I am troubleshooting a Gas furnace. I am trying to eliminate the fan/limit control and a 24 volt electric controlled switch. I am getting 24 volts to the switch. I have replaced a thermostat for this unit. There is an auto/manual switch on the fan/limit switch. When this switch is in the manual position the blower turns on, but the burner does not ignite. When the switch is in auto the blower does not turn on. The igniter flame is lit. The burners will not light. Any suggestions? Thanks

You might have the thermostat wired incorrectly. Did the unit work before you changed the thermostat?

DO NOT eliminate the fan limit control. The limit is a safety device.

hvac1000. Thnak you for your reply. The unit did not work prior to the chaange of the thermostst( it was old so I thought it might be the problem) it is wired correctly. It only has two wires a white and red one. So I pretty sure it is not the themostat. I am really leaning towards the fan limit switch but I don't understand the operation principles. Would this limit cause the burners not to fire up?

You should have a high and a low limit switch. The low limit switch turns the fan on and off. The high limit switch turns the burners off if the furnace gets too high. If it sticks open, it will keep the burner from lighting. If so, replace it.

If the burners then light, but the fan doesn't come on once the furnace warms up, either the low limit switch or the fan relay is bad.

24 volt power flows from the thermostat to the high limit switch to the gas valve. It flows to the low limit switch and then to the coil of the relay that switches the 120 volts to the fan.

Sounds like a older furnace. As labman said that is the way it works. BUT you might have a combo switch and no relay. Usually these devices are Honeywell or White Rodgers. They usually work off 120 volts but I have seen a few that had the fan switch at 120 volts and the limit at 24 volts. The one you have might also have a white push pull button on it to manually turn the fan on.

hvac1000/Labman . Thank you for your reply. The unit did not work prior to the change of the thermostat( it was old so I thought it might be the problem) it is wired correctly. It only has two wires a white and red one. So I pretty sure it is not the thermostat. I have mor information on the limit switch. It is a Honeywell combination furnace controller type L498B. The blower will run in manual mode but not in auto. But the burners will not light.

If it looks like this then it is the newer universal replacement shown below.

http://customer.honeywell.com/techli...0s/69-0117.pdf

Also BE SURE to get the same length element tube. That is the part that sticks in the furnace. This is important. They are different lengths for the different way furnaces are constructed.