Use a (thermostat controlled?) flueless gas fire as the heater in a passivhaus.
Upsides: Simple. Visible flame/focal point. Cheaper than central heating. Practically 100% efficient. Low power is OK for this application. Provides humidity!
Downsides: Emissions (esp. particulates). May not be safe/allowed as MHRV does not provide passive ventilation. Still need to provide hot water.
If not, a small balanced flue one may be good, although far less efficient.