The link below will give you some general ideas with respect to furnace sizing. Using these general rules of thumb, along with the information that you have provided in your inventory, it seems that your furnace is likely properly sized to heat your home, but is not oversized. Years ago, it was common to oversize furnaces, to make sure that there was enough heat on the coldest day of the year. This way, the installer didn't risk a complaint if the furnace was not able to keep up. These days, though, the industry has recognized that an oversized furnace will cycle on and off more frequently, leading to problems with comfort, efficiency, and lifespan. For this reason, furnaces tend to be smaller than they used to be, and are sized to be just big enough to maintain the right temperature inside the homeon the coldest day of the year. If perfectly sized, the heat delivered by the furnace will exactly balance the heat being lost through the walls, ceilings, windows, and doors, and will run constantly (60 minutes out of every hour) at the minimum design temperature. If this is the case, the furnace will only be able to maintain the temperature in the home, but will not be able to exceed the amount of heat being lost, and so will not be able to raise the temperature in the home. In the example that you have described, the outside temperature is certainly not the "coldest day of the year" design temperature, but a furnace that is properly sized will still take some time to bring the house up three degrees. Keep in mind that even if you kept the thermostat set at 68 degrees, the furnace would still be running occasionallywhen the temperature was 25 degrees outside. If, for example, the furnace came on five minutes at a time every twenty minutes, that's a total of fifteen minutes per hour just to maintain the temperature. If the furnace is running 45 minutesto raise the temperature, fifteen minutes of that time is devoted to balancing the heat loss in the home, while the remaining thirty minutes is devoted to raising the temperature. The colder it gets outside, the longer it's going to take, because more and more of the furnace capacity is devoted to offsetting heat loss. Eventually, once the furnace capacity is reached just maintaining the temperature, there will be no capacity left over to increase it. In summary, the situation that you have described is normal, and is the result of modern, efficient heating design.