ERV vs HRV vs air exchanger

Do not run your HRV or ERV off its humidistat if you have a whole-house humidifier

Cold-winter homes in most of the country often run a whole-house humidifier to fight dry air, so this rule matters here. Most ventilators have a humidity control called a humidistat or dehumidistat (some labels say “RH”). When indoor humidity rises above its setpoint, it kicks the ventilator to high speed to exhaust humid indoor air and bring in drier outdoor air. NRCan and Lifebreath both describe it exactly that way. It is a useful feature in a home that has no humidifier. In a home that does, the two controls work against each other:

1. They chase each other in a loop. The humidifier adds moisture until the house reaches its setpoint. The ventilator’s humidity control sees that humidity and goes to high speed, exports the moisture you just paid to add, and pulls in dry outdoor air. The house drops below the humidifier’s setpoint, and the humidifier starts again.
2. You pay for the same water twice. Humidifiers use substantial energy to evaporate water, and running the fan on high adds fan energy and more incoming air to heat. The core recovers only part of that heat, and you also draw more mineral scale through the humidifier.
3. Humidity is the wrong signal for fresh air. A humidity reading says nothing about carbon dioxide, cooking fumes, VOCs or stale air. The ventilator can sit at low speed for days when humidity looks fine while the air quality slips. Code ventilation rates are set by house size and bedrooms, not by humidity.
4. Cold weather makes it worse. Humidified indoor air carries more moisture into the core, and in freezing weather that moisture is what turns to frost on it, so you can trigger more defrost cycles and lose fresh airflow when you need it.

What to do instead

• Run the ventilator on its own schedule: continuous low speed, or a timer or intermittent mode that delivers the airflow your home needs (see the formula above).
• Turn the ventilator’s humidity boost to its highest setting or to off. The exact label varies, so check your manual.
• Let the humidifier’s own humidistat hold the indoor humidity, ideally one that adjusts to the outdoor temperature. Lower the target as it gets colder, and if you see moisture on the windows the target is too high.
• If you are humidifying constantly, consider an ERV. A good ERV returns part of the moisture in the exhaust air, and one building-science discussion points out that this can save much of the energy a humidifier would burn.
• If you do not have a humidifier and your winter problem is too much moisture, the ventilator’s humidity control is helpful. The colder the outdoor air, the more moisture a ventilator can shed.
• In summer, neither an HRV nor an ERV is a dehumidifier, and Lifebreath’s FAQ says so plainly.

This explains how the controls interact, based on how manufacturers describe the humidity function. It is not a manufacturer prohibition, so check your own manual for the exact wording on your model.