We have a gas furnace that is acting up. When you turn it on, the heat just keeps running. If I flip the switch on the thermostat to off, the heat turns off, but the fan stays on until I shut the system off entirely. We replaced the thermostat earlier this year because the a/c was doing the same thing, and that seemed to have solved the problem. The first night we ran the furnace we didn't have this issue, but it started within about 12 hours and has continued ever since. The a/c still works perfectly, and the thermostat still clicks and turns off the "heat" icon when the correct temperature is reached, but the furnace doesn't respond. If at all possible, we'd really like to be able to fix this ourselves, if anyone has any idea what it could be?