My Furnace lites and blows and acts as though it's running fine, but the heat won't go beyond about 65 degrees, even though it's set for 70. It runs all the time. We increase the gas usage to get it to run hotter but that hasn't helped. (Coldest weather in Washngton State in years, of course) Any ideas? I'm thinking we should replace the thermostat even though it seems fine.