Flagship Guide · Part 1 of 5

Protecting Your Home from Wildfire Smoke: A Filtration Upgrade Guide for HRV & ERV Owners

A DIY guide to adding automated, multi-stage smoke filtration to your existing ventilation system — for a fraction of what a premium system costs.

Disclosure: This guide contains affiliate links. If you buy through them, I earn a small commission at no extra cost to you. I only link products I'd use in my own install.

Disclaimer: This guide involves modifications to your home's ventilation system and low-voltage/line-voltage wiring. Ductwork changes affect house pressurization, which matters if you have combustion appliances (gas furnace, wood stove, gas water heater). If you're unsure about any step — especially electrical work — hire a licensed electrician or HVAC technician. You are responsible for compliance with local codes.

This guide is in five parts. Part 1 covers the problem and the design. Parts 2 and 3 get you to a specification you can buy. Part 4 is the build. Part 5 is running it. Each part takes about four minutes. If you only read one thing first, make it the self-check tool — it tells you in three steps whether your unit can take this at all.

The Problem

If you have an HRV or ERV, it’s pulling outdoor air into your home 24/7. That’s exactly what you want — until wildfire smoke arrives. Then your ventilation system becomes a smoke delivery system.

The standard advice is “turn off your HRV or ERV during smoke events.” That can reduce smoke entry, but it also reduces outdoor-air ventilation. During a prolonged event, CO2 and other indoor pollutants can build up, so the practical goal is to provide filtered outdoor air rather than treating ventilation as simply on or off.

The better answer: filter the incoming air properly, and automate the switch so your system protects you without babysitting.

Why Your Current Filter Isn’t Enough

Wildfire smoke particles are small — the bulk of the mass sits between 0.1 and 0.7 microns, peaking around 0.3 µm, which happens to be close to the hardest particle size for any filter to catch.

The filter supplied with an HRV or ERV is often primarily intended to protect the equipment, not to provide high-efficiency smoke filtration. The exact filter and efficiency vary by manufacturer and model, so check your unit’s documentation rather than assuming a particular MERV rating.

MERV-13 has a minimum composite efficiency of 50% in the 0.3–1.0 µm test range. That is a rating floor, not a statement that every MERV-13 filter removes only 50% of wildfire smoke. Actual performance depends on the filter and operating conditions. The distinction matters because an HRV/ERV intake is a single-pass application: air passes through the filter once before entering the occupied space, whereas recirculating filtration gets repeated opportunities to remove particles.

And there’s a second problem no particle filter solves: wildfire smoke carries gas-phase pollutants — VOCs, benzene, acrolein, formaldehyde — that sail straight through HEPA. For most of those, you need activated carbon (with one caveat about formaldehyde covered in Part 2).

The Solution: A Filtered Bypass Loop

The design in this guide adds a multi-stage filter bank on a bypass loop around your HRV’s intake duct, with motorized dampers that switch between two modes:

Normal mode: air takes the straight, unrestricted duct run. Your HRV runs at full efficiency with minimal static pressure. This is 95%+ of the year.
Smoke mode: dampers switch, forcing incoming air through a three-stage filter bank — pre-filter → activated carbon → high-efficiency particle filter — before it reaches your HRV. Smoke mode is also designed to run at reduced airflow (more on why in Part 3 — it’s what makes the whole thing feasible).

The switch can be manual (flip a switch) or automated based on a local outdoor air-quality sensor. The damper arrangement can be designed to fail toward filtration on loss of control power: if the relay or control power is lost, the dampers return to the filtered path. Verify the actual damper behavior and the manufacturer’s wiring requirements for your specific components.

The bypass loop. Two dampers decide which path the incoming air takes. The filters sit on the loop rather than in the main duct, so the HRV sees no added resistance the rest of the year — and the bank only accumulates dirt during the events you built it for.

Why filters on the bypass, not the main run? Because smoke mode is the exception. The straight run gives your HRV ideal operating conditions the vast majority of the time, and the filter pressure drop only comes into play when you actually need it. Filter life also improves dramatically since air only flows through the bank during events.

What About Defrost? (If Your Climate Freezes)

If your climate freezes, your HRV/ERV has a frost-control or defrost strategy, and its behavior can matter during a smoke event. The details are manufacturer- and model-specific, so check the sequence for your unit.

Exhaust-only defrost can reduce or stop supply airflow while exhaust continues. When that happens, the house can become negative relative to outdoors, increasing uncontrolled infiltration. During a simultaneous cold-weather and smoke event, that infiltration can bypass your filter. The exact sequence and duration depend on the unit.
Recirculation defrost uses indoor air to protect the core and can avoid the particular negative-pressure mechanism described above. It still does not guarantee zero infiltration or zero smoke entry; follow the manufacturer's sequence.

If you are choosing a new unit for a freezing climate, recirculation-based frost control may be advantageous for smoke-mode operation because it can avoid the negative-pressure period associated with some exhaust-only strategies. Compare the actual defrost sequence in the manufacturer’s documentation rather than choosing on the label alone. The Panasonic and Broan examples discussed here are product-specific and should be checked against current manufacturer documentation before purchase.

If you already own a unit with an exhaust-only strategy, the filtered bypass can still substantially reduce smoke entry during normal operation. During defrost, however, the unit may temporarily change its airflow balance. Your indoor recirculation filtration (Part 5) provides a second line of defense.

Part 2 →Choosing your filters