Does the heater blow warm air at idle with the engine warmed up? Yes or no?
If yes, check the coolant level in both the the reservoir and radiator with the engine overnight cold, and top up both as required. Then drive the car for several warmup-cooldown cycles. If after a second level check, the radiator is low on coolant, a leak is indicated, which may be a leaking head gasket. The reservoir level may or not change after this. A rise in reservoir level at the second check is a strong indication of a leaking head gasket.
The thermostat should have been replaced in the same orientation as when it came out, that is match the spring location and weep hole position at reassembly.