Need furnace advice
 

I'll call a technician if I can't figure this out first.
My furnace won't come on, I know the thermostat works as the fan comes on when put in that mode. When switched to cool the air conditioner starts. I pulled and cleaned the flame sensor then reinstalled but nothing. I did a continuity test on the ignighter and it's good. Asking if there is anything else I can test before calling in a pro.

I went through this recently. Tech was called as I lack the skills and wife has some barter type money she gets as a bonus. There is a great HVAC guy who takes the stuff. I have an old Honeywell electronic air cleaner that has bad inserts. They got clogged and the air flow was restricted. Furnace sensed that and shut down. A good cleaning of everything got it going again. I'm ordering paper elements to replace the stupidly expensive inserts that he said really don't work better than good paper filters.

Another thing that might need checking...the condensate line. Mine was plugged once, from little use, and wouldn't allow the furnace to run. Just an idea..easy to check.

Is it high efficiency? The chimney fan has to start first...

Good call. I changed it a few years ago. Will check it in the morning.

It is high efficiency . The fan does not come on.

Change the battery in the thermostat. It may have enough power to start the unit but not enough to sustain it running.

No battery. It's powered by the furnace which is plugged into a 110 outlet.

Find the pressure switch line to the blower motor. Its like a piece of vacuum tubing. Pull it off the blower motor housing and clean out the nipple it connects to on the motor housing with a small stiff wire. Very common issue.

On my furnace the white wire comes form the thermostat (26 Volts AC) and turns on the relay that runs the inducer blower and powers the ignition module. The 26 Volts then goes into a centrifugal switch on the inducer blower, through the limit switch, through the flue pressure switch and on to the ignition module that starts the ignition sequence. The wiring diagram should be on the door.

A meter that measures 26 Volts AC relative to chassis would be very handy

If it's a close combustion model, check the fresh air intake and exhaust to make sure they aren't clogged- maybe a rodent decided to nest in it over the summer. Mine used to suck in snow and stop working.

Following the wiring diagram should help you figure out how to test each sensor.

One more thing, some furnaces have a blinking light on the board that will blink out trouble codes.

Sounds like you may have a problem with one of the safeties.
First, have a look at your thermostat and if it has switches on it for "Cool/Auto/Heat" make sure that switch is set to Auto or Heat.
Second, what make of furnace? Some have their terminal boards numbered differently.
Third, do you have an electrical meter?
If you do, set it for A/C Volts and when the furnace is NOT calling for heat (turn the T'stat down to a low setting), check for voltage between the R terminal (24 VAC) and W1 (first stage heat).
If there is voltage potential of 24 VAC shown on your meter, try to jumper these two terminals together and see if the furnace starts.
If it starts, it is the thermostat.
If there is no voltage potential between R and W1, then you may have a safety open.
usually there is an over temperature switch with a manual reset button in or around the closed combustion area of the burners or at the exit of burner section on a mid efficient furnace.
Try that first and see what happens.
Oh ya, and RWest is correct in that most of the printed circuit control boards will flash out a failure code. Look at the back of one of the removable panels or the owners manual for what the different flash codes mean.

If this is a high efficiency unit, it will have at least two leds that blink trouble codes. They will tell you what is wrong. You don't say if it is gas or oil, but I'm assuming gas. Since it doesn't come on at all, if you opened the unit (removed a panel) you may have tripped the interlock. Make sure they are properly reinstalled. Finally, when you figure it out all parts can be purchased online for a fraction of the retail price. I bought a new control board for one of mine several years ago for 1/2 the price the hvac guy wanted. Good luck!

It has three lights on top of the control board. One orange and two green. The green ones are on but the orange one flashes when I push in the interlock button. This button is usually depressed when the panel is on. When I depress this button the unit has a faint humming sound..
It's a Ruud/ Rheem LPG unit.

^^^^ That may mean the capacitor is blown on the main motor if you get a hum. Or did you say it works for AC? There are several safeties that have to all agree for you to get a flame and start the motor. The guys that mentioned the LED trouble codes are correct. There should be a sticker on the panel someplace that describes the fault code. Start there. Also, when you remove the inspection panel, it wont turn on. You may need to hold the switch so it thinks the panel is on. If the LEDs arent blinking, turn the unit off with the switch on the outside of the unit, have the thermostat calling for heat, and then turn it back on while holding the panel-removed-cutout-switch. This will reset the codes and then see what they say. If you cant see the LEDs, there is usually a small window on the panel that allows you to see them with the panel on.

In the past I could get the unit to fire up by hold the button in with my finger without the panels on. I'll try turning it off and on as soon as it gets light here.
Thanks for all the advice gentlemen.

A high efficiency furnace also has a water trap in the drain assembly somewhere If the water in the trap evaporated over the summer ,they won,t start. If you have the owners manual or access to it online check this out I have had it happen twice over last 10 years at season startup cost me a service call firs time ,easy fix second time

That is the door switch and it prevents the furnace from running without the blower door.
The humming is the transformer. It hums because it has forgotten the words....
Turning the unit off and on again typically resets the board and it will not necessarily show the fault after the reset.

Edit: Looked up a generic Rheem/Ruud wiring diagram and it looks as though there are two manual reset limits then two automatic limits in series with the W1 heat terminal. All these need to be in there normal position (closed) before the unit will try to fire the heat. Finding which one is open could be as simple as pressing a small pushbutton on the limit or finding 24 VAC potential across the switch when it is calling for heating.

I have three LED lights,one orange and two green. The green ones stay on after resetting but the orange one doesn't light. I found the units manual and will goe through it.