Thanks 波大！回複見內。

Fan Limit Switch is configurable. 

Steps to change a specific limit:

1) disconnect power;

2) remove the switch cover

3) while holding down the dial panel with one hand, move the limit notch to the wanted temperature limit. 

Pictures below:

1. Fan Limit Switch 外觀

2. 修改前設置

3. 修改後設置

Tenants mentioned it has been working like that since they moved in a few years ago.

They mentioned the furnace behavior and wanted it to be checked due to another unrelated reason.

The filter was replaced 4 weeks ago. 

Some where on the internet, I saw the range can be set within 100F - 150F.

In addition, I noticed during heating process, the inside furnace temperature is always slightly above 150F. 

How often the furnace burner and air duct should be cleaned?