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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 21995
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I have (2) Lennox LF24 commercial gas heaters with the same

Customer Question

I have (2) Lennox LF24 commercial gas heaters with the same issue. Units turn on and ignition lites but no fan. Control board lites says the board is working. Is the control board the issue?
Submitted: 7 years ago.
Category: HVAC
Expert:  BillyHvac replied 7 years ago.
If you have a meter the best way is to see if when the board says it is energizing the fan if it has voltage on the terminals going to blower. Or if the blower is turned on by heat instead of a circuit board timer you could see if power is leaving the heat activated blower switch.
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Customer: replied 7 years ago.
There is no power coming out of the fan terminals when checked with a DVOM.     The board blinks slow with power applied, then faster when starting, after ignition starts( I can feel heat but no blower) unit lite blinks 4 times . The code says that is "open high limit or flame sensor."   Can it be cleaned or is there a reset ? The furnaces are Lennox LF24-200A-2
Expert:  BillyHvac replied 7 years ago.
Ok, sorry for delay but I wanted to make sure before I shot my mouth off. Your circuit boards are defective. Once the gas valve energizes the timer on the board starts and turns on the blower. If you have had any power surges or generator use this would contribute to an early failure. I appreciate your extra payment and more importantly your patience. I am available for follow up.
Customer: replied 7 years ago.
Would these be the 19W94 boards??? They cost over $250.00 each......No other possibilty?????
Expert:  BillyHvac replied 7 years ago.
Yes those are the boards and yes there is another way to make the unit work. The flame rectification and limit is obviously doing their job by shutting down. Therefore the only thing you really have to do is get the blower to kick on. I do not know how far you want to go on this but you could put in a $10 relay to turn fan on when gas valve makes or even throw in a $20 delay on make timer. I am not an advocate of circumventing controls and part functions, but really in this case you just want the blower to turn on. You coul even put in an older version heat activated blower switch. This all depends on how creative you want to get.
Customer: replied 7 years ago.

Thank you but no. This are in my warehouse and have commercial renters.......No a good option but thank you for the advice.    
Expert:  BillyHvac replied 7 years ago.
I understand totally. I commend you for wanting it the right way.
Customer: replied 7 years ago.
The installation instructions for the 19W94 board are for the 80MGF(X)-1/3 and 80MGF(X)/G24M(X)-2/4/6 UNITS. My model in the LF24-200a-2 units.    They where installed in 1997. I do not know which wiring diagram to follow. Any ideas?????
Expert:  BillyHvac replied 7 years ago.
can you obtain the number off the previous circuit board?
Customer: replied 7 years ago.
turns out that my boards are the 19M54 part number. I do ot know if they are a direct fit or take slight modification in wiring due to upgraded board> DO YOU KNOW anything about them?
Expert:  BillyHvac replied 7 years ago.
Yes I have dealt with them. I simply need to know what the original control was. 43K49, 43K4901, 19M54, 19M5401??

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