I cant see your wire diagram but trace the circut your talking about. I believe it usually goes through a series of safety switches. Put your VOM across each individual switch. If you get 24 out from the board the first 1 you check and get 24v is open and not feeding back to the board is out. FIND out why. the switch could be bad but it may be indicating a more involved problem, just look aroeund for chared wires or tell tell signes of flame roll out or if its an open (fuse link) suspect what can cause an over heat condition anythign from a bad board or too long an off cycle before the indoor fan comes on ect. PONIT IS Once you come across the cause may not be the reason.
Please grade me accordingly and if you need more I need more info from you.
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Transformers typically supply 24v to the circut board, controls, and safety switches. 120v supplies the fan motor. limit switch is a safety switch and is part of the startup/ignition sequence. If the limit switch is tripped then there is a problem with your furnace that should be managed by a qualified professional in your area, or risk damaging your furnace, and perhaps causing a fire. Hope this clears things up.
Yes. There could be a couple of high limit safety devises in the circuit that can open the circuit to the gas valve. Pending your furnace type If your thermostat is calling for heat and the furnace tries to start your transformer is working. High limit safety controls are normally closed. If you have a open limit switch you will not have any continuity through it using an ohm meter or if you test it with a volt meter you will have 24 volts across the terminals of the safety devise. Other problems can be pressure switches, hot surface igniters and control boards. most newer furnaces have a control board that will flash a light code to help trouble shoot the system. Count the flash pattern and look on one of the furnace doors for the flash code directory.
Check gas pressure, inspect manifold & make sure there is not a blockage.Make sure gas is present and main gas shut off valve to furnace is on also check the knob on the 24 volt gas valve and see if it is in the on position , check the MV terminal on the gas valve to 24 volt common with a volt tester to see if you have 24 volts present, if you do have 24 volts at gas valve, replace valve. if you do not have 24 volts at gas valve check high limit control & roll out limits for a open circuit, Good luck.
Definitely a furnace issue. The gas valve is not opening if you see no flame. I have seen the igniter glow but not send enough voltage to the circuit board to make the gas valve open. If this is the case you should replace the igniter. It is very fragile so please use care when replacing. Check all connections at the circuit board as well as the gas valve. Check your gas supply. Check to see if you get 24 volts AC to the gas valve. If no, replace the circuit board. If yes, replace the gas valve once you confirm that you have gas to it. If you get a pilot flame only, you will need to clean the burners and flame tray that spreads the flame from one burner to the next.
Assuming the inducer fan and pressure switch are working fine, your next step is to test the voltage coming to the gas valve. There should be 4 wires going to the gas valve, one is black. This is the 24 volt power side. Unplug the molex connection at the gas valve and while the inducer is running test for 24 volts AC. If you get voltage you have a bad valve. If no voltage you have a bad circuit board. Test the safety circuit and roll-out sensors to make sure all switches are closed. Check for continuity across the switches.
At the furnace, jump the R and W (or T and T) terminals on the circuit board or gas valve. Make sure the door switch is closed with some electrical tape. By doing this we are forcing the unit to start. If it does fire, you have either a bad thermostat or a broken wire to the thermostat. If it does nothing, look for a bad fuse, check for 24 volts going to the gas valve, an open limit switch or 120 volts at the transformer.
Yes you are right the control voltage of your furnace is 24 volts. Really without a load on the transformer usually runs 28 volts then you put a load on it and it drops down to 24-26 volts. Check the voltage between the R terminal and the W terminal with the selector switch in heat mode. Or move the thermostat switch to heat. R to C would do it also. You could also do the voltage check right at the transformer one side of the transformer is 120 volts and the other side is 24. Look for the black and white wires that is the 120 volt side the blue and yellow or off colors is the secondary side.
Now if it checks low say 24 or 22 your gas valve needs min. 24 volts to hold open the valve that allows the gas to flow into the burners. So if one of the symptoms is related to burners come on for about 3 to 5 min. then shut down and the fan keeps going it is loosing the necessary voltage to hold open the gas valve.
One more thing make sure the air filter is clean as this will cause the heat to build up in the heat exchanger and hit the high limit safety device and cut the gas off. Until the air cools enough to allow the high limit to reset then the gas valve will open and start the process all over again. Dirty A-Coil will do the same thing as the air goes through the coil to get to house. let me know how it goes ken
either the gas at the outside valve is off, or the gas valve at the furnace may be off, or the switch on top of the gas valve in the furace may be off. if they are all on, then you may not be getting a call for heat from the stat to the furnace control board. check to see if you are getting a 24 volt signal from the stat to the w terminal and common terminal on the furnace board. if you are then check to see if the voltage is arriving as 24 volts to the wires at the gas valve itself . if it is then the gas valve is possibly bad. if it is not then the board is possibly bad.look for loose or broken connections on the low voltage wiring. most unusual that the board does everything else but send power to the gas valve. you need to zero in on the valve i think and start by turning off power and that should restart the sequence of operation and you can observe it step by step . if you are not getting a call for heat then the stat may be bad be sure you are in heat , fan in auto and temp pushed up to max. if no 24 volts is sent to furnace board white and common low voltage board then stat is bad. remove it if you think its faulty and jumper the white and red wires together on the subbase. that duplicates what the stat normally does. if the furnace lights its the stat. having said that i realize the stat has to be sending a heat signal or the other stuff would'nt start. so..... back to the furnace board. check that 24 volts in from stat and 24 out to valve and that will put you on your problem item. good luck
OK, if the furnace is not lighting for even a few seconds, you have either a bad gas valve, bad wire, bad control module or no gas. The pressure switch is not the issue. Check all wires to the gas valve and look for 24 volts across them when the ignitor glows and you hear the click. Then test for 24 volts to ground. It could be a bad ground wire.