• Commercial Space Heating
  • Small Industrial and Industrial Office Space Heat



Space heating is the ‘bread and butter’ of many gas utilities’ annual sales volumes. The gas furnace is the heating system of choice in over 90% of small space heating applications in northern climates. Its simple operation, low first and operating costs, and availability from numerous sources makes it an easy choice.

Southern climates may have all electric rooftop HVACs. Other applications such as office areas contained within an industrial plant that has it own HVAC system (mainly for cooling) may also have electric heat. Electric heat is significantly more expensive to operate than gas, but the equipment has a lower first cost. Therefore, if the heating load is small and/or the climate is mild, all electric may be the best choice.
ROOFTOPHVAC_2Gas furnaces are available in sizes from about 30,000 to 500,000 BTUs. The most common size in residential styled furnaces (the vertical standing box) is around 100,000 BTUs. The residential styled units max out at around 200,000 BTUs with a cooling section of about 5 tons. (400 CFM/Ton = 2,000 CFM total fan air flow.)

The most common size in commercial rooftop units is around 250,000 BTUs. Rooftop units tend to be sized for air conditioning loads. It takes a larger fan to move cooled air than heated air. Therefore, a combination rooftop HVAC unit may have 10 – 20 tons of cooling (4,000 – 8,000 CFM) with a heating section of 250,000 BTUs (3,000 CFM), or, a heating section that is over-sized for the heating load. New units use variable speed fans between heating and cooling modes. Older units just over-sized the heating section, which resulted in uncomfortable hot/cold cycles and inefficiencies.

All furnaces should only require standard gas delivery pressure of about 7″ or 0.25 psi to operate. However, elevated delivery pressure is sometime requested to reduce over-the-roof gas pipe size.


  1. Air Conditioning Compressor
  2. Cabinet
  3. Air Conditioning Heat Exchanger – Condenser
  4. Fan and Drive Motor
  5. Controls
  6. Heating Heat Exchanger
  7. Gas Burner



Ruud Commercial Products


Sources: Text Bob Fegan 1/2009; new unit pictures Ruud web site 4/02; rooftop unit photo by Bob Fegan;

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