A direct air connection for a woodstove is often framed in the context of thermal efficiency. The idea is that the chimney isn’t sucking warm air out of your house and drawing cold air in. It’s a logical idea but it doesn’t really work in an airtight building unless you have an energy recovery ventilator.
Whether it’s a fan or an open window every small living space needs some amount of airflow to keep CO2 levels down. From this perspective the draft of the woodstove is actually assisting the ventilation system and if you remove that draft you have to increase the ventilation rate by an equal amount to keep CO2 levels in check. So there’s no real thermal gain, but thermal performance isn’t the only reason to have direct air. What you do get is more control of your ventilation system. This reduces the number of passive vents that need to be installed AND the potential for smoke backdrafting into the living space when fans are turned on.
To get thermal gains from a direct air feed the stove needs to be paired with an energy recovery ventilator, which balances intake and exhaust air and recovers some of the heat by mixing the flows in an energy exchanging core. The only caveat to this strategy is that the most common tiny house energy recovery ventilator, the Panasonic Whisper Comfort, has a feature that shuts down the intake side of the fan to prevent the core from freezing below freezing temperatures. If this happens while you have a woodstove going regardless of where the air is coming from it will suck smoke into the living space, possibly while you were sleeping, possibly killing you. For this reason I recommend that anyone living with a woodstove install at least some direct air vents (even with an ERV), and also a carbon monoxide detector.
If we were just going to be a passive or fan vented space I could’ve gone either way on the direct air, but installing it gives more control and the opportunity to experiment with ERVs. Also, I just like tinkering with stove parts and Tiny Woodstove makes such nice stuff!
-Brian
