During the daytime, the thermostat for the heating unit works just fine. But at night, without touching the thermostat, the temperature creeps up. Why does it do this?

Is it a programable thermostat? Whats the temp diff your talking about 1 or 2 degrees or more?

It is about 3 to 5 degrees. The temperature outside is about 52 degrees now and the temperature inside was bout 74 when we turned the unit off. The thermostat was set at about 68.

If its a older mercury bulb thermostat you can set the heat anticipator on it and it may help. If you remove the front cover of the T-stat you will see scale with a arrow pointing at the numbers move it to the smaller side of the scale(shorter cycle). Make sure you do not have a large hole behind the T-stat where it mounts on the wall, it may throw the temp reading off once it cools off out side from cold air penetration on the inside of the wall.

we hope the stat isn`t mounted to the inside of an outside wall.,will cause stats to go crazy,most of the time