Thread by @CorsIAQ

1/ Been hearing of some push back by school districts over fears that fan motors on CR-Boxes may overheat and be a fire hazard. This thread addresses those concerns.

2a/ Resistance on the fan motor was considered in design of the original CR-Box. Clearly, resistance is greater if just one filter is duct-taped to a fan, and I wanted to get away from that simpler concept, even though such a design can be quite effective in right-sized spaces.

2b. Note that the 1-filter on fan design should be worst-case scenario for resistance (assuming similar filters as used in a 4 or 5-filter CR Box), yet Underwriters Lab (UL) tested this design and concluded that under normal and blockage conditions ...

2c. "Overall, all measured temperatures of the fans under the conditions described in this study were lower than the maximum allowable temperatures in UL 507, the safety standard for electric fans. Full report here: chemicalinsights.org/wp-content/upl…

2d. They did indicate that more tests might be completed under extreme temperature conditions, e.g., 40 oC indoor temperature for this worst-case single filter on fan scenario.

3/ With each successive parallel MERV-13 filter, the resistance on the motor decreases, flow rate through the fan increases, and thus effectiveness of the system in terms of clean air delivery rate (CADR) also increases.

4/ With a 4 or 5 filter design the resistance is so low that the flow rate through the fan is VERY close to the flow rate for a box fan with no filters attached. I have done testing of pressure drop across filters for about a dozen CR Boxes (20” x 20” x 2” MERV-13 filters).

5/ For the 4-filter CR-Box the pressure drops ranges from only 4 to 8 Pa (highly consistent results across different 4-filter CR-Boxes) for low flow to high flow settings. These are extremely low pressure drops, folks.

6/ The key to these very low pressure drops (and low resistance on the fan motor) is that incoming air flow is spread across 4 filters in parallel & the filters are pleated leading to a very high surface area. This yields a very low air speed normal to filter surface.

7/ Much lower air speed = much lower pressure drop. In fact, trying to measure the incident air speed on the inlet of the filters is difficult, even with a good hot wire anemometer. It's easy to measure or feel the very high air speeds exiting the CR-Box, but not entering!

8/ That very fact was built into the conceptual design of the CR-Box to maximize flow, effectiveness (as CADR), and minimize resistance on the fan motor.

9/ We have also tested a CR Box after 15 months of operation (over 3,300 hours of actual operation) with little change in the pressure drop or effectiveness of the system.

10/ Now, back to resistance on the fan motor when it is hot inside. As stated above, resistance on the motor was included in the original CR-Box design and is one reason that multiple parallel filters were selected.

11/ People do use fans during heat waves to stay cool and the CR Box with a 4 or 5-filter design exerts a very small additional resistance on the fan motor.

12/ So, I don’t see it being all that different than someone just using a fan to keep cool, which I don’t think we would want to discourage.

13/ Further, people have been using CR Boxes in hot climates, particularly from India to the Philippines and Mexico for the past three years with no fire-related incidents that I am aware.

14/ Having said that, it is important to use new/newer fans when building CR-Boxes. Avoid the old one found in your parent's or grandparent's garage, e.g., those over 10 years old.

15/ I think it's also important that we test and report results for extreme conditions to provide greater confidence and acceptance of the CR-Box. So, hoping others will test in environmental chambers to 35-40 oC (heat wave conditions with power outage).

16/ Measurement motor temperature & we will do the same. Report!