My furnace was running on and on and not keeping up with the thermostat so I called Northern Air Systems. The gentleman who responded did so within an hour or so and after some preliminary discussion and inspection of my furnace he was able to troubleshoot why this was happening. To fix it required a mere filter change. He explained the situation as since we were using tap water in our newly purchased humidifier, it caused mineral deposits to be collected by the filter to a point of restricted airflow which caused the furnace to "over heat" and trigger it to shut down. Once it cooled it would start right up again. He changed the filter and I've had no further issues since. The cost was in line with the services provided.