Gas Furnace Control Board
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- เผยแพร่เมื่อ 7 ก.พ. 2025
- Gas Furnace Control Board
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Not a hvac tech but im pretty sure the board needs 24v ac supplied at the terminals somwhere near the relays. The terminal block you were hooking to is the send and rerurn for the t stat.
R (red is 24v ac from the board to the stat)
W (white for heat)
Y ( yellow for cooling)
G (green for fan)
C (common ground if your tstat needs to be externally powered.
The t stat sends power from red back to w g y
That is correct!!! A very common issue with furnaces is actually the thermostat. I have gone through 3 of them in the past couple of years. The thermostat actually has relays/contactors inside that route the 24 volt power from R to G and either W or Y (depending on if it's calling for Heat or Cooling) and if that relay/contactor goes bad, it will send a faulty signal and possible power spikes/harmonic distortion in the a/c sine wave and cause damage. usually it won't damage the furnace control board, but will definitely cause the furnace to not work properly until the thermostat is replaced.
Like many have said it's 24v ac. I service commercial laundry equipment in Laundromats. This sometimes includes cleaning and checking roof ac package units. The transformers used to power the control boards and thermostats are indeed 24v ac. I'm also thinking that those small electromechanical relays would be a good place to start for diagnosis.
Red = 24VAC (L1 from TFMR)
Blue = Common (L2 from TFMR)
White = Heat (begins heat sequence)
Green = Fan (energizes fan relay, unnecessary in Heat mode)
Yellow = Cooling (energizes outdoor A/C contactor)
There are no large rectifiers in this equipment to keep things simple, so everything runs off of a 24VAC transformer in the furnace or air handler. That's one reason you don't see many EL caps on your board. Modern thermostats and other boards with microcontrollers may have local rectifiers and regulators, but these don't require much current.
On most gas, forced-air systems there are 2 main modes plus a Fan On mode:
Heat On = Red connects to White (heat sequencer, which automatically controls Fan).
Cool On = Red connects to Green (fan relay) and Yellow (A/C contactor).
Fan On = Red connects to Green (fan relay).
Yellow is not needed in the furnace, so the yellow terminal is just that; a terminal to connect the yellow from the thermostat to yellow from the outdoor unit. An A/C-only outdoor unit just needs 2 wires: Yellow and Blue, but the actual wire colors for 2-strand were always red and white.
Notice that Blue (COM) is not normally used, so older gas systems with mercury thermostats did not even include a Blue wire to the thermostat (4-wire system with RWGY). These days we use electronic thermostats that require power, so they need Blue. While this is technically 5 wires for a gas furnace, most contractors will just run 6-strand (RBWGYO) as that's what we've always used for heat pumps anyway.
With a heat pump, you add an Orange/Brown wire to engage the reversing valve in the outdoor unit; this switches between cooling and heating and the outdoor unit interrupts this signal to defrost itself when necessary.
Paul. I just finished changing out the control board in an electric stove. It had five relays on it and the fix was to reheat all the solder joints on the coil pads and the contact pads.
I see comments are talking about control board being mounted on blower housing being a problem. Same thing happened to my furnace. As blower gets dirty overtime, it starts to get out of balance and control board starts to vibrate which is not good. Relay points arch and PCB traces crack etc. I replaced all the relays on my old board and jumped any bad traces and I had a new replacement for a fraction of the cost of a new one. Blower needs to be removed and cleaned if it's vibrating from being out of balance from being dirty. This is a good lesson for replacing filters at least monthly.
Hi I am a hvac tech. First thing I can tell you is placing 24 volts of either ac or dc between r and w will not do any thing. R is power out of board 24 volts ac. The thermostat acts as a switch. W is the voltage back in or what is known as call signal. So that being said. There is a connection on the board which is labeled for 24v transformer sec1 and sec 2. The c on the board is know as common so when 24 volts are hooked to transformer secondary connections there should be 24v ac across r and c terminals. Here is a sequence of operation for heat. When heat call is applied to w first the board looks to see if pressure switch is open. 2nd the induce draft fan motor activates not sure which relay (is controlling 120v ac to turn on draft motor 3rd board checks pressure switch to see if it closed. 4th hot surface igniter is actived another relay controlling 120v. 5th after hsi is glowing bright the gas valve is actived. 6th there is a flame sensor which reads ground thru moisture in flame to tell board there is fle present. 7th after preset time another relay actives blower relay. 120v contol relay. There are safety switch in wiring also which did not discuss. But they are all heat actived. Snap disc which open at a predetermined temperature. Hope this helps. I enjoy your videos started watching them to understand more about electronics so i could better diagnose and understand what is going on with in these controll board. Thanks
The object of modern updated systems is to ensure that the home owner can NOT work on them. Cars, phone,ect.
First thing I would check is for annular cracking around the relay load pins (with a loupe)
These boards are notorious for cold joints around the molex connectors and terminal blocks. They take a beating from the wires vibrating. For those watching, it needs to be tested with 24v AC.
Get a schematic of your furnace.
Build a setup that looks like your furnace, you can use light bulbs to simulate fan motor, igniter, and what not. Switches where you clearly can se if the are on or off to simulate signals like thermostat, overheating protection etc. A flame sensor can be easily be made from resistors, look at the datasheet for the flame sensor for your device. Some flame sensor may be the type that is an electrode sticking into the flame, it is "measuring" a current around 50-150 uA when the flame is present (It is telling how clean the burning is too)
The tricky part is to understand the operation of the controller board, especially the safety functions (a lot of stuff is going on to ensure that your house is not going up i flames because of a gas explosions, so a lot of ventilation is taking place just to be on the safe side) may not give much sense if you only focus on "get the thing working".
Often the operation of the board is described on a sticker in the furnace (same with the schematic).
Some board are universal over different types of furnaces, so switches/jumpers on the board must be set correctly, often some info can be on the board itself, or Google may come in handy
Very interesting. This makes sense, especially for those who service equipment with control boards
get well soon and have BLESSED HOLIDAYS!
I'm a heating engineer in UK and super interested in this because there is no actual way for us to determine why a PCB has gone bad unless its super obvious. Essentially we test every other component, fan, gas valve, thermistors, electrodes, pump etc and follow a logic chart which directs us towards a bad board. 24V A/C is pretty small, our boards all require 240V A/C and have transformers for the low voltage parts.
Essentially the appliance takes in data from thermistors, high limit stats, pressure switches etc and only then enters ignition sequence. The first things to run are the pump and the fan. The fan in older style appliances (non condensing) pulls a pressure switch which allows sequence to continue. The gas valve opens and a spark electrode ignites the flame. An A/C current is passed through an ionisation electrode which because of the flame gets partially rectified resembling a DC current, the appliance then knows a flame is present and keeps the gas valve open until the demand for heat is satisfied. I doubt very much that a room stat can cause a board to go bad, after all its just a switch. C, NC and NO.
From what I've seen on youtube American appliances are similar in most respects. Please when you are felling better can you make a vid of testing that PCB, it would be most appreciated.
Warmest regards to you and all the ones you love, your fan Robbie, Glasgow, U.K
It's 24vac coming in and the relays are 24vac also.
Sounds like when one is kicking in that there's a voltage drop across the coil but I'm not there you can test for that probably with an ohm-meter across the coils of the relays and see which one is different.
Hey Paul. A retired hvac guy here.. The best way to check that board is to have it installed on the furnace and check it in operation. You said something that makes me think it may have been the Thermostat. On a call for heat you should have 24 vac across R and W on board terminal wiring strip. If you had 16 vac, that tells me there maybe something wrong the Tstat. The Board doesn't like that weird voltage and will either work erratically or not at all. That's my opinion for what's worth, but then again, I wasn't standing in front of your furnace. All the best in 2023,,,
That's exactly what I was thinking. Those stupid thermostats now have relays/contactors in them to call for Heat or Cooling and those contactors/relays are super cheap and burn up/get corroded from sparks jumping when switching on and off.
@@johnbrizendine4425 True... a quick jumper applied over R and W would tell you if this is true..
@@daveoswald5375 Yep, I have had 2 cheap digital thermostats have the relays inside go bad in the past couple of years, and that's exactly how I was able to diagnose the issue, that and measuring the voltage across the red and ground and then across the relay inside the thermostat. Seems like everything is being made cheaper and cheaper and just doesn't last anymore.
@@johnbrizendine4425 Yes, I agree.
Yep, I been there, reverse engineered and drew up a schematic for the version of white rogers gas furnace PCB my unit uses. The common failure mode on these is micro fractures in solder joints due to the board being mounted o the air handler squirrel cage. vibrations were taken into consideration which is why the board is coated with a resin coating that smells like pine sap when heated with a soldering gun.
My furnace no longer runs using this control system since I engineered my own system based around an ESP wifi device as the main MCU and a series of SSR's and sensors.
An ESP WiFi device as the main MCU with SSR's and sensors sound like a very neat project. Have you ever done any demonstration videos or tutorials for it?
@@JonathanDeWitt1988 no I haven't, I don't usually think about that sort of thing when I do stuff like that. My thing is when I have a problem I find the cause and then engineer and fabricate a solution. sometimes those solutions fail horribly but usually they turn out working better than the factory version. Besides that, I no longer live in that home with the gas furnace. I did revert the furnace back to factory spec with it's original PCB when I sold the house. and moved. I have an electric system now and it in itself is way over engineered for what it does but it has not given me a reason to go into messing with it yet. If it isn't broke, don't fix it.
So sorry to hear you got super sick. I hear the flu bug this year is a real whopper. Glad you are feeling better. Thanks for the video and hope you can repair the board. Take care and have a wonderful holiday season with your family and friends.
24v AC is required for these HVAC control boards, using AC simplifies design as there is a tap off on the or a transformer in the unit which removes the need to have AC to DC conversion, DC conversion happens locally on the control board(s) for the MCU(s) & relays.
I'm Glad you are feeling better, I miss your video's
Paul, Sorry to hear you got the flu. I contracted "A" on Tday, 4 days later I felt better, went to work, and then it rebounded with upper respiratory stuff. Weird cause all 3 times with the vid I was down a couple of days.
"I know a little about electronics". That is fun-ny!
@ 6:08 a little upper right from center looks like a cold solder joint. I had a problem with the board in my furnace and all it was, was a bad joint on one of the spade joints. Looks like it might be the two pin Molex connector. Take care!
All units have the schematics somewhere in plastic sheath attached to the panel. Use it to ohm out the circuit. Often there are bad solder joints that arise from the air handler vibrations over 10 years. Can spot them often with strong magnification.
AC Service Tech channel is a good source for the overall operation of the furnace sequence. He doesn't go deeper than isolating it down to a bad board after the external components are confirmed. This looks like a standard 24vac signal system. The mains go into an around 40 VA 24vac secondary transformer then to the board, through the fuse. R is hot, C is common, the thermostat switches R to W for heat, Y for compressor, G for fan, etc. C may not even be wired to thermostat if it uses batteries.
On the board however, you do have mains power, but that may only be feeding the relay outputs. Likely there a rectifier to 24 DC to operate the relay coils and chip. If your system used flame rectification for flame proving, then there will be an inverter sending out 80-150 vac to the flame sensor probe with the board looking for around 10 microamps DC via the chassis ground. You have a XFRM-H which may be the transformer hot with the return commoned to the chassis and through the board grounding. If so, the 24vac is what powers the board, sends out the 24vac on R and the senses the return on the W, Y, G, etc.
Merry Christmas and a Happy New Year.
It's 24-volt AC but put the 24 volts on the R and c terminal to power the board with the 24 volts. Next, put a jumper wire from the R and W terminal to activate the heat relay. For cooling, the jumper would be from R, Y, and G terminals. For the fan only it would be from the R to the G terminal. You will hear the relays click, and from that, you should be able to test if the contacts are good. Its almost always the relays.
Terry is correct. The R and C terminals would receive the 24v power with the board out of the furnace. The 24 vac would normally come through the large molex plug above the fuse.
Get well soon
If we have information on what the thermostat control signals should look like, there's no need to go out and get one. Do we get to assume the voltages and current limits on this unit fall under an industry standard?
Merry Christmas. Sounds like the Furnace tech was just replacing parts. Im sorry for that.
Pull the relays and open it to check the contacts for carbon build up or pitting. That would cause a drop in voltage. You would have a 120v transformer for your furnace that steps it down to 24VAC. That capacitor may be a Lisle brand and they last a long time. Haven't had any I tested go bad. I rebuilt my furnace board since it is 20 years old. Replaced all the caps and re flowed the solder joints on connectors and any relays that had suspicious solder joints.
Plenty of useful comments here already so I won't add any additional value here, however I will say I used to (once at the start of the season) blow out the inducer motor tube and gently suck to make sure the pressure switch closes on my old furnace. However, the issue you described seems to be with the board and possibly the thermostat (like the tech said). Test all the relays. Good luck and this will be a good little project to work on since you now have heat ($700 later!). Continue to feel better!
Does the Inducer motor turn on? PAUL did you troubleshoot which Relay is not turning on?
I'm no HVAC engineer, but there may be a slight hint in what the engineer said. 24V then 0V then 16V may indicate a faulty thermostat (if this was pure mechanical then dirty contacts etc) or something on the board is dragging the 24V AC down. A follow up video would be interesting (perhaps after a test by one of the commenters offering to help)..
My furnace experience just cost me $5000 (for the cheapo model). Amazing what the price of things are nowadays.... Get well soon....
The fuse that you were looking for is good. Nevertheless, the micro filment inside wasn't separate
I used to work in the HVAC industry several years ago, and you need 24 volts AC. I could go through the 'cycle', but it would take too much typing on my phone, I'm currently with no internet other than the little bit I can get on my phone at the moment. Pretty simple actually. The startup procedure is set up to be 'safe'. 😉
line and neutral on upper left of board, probably 24 ac. short w&r should call for heat, IE inducer, when it sees vacuum (low or high state), starts burner, temp sensor starts blower.
I'm an HVAC contractor in California. Send it to me and I can test and repair if financially viable. I have a test jig I built for just this purpose. You won't hear any relays click with the molex plug removed. The most common problem with these boards is the relays and the flame sensor circuit. They DO require 24 volts AC but will work fine with 18V. They also produce generic universal replacement boards much cheaper than the original manufacturer. Also, if you can, send me the old stat. Bad stat's are not that common.
Email me arduino0169@gmail.com
That is a white Rogers 50A55-743, I have the same board, same code. Inducer motor fires up, failure to release gas. I feel this pain.
Is that orange disk a varistor?
24 volt power will go to bard at terminals above relays. that will send 24 volts down to r and c terminals. then you can jump r to w, but since there are no limits connected you will need to know what pins to jump at the 8 pin connector. I am a retired HVAC tech with 40 years exp.
Should be able to tell what pins need jumped from furnace wiring diagram. if you just put 24v to board with no pins jumped led should flash a code.
The electrolytic looks good from the outside, I'd leave it alone. Let's see if there are schematics for that board.
Then measure resistance of coils on relays more than likely one is burnt.
You have to try 24 volts ac at terminal R and c .then you will see it will power up or not
Add power then tap on relays too see if one is stuck.
My heater isn't working either! Someone said I need to put wood in it as it's a wood burning stove! REALLY???
Hope you get your flu under control and have a great Christmas and New Year!!
Happy christmas to you all Paul
Paul, Whats the value for C3? I have one that is burned out from a friend of mine
There should be 2 spade connectors labeled xfr for transformer
The terminals are just pass-through. In order to power the relays you have to get power to that multi pin connector.
I been HVAC tech in Okieland for 20+ years and I'm gonna guess it's got broken solder connections, bad caps and / or bad contacts in the relays.
Or it could have been corrosion on the wire connectors. Sometimes you can just pull all the wires and put everything back together and it will go anther ten years.
I have seen them run fine for 10-15 minutes and then start acting up, cutting on and off.
Mine would run 10-12 hours and then just click.
Not hvac guy, but I always thought they were 24 VAC.
There are many neutral connectors. Suggests 24v ac.
Nice video 👍
Last one I fixed it was bad connection behind 12 pin connector.
yeah flu hit my house this week. hope you get well soon too
guess sounds cheap. lol you made a logical assumption 😂
Your HVAC guy most likely sold you a thermostat that didn’t need to be replaced. But if you went from a mercury bulb to a digital, why not.
Sled down the hill, Weeeeee!
Glad you are getting better.
$700 for all the guessing, ouch.
You can probably purchase those relays, replace them all and have a newish board.
Merry Christmas to you, Blake, Doggly and the rest of the family.
you spend the money because they know how to solve your problem. Everyone is wiser after the fact ;)
You are not alone with the flu….if he thought it was the thermostat did he jump out r to w? R is 24vac out and then a relay (or old school thermostat) is what kicks on heat… I am an hvac controls tech… more industrial type stuff though, but troubleshooting you always cut it in half, ie cut out the thermostat… still doesn’t work? Safety circuit? Not tripped still doesn’t work? Then start looking at this board but you see that’s like 30 mins of troubleshooting…. Not 700 bucks lol
Just by the micro-controller it says "24VAC"
Lol just fixed mine last year. Ya a relay and 1 diode on mine lol.
I tell you, that is the Worse. When you end up spending money in something you know you can fix for cheap or free. All because it happens to be an “urgency”. But it is what it is.
Fix that board and sell it and get some of your money back.
Get well soon!!
Not working and no codes I bet thermostat got hit after relay shorted. Board should try to start furnace 3 times before it goes into safe mode. If no codes usually thermostat malfunction.
If you feel better and, If you haven't, and you have some handy, snap some toroids on the thermostat wire. When I operated amateur radio at the old house with a pulse21 gas furnace I think it was called, I noticed that after getting on the air, went to bed, got up to go to work and the house was cold. Came home, and the house was VERY cold, like "I might just bust a pipe" cold. Turned the thermostat off and on again, everything was fine. The next weekend, the same thing happened. And then I understood it. So I put some toroids on the line both at the thermostat and at the furnace because they are cheaper than heat tape and especially busted pipes, and that was that.. All the Best! 73 DE W8LV BILL
Just cost me 800.00 for pump and control board and service man.
when your down then something just happen to happen .
Feel better man, it’s a nasty one.
Probably a relay
BS on the thermostat
So sorry to hear you got super sick. I hear the flu bug this year is a real whopper. Glad you are feeling better. Thanks for the video and hope you can repair the board. Take care and have a wonderful holiday season with your family and friends.