — a modulating damper that senses the outside air temperature and increases the intake air up to 100% in order to provide free cooling when possible. 

A power exhaust can be, but seldom is, added as an option to a rooftop unit. Independent general and toilet exhaust systems are usually more cost-effective. 

If a power exhaust is added, however, a barometric relief valve on the rooftop unit will be required so that the outside air injected by the system does not excessively pressurize the building.


Zone Control – Not Always Needed

Commercial buildings with larger spaces, more rooms or uneven or non-uniform window placements have a greater heating and cooling load, which makes zone control in this type of space more important. 

In smaller strip shopping centers spaces of about 1,500 square feet where a single zone can be used, builders should not use forced air. 

However, if a tenant in a strip center has combined multiple spaces, builders can configure the ductwork from each single zone rooftop unit to provide interior and exterior zones. 

For the next level of improvement — spaces requiring six to about 12 zones — builders should add zone control devices in the ductwork. Here, the unit discharges air at a specified temperature, and zone dampers in the ductwork modulate the air volume for temperature control. 

When zone dampers are used, each mechanical unit must have a bypass damper to short-circuit the supply air to the return in order to relieve pressure caused when dampers restrict all the airflow at the same time. Such a system also requires that a system controller coordinate the unit and devices. 

In colder climates, builders frequently add radiant heating near windows and in vestibules to supplement the heating in these zones. In the past, builders used electric reheats in the ducts, but most energy conservation codes are restricting or prohibiting using these coils in order to avoid cooling and then reheating the same air.


Avoid Common Errors Along the Way

In addition to understanding the fundamental differences between residential and commercial HVAC systems, residential builders considering diversifying into light commercial construction should avoid the following common errors when installing commercial systems:

  1. Don’t use residential systems for commercial projects. Residential HVAC systems are not designed for, nor capable of, providing the outside air required by code and therefore have no place in commercial structures.
  2. Outside air intakes should be included when installing RTUs. RTUs without outside air intakes are suitable for cooling a warehouse, but not an occupied building.
  3. Don’t forget about general exhaust. Once you have chosen the exhaust required by code for your building, include appropriate compensating features such as a barometric relief valve, so that the entire system work in concert.
  4. Roof alterations are tricky, can be costly, and must be managed. 

More and more commercial flat roofs are proprietary systems that specify exactly how each roof curb and penetration must be configured in order to maintain its warranty, so builders shouldn’t simply poke through the roof and then caulk the penetration with roofing cement. 

Also, since curbs and roofing are both set on the deck, the curb height must include the thickness of the roofing with insulation as well as the required height above the top of the roofing. If the roof slopes more that 2% (1/4-inch in 1 foot) the curb must be altered to keep the top level.  

With proprietary systems, builders must use portals to accommodate the control wiring, power wiring and gas piping. In addition, they will need to have a factory-certified technician make all roofing patches.
  5. Metal roofs may require structural reinforcement. Metal roof structures are designed to accommodate only a little extra load, so installing additional rooftop units may require structural reinforcing. 

One common solution when working with metal roofs is to place the compressor-side of the new rooftop unit — the heavy end — over a beam. The installation may also require that you have angle iron reinforcing the joist web. 

In some municipalities, any additional structural work must be designed by a structural engineer or architect.
  6. Zoning is important for customer satisfaction. 
The opportunities for zoning in the smallest of commercial structures are limited, as described above. However, keep in mind that the customer may want to you to make some attempt at zone control or he may think that the building’s heating and cooling just doesn’t work correctly.
  7. Disconnects become more costly and tricky to order. For the smallest rooftops 
the disconnects needed are similar to those in residential construction. However, as the unit size increases, the requirements and costs of disconnects change dramatically. 

That’s because in commercial construction disconnects must be specifically selected for their characteristics and over-current protection. Either the electrical or HVAC contractor can provide disconnects, but builders should be sure that the disconnects are the correct ones for the job. Depending on their characteristics, they can increase costs substantially and take valuable construction time if they need to be replaced.
  8. Forgetting to install service receptacles and/or lights can be expensive. A 120-volt service receptacle is generally advisable — and frequently required by code — for each rooftop unit. Builders can either purchase the service receptacle as an option with the HVAC unit or field-install it separately when installing the unit. 

What builders don’t want to do, however, is overlook adding a service receptacle after installing the unit. Inspectors will notice that the service receptable is missing during final inspection — and that will cost you time and money.

Plan an Entire Building System, Not Piecemeal Installations

Ordering and installing all the rooftop units needed for a building is a complicated task that requires planning the entire building system, not simply installing each HVAC rooftop unit as needed. The pointers above can help you achieve satisfactory, and cost-effective, results.

Improper planning, on the other hand, can leave you with a very costly solution you may not be able to ignore — tearing off the roof unit and buying a new one.

Also, don’t be overwhelmed if you find that your system requires a factory-certified installer to install it. Acquiring the certification is usually as simple as having your installer attend a free seminar provided by the factory.


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