With the rising cost of the oil barrel, heating costs invariably follow suit.  Old, inefficient gas heaters will waste up to 40% of the heat through the flu.  Other efficiency considerations would be the insulation factor behind the walls and as well, any air exchange via windows and doors. 

In September 2007, my friend Al purchased a 40 year old home.  He complained about a gas furnace which would turn itself off and on alternately every few minutes while heating the house.  It was both chilly in the home and costly.  I quickly traced the symptom to the heating chamber, where a firewall mounted heat activated disconnect switch was shutting down the gas valve when the chamber temperature exceeded about 180 deg. F.. When the chamber cooled to about 140 deg. F., the heat switch would cool enough to reconnect and reactivate the gas valve and blower.  Thinking that the switch was simply a bad one, we changed it out.  The problem persisted, which compelled me to study the distribution of air flow to determine what was creating such a high temperature in the chamber and plenum, causing the shutdown of the safety switch. 

What I found was so elemental that I was almost embarrassed for missing it in the first place.  The furnace was first of all built into a tiny closet in the garage (since there was no basement) and the unit itself was only producing 80,000 BTU's, where it should have been at least 100,00 BTU's for the total square footage of the home.  Secondly, due to the small space and the fact that there were beams and supports to navigate, the plenum was built too small.  Finally, and more importantly, the amount of air flow was restricted by the fact that only two 6 inch ducts ran from the plenum to feed 7 rooms, including three bedrooms on the second floor. 

This is monumental, there must be one duct to each room and the plenum must be custom built to accommodate the number of ducts according to the amount of airflow. Not only did the restriction create a back-flow of heat causing the safety switch to shut the whole system down, but it facilitated a real danger of carbon monoxide poisoning.  Consequently, Al and I determined that the best place for the new system would be located in the attic, where the ducting could be fed to each room on both floors.  He also decided to go electric, which in my opinion saves cost by not using an exhaust flu that looses heat.