It is now late winter here, and the sun is gaining strength, climbing higher in the sky, and days are getting longer.
BUT, it is still winter, snow is still on the ground, and the average March will still get 7 inches ( 180mm) of snow in our region.
We are fortunate to have a large sunroom along the south face of our house, gaining warmth, and a good place to start garden seeds.
When we have sunny days, and when it is warm enough in the sunroom, we open the sliding door, and let warm air enter to replace burning wood for the day.
How to know when it is warmer in the sunroom than the house? I have a very sensitive temperature differential sensor, and when it signals good conditions, we open the door, and monitor the sensor to know when to close the door in the late afternoon.
It's a bit low tech. I taped a small length of black thread to the top of the door jamb, and if the thread starts being blown in to the house, I know that warm are is moving in to the house in the upper half of the doorway, and relatively cooler air is flowing from the house out in to the sunroom in the lower half of the doorway. When the thread is no longer moving, or even showing flow out in to the sun room, it's time to close the door.
Yay for the buoyancy of warm air.
hopefully you can see the thread in this shot, showing warm air entering the house.