Gas and Oil Home Heating Furnaces - Decreased air-flow at vents

I added 3 ducts in my finished basement, which are kept closed almost all the time. My central gas heat furnace is 2 yrs old. I've noticed a decrease in air flow out of most of the ducts. I keep the filter clean and chk'd the dampers. Would an aux. blower built into the system help?

Anytime you add supplies to an existing system you will loose air somewhere else. Even with vent closed, you still will lose air/static pressure. Remember, the system was not designed for the basement. In line fans for supply runs can help, but and oversized blower may do more harm than good.

Many furnaces have a multi-speed motor. Choosing a higher speed may help. The install instructions are helpful. The temperature increase should be 35 degrees F or more (rule of thumb...) depending on the make and model. It is best to consult the instructions. Calling the manufacturer is an option also.

Adding on new ducts will definitely reduce the air flow to the remainder of the home. The only option is to increase the fan speed to supply more air to the system. If the heating unit is quite old, you may have a fan belt connected to the blower pulley from the motor. If this is the case, larger pulleys will be required which can be a bit of a pain. If your fan is a direct drive, there are usually at least 3 speeds (or more) to choose from, each having a different coloured wire for each speed. Care should be taken with increasing the fan speed too much creating too little temperature rise in the furnace which will eventually shorten it's operating life. The minimum and maximun temperature rise is usually listed inside the furnace on the units rating tag. Temperature rise is the difference between the incoming air temperature (return air) and the outgoing air temperature(supply air). Hope this helps.