7.3.5 Student Book © 2005 Propane Education & Research CouncilPage Identifying the Function and Characteristics of Draft Fans & Controls High efficiency appliances remove so much heat from the fuel gases that draft fans are needed to move the products of combustion through the appliance heat exchangers and venting system to the outdoors. In this module, you will learn to: 1.The function and characteristics of draft fans in high efficiency appliances 2.The function and characteristics of the pressure switch in the draft fan system

7.3.5 Student Book © 2005 Propane Education & Research CouncilPage 1 Identifying the function and characteristics of draft fans in high efficiency appliances In the Basic Principles and Practices of Propane study course that you completed to start your propane certification studies, you were introduced to the phenomenon known as “latent heat of vaporization.” In the section that discussed physical properties of gases, it was stated that as water is heated, a thermometer will show the result of heat added to the water as a burner is turned up until the water reaches its boiling point. If the thermometer reads 212°F (100°C) at atmospheric pressure at sea level, no matter how much additional heat is added, the thermometer will not show a higher reading. The only observable outcome of the added heat is an increase in the boiling action. Scientists and engineers refer to this added heat as “hidden or latent heat” of vaporization.

7.3.5 Student Book © 2005 Propane Education & Research CouncilPage ? Combustion of propane or any fuel produces water vapor. This “boiled” water is a hot flue gas, and in conventional gas appliances, provides the energy for venting of the products of combustion. High efficiency furnaces and boilers use a second heat exchanger to recover the latent heat of vaporization by condensing the water vapor. As a result, the venting energy is reduced and must be replaced by a draft fan.

7.3.5 Student Book © 2005 Propane Education & Research CouncilPage 2 Figure 1. Diagram of a Condensing Furnace

7.3.5 Student Book © 2005 Propane Education & Research CouncilPage 1 The purpose of the draft fan is to ensure that the products of combustion will move through the primary heat exchanger, and through the secondary heat exchanger, and ultimately to a location completely outside of the building to be vented to the outside atmosphere. Most condensing appliances are readily identified because their venting systems and make up air intake piping are usually made up of PVC (or some other type of approved plastic) piping sections and fittings.

7.3.5 Student Book © 2005 Propane Education & Research CouncilPages 2 & 3 Identifying the function and characteristics of the pressure switch in the draft fan system Figure 2. Pressure Switch Assembly A pressure switch is connected to the draft fan by a sensing tube. The pressure switch confirms that the draft fan is producing a draft pressure sufficient to move flue gases through the heat exchangers to the outside atmosphere. In most condensing furnaces, the draft fan pressure switch is wired in series with a flame roll out switch, the blower fan limit switch and the ignition control module. This arrangement ensures that the draft fan is providing the required venting pressure or the entire system is shut down.

7.3.5 Student Book © 2005 Propane Education & Research CouncilPage 3 To test the operation of draft fan pressure switches, read and follow manufacturers’ instructions. Usually the instructions will call for the insertion of a tee fitting into the sensing tube, with the tee outlet tubing connected to a water column manometer. The manufacturers’ instructions will give the manometer pressure range required for normal operation. NEVER suck or blow on a pressure switch sensing tube—you may damage the pressure switch. Technical Tip A typical draft fan will supply air pressured to approximately 19 inches water column. Be sure that the manometer used to measure pressure is capable of measuring that pressure; typical u-tube water column manometers used for checking low-pressure gas regulator output may give the technician an unscheduled shower.

7.3.5 Student Book © 2005 Propane Education & Research CouncilPages 4 & 5 Time to See If You Got the Key Points of This Module… Complete the Review on page 4. See if you are ready for the Certification Exam by checking off the performance criteria on page 5.