Difficulty understanding furnace circuit - Page 1
Forum section: electronics / projects

author: zphaze
author_rank: Contributor
timestamp: January 16, 2025, 02:49:09 am
content: Hi,I'm not too familiar with this kind of schematic and would like perhaps an explanation of what does what and what triggers what.K1 and K2 are fan relays to control the blower motor. There is a 24V transformer to power low voltage equipment such as the thermostat that controls the heat stages (2 heat stages each controller by "time delay relays" R1 and R2 which power heating elements).I don't understand how K1 and K2 work. The symbol should tell me something I suppose, but it doesn't. What does ports 1 thru 6 do? I understand the 24V is probably the coil to control relay on/off... But then why is there a diagonal bar between ports 6-5 on K1 & K2? Are ports 6-4 on K1 shorted?Does R2 controls also motor going on through K2?!Can the speed wires on the motor only be under tension one at a time or can e.g. yellow and blue be "hot" at the same time.Is all the black wires "neutral"?!Sorry if I look confused.Full manual is here :https://images.webfronts.com/cache/frlogaiivpff.pdfSee page 9 my model is 21B20M.

author: moffy
author_rank: Super Contributor
timestamp: January 16, 2025, 03:27:22 am
content: The K relays have one NO(normally open) and one NC(normally closed) contact. The line through what looks like a capacitor in K1 is the symbol for an NC contact, the other inside the relay that looks like a capacitor is the symbol for a NO contact. The little circle with the 240V AC inside is a 240V AC coil that activates the relay. The motor has a run capacitor that helps create a phase shift or pseudo second phase so that the motor can rotate.

author: coppercone2
author_rank: Super Contributor
timestamp: January 16, 2025, 03:27:34 am
content: it looks like its AC coupled

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 16, 2025, 04:31:50 am
content: That is known as a wiring diagram, not a schematic. It attempts to show things with a physical representation of the components rather than a more abstract drawing with symbols only which is what a schematic is.K1 controls fan on/off. K2 controls low/high speed.I believe there is a mistake in the drawing as the heater elements are only connected to L1 with no connection to L2 so I don’t see how current could evey flow in the heaters. It appears to me that the red wires on the right side of B1 and B2 should connect to L2, not L1.There are no neutral wires anywhere, only L1 and L2. You should have 120V between L1 (black wires) and ground, the same between L2 (red wires) and ground. Between L1 and L2 should be 240V.Yes, both fan wires will be “hot” when in high speed with both sets of elements on. This occurs when you have 24VAC as measured between C and both W1 and W2 of the control terminal strip.Could you please tell us why you’re troubleshooting this? As in what’s wrong? What isn’t working and we could perhaps help with more than just explaining connections.

author: bdunham7
author_rank: Super Contributor
timestamp: January 16, 2025, 04:37:59 am
content: Quote from: zphaze on January 16, 2025, 02:49:09 amThe 24VAC is the coil that energizes the relay. The two vertical lines represent contacts that are normally open and then close when the relay is energized. The NC designation and the diagonal symbol indicate contacts that are normally closed and open when the relay is energized.On the fan motor, only one of the R/Y/Bl (presumably red, yellow, blue) wires should be energized at any time. The inter-operation of K1 and K2 ensure that. If K1 is energized, the blue wire is hot and no power goes to K2 and thus R or Y. If K1 is not energized, then power goes through its closed contacts to K2 which then selects which of the R or Y wires is energized.

author: zphaze
author_rank: Contributor
timestamp: January 17, 2025, 04:45:29 am
content: Quote from: WattsThat on January 16, 2025, 04:31:50 amIt seems you're right there was a mistake in those schematics. I pulled another PDF manual from a website that seems corrected.See here :Regarding what I'm trying to troubleshoot, my blower motor stopped working when I came back to my house in the holidays (7 days vacant). When entering there was a burnt smell into the whole house and motor wasn't working anymore. The motor I had only lasted 6 years. It was a 115V rated motor, yet I have a 230V furnace... it was changed in 2018 from a service call by the previous owner. But I just want to understand the wiring diagram better because I'd like to be more familiar with it.Quote from: bdunham7 on January 16, 2025, 04:37:59 amYou seem onto something.I don't see how K1 and K2 are mutually exclusive but I might be confused. I don't understand much what the G terminal does (call for fan). I suppose it's a relay "shorts" to the 24V "neutral" of the transformer? Or is it the other way around?Say thermostat turns on and asks W1 and W2 (stage 1 and 2 heat) to turn on.24V common out (C) is connected to K1-3 and (G) is neutral so K1 "closes" which means speed is MED-HIGH (blue is hot?).But then W1 and W2 are both at 24V neutral the HEAT light would be on... so if K1-3 is still 24V+ then R1 turns on because both of its coils are between K1-3 at 24V+ and W1 is at 24V-. But then R1-A1 and R2-A2 are shorted and R2 coil is energized because W2 is at 24V- and R1-A1 is 24V+.Since R2 is on, R2-M2 is at L1 which means K2-3 is at L1 and K2-1 is at L2 so K2 turns "on" its coil being energized. K2-6 and K2-5 are disconnected but K2-4 and K2-2 are shorted but K1-6 is open-circuit... so at the final only M blue wire is hot and motor runs MED-HI yet again.But... when R1 is ON, and K1 is OFF (coil off), K1-5 is at L1 which means Motor-Red is also ON. But in which circumstances would that even happen? (G) would have not to be at neutral and never ever would the thermostat call the heat without the fan, no? I'm confused, it's almost like it's not going to happen ever...Do you guys agree or I'm missing something.

author: bdunham7
author_rank: Super Contributor
timestamp: January 17, 2025, 05:39:03 am
content: Quote from: zphaze on January 17, 2025, 04:45:29 amI'm not sure. Let's go through the operation of each mode.If no functions are called, then K1 and R1 are deenergized because their coils are connected to 24VAC common but not power. R2 is deenergized because its coil is not connected to common although it is connected to 24VAC power if the season select switch is closed. K2 is deenergized because the coil (3) is not connected to L1 through R1 but the coil (1) is connected to L2. If the continuous ventilation switch is closed, the fan will get power to the red wire through K1 (5-6) and then K2 (6-5) and run on LOW.If CALL FOR FAN is on (the thermostat applies 24VAC to any function it wants on) then K1 is energized and the motor receives L1 through K1 (2-4) and runs on MED-HIGH (blue wire). Note that the call for fan function is typically independent of the heat functions. If the CALL FOR FAN function is on, then the fan will always run on MED-HIGH no matter what else happens. K2 will have no further effect because it is out of the circuit when K1 is energized since it receives L1 through K1 (5-6) when K1 is off.If CALL FOR HEAT 1 is on, then R1 will be energized, turning on heating elements 1 and 2. In addition, L1 power will go from R1 (M1-M2) through K1 (5-6) and K2 (6-5) to the red wire, running the fan on low. Finally, the common side of the R2 coil will be connected through R1 (A2-A1). If the season select switch is closed, R2 will be energized and the system will behave as if CALL FOR HEAT 2 was on.Note that CALL FOR HEAT 2 would not be on with CALL FOR HEAT 1 off--that is how the thermostat must work and in this system, a CALL FOR HEAT 2 by itself would not do anything.If CALL FOR HEAT 2 is on, or CALL FOR HEAT 1 plus season select as noted above, then R2 energizes and provides L1 power to heating elements 3 and 4 and also to K2 (3) which energizes it. Power this is routed through K2 (4-2) to the yellow wire and the fan runs on MED-LOW.Hopefully that makes some sense.

author: zphaze
author_rank: Contributor
timestamp: January 18, 2025, 12:04:53 am
content: Thanks a lot for your help, I think we're on the same page regarding how the circuit works.One thing though I still wonder...Does anybody find this "normal" I had a 4 speed 1/3hp motor115vrated 5kcp39hg from Genteq?The furnace is clearly 240V with no 120V transformer anywhere. It lasted 6 years from the previous call made from the past owner.Isn't that strange that it didn't fail earlier?Or are 115V motors fine running 230V? (I doubt it's fine I mean... the windings are seized for 120V operation, the speed won't increase as these are PSC/Induction motors but the torque would... and the current as well to a certain degree, no?)

author: bdunham7
author_rank: Super Contributor
timestamp: January 18, 2025, 12:23:33 am
content: IDK how long the motor would last on 240V, I'm guessing not long but who knows. You can set this up for a 115-120V motor by simply connecting the "O" wire (most likely white, not orange) to the neutral instead of L2. I'd check to see where that wire is actually connected and if it is to L2 and you are using a 115VAC rated motor you might want to change either the motor or the position of the wire.

author: MathWizard
author_rank: Super Contributor
timestamp: January 19, 2025, 05:27:03 am
content: So how long do the high voltage contacts last in old furnaces ? How much worse are newer ones thes days ?How do they compare with say the HV switch in a floor heater I have. IDK how old it was, but the 2 contacts were fusing together a bit. I had to sand them flat again.And If I was using it all the time, I'd have to do it regularly, and I think their was pitting too. So it wouldn't last decades, like I'm thinking old furnaces switches did.