If your window is leaking and letting in heat, it may be because it is not properly adjusted. A simple adjustment of the window hardware, especially the locking pins (also known as eccentric pins), can often quickly solve the problem.

What you will need:
A suitable Allen key (usually size 4)
Or a wrench or pliers if the pins are shaped differently
Detailed instructions
Open the window fully. You will then see the locking pins on the side of the sash. These small metal pins slide into brackets on the frame when closed and press the window firmly.

Look for the pins on the edge of the window. There are usually several of them, often on the side, and sometimes on the top or bottom.

Adjust the seal. Most modern windows have eccentric (off-center) locking pins. This means they are oval or round with a small notch.

To increase the seal (to press the window harder), turn the pin with an Allen key so that the notch points inward, toward the window frame.
To decrease the seal (if the window is difficult to close), turn the pin outward.
Check the result. Close the window and test the seal. If you feel a draft, you may need to adjust the pins further.
Additional tips

Adjust window hinges: Sometimes the problem is not the seal, but the window hanging at an angle. The hinges at the top and bottom of the frame can also be adjusted with an Allen key to correct height or side play.
Check the seals: If the seals are porous, cracked, or brittle, even the best adjustment will not help. In this case, the rubber seals will need to be replaced.
Maintenance: Oiling the hardware and seals occasionally will ensure that the window stays smooth and the adjustment lasts a long time.
With these steps, you can quickly fix most window leaks and keep your home warm.