unit heater cold

Jack_6

I recently purchased a small unit heater to heat my basement but the unit never really stays hot. The problem is that the unit intially gets hot, but starts blowing cold until the steam vent cools enough to open and the cycle repeats. What do you suppose I do? When I unscrew the steam vent and loosen it to have a constant leak the unit heater stays hot, but I am sure that is not the correct solution. The unit is piped with steam entering from the top, I installed the steam vent in a tee on the bottom return side. Do them make any thermostaticaly controlled steam vents that I can clip to the unit heaters fins?