But there’s more to it than that. Although many buildings do rely on constant air volume, many more owners and operators are looking for variable air volume (vav) systems that can ensure a building is receiving the proper amount of outside air at any given moment.

Some systems, however, can be complicated to set up and may need regular “tweaking” to keep ventilation rates where they should be.

To deal with this issue, one manufacturer has introduced an outdoor air measurement system for its applied rooftop systems and air handlers. McQuay’s DesignFlow™ precision outdoor air control system responds directly to the total volume of air flowing through the outdoor air intakes and has been found to keep its accuracy to within 2.5% of its initial settings.

What It Means for Contractors

For contractors, the Design-Flow system means that they can feel comfortable setting up the vav system and knowing that it will still be accurate well into the future. With other systems, contractors may set the minimum outside air at 10%, 15%, 20%, or whatever is called for.

However, as soon as the variable-frequency drives (vfd’s) start responding to the duct static pressure, the total volume of return and outside air goes down, and it becomes necessary to compensate for this reduction in outside air by opening up the outside air dampers.

“On a light-load day, you’re supposed to have just as much outside air as you did on a 95üF day. We help ensure that that happens,” says Dave Norrbohm, applied rooftop product manager, McQuay International, Plymouth, MN.

It is true that a vav rooftop system is going to take a little longer to set up than a constant-volume system. That’s because in a constant-volume system the design airflow is fixed, and there are obviously no vav boxes. During the balancing of the building, all that’s necessary to do is set the return airflow at the design airflow minus the constant building exhaust. Once it’s set, that’s usually it.

Balancing a vav system is more difficult. The vav box is an added device that varies the total volume of air (a combination of return air and outside air). It also becomes more crucial for the contractor to provide comprehensive start-up and balancing of the building. Balancing is important, because if it’s overpressurized or underpressurized, there may be IAQ problems or wasted energy.

“A real good start-up needs to be performed. You can do a quick start-up or fan balance and get it pretty close, but it gets to be critical to balance a building so that at all operating conditions — from minimum outside air to full outside air — the building is properly pressurized. That can get tricky,” says Norrbohm.

The actual installation of a rooftop unit that includes the factory-mounted DesignFlow is fairly easy. All it takes is a leveling procedure. And the DesignFlow doesn’t require any regular maintenance.

The system is not meant for older rooftop units, as it requires McQuay’s “MicroTech II” control system. Also, older rooftop units were not manufactured to handle the greater amounts of outside air required today. The unit heating (and to a lesser extent the cooling) would need to be carefully analyzed to ensure that they can compensate for the additional outside air. Otherwise, building occupants would probably start complaining about all the cold air coming out of the diffusers in winter.

Another possible option is to use makeup air units. “For example, instead of retrofitting 10 existing rooftop units on a building to meet minimum outside air quantities, you could possibly add one makeup air unit. The makeup air unit will meet the fresh air requirements and the other units will take care of the comfort portion,” says Norrbohm.

IAQ Considerations

“We have tried to leave no stone unturned as far as IAQ is concerned. It’s demanded by our customers, and we feel we’ve met those demands. There isn’t anything that our customers have asked for that we haven’t been able to provide,” says Norrbohm.

To handle other IAQ issues, McQuay offers a range of factory- or field-installed dehumidification and humidification equipment, as well as sound attenuators. Norrbohm is quick to point out that IAQ issues can also stem from noisy hvac equipment, and sound attenuators can make sure that type of IAQ problem does not exist.

With all these bells and whistles available to ensure proper ventilation and IAQ, it’s almost certain that you can find the rooftop unit for just about any application.

Sidebar: Energy Savings Also an Issue

Due to ASHRAE guidelines, more outside air is being brought into buildings, and the mixture of outside and return air can easily drop below 55°F. When it gets this cold, it becomes necessary to heat the air back up to an acceptable discharge temperature. On a moderate day, the amount of heating required may fall well below the minimum that a 3:1 turndown or two-stage burner can provide, resulting in overheated supply air (and wasted energy) or cold drafts (if the burner shuts down to pilot).

According to McQuay, its SuperMod burner, with a 20:1 turndown, can turn down to as little as 5% of its rated capacity to provide the right amount of heat, saving energy and improving tenant comfort. It is particularly effective in vav and makeup air systems, the company said.

“Everybody wants to meet the new ASHRAE standard, but bringing in more outside air costs money. Because of that, we developed the DesignFlow to accurately measure the outside air coming in and the SuperMod burner to efficiently treat that air, without sacrificing tenant comfort,” says Dave Norrbohm, applied rooftop product manager for McQuay.

Publication date: 06/25/2001