If you need to have a new furnace installed, your furnace contractor will generally provide you with a quote before they proceed with the work. Sometimes, this quote will come back with the assertion that you'll need some modifications to your ductwork before the new furnace can be installed. Since this generally comes with an added cost, you may wonder why the ductwork needs to be modified and what the modifications may involve. Here are a few likely possibilities.

Your current ducts are not the right length to connect to the right furnace. 

The furnace heats air, and then the blower unit pushes the heated air out into the ducts. The main duct has to connect to the blower unit where its opening is located. If your old blower unit is a different size or has its opening at a different spot than the new unit, then the HVAC contractor will likely need to install a new duct. In most cases, they will be able to add an extension to the current duct or simply trim off part of the old duct. So, this is not extensive work, but it is work that your HVAC contractor has to do and bill for.

Your furnace needs to be moved.

This situation arises a lot when homeowners have an old furnace replaced. The old furnace probably vented through a chimney. New furnaces, on the other hand, tend to vent out of PVC pipes. If there is not a great place to put the vent with your furnace in its current location, then your HVAC contractor will need to relocate the furnace. They'll then need to run ducts to the new location.

Your old ducts are in bad shape.

Ducts do tend to last a while because they basically just sit there while air blows through them. But they do not last forever. If your ducts are dented, bent, or leaky, your HVAC contractor may not feel comfortable connecting them to the new furnace. Doing so could lead to leaks and inefficiency. So, your HVAC contractor will run new ducts to ensure your new furnace is able to function at its best.

If your HVAC contractor tells you they need to modify your ductwork before installing a new furnace, there is usually a good reason for this recommendation. Chances are, one of the above situations apply. Your HVAC contractor can tell you more about the type of work they need to do.

Contact an HVAC contractor for more information on heating services.