Warren Technology Warren Technology

  Air filtration is one of the most important components of Indoor Air Quality (IAQ), yet may be one of the least understood areas of the HVAC industry. Every building's air filtration objective should be to remove as many harmful airborne contaminates at the lowest possible material and fan energy costs. Most building owners and facility managers recognize the need for more efficient filtration to remove sub-micron particles in addition to the dust filters they typically have in their present systems.
  Why Sub-micron Filtration?
  Most harmful airborne particles are smaller than one micron in size. Most HVAC central fan filtration systems do not remove these particles because the filters used are only capable of removing larger particles such as dust. Currently, without the application of Uni-Guard technology, in order to remove sub-micron particles, special high efficiency sub-micron filters are usually installed in place of, or in addition to, the existing dust filter(s) at the inlet of the central fan. Because of their smaller air passages which are necessary to collect sub-micron particles, these filters have larger air pressure losses than the conventional dust filters. Therefore more fan horsepower is required to provide for this increased pressure loss. Also the sub-micron filters are usually larger in size. Therefore special framing and/or additional space is required to accommodate them. The costs to retrofit systems, system down time, and the high operating costs (increased fan horsepower) as described the above often make this approach undesirable. Uni-Guard overcomes these problems through its unique design, and also increases the overall system effectiveness.
  Overall System Filtration Effectiveness
  The ability of a complete filtration system (which includes not only the filters, but the air moving fan, duct work, and air distribution devices) to remove particulates is the principle critical concern in the design of a central fan filtration system. Due to the inadequacies of current (without Uni-Guard) system's air circulation to "sweep" the contaminates to the filter(s), and/or the proper placement of the filters in the system, the actual overall system filtration effectiveness is seldom equal to the individual efficiency rating of the filters used in the system. The individual filter efficiency rating is only one of many factors which must be considered to determine the overall system filtration effectiveness. With a given quantity of re-circulated air flow in a central fan system and all other variables held equal, the overall system filtration effectiveness is increased with:
  1. Higher individual filter efficiency rating of the type of filter(s) used

  2. Greater quantity of filters used

  3. Decreased face velocity of air entering the filter(s)

  4. The effectiveness of the re-circulating system to "sweep up" and transport the contaminates to the filters

Only Uni-Guard filter-diffuser units are designed to take all of the above into account by making it possible for a system to use a large number of low cost high efficiency sub-micron filters with low face velocities, and also provide high performance air diffusion to "sweep" the contaminants to the filters. With Uni-Guard, the low pressure loss (a result of the low face velocity through the filters) reduces fan energy cost when compared to conventional sub-micron filtration systems.



Filter Type Mini-Pleat Rigid Cell Mini-Pleat
Filter Cost $30 $80 $30
Filter Size - W x H (in) 24 x 24 24 x 24 24 x 24
Filter Depth (in) 2 10 2
Quantity of Filters in System 5 5 50
Modifications to AHU Unit - Required NO YES NO
Larger Fan Motor H.P.- Required YES YES NO
CFM / Filter 2,000 2,000 200
Filter Face Velocity (fpm) * 500 500 50
MERV Rating @ Face Velocity 14 16 16
Sub-Micron Filter Efficiency ** 85% 95% 95%
Avg Filter Resistance (in.W.C.) 1.1 1 0.06
Fan Energy Cost per Year - Increased $1,425 $1,320 $90
Filter Total Cost $150 $400 $1,500
Filter Life in Months 4 6 30
Filter Cost per Year $450 $533 $600
Total Increased Cost per Year $2,875 $2,120 $690
* Denoted KEY FACTOR for Increased Efficiency at Reduced Operating Costs
** ASHRAE Standard 52.2 Range No. 1 (0.3 to 1.0 microns)


1. All systems have MERV 2 (dust) pre-filter at fan inlet.
2. Operating costs are based on 24 hr. / 7 days / 365 days of year operation, $0.80 per KWH, motor-fan-drive efficiency = 58%
  Occupant Thermal Comfort
  Thermal comfort provided by room air diffuser devices is also an important component of IAQ, and is sometimes overlooked in the design of HVAC systems. In addition to enhancing overall system filtration effectiveness by high performance air diffusion, Uni-Guard filter-diffuser units provide draft-free uniform air motion within the space which allows the occupants to "feel" comfortable at higher thermostat settings, thus providing energy savings. A one degree F difference in thermostat setting can result in as much as 10% cooling energy savings.
  How Uni-Guard Works
  The unique design of Uni-Guard filter-diffuser units maximizes the positives and eliminates the negatives as described above. Fan energy costs of sub-micron filters is reduced by low air face velocity through the filters. Therefore increased fan horsepower is, in most cases, not required. Because the space where the filter-diffuser units are located (ceilings) is virtually unlimited, more filters may be used than in conventional systems to increase the overall system filtration effectiveness. This eliminates the requirement for additional space at the central fan location or special framing. After the air has been filtered by the Uni-Guard filter-diffuser units, the air velocity is increased by evenly spaced diffuser outlet jets to provide high performance air diffusion necessary to enhance overall system filtration effectiveness, and draft-free energy efficient thermal comfort. Conventional dust filters at the inlet of the central fan are retained in the system to ensure extended life of the sub-micron filters (located downstream) and to protect the equipment (fan, cooling and heating coils, etc.) in the system.
  Individual Filter Ratings
  Individual filter efficiency ratings vary depending on various methods of testing and only indicate a given filter's efficiency when tested under certain ideal conditions. Because of the many ways a filter can be applied in systems, often less than ideal, these ratings cannot indicate overall system filtration effectiveness; however, individual filter efficiency ratings can be used to compare the relative effectiveness of filters in any given application. The current and only standard that concerns sub-micron filtration (other that HEPA) is ASHRAE Standard 52.2 1999 which comprehensively rates a filter's ability, under certain conditions, to remove particulates over the entire range of sizes, including the harmful smaller sub-micron particles. This rating method is termed the Minimum Efficiency Reporting Value (MERV). MERV ratings range from 1 to 20 (see Table 2). Uni-Guard filter-diffusers accommodate any MERV rated filter.
Table 2. Minimum Efficiency Reporting Value (MERV) Parameters

Composite Average Particle Size Efficiency % in Size Range

  Range #1 Range #2 Range #3
MERV 0.30-1.0 1.0 - 3.0 3.0 - 10.0
1 n/a n/a < 20% > 10 residential,
light pollens,
dust mites
2 n/a n/a < 20%
3 n/a n/a < 20%
4 n/a n/a < 20%
5 n/a n/a 20 - 35% 3.0 - 10 industrial,
6 n/a n/a 35 - 50%
7 n/a n/a 50 - 70%
8 n/a n/a > 70%
9 n/a < 50% > 85% 1.0 - 3.0 industrial,
10 n/a 50 - 65% > 85%
11 n/a 65 - 80% > 85%
12 n/a > 80% > 90%
13 < 75% > 90% > 90% 0.3 - 1.0 hospitals,
smoke removal,
14 75 - 85% > 90% > 90%
15 85 - 95% > 90% > 90%
16 > 95% > 95% > 90%
17 >= 99.97%     < 0.3 HEPA clean rooms,
18 >=99.99%    
19 >= 99.999%     < 0.3 ULPA
20 > 99.9999%    
This table shows the composite average particle efficiency (%) by size range, of the three size ranges, for ASHRAE Standard 52.2 minimum Efficiency Reporting Value (MERV).

In addition to using the MERV number as a general guideline, exact data can be taken from test reports of a filter that has been tested to ASHRAE Standard 52.2. This table can be used to choose the proper filter for a specific application.
  Most existing central fan systems may be upgraded with Uni-Guard. Some examples are as follows:
  1. Constant-volume, low-efficiency filtration systems are the most common type of systems and can be upgraded with Uni-Guard to sub-micron filtration efficiency with little or no increase in operating costs. In some cases where a system may need to be re-balanced, there is only one diffuser that determines the critical pressure loss path and the required fan horsepower for the system. In all other, noncritical paths (run-out ducts to diffusers in the system), the balancing dampers are partially closed to create pressure drops to those diffusers to achieve the required air flow. These dampers can be adjusted towards the open position to offset the small additional pressure loss of each sub-micron filter in the filter-diffuser unit.

  2. Conventional constant-volume sub-micron filtration systems, where the filters are located at the central fan inlet, can be retrofitted to substantially reduce operating costs (fan horsepower) by eliminating the high- pressure loss, high-efficiency filters (bag filters, etc.) and replacing them with low-efficiency, low-pressure loss dust filters. Overall system filtration effectiveness will be increased by the use of Uni-Guard filter-diffuser units with sub-micron filters which cost less than conventional filters with equivalent efficiency. Energy savings are proportional to the pressure loss of the existing conventional sub-micron filtration system, which is typically 1.0 in. WG (average), compared to a Uni-Guard system where the pressure loss is typically less than 0.1 in. WG (average).

  3. VAV systems using constant volume fan terminal units are suitable for Uni-Guard systems because, in most cases, only the primary air is filtered. Therefore, the amount of filtered air to the room is reduced in proportion to the volume of primary air delivered to the room(s) at any given time. This often results in room filtration levels which are less than 10% of the primary air system filtration level. For example, if 30% filters are installed in the primary air system, then only (0.1 X 0.3) = 3% room filtration efficiency can be expected during light cooling or heating modes. By using Uni-Guard, both primary and return air passes through the sub-micron filters regardless of the primary air volume rate.

  4. VAV systems (single duct) are particularly suitable for Uni-Guard because the filtration efficiency, in fact, increases due to lower face velocity of the secondary air through the terminal unit and filter when the primary air volume is reduced.

  Protection Against Bio-terrorism Attacks
Create a Safe Room

Individual "safe rooms" can be created in any existing building with the use of Uni-Guard at minimal cost and inconvenience. Central fan systems by, their very nature, provide a devastatingly efficient distribution method for bio-terrorist attacks. Pathogens placed in the return air or outside air openings can be spread through an entire building in a matter of seconds. Uni-Guard units with MERV 16 or HEPA filters can provide protection to the room(s) against bacteriological pathogens such as anthrax. Uni-Guard units with ULPA filters provide protection against viral pathogens as well as biological pathogens.


Installation of Uni-Guard ceiling filter-diffuser units is simple. Old conventional ceiling diffusers are removed and replaced by Uni-Guard units in a matter of minutes. No architectural or duct work modifications are required. The units may be installed one at a time to suit budget or time constraints without having to shut the system down. The pressure loss of sub-micron filters when used in Uni-Guard systems is low, and often is no more than the pressure loss of the existing conventional diffuser/damper assembly. For this reason, re-balancing is seldom required when retrofitting a system with Uni-Guard. Most existing systems have provisions to increase the fan speed with the use of adjustable pulleys or fan speed controllers. These may be adjusted to allow for any small increase in static pressure that may be needed for Uni-Guard sub-micron filters. The AHU fan speed is simply increased in proportion to the increased static pressure requirement. For example, if the maximum (critical path) increase in static pressure is 0.06 in. WG, and the existing external fan static pressure is 1.5 in. WG, the fan speed will only be increased approximately 2% (fan law).

If selective (partial) Uni-Guard filtration is used, or in Uni-Guard systems containing filters with various different efficiencies and pressure losses, then air re-balancing may be required for optimum results. However, re-balancing can be minimized by simply measuring the existing velocity pressure at each run-out duct to be retrofitted with a Uni-Guard unit. After the unit is installed, the balancing damper can readjusted to duplicate that same value necessary to produce the original air flow.

Installations are often performed by building maintenance staffs; however, Warren Technology offers turnkey installation in most areas nationwide.

  Model Selection

Uni-Guard Filter-Diffuser units are comprised of three elements: 1) plenum casing, 2) filter, and 3) diffuser plate.

1. Select casing type:
A. Flush mount (T-bar ceilings)
B. Surface mount (T-bar, gypsum, or plastered ceilings)
2. Select filter type:
A. Sub-micron filters ( ASHRAE MERV 12-16) are recommended for general applications. Tests have shown that sub-micron filters in re- circulated air systems will produce system filtration effectiveness which approaches HEPA levels. It is generally agreed by experts that the additional expense for HEPA filters cannot be justified in many commercial applications.
B. HEPA filters are recommended for health facilities where immuno-compromised patients cannot tolerate even small amounts of bacterial or fungal contamination and non re-circulated air is utilized.
3. Select diffuser plate configuration:
A. Air flow capacity
  1. Low - 180 cfm nominal
  2. Medium - 300 cfm nominal
  3. High - 450 cfm nominal
B. Air flow patterns
  1.  8-way throw (standard)
  2.  5, 3, 2, or 1-way throw (options)
Special models are also available. Consult Factory for details.
  Filter Costs and Filter Life Expectancy

The cost of sub-micron filters are typically less than conventional high-efficiency sub-micron filters, such as bag type, or 10" deep pleated filters. In a Uni-Guard system, for a given contamination rate, the overall system filtration effectiveness is greater for all types of filters and their useful life is longer than conventional systems because there is up to ten times more filter surface area to collect particulates. Therefore, for a given environmental application, the filters in a Uni-Guard system will last longer than filters of equal performance, used in a conventional system. In a typical Uni-Guard system, the sub-micron filter life is approximately 24 to 30 months, and the dust pre-filter life is approximately 2 to 3 months.

  System Design

Filtration system analysis and design assistance is available from Warren Technology. Computer analysis of existing systems and recommendations for up-grades, including life cycle costing, is available.

  Uni-Guard is a revolutionary concept that provides superior sub-micron air filtration and draft free thermal comfort at lower costs than conventional systems. The concept can be applied to a single room or zone(s) within a building or to an entire building. Uni-Guard is simple to install in most central fan systems in existing and new buildings.
Occupants will  SEE and FEEL the difference.
You may also measure the difference immediately after installation with a Laser Particle Counter available from Warren Technology (sale or rent with refundable deposit). Contact factory for details.
See Uni-Guard in action!
• Introduction
• Why Sub-micron Filtration?
• Overall Filtration Effectiveness
• Occupant Thermal Comfort
• How Uni-Guard Works
• Individual Filter Ratings
• Applications
• Safe Rooms
  Bio-attack Protection
• Installation
• Model Selection
• Filter Costs and Life Expectancy
• System Design
• Summary
Case Studies & Testimonials
Specifications & Drawings
NIOSH Guidance for Filtration
and Air-Cleaning Systems to Protect Building Environments