Photo courtesy of the Brian McGowan

Servidyne calculates estimated effects of installing MERV-13 filters and an academic study measures results from 15 rooftop units–results show increase of between $0.01 to $0.02 per s.f.

What can we forecast for MERV-13 energy costs as we prepare for the reboot and adapt our building to this ongoing COVID-19 crisis? While MERV-13 filters are not the first step to re-entry, it’s an important question. To help, Servidyne ran some estimated calculations showing the energy impact of switching from a MERV-8 to a MERV-13 filter in a typical office building, and addressing the associated additional pressure drop of 0.3″, run about 0.30 kBTU a square foot a year.

The MERV-13 energy costs, in dollars, ranges between $0.01 and $0.02.

For small and medium-sized office buildings with rooftop air conditioning units, the increase in energy use from switching from MERV-8 to MERV-13 is higher.

Marwa Zaatari, Atila Novoselac, and Jeffrey Siegel of the Department of Civil, Architectural, and Environmental Engineering, University of Texas at Austin, and the Department of Civil Engineering, University of Toronto, measured the effects of the filter swap. They studied the effects on system airflow, filter and coil pressure drop, fan pressure changes, and power draw, and modeled the cooling capacity and compressor power for four filter pressure drops in 15 rooftop units equipped with and without fan speed controls.

Box plots showing the impact of replacing MERV 8 with MERV 11 (left), 13 (middle), 14 (right) on flow, total power (compressor and fan), fan power, CADR PM2.5 and CADR PM10 for five MERV 8 base cases for face velocity of 0.9 m s−1 for units equipped with no fan speed control.

For the 14 units without fan speed controls, the switch to MERV-13 resulted in a 2% to 4% increase in energy use during cool mode and a decrease in energy during fan-only mode. Of the 15 rooftop units tested, one had fan-speed control. For that unit, after a MERV-13 filter was installed, energy use increased in fan-only mode by 11% to 18%.

An ASHRAE research project found that gaps of less than a half inch can result in up to a quarter of the air bypassing the filter, effectively reducing the performance of a filter from MERV-15 to MERV-8. – Meghan McNulty, PE, Servidyne

And a final note–Servidyne points out the importance of proper filter fit. The first step to improved air filtration, even if you can’t upgrade filters, is to check for field edges without gaps in filter alignment, and in so doing address any bypass.

 


Resources

Buildings & Environment | The relationship between filter pressure drop, indoor air quality, and energy consumption in rooftop HVAC units

 

About Millen Paschich

Millen has been thinking about the built environment since he was four, when he started with site walks on residential construction projects. He began his career at Cambridge Associates and has an MBA from UCLA. Talk to him about bicycling, buildings, businesses, and green chile burritos!

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