Thursday, July 21, 2011

MERV13

There has been a lot of discussion about the impact of LEED Credit EQ5 and the necessity of a MERV13 filter on recirculated air devices (such as fan coils, Fan Boxes and even Chilled Beams). A MERV13 filter is likely to seriously degrade the performance of these devices. After a lot of back and forth, the following is the revised language which has been proposed, and is assumed to be adopted immediately. The wording is the same for Retail Commercial Interiors and Schools.

The impact is that to gain the point, only outdoor air must be MERV13 filtered, somewhere in the path between the outdoor air inlet and the occupant.

NC - EQ Credit 5: Indoor Chemical and Pollutant Source Control
1 Point

Intent
To minimize building occupant exposure to potentially hazardous particulates and chemical pollutants.

Requirements
Design to minimize and control the entry of pollutants into buildings and later cross-contamination of regularly occupied areas through the following strategies:

--- Employ permanent entryway systems at least 10 feet long in the primary direction of travel to capture dirt and particulates entering the building at regularly used exterior entrances. Acceptable entryway systems include permanently installed grates, grilles and slotted systems that allow for cleaning underneath. Roll-out mats are acceptable only when maintained on a weekly basis by a contracted service organization.

--- Sufficiently exhaust each space where hazardous gases or chemicals may be present or used (e.g., garages, housekeeping and laundry areas, copying and printing rooms) to create negative pressure with respect to adjacent spaces when the doors to the room are closed. For each of these spaces, provide self-closing doors and deck-to-deck partitions or a hard-lid ceiling. The exhaust rate must be at least 0.50 cubic feet per minute (cfm) per square foot with no air recirculation. The pressure differential with the surrounding spaces must be at least 5 Pascals (Pa) (0.02 inches of water gauge) on average and 1 Pa (0.004 inches of water) at a minimum when the doors to the rooms are closed.

--- In mechanically ventilated buildings, each ventilation system that supplies outdoor air shall comply with the following:

1) Particle filters or air cleaning devices shall be provided to clean the outdoor air at any location prior to its introduction to occupied spaces.

2) These filters or devices shall be rated a minimum efficiency reporting value (MERV) of 13 or higher in accordance with ASHRAE Standard 52.2.

3) Clean air Filtration media shall be installed in all air systems after completion of construction and prior to occupancy.

--- Provide containment (i.e. a closed container for storage for off-site disposal in a regulatory compliant storage area, preferably outside the building) for appropriate disposal of hazardous liquid wastes in places where water and chemical concentrate mixing occurs (e.g., housekeeping, janitorial and science laboratories).

Authored by: Dan Int-Hout, Chief Engineer Krueger

Thursday, July 7, 2011

Rules of Thumb

Over the years, Engineers develop “rules of thumb” for quick estimates of engineering issues. We manufacturers often do the same. Here are a few of the ones I use in looking at air distribution issues:

Buildings cost about $2/sf/year for energy. This is of course, a wild guess. I’ve been using it for 20 years. It is likely less than this in modern buildings, but I’m not getting much push back from engineers. If you take 1 watt/sf * 8670 hours/year, you get 8.6 kwh/sfy. At $0.10/kwh, that is less than $1/sfy. I said this was rough. I recently visited a building in Canada that claimed less than 1W/sf HVAC load.

Currently, a commercial building HVAC system’s first cost is about $30/sf. It was $15-20 when I started in this end of the industry, about 1980.

Building loads are typically designed at 1 cfm/sf. With 55F supply air in a conventional overhead mixing system, that comes to 22 BTUH/sf load. I suspect most office building loads are 1/3 of that most of the time.

The ASHRAE 62.1 default ventilation rate is 17 cfm/person. At 55F (typically required to maintain a minimum of 60%RH), that is likely more BTU’s than a person generates, and likely has enough left over to handle his computer.

Most diffuser reported NC’s are 5NC below what you should expect. There is an ASHRAE research project that will provide more definitive data, but catalog data is collected with 10 diameters of straight duct, not the typical flex inlet. In addition, it assumes 10dB room attenuation in all bands, which is also unlikely. I recommend adding 5 NC to all manufacturers ceiling diffuser data.

If I can just barely hear a diffuser in an operating environment, it’s about NC=35. Specifications requiring NC 25 or less are typically too conservative, except in very quiet environments. Remember, however, that you add 3NC each time you double the number of similar diffusers when they are all in hearing distance.

75fpm throw is affected by temperature by about 1% /degree delta-t. This rule of thumb allows calculation of jet projections at differing temperatures. Cold air falls further, hot air falls less, compared to isothermal throw, which is what most manufacturers catalog. Along a ceiling warm air travels further.

I’m sure I’ll think of some more. Stay tuned….

Authored by: Dan Int-Hout, Chief Engineer Krueger