Thread by @joeyfox85

PUTTING MERV-13 FILTERS IN AIR HANDLING UNITS 🧵

Many schools have replaced the standard MERV-7 or 8 with MERV-13 filters in the ventilation systems. It's a simple method to filter viruses from recirculated air, but doesn't always work. Here are some issues to consider:

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MERV-13 filters are efficient at filtering out aerosols that contain viruses unlike MERV-8, but they also create a greater pressure drop and restrict flow. This is why ASHRAE recommends units to be rebalanced when filters are upgraded:

ashrae.org/technical-reso…

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What happens if this isn't done? Air handling units could have 2 purposes: ventilation only or ventilation + temperature control.

If it's ventilation only, you will be providing less fresh air to the space. There will still probably be a net benefit, but this isn't ideal.

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If the air handling unit is used for ventilation + temperature control (like most roof top units), then you can have a problem with insufficient heating/cooling on very cold and hot days - just like if you restrict the flow to your furnace by not changing the filter.

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There's a possibility of gas heating or DX cooling coils getting damaged without sufficient airflow.

Some ventilation systems provide 100% outdoor air. Using a MERV-13 filter here will not help mitigate transmission. You only need to filter recirculated air for viruses.

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Switching from MERV-7/8 to MERV-13 is very easy, but I'm skeptical most schools rebalanced their systems. If a school board tells you they switched to MERV-13 as an excuse to not do anything else - ask if they rebalanced the system to ensure flow wasn't restricted.

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