Replymistake....new module is 780-845
Professional Member
friend of mine had to replace his ignition module on Lennox unit model # G20Q3E-75-4
the original module was SP745 robert shaw
the replacement part they gave him is also Robert shaw 80-845
he said he went wire for wire but he said the blower wont shut off even disconnecting tstat, the blower stays runnin. I havent been there ...Im just throwing this out here to see if anybody knows if it could be MODULE related. I told him to check contactor to see if its stuck in. But not being there, i couldnt really say much else.
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Professional Member
mistake....new module is 780-845
“If I have seen further than others, it is by standing upon the shoulders of giants.” ~ Sir Isaac Newton
I'm learning all the time, the tombstone will be my diploma!!!
Did you ever stop to think, and forget to start again?
Professional Member*
Was the blower runnign constant with the old module?
Look really close at the limit switches/connections, some modules will run the blower when a limit opens up.
Global Moderator
If your friend accidentally crossed the "W" and "limit" wires on the BCC2 control board, it will cause the fan to run continuously.Originally Posted by Tommy1010
Instead of learning the tricks of the trade, learn the trade.
Professional Member*
ignition module has NO control over fan operation. fan is controlled by blower control board.
if limit switch is open fan will run constantly and no power will be supplied to thermostat or ignition module.
Professional Member*
Nope
No Fan Control
Tell him to follow the voltage.....
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When the thermostat calls for heat, the 780-845 control simultaneously initiates ignition sparking and opens the pilot valve portion of the gas valve. Pilot flame recognition stops ignition sparking and opens the main valve portion of the gas valve. Pilot burner flame is continuously monitored at a synchronous frequency for the duration of the heating cycle. Should the pilot flame fail during the heating cycle, the control will shut off the main valve until the pilot flame is established.
The 780-845 lockout ignition control provides 90 seconds of spark, followed by a 6 minute time delay (purge) between ignition attempts. After three tries, if no pilot flame is sensed, the control goes into a one-hour lockout period. At the end of the lockout period, if the demand for heat is still present, the unit will repeat the three tries for ignition.
During lockout, you may override the auto-system, and manually reset the unit at the thermostat. The green diagnostic LED will be on continuously for normal operation. The LED will be off for internal lockout and will flash continuously for problems external to the control (ie. not sensing flame).
Global Moderator
Another thought - has this G20 unit been modified with the OPTIONAL "Continuous Fan" switch?
If it has, and your buddy is familiar with the BCC2 control board, and capable of troubleshooting SPDT switches, I would have him check that also.
If you are heading over to look at the unit, let me know, and I will get a manual with wiring diagrams off to you via email.
Instead of learning the tricks of the trade, learn the trade.
Regular Guest
Make sure you check the heat exchanger
Professional Member*
Now this is why i like this site...VERY good information....
Professional Member
I agree, I suspect a bad control board if the blower is running constantly. Does he have a himidifier? I know with the aprilaire 400 they want you to wire it in such a way that it humidifies even without the system calling for heat. Simple fix but just make sure you checkout everything befor condemning parts. Let us know how it turns out.
Professional Member
We are Lennox dealers, I find bad hi limit switches often that cause this issue. With the furnace cool, check the high limit. I replace oil high limit switch with Texas Instruments brand much better luck with them.
Professional Member
Lennox started using a universal board a short time ago and sending four different wiring harnesses to replace on the different furnaces. I had to replace a board on a g61 and the original board used 11 of the 12 pins on the mullinex. New board used all 12. Blower always ran till I figured out the old harness wouldn't complete the limit circuit. Changed harness and shagged a$$ outta there
Professional Member*
Why were you given a WR control for that furnace?
Should be a black honeywell, 30w33 is the part number from Lennox. The directions make this a brainless swap out.
Comes with a new flame sensor and ignition wire among other things.
Professional Member*
That's not a universal board, it's an update to bring the dash 6 and below G61 up to the dash 7 and later style board/igniter.
Same with the G32.
Think that's bad? Just wait til you have to start carrying parts for their new furnaces.
The ML193 is up to -4 and has 3 boards, none are cross compatible.
Then there's the pressure switches
New Guest
I had this exact problem when changing out the Ignition control on a Lennox G20 Whisperheat. The original module was a Robertshaw 735L. I replaced it with a Robertshaw SP845 780-845. The 735L was starting to buzz pretty bad so I sought out to replace it. After replacing it with the SP845, the ignition was perfect and the buzzing went away. I was high fiving myself. After the furnace shutdown, the fan would not stop running. Checked all connections which were fine. Tried swapping the limit and W leads as mentioned earlier on the BCC, no fix. The BCC board was the original LB-63622A. Fortunately I had the updated BCC ( 48K98) as a spare. I figured the SP845 is not completely compatible with older BCC boards so I installed the 48K98. Bingo Bango, using the SP845 with the 48K98 shutoff the fan properly after the heat cycle ended. I'm sure your buddy fixed his furnace one way or another since its and old post but this is good info for others seeking help on the internet.
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