I am looking into a new heater. The house is about 4000 sq ft (2000 on top and 2000 on bottom). The house was built in about 1910 and originally had a gravity furnace.

A contractor told me that because the ducts in my house are located on the interior walls (typical for a gravity furnace) that a two speed furnace really wouldn't be very helpful. He said that two speed furnace are effective when the ducting is located on the exterior walls and the low speed can be used to keep the temperature constant.

Does this sound right? All the advantages of two speed furnaces sound compelling but I don't want to buy one if it won't work for my house.