HVAC FAQs

A: Make sure to buy a HEPA filter certified by the state of California and by AHAM. Specifically, see some filter recommendations from CU Professor Shelly Miller. Avoid plasmas and ionizers. Feel free to email Paulo Tabares if you have any questions: tabares@mines.edu.

A: It is important that you buy a portable HEPA filter that is rated for a similar space to where it will be located. For example, don’t buy a portable HEPA filter rated for a 100-square-foot room if you are going to install it in a 300-square-foot room. Why? It is important that all the air in your office/room goes to the filter at least twice an hour (or 2 ACH).

A: Charcoal filters are typically installed as a prefilter to capture larger particles and gases in the space. It extends the lifetime of your HEPA filter, but the HEPA filter is the system that is filtering small particles. While they are not required, they extend the lifetime of the HEPA filter.

A: A portable HEPA filter will add some background noise, but most portable HEPA filters have 1-3 speeds that you can vary. The added noise should not be loud enough to impact your work.

A: AHAM seal/certification provides a clean air delivery rate (CADR) that characterizes the volume of filtered air that a specific air cleaner can deliver. CADR is calculated and reported by three different particles sizes: smoke (smallest), dust (smaller) and pollen (small). The CADR most relevant for COVID-19 are smoke and dust. This also helps you size your filter, by using the AHAM 2/3 rule: Size your unit using a CADR at least 2/3 your room’s area. So, if your room is 10 feet by 10 feet (and 8 feet tall), find a filter with a CADR of 67 or higher. The AHAM seal also includes recommended room size.

A: Yes, however, it is important to mention that vaccination is the first action we all must take. Building heating, ventilation and air conditioning (HVAC) have a role in aerosol transmission by disturbing their transmission pathways inside buildings. HVAC systems can help dilute and filter these aerosols to some extent.

A: What building HVAC systems can do is to reduce the aerosol load (amount of virus in the air), which in turns reduces exposure, thus reducing the overall risk of getting sick. This can be done by the following methods:

1. Ventilation: Supply cleaner, outdoor air with the main goal to dilute indoor air contaminants and aerosol load. This could be done by simply opening a window or by the mechanical system, using fans to bring more outdoor air to the indoors.
2. Filtration: Use filters that can be either in rooms or inside HVAC systems (air handling units) to filter out particulates. ASHRAE recommends using filters rated MERV (Minimum Efficiency Reporting Value) 13 or higher or using HEPA filters (best in class). HEPA filters can block particulates as small as 0.01 microns. MERV 13 is not as efficient as HEPA but combined with other measures, can be very effective.  Most existing HVAC systems cannot be retrofitted with HEPA filters due to size and the resulting restriction of airflow.
3. Filtration with high flow rates to ensure air passes to MERV13 filters at least 4 times an hour.

A: HVAC stands for heating, ventilation and air conditioning. These systems process outdoor air so we can be comfortable inside. It consists of fans, pumps, heat exchangers, dampers, filters, smart controls and many other subcomponents that add heat (warm up) or moisture (humidify), remove heat (cool down) and/or moisture (dehumidify), and remove contaminants from the air before the air enters our buildings. In many buildings around campus, most of these subcomponents are inside huge boxes called air handing units (AHU) that you might see on top of our buildings.

A: Facilities and the COVID HVAC Committee have been following guidelines from the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). ASHRAE has developed a taskforce consisting of building, HVAC, and indoor air experts that are keeping track of (and also publishing) the latest research on SARS-CoV-2. Some of these members have been in major news outlets and authored scientific articles about HVAC, indoor air, and SARS-CoV-2.

At Mines, Professors Paulo Tabares, Alina Handorean and Mahadevan Ganesh have developed a data collection and probability evaluation framework that facilitates obtaining and processing data from Mines’ HVAC building automation system. This, combined with a room database, provides an environment for modeling probability of potential aerosol infection. We will continue monitoring HVAC system utilizing this framework.

A: Yes, opening your window will allow outdoor air to get into your office, thus improving ventilation. Some offices and dorms around campus can do that, and you can certainly do this. However, we just ask that you close your window when you leave your office/dorm. Leaving an open window unattended can let rain/snow inside and in winter produce cold drafts or even freeze some pipes which can cause major problems.

A: For most buildings, leaving a classroom door open is not recommended and in some cases may be against fire code. Also, more fresh ventilation air is provided to classrooms than to transient spaces like hallways, so the effect of leaving the door open might be to actually get less ventilation air in the classroom.

A: While HEPA filters are best in class, it takes a lot of energy (and huge fans) for air to pass through them, as there is a significant pressure drop across them, which fans must overcome. Most building HVAC fans were not designed to handle this pressure drop, so using HEPA filters will substantially reduce airflow in buildings, reduce the capacity of HVAC to dilute contaminants and overall increase risk. Also, in a lot of cases HEPA filters simply won’t fit inside the air handling unit.

A: No. If you are sick, you should notify your supervisor and stay at home.