Dragos,
I expect your issue may be that the supply air temperature is equal to the heating set point. The HVAC system will need to supply warmer air to the space to offset the heat loss through the walls. A supply air temperature of about 30 C is common for heat pump or condensing furnace systems, though lower temperatures may be possible with a very well insulated house.
You may also choose to keep the ventilation system neutral and heat the space directly, for example with a radiant floor system.
Ben Heymer, PE