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ASHRAE and MERV Rated Filters: Which Adjustment Do I Use for a Filter Series?

 
Question:

Hilary asks: ASHRAE 90.1 Table 6.5.3.1.1B lists the Pressure Drop Adjustments for various devices. There are differing interpretations in my office about the filtration listings if you're using both pre- and final filters. One camp believes that you pick the highest MERV filter you are using in the system and use that adjustment. The other camp believes that you can add the associated adjustment for each filter in the series. I checked the User's Manual but it does not address filters in series.

Answer:
Advice provided by: David M. Pratt, P.E., CEM, LEED AP, MWH
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Hi Hilary, thanks for your question concerning air filters and their associated pressure drop adjustments.

Air Filters and LEED IEQ

Air filter selection is important in a LEED project because it can affect two potential credits in the Indoor Environmental Quality (IEQ) category: IEQ Credit 3.1 (Construction Indoor Air Quality Management Plan—During Construction) and IEQ Credit 5 (Indoor Chemical and Pollutant Source Control).

IEQ Credit 3.1 and Filtration

IEQ Credit 3.1 requires that filtration media with a minimum efficiency of MERV 8 be used at each return air grille if permanently installed air handlers are to be used during construction.

However, do note that the LEED for New Construction Rating Guide states that “If possible, avoid using permanently installed air handlers for temporary heating/cooling during construction.” This is for various reasons, mainly because you will have to take extra precautions to protect the HVAC system from dirt and dust, and also because it will unnecessarily activate the warranty of the HVAC system.

IEQ Credit 5 and Filter Requirements

IEQ Credit 5 requires the installation of air filtration media with a MERV rating of at least 13 after permanent construction is complete.

Pressure Drop Adjustments: Highest MERV Rated Filter

Your question is really more of an HVAC design question that a sustainable design question, so let me put my mechanical engineer hat on for a moment.

I understand where both camps are coming from. The camp that believes you add the static pressure adjustment for each filter is using the total pressure method for fan selection, where you add up the friction losses of all the duct runs, fittings, diffusers, and filters. This is a good thought process and is typically how one would determine total pressure in an HVAC system.

That said, you should add the friction losses of each of those components EXCEPT the multiple filters. Adding multiple filters will increase the total filtration surface area, but will not affect the final system pressure drop. Designing more filtration surface area into your HVAC system will actually lower the total static pressure loss, which will in turn reduce the amount of energy used to operate the fan.

Therefore, you should use the adjustment value from ASHRAE 90.1-2007 associated with the pressure drop of the highest MERV rated filter in the system.

I hope this helps! Best of luck.

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Expert Advice and Comments

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Tue, 02/14/2012 - 14:05 — Tony Kelly (not verified)

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ASHRAE 90.1 Table 6.5.3.1.1B

Tue, 02/14/2012 - 23:29 — Berni Baier (not verified)

When designing an application, both the pre and the final filter pressure drop need to be taken into consideration. That being said, there are numerous filters that claim low initial pressure drop and are promoted as "energy saving" filters. Initial pressure drop should NEVER be used as a guideline for energy reduction. AVERAGE pressure should always be used. You could install a filter with an initial pressure of .25" w.g. but climb to 1" w.g. in a short period of time. You could then install a filter with an initial pressure of .3" w.g and reach .6" w.g. within the same period of time. The latter would have a lower average pressure drop. In order to achieve a lower pressure drop and save energy, the following factors need to be met:
1) Must be using a VFD fan, Constant fans with lower pressure drop filters will supply more air but not necessarily use less enery
2) Do not run the filters to the maximum pressure drop. Always change them out at twice the initial pressure. This is the point were most filters begin to climb dramatically in pressure
3) If possible, install the prefilters (if they are the pleated style) in a "V" shape. This will drastically reduce the pressure drop and increase the life of the filter
4) Use mini-pleat V-bank final filters which have the lowest pressure drop of any final filter and the longest life

Good luck

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Tue, 02/21/2012 - 04:28 — John Watson (not verified)

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