Furnace Blower won’t start in Auto mode

______________________________________________________________________ GRATEFUL for any suggestions and help from ALL the experts on this forum, expecially XENOS and Fireweaver67 Problem Symptom: Setting thermostat on Heat and fan on Auto, causes a burning smell to come from the registers but NO heated air Equipment Affected: Comfortmaker RPJ II gas furnace installed in Aug’98 by InterCity Products. InterCity model no.GUI100A012FIN using White Rodgers 50A50-112 furnace control unit and White Rodgers Thermostat model 1F80-51 Problem Description/Observation: With thermostat set on Heat and Fan set on Auto, the following sequence of events were observed. 1. Draft Inducer motor starts. 2. Igniter lights up. All burners fire up. 4. Few clicks are heard coming from the Contol Box,however Blower does not cycle on. 5. Gas burners operate for about 4-5 minutes before shutting off. 7. Control unit LED flashes 4 times – code for High Limit Switch Open Troubleshooting/ Diagnosis steps: I have tried the follwing to identify cause of the problem. i) Set thermostat Fan to ON. Blower motor did not start. So I suspected the motor start capacitor. But Capacitor tests OK at the local Grainger store, where I had gone to buy a new one. ii) Suspect bad motor. Dismantled motor. Tested each speed coil with exisiting capacitor. Motor works fine. iii) Suspect motor may not work under load. Installed blower on top of shaft and run motor. Works OK. iv)At this point suspect Thermostat. Check and find 24V AC across RH-W, RH-Y and RH-G ( RH jumpered to RC). v)Replace Thermostat with RiteTemp model 8022c from Home Depot.Test New Thermostat: Jumper thermostat terminals RH-W – Gas furnace fires up Jumper thermostat terminals RH-G – Fan/blower turns on Jumper thermostat terminals RH-Y – Cooling compressor turns on. Furnace starts ok with thermostat on HEAT. Cooling compressor AND blower fan both start when thermostat set to COOL. FAN turns on with thermostat set to ON, however FAN still does NOT cycle on in Heat and Auto Fan mode. viii) Try to start furnace in the following sequence: 1. Set thermostat to Heat and fan to ON. 2. Fan starts and draft inducer motor starts. 3. Ignitor lights up. 4. Lights up first burner, however due to the draft caused by the blower motor, the flame goes off. 5. Smell gas. ABORT test. ix) Start the furnace in the following sequence: 1. Disconnect CIRC & HEAT leads of blower motor. 2. Connect CIRC & HEAT to separate power source (via extension cord and switch) 3. Set thermostat to Heat and fan to ON. 4. Furnace heat cycle starts. Draft inducer motor starts. 5. Ignitor lights up. All burners fire up. 6. I wait 90 seconds and turn fan on(from separate power supply) 7. Blower starts and hot air starts coming out of floor registers. 8. Heat house first time in two days. 9. Furnace shuts off after target temp is reached. 10. manually stop blower motor. At this point I’m a bit stumped. At this point I suspect the control box, however since the fan comes on in manual mode, can’t think how the control box could be bad.. GRATEFUL for any suggestions and help from ALL the experts on this forum, expecially XENOS and Fireweaver67 Zorro Replies: fireweaver67 “Re(1):Furnace Blower won’t start in Auto mode” , posted Sun 30 Oct 20:06: ______________________________________________________________________ I think I’d start by verifing 115v to the motor, from the board, when a call for heat is present and the T-Stat was set to auto. If the limit opens and theres no voltage to the motor, then its the board. I AM CANADIAN………EH!!! [this message was edited by fireweaver67 on Sun 30 Oct 20:24] zorro “Re(2):Furnace Blower won’t start in Auto mode” , posted Sun 30 Oct 23:55 ______________________________________________________________________ quote: ______________________________________________________________________ I think I’d start by verifing 115v to the motor, from the board, when a call for heat is present and the T-Stat was set to auto. If the limit opens and theres no voltage to the motor, then its the boa … ______________________________________________________________________ I have a White-Rodgers 50A50 – 112 Total Furnace Controller on my furnace. I couldn’t find any wiring diagrams on the White-Rodgers web site as this is an obsolete model. I had my digital multimeter on the blower HEAT & CIRC connectors while the thermostat was set on Heat and Fan set to Auto. The furnace went through it’s cycle until the high limit switch opened and shutdown the gas. However no voltage ever showed up on the blower terminals CIRC & HEAT either during the furnace ignition cycle or after it. Zorro fireweaver67 “Re(3):Furnace Blower won’t start in Auto mode” , posted Mon 31 Oct 00:16: ______________________________________________________________________ I think you’ve found your problem. Another thing that makes me think its your board is that your fan doesn’t come on once the limit has tripped and you’ve proven, with all your hard work, that the fan motor does indeed work!! I AM CANADIAN………EH!!! [this message was edited by fireweaver67 on Mon 31 Oct 00:19] zorro “Re(1):Furnace Blower won’t start in Auto mode” , posted Sun 30 Oct 18:42 ______________________________________________________________________ Thanks for such prompt posts.. I’m a little less certain that the Control Box is the problem. Here’s why… When put in manual mode (fan -ON)from the thermostat, the fan blower comes on, which proves that whatever circuitry inside the control box is responsible for switching 120V to the fan blower leads, is working normally. Wouldn’t the same circuitry be responsible for energising blower leads when thermostat Fan setting is set to auto ? Zorro Xenos “Re(1):Furnace Blower won’t start in Auto mode” , posted Sun 30 Oct 18:20 ______________________________________________________________________ I have to agree with Ace. Xenos. The best way to escape a problem is to solve it. acefurnacefixer “Re(1):Furnace Blower won’t start in Auto mode” , posted Sun 30 Oct 17:59 ______________________________________________________________________ Man I wish some of my techs was that through…..sounds like what ever tells the blower to come on is bad…….the main board is #1 suspect. Yes! I CAN fix that! zorro “Re(2):Furnace Blower won’t start in Auto mode” , posted Sun 30 Oct 19:03 ______________________________________________________________________ quote: ______________________________________________________________________ Man I wish some of my techs was that through…..sounds like what ever tells the blower to come on is bad…….the main board is #1 suspect. … ______________________________________________________________________ Thanks for such prompt posts.. I’m a little less certain that the Control Box is the problem. Here’s why… When put in manual mode (fan -ON)from the thermostat, the fan comes on, which proves that whatever circuitry inside the furnace control box is responsible for switiching 120V ac to the blower motor leads is working fine. Wouldn’t the same circuitry be responsible for switching the 120V for the fan blower when thermostat is sets for Fan Auto.? Zorro zorro “Re(2):Furnace Blower won’t start in Auto mode” , posted Sun 30 Oct 18:58 ______________________________________________________________________ Thanks for such prompt posts.. I’m a little less certain that the Control Box is the problem. Here’s why… When put in manual mode (fan -ON)from the thermostat, the fan comes on, which proves that whatever circuitry inside the furnace control box is responsible for switiching 120V ac to the blower motor leads is working fine. Wouldn’t the same circuitry be responsible for switching the 120V for the fan blower when thermostat is sets for Fan Auto.? Zorro Xenos “Re(3):Furnace Blower won’t start in Auto mode” , posted Sun 30 Oct 19:58 ______________________________________________________________________ The furnace controller has 2 different fan relays, The cooling one ( high speed) is powered from the G terminal on the thermostat. The second, Heating speed, is controlled for a timing logic in the controller. What make and model is you unit as I may have a wiring diagram here. Xenos. The best way to escape a problem is to solve it. zorro “Re(4):Furnace Blower won’t start in Auto mode” , posted Sun 30 Oct 23:57 ______________________________________________________________________ I have a White-Rodgers controller Model No. 50A50 -112 Z zorro “Re(5):Furnace Blower won’t start in Auto mode” , posted Thu 3 Nov 15:28 ______________________________________________________________________ It was the furnace control box. I replaced my White Rodger’s 50A50-112 furnace conroller with their 50A55-843 model. The blower motor now cycles normally in Auto mode. Many thanks to everyone for their help. Zorro

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