1. 7.3.4 Student Book © 2005 Propane Education & Research CouncilPage 1 7.3.4 Identifying the Components and Operating Characteristics of Limit and Fan Controls A vital safety component of warm air furnaces and warm water boilers is the limit control. In this module, you will learn to: 1.The purpose and operating characteristics of limit and fan controls

2. 7.3.4 Student Book © 2005 Propane Education & Research CouncilPage 1 Identifying the purpose and operating characteristics of limit and fan controls The purpose of a limit control is to monitor the heating system for overheating conditions; it turns off the heat source before a dangerous condition arises. Most limit controls are constructed of a normally closed (N/C) single pole/single throw (SP/ST) heat sensitive electrical switch. Physical Description— The more economical limit controls have a fixed temperature setting with temperature sensed by a bimetal device. Figure 1. Disc Bimetal Limit Control

3. 7.3.4 Student Book © 2005 Propane Education & Research CouncilPages 1 & 2 Circuit Operation— The limit control is a heat sensitive electrical switch. It breaks the circuit to the heat source in the event of an overheating condition. The limit control can be placed in several different areas of the system's circuit. Temperature Settings— Most limit controls are set to open their contacts at 200° F, although the manufacturer determines the exact setting. For example, a limit control operating with a differential of 25° F: This means that at 200° F the bimetal forces the contact open. The contact will not close until the temperature drops to 175° F.

4. 7.3.4 Student Book © 2005 Propane Education & Research CouncilPages 2 & 3 The purpose of a fan control is to monitor the heating system and activate the fan or blower after sufficient heat is available for distribution. Figure 2. Combination High Limit and Fan Control (The Bimetal Sensor Extends Into the Heat Exchanger) Most fan controls are constructed of a normally open (N/O) single pole/single throw (SP/ST) heat sensitive electrical switch.

5. 7.3.4 Student Book © 2005 Propane Education & Research CouncilPage 2 Physical Description of a Fan Control. The fan control completes the circuit for the blower. The two most common styles of fan controls are (1)heat sensitive fan controls and (2)time delay relay controls. Heat Sensitive Fan Controls. Heat sensitive fan controls sense temperature by means of bimetals. Remember, fan controls are normally open (n/o) contacts. As the bimetal detects heat the contacts begin to close. A complete circuit is made when the contacts close completely. The fan control also provides a delay on the blower. When the walls of the heat exchanger are still hot there is available warm air to circulate. As the heat is conducted away from the heat exchanger the bimetal detects the temperature drop and breaks the circuit by opening the contacts.

6. 7.3.4 Student Book © 2005 Propane Education & Research CouncilPages 3 & 4 Fan Time Delay (Timer) Relay Controls. The major advantage of the time delay relay is that a heat sensitive fan control does not necessarily have to be used in the circuit. Time delay relays are normally constructed of a flat bimetal with an attached contact, a stationary contact, and a length of high resistance wire. The style of bimetal and the type of resistance wire determines the delay time of the time delay relay. Time delay relays are commonly available with time delays from 15 seconds to 300 seconds, though 120 seconds is a widely used time delay relay. A time delay relay will commonly delay the opening of the contacts (sensor cool down period) as long as it delayed the closing of the contacts.

7. 7.3.4 Student Book © 2005 Propane Education & Research CouncilPages 5 & 6 Time to See If You Got the Key Points of This Module… Complete the Review on page 5. See if you are ready for the Certification Exam by checking off the performance criteria on page 6.