Old Gas Furnace Control Circuitry question

Old 02-15-13, 07:28 PM
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Old Gas Furnace Control Circuitry question

If I understand the furnace operation correctly: when heat is called for, the blower motors starts first. After ruining for few seconds to min, the Gas valve open and furnace become operational. When temp has reached to a desired level the thermostat turns off the switch (open) which closes the Gas valve while the motor is till running to cool off the heat exchanger. After few sec to min the fan turns off. So, they start in opposition (first ON last OFF). This is the operation principle, however way the control circuitry is designed, whether by old limit SW mechanism or by modern electronic circuit board. The old furnace may have less safety features than the modern one and may have a pilot. Now coming to my question, I have an old Lennox furnace which is operated by limit switch and has pilot. I have attached the control circuitry for your review. If you see the circuit, the thermostat switch has two loads in series (Gas valve coil and limit switch coil). But how can that be? Two loads putting in series will divide the 24V. I am sure the valve coil and limit switch coil both operates at 24V. My second question is, the circuit shows that the gas valve is ON immediately after thermostat closing the switch, which is contrary to the operational principle I described (unless I am wrong on that assumption) So I am lost here. My second question is, what tells the control, now is the time to open the GAS valve after the blower motors is operational for few sec to min? And thirdly, at the end of the cycle what tells the control, to turn off the Blower motor to turn off. (My hunch is some form kind thermo-couple, but did not see any in the furnace). I called Lennox technical support but did not get any answer to my level of question. Any thought from experts? Thanks in advance.
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Last edited by hvactechfw; 02-16-13 at 08:48 AM. Reason: Removed Email address - not allowed
Old 02-15-13, 08:22 PM
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Your image of the wiring diagram is way too small to read and when copied and enlarged it loses too much detail to read. In other words, the diagram is useless as far as my being able to offer specific answers.

IF this is an "old" furnace with an atmospheric burner (no induced draft blower) then most likely there is no timing involved for the main blower operation. The room thermostat will be in series with some limit controls (normally closed) along with the transformer and gas valve. When the thermostat "calls for heat" the gas valve opens, provided all the limit switches are satisfied, and the flame heats the heat exchanger. Within the heat exchanger is a thermostatic switch that starts the blower motor when the heat exchanger reaches the set temperature of this switch. The blower runs continuously and when the room thermostat is satisfied it opens the circuit to the gas valve, shutting off the flame. The blower continues to run until the heat exchanger cools below the set point of the thermostatic switch and then the blower stops. Everything is now set for the next cycle.

That is the basic circuit, yours MAY have some refinements.
Old 02-15-13, 08:24 PM
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Welcome to the forum. Remove your email address also.

A little confusion here.

When the thermostat calls for heat......the gas valve opens and burner fires.
There is a combination fan thermostat/limit switch.
When the heat exchanger gets up to heat.....the fan thermostat closes and the blower runs.
When the thermostat is satisfied.......the gas valve closes and flame goes out.
The blower will continue running until the heat exchanger is cool.

Below is picture of fan thermostat/hi heat limit control. Look familiar ?
You'll only see the silver box.....the probe is in the plenum.
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