Gas and Oil Home Heating Furnaces - Heating

What is the best way to clean the heating side ducts on a two story home? No matter how much we clean the house in the morning there is a layer of dust on the dining room table. The duct work dates back to the 1940's &50's. Since my wife and I both suffer from allergies they get worse in the heating months. I've thought about using a chimmy sweep tool to shove into the straight runs from the basement to disloge the accumulated dust, as there would not be much of a way to clean the horizontal ducts without completly dimantling them. Any help would be great!

Forced air systems are more or less a closed loop system. The returns take air to the system and clean the air first through a filter, then the air is heated and redistributed throughout the home through a supply ducting system. Therefore the air is supposed to be cleaned before it's heated and distributed. The dust that you are referring to may not be coming from the furnace but if it is, then your ducts have a breach where the dust is by passing the filter or the filter is not doing its job. Sometimes people put the wrong size filter in which allows the dust to pass it. If your ducts are sealed properly all the dust would be captured by the filter and the supply ducts would be clean. That's why I don't recommend duct cleaning.

Standard filters only stop about 10% of what goes thru them. You could be moving fine dust thru you duct system.
Try using a good pleated filter. This will stop about 65%.
You should see a reduction if it is a duct problem. you may have to change the filter every 30 days because the collect more dirt.