This Houston office building's IAQ took a massive turn for the better when its old HVAC filters were replaced with filters with much higher MERV ratings and de-mister filters that help eliminate wetness on the intake of the outside air handler and filters.

MetroNational is a privately held billion-dollar real estate investment, development, and management company. The indoor environment of this Houston-based company's properties plays an important role in the welfare of tenants and in its ability to market its buildings.

In an effort to improve the IAQ of the company's buildings, MetroNational chief engineer Danny Rodriguez, RPA, SMA, monitored temperatures and CO2 and humidity levels and evaluated the HVAC system at Memorial City Place - a 10-story, Class "A" office building with 152,000 sq ft of lease space in Houston. A professional laboratory also conducted tests for yeast and mold.

Based on this research, Rodriguez decided to upgrade the building's HVAC filtration system to a higher IAQ standard as the first of many of the company's buildings to receive such upgrades.


Memorial City Place has two 200-ton and one 100-ton air cooled Carrier Flowtronics chillers, one conditioned outside air unit and electric side-pocket heating to serve 10 air handlers. Originally, a single pad filter made of 1-in. thick polyester media - laid over the face of the intake -- was the only filter on the AHU. Although this MERV 4 filter removed larger particles, it did little to remove particles of less than 10 micrometers, which stay suspended in the air longer and have the greatest effect on IAQ.

"We raised the filtration efficiency to MERV 11 as recommended by our HVAC contractor, Air Tech Commercial Services," said Rodriguez.

Air Tech removed the existing air filter assembly and designed a front-loading filter holding frame assembly containing a heavy ¼-in. by ¾-in. rubber gasket to seal the eight filters within each air handler, preventing air bypass. Snap clips hold the filters in place. Eliminating the need for pan tubs, Air Tech installed bottom drains in each condensate drip pan to prevent rust, corrosion, and fungus growth.

Air Tech replaced the 1-in. polyester media filters with 2-in. deep-pleated MERV 7 pre-filters and 6-in. deep pleated MERV 11 final filters. The pre-filters extend the life of the more expensive final filters. The filters' pleated design increased surface area and reduced pressure drop, which improved particle removal efficiency without significantly increasing energy costs.

To prevent condensed moisture - from morning rain and fog - from entering the building, the firm installed M-81 de-mister filters. The new filters helped eliminate wetness on the intake of the outside air handler and the outside air filters, which could have impeded the original filters from functioning as designed, and discouraged the growth of mold, mildew, and bacteria.

With the original 1-in. polyester media filters, building engineers replaced filters on a PM schedule every four months. With the new assembly, the MERV 7 pre-filters are changed every four months, and the MERV 11 final filters are changed every 12 to 18 months, or upon reaching a final pressure drop of 1-in. w.g. The filter frame snap-locks allow easy removal and replacement of filters.

Because dust no longer gets into the system's coils and drain pans, annual cleaning of these items is no longer necessary.

"Before the upgrade, we took pressure drops across the coils to see if they were dirty," Rodriguez said. "We cleaned coils annually - an all-day effort of two engineers for each air-handling unit. With the upgraded filtration system, we will clean the coils about every three years. These combined improvements add up to approximately $10,000 per year in savings."


The upgrade to a MERV 11 final filter resulted in significantly reduced particle counts: From 13,130/cu ft to 430/cu ft in the 5-micrometer range; and from 26,710/cu ft to 2,730/cu ft in the 1-micrometer range.

This IAQ improvement represents a significant reduction in airborne mold, allergens, and other irritants. "Tenants have reacted favorably to the upgrade and have expressed gratitude to MetroNational for creating an exceptional indoor environment," Rodriguez said.

The air filtration/IAQ upgrade at Memorial City Place won the National Air Filtration Association's Clean Air Award, which recognizes organizations for utilizing effective air filtration in maintaining a clean and healthy indoor environment.

"We are very excited about winning the Clean Air Award and have instituted similar upgrades at other MetroNational buildings," concluded Rodriguez. "Now, MetroNational and our tenants are more alert about their indoor environment."

He added, "MetroNational always strives to be at the leading edge of technology in the office building industry. This upgrade helps us do that, while creating a better indoor environment for our tenants and employees."