HVAC Systems, Ventilation, Filtration & COVID-19

As we approach two years of dealing with travel restrictions, shutdowns, zoom meetings, health concerns, masks, sickness, deaths, etc., it is an appropriate time to look closely at the buildings we live in and how building design, building codes, and specifically HVAC design and operation impacts the spread of indoor pollutants, viruses, bacteria, and moisture. While this blog post is primarily based on residential buildings, indoor air quality in commercial buildings is equally important. This post will identify several factors to be considered in buildings.

Mechanical ventilation as defined in the Adopted Codes and ASHRAE Standards is a key component to improving indoor air quality. A few important definitions include:

VENTILATION

Defined as; The natural or mechanical process of supply conditioned or unconditioned air to, or removing such air from, any space.

Ventilating a space is accomplished by removing a portion of the air contained within a building and replacing it with fresh air from outside. Through ventilation fresh air replaces air contaminated with smoke, odors, allergens, carbon dioxide, moisture, mold, viruses, bacteria, and other pollutants. We might do this with natural ventilation, by opening windows and doors to allow air to move in and out of the building; or with mechanical ventilation, where one or more fans is used to exhaust air out and/or supply fresh outside air into the building. The purpose of ventilation is to maintain air quality. The residential mechanical code allows use of any of the following strategies:

• • • Exhaust fan, running continuously, removing air, which is replaced by air leaking into the building.
    • Supply fan, either part of the HVAC system or an independent fan, drawing air from outside, discharging into the building, creating a positive pressure, which results in the excess air leaking to outside.
    • Balanced ventilation, with a supply fan and an exhaust fan, removing and replacing air, not relying on air leakage. These systems include HRV/ERV systems, which recover heat energy from the discharge air. Where buildings are constructed extremely tight, with ICFs, SIPS, or extensive use of closed cell spray polyurethane foam, or other advanced air sealing techniques, balanced ventilation is required as air leakage is limited.

If the IECC (Energy Code) was not amended in Utah back in 2012, continuing into the current 2015 residential energy code, every home in all twenty-nine counties in the state would include continuous mechanical ventilation. Building science clearly supports the critical need for mechanical ventilation in new homes and other buildings. Legislation may modify the code requirement; however, it does not change the science. It is time to implement mechanical ventilation in every building constructed moving forward.

The design of the ventilation system may include high efficacy filtering of the outside air as it is drawn into the home, and the filtering of indoor air circulated within the home by the HVAC system.

The next question might be what is the CFM requirement for a home? The 2015 International Residential Code provides direction based on the square footage of the house and the number of bedrooms, as detailed below:

It is assumed one occupant per bedroom, plus one for the master bedroom. For homes where it’s known to have more than one occupant in other bedrooms, an adjustment should be made.

AIR FILTRATION

Defined as; A device that removes particles, mist, and other contaminants from the air.

Air filter efficacy is commonly rated based on the Minimum Efficiency Reporting Values or MERV rating, developed by ASHRAE. The higher the number, the better the filter is at trapping specific types of particles. MERV filters ≥ 13 are considered efficient in capturing airborne viruses.

As filters with a MERV 13 rating or better are effective in capturing viruses, many homeowners may be replacing their filters with these ‘better’ filters. Unfortunately, a 1” pleated MERV 13 filter is typically too restrictive for airflows greater than 800 CFM, reducing airflow to the point coils might freeze, AC efficiency is reduced, comfort is decreased, and equipment life is jeopardized.

A photo was recently taken at a local home center of a package of 1” pleated high efficacy filter. These filters will effectively remove contaminants, but with a serious negative consequence, greatly reducing airflow.

Flipping the filter over to the back side (photo below), we find the initial resistance (often also referred to as pressure drop) listed. Notice the resistance at 820 CFM is 0.21” wc. and at 1390 CFM, 0.44” wc. The typical ACCA/ANSI Manual D friction rate worksheet submitted for review in Utah is designed with a 0.10 to 0.15” wc. filter resistance/pressure drop. Filter resistance/pressure drops higher than 0.20” typically do not leave sufficient available static for proper airflow through the evaporator coil, duct system, registers, and grills. These 1” pleated filters may work effectively with smaller 1.5 & 2.0-ton systems, with airflow ≤ 800 CFM, but not for higher airflow conditions. Homeowners should contact a knowledgeable HVAC contractor/service technician for guidance on the use of 1- and 2-inch pleated filters. 

There are many air filter products available with low pressure drops, rated at MERV 13 levels or higher. This is accomplished using pleated filters 4” or greater in thickness, providing a larger surface area, hence a lower pressure drop across the filter. The concept behind pleated filters is creating a larger surface area. If a 1-inch pleated filter is pulled apart, with the media spread out flat, you will have a flat sheet of filter media, significantly larger than the filter size. The larger surface area, the lower the resistance to airflow. If you take an identical sized 4-inch-thick pleated filter apart, laying it out flat, the surface area will be 4 times larger than to 1-inch pleated filter. An example of a thicker is a Aprilaire Model 213 filter with less resistance/lower pressure drop as indicated in the chart below. Notice the pressure drop or resistance at 1400 CFM is 0.20” wc., while the 1” pleated filter has a pressure drop or resistance of 0.44 at 1400 CFM – more than double that of the thicker filter. Most HVAC system manufacturer’s offer similar large pleated and other types of high efficacy filters. Please see a HVAC design professional for further direction on high efficacy filters.

CONTINUOUS VENTILATION

Finally, if the fan in a ventilation system (including HVAC system integral with the ventilation system) is not operating continuously, ventilation is not provided, and air filtering does not occur. Recent Federal Standards require ECM (high efficacy) motors in all whole house ventilation systems, whether it is a furnace, air handler, or continuously operating ventilation system. The Utah Energy Code Quick Guide includes the following table defining ventilation fan efficacy requirements.

This Table was proposed and approved for the 2018 IECC and IRC codes to ensure the operational cost for a continuously operating ventilation system is low. The 2019 Utah Legislature wisely adopted it as an amendment to the current 2015 IRC. For comparison, the standard budget 50 CFM fan installed for many years has an efficacy rating of 0.4 CFM per watt, almost 4 times the energy consumption per CFM.

Summarizing:

Improvements in building practices and the minimum requirements of the code have reduced the natural infiltration rates through building envelopes. With these tighter buildings and an increased understanding in building science, the following items should be considered when air leakage is reduced:

1. 1. Improved occupant comfort by eliminating drafts, especially when extreme temperatures occur or high winds are experienced.
   2. Lower heating and cooling expenses.
   3. Smaller heating and cooling loads, reducing equipment sizing requirements
   4. Build tight and ventilate right – introducing the right amount of ventilation air, but not too much.
   5. A mechanically ventilated building reduces pollutants, viruses, bacteria, and excess humidity in the home.
   6. If the 2015 IRC was adopted without amendments, every new home built in Utah would already include continuous mechanical ventilation. Each of those homes would enjoy cleaner, healthier air, and improved comfort.
   7. Air filtration with MERV 13 or higher filters are very effective in removing undesirable contaminant in a home; however, such filter system must be designed and installed by professionals experienced in duct design.

This discussion has briefly touched on the subjects. Please feel free to reach out if you have questions or desire further discussion.

– Dr. Energy