Forced Air Gas Furnaces

Forces air gas furnace is a central heating system used to heat a home or building. All furnaces work on the same basic principles and have similar parts:

  • Thermostat
  • Pilot light or electric igniter
  • Gas valve
  • Burners
  • Heat exchanger
  • Fan
  • Exhaust

The pilot light ignites a series of burners inside the combustion chamber that then enters the heat exchanger, where the heat transfers to the air in the home with the use of a fan to heat the home to the set point of the thermostat.

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There are 3 types of forced gas furnaces I come across inspecting homes:

  • Low efficiency furnace 78% or less
  • Mid efficiency furnace 80% – 89%
  • High efficiency furnace 90%-98% (Condensing)

When looking at the efficiency it is easy to understand if you think in these terms:

  • In an 80% efficient furnace 20% of the heat is loss in the combustion and goes out the chimney.

Low and Mid efficient furnaces have one heat exchanger usually 1 gas valve setting, and standard fan adjusted to one speed for heat, cool or just fan. They require a chimney to vent the flue gases to the exterior of the home, with temperatures of up to 300-350 degrees.

INTERNAL DRAFT DIVERTER

High efficiency condensing furnaces will have two heat exchangers, the standard heat exchanger as well as a second heat exchanger the flue gases are run though to extract as much heat from the system as possible. This process does cause condensation that is why they are called condensing furnaces. Because of the secondary heat exchanger these furnaces use PVC exhaust pipes with a temperature of about 100 degrees.

CONDENSATION IN HIGH EFFICIENCY FURNACES_pers

High efficient furnaces also come with several options to choose from to make the furnace even more efficient:

2-Statge gas valve

  • 2 – stage furnace will operate in its low-fire stage during periods of mild weather (pushing less air through the ducts and less combustion gas through the flue), and automatically ramp up to its second high-fire stage during extremely colder weather.
  • This will cause the home to experience fewer and much less severe temperature swings in the home, which will result in greater over-all comfort for you and energy savings.

Variable Speed Motor (ECM) – This is the blower motor in the furnace, unlike the traditional motor is saves you money and adds to the comfort of your home:

  • The motor converts AC current into DC current which is cheaper to run
  • When starting it will ramp up to speed and ramp down to prevent a large use of power at the start
  • When sit to fan mode it will operate the fan at 1/8th the cost of the traditional fan.
  • With the fan running the air is constantly circulating preventing cool or hot areas and giving the home a consistent temperature throughout the home

With a high efficient furnace, I recommend to not turn the thermostat below 60 degrees because of the following possible results:

  • With return air that cold, flue gas will condense in the primary heat exchanger causing rust & corrosion.
  • Newer, more efficient heat exchangers are, by necessity, thinner than they used to be, and if you set the thermostat too low in heating mode, the increased temperature differential between the returned air and the heated air increases the effect of the cool/hot contraction/expansion cycle of the metal in the heat exchanger, which significantly shortens its life

In the event that the furnace is not operating here are a couple recommendations to check before calling an HVAC specialist.

  • Turn off the main switch to reset the control board, remove the air filter and turn back on. If the furnace starts most likely a dirty air filter, install a clean filter. If this does not work check the exterior of the vent pipe for a blockage, if blockage is found turn off main switch then turn back on to reset control board and see if it runs normally.
  • If neither of these works. Call a licensed and insured HVAC contractor for further evaluation and repair or replace as warranted.

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