The ERV cools and dries the incoming outside air during the summer months and heats and humidifies the incoming outside air during the winter months.

Which do you need for your applications, a stand-alone or bolt-on ERV? According to a survey of some manufacturers, that depends on many factors.

"Both stand-alone and unitized ERVs have their place in the market and are designed to meet many types of applications," said Brian Wathen, commercial product manager for York International (Norman, Okla.)

Stand-Alone Systems

In the eyes of Aaron Gotham, general manager for ERVs for Greenheck (Schofield, Wis.), most applications favor the stand-alone system, although the bolt-on ERV (or other integrated ERV and air conditioning products) can be a good solution for systems without exhaust, or where the exhaust airstream is remotely located.

"In most cases, a system design that incorporates the stand-alone ERV is superior to the bolt-on system," said Gotham. "The fundamental reasons for this can be found by taking a look at more than just the outdoor air requirements. If we consider both the outdoor air and exhaust air requirements of commercial buildings, the advantages of the stand-alone system become apparent."

Gotham commented that the stand-alone ERV system provides the most efficient method of ventilating a building "because it steals energy from air that has to be exhausted anyway.

"The bolt-on ERV is not tied into the exhaust system, allowing the conditioned air to escape into the atmosphere.

"In many commercial buildings, the exhaust air code required is roughly half of the outdoor air volume. This means compared to the bolt-on unit, the stand-alone ERV can recover energy from twice as much air leaving the building. The net impact is nearly double the effectiveness for the stand-alone system."

In his estimation, stand-alone ERV systems inherently remind the HVAC engineer to perform the building balance check, "because the outdoor air and exhaust air volumes are located on the same equipment schedule."

Gotham also maintains that the installation cost of a stand-alone ERV system is typically less than the installation cost of a bolt-on ERV. "With the stand-alone ERV, the ERV is performing the function of an exhaust fan, eliminating the need to purchase one," he stated.

"No additional roof or wall penetrations are needed for stand-alone ERVs. The same penetrations that are needed for the ERV were needed for the exhaust fans as well. In some cases, the ERV takes the place of several exhaust fans, and penetrations are reduced.

"Additionally, because the stand-alone ERV has a greater system efficiency (recovering energy from more exhaust air volume), more air conditioning equipment downsizing is available," he said.

"This is another first-cost advantage compared to bolt-on ERVs when a system includes exhaust air."

According to Russell Smith, sales manager at Berner Energy Recovery Inc. (New Castle, Pa.), there are still more cost-saving advantages to the stand-alone system.

"When introducing 100-percent outside air by incorporating a stand-alone energy recovery unit, one can design and limit the amount of impact to an existing HVAC system by implementing and making use of an independent set of coils within the ERV," said Smith.

"These coils can then produce the desired conditions, which can be ducted directly to the space."

Wathen pointed out that several key items need to be considered when applying either a stand-alone or bolt-on unit.

"Stand-alone ERVs are de-signed to supply fresh air into a space external of an HVAC unit," he said. "This type of ERV will supply a predetermined amount of fresh air into the space. Supply ductwork is usually required and in many cases the stand-alone ERV is tied to the air conditioning system's supply ductwork."

Benefits Of Both

Wathen continued: "Most stand-alone ERVs that have their supply air connected to the return of an air conditioning system have them connected to multiple air conditioning systems. This means that the ERV must be balanced with all of the air conditioning systems operating, and multiple balancing dampers adjusted accordingly. All of the air conditioning systems must operate with continuous blowers or air balance for all systems will vary."

In his estimation, a bolt-on system has the advantage of running when the air conditioning unit operates. "This allows the ERV and air conditioning system to remain balanced," he said.

As for the efficiency, Wathen contended that both the stand-alone and the bolt-on system can have the same efficiency depending upon the efficiency of the wheel. "The benefit of the bolt-on system is that it provides precooling or preheating for the air conditioning unit," he said, "thus making the unit more efficient."

That's not to say that Wathen believes in the superiority of one over the other. "Stand-alone models will allow fresh air to be supplied to large common areas and can be tied into existing air conditioning ductwork, thus becoming a separate entity from the air conditioning units themselves," he said.

"They can also be used on applications that do not have an air conditioning system." Bolt-on models are designed to allow fresh air to be supplied to the space in either large common areas or individual spaces served by individual air conditioning systems, he said.

"In most cases, return and supply air ductwork is provided from the air conditioning system," he added, "thus having a lower ERV installation cost over a stand-alone model that requires separate ductwork in many cases."

Does a stand-alone ERV inherently remind the HVAC engineer to perform the building balance check? Wathen doesn't think so. "A stand-alone system is difficult to determine a good air balance," he said. "The fact that it has an independent duct system builds in a typical 10-percent leakage, which requires the balancing contractor to ‘eliminate' the true amount of air.

"Also, most stand-alone ERVs that have their supply air connected to the return of an air conditioning system have them connected to multiple air conditioning systems," he said.

"This means that the ERV must be balanced with all of the air conditioning systems operating, and multiple balancing dampers adjusted accordingly. All of the air conditioning systems must operate with continuous blowers or the balance for all systems is shot.

"The bolt-on system has the advantage of running when the air conditioning unit operates. This keeps the system balanced."

In the end, Wathen said the overall goal of an ERV is to supply fresh air into a space without greatly increasing the sensible and latent load in the space. "Both stand-alone and bolt-on ERVs provide this function.

Sidebar: One Example - Stand-Alone System vs. Bolt-On System

To back up his point concerning the stand-alone ERV, Aaron Gotham of Greenheck provided a comparison based upon the following criteria:

A commercial building requires 3,000 cfm of outdoor air based on ASHRAE Standard 62.

Based on codes, the minimum exhaust is 1,500 cfm from areas such as restrooms and conference rooms.

The building is to have a slight positive pressure and specifies total exhaust/relief air of 2,700 cfm (10 percent positive).

Outdoor air design is 95 degrees dry bulb (db) / 78 degrees wet bulb (wb) and room air is specified at 75 degrees db / 50 percent relative humidity.

Gotham drew the following conclusions:

Stand-alone system - "Because we can exhaust the restrooms and conference rooms with the ERV, we can recover energy from the total 2,700 cfm of exhaust/relief air.

The resulting effectiveness at 3,000 cfm outdoor air and 2,700 cfm exhaust air is 75 percent. This pre-conditions the outdoor air from 95 degrees db / 78 degrees wb to 80 degrees db / 67 degrees wb. The air conditioning load is reduced by more than 11 tons."

Bolt-on system - "Because we cannot recover air from the restrooms and conference rooms, there is only 1,200 cfm of relief air left to recover energy from. The resulting effectiveness of 3,000 cfm outdoor air and 1,200-cfm exhaust air is only 40 percent. Outdoor air is only preconditioned to 87 degrees db / 72.5 degrees wb and the load is only reduced by 6 tons."

- Mark Skaer

Publication date: 01/17/2005