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Lennox G24M3x-75Awith RAM-3MC5 control. LED flashes slow…

Customer Question
Lennox G24M3x-75Awith RAM-3MC5 control...

Lennox G24M3x-75Awith RAM-3MC5 control. LED flashes slow steady indicating normal no heat demand. However I measure 24volts on white thermostat wire input.

Contractor's Assistant: Thanks. Can you give me any more details about your issue?

No Heat. AC OK Fan Only OK

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Submitted: 2 years ago.Category: HVAC
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Answered in 18 minutes by:
1/20/2016
HVAC Technician: Rick, HVAC Supervisor replied 2 years ago
Rick
Rick, HVAC Supervisor
Category: HVAC
Satisfied Customers: 24,631
Experience: 40+ yrs. experience as a licensed oil & gas technician.
Verified

Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.

Sounds like your furnace isn't responding to a call for heat. Could be a problem with the thermostat or thermostat wiring. First thing to try is to connect the leads on thermostat terminals R & W together. Does the furnace respond? If not then go to the thermostat terminal strip on the furnace and jump the R & W terminals. After making sure the blower compartment panel is securely in place does the furnace respond?

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Customer reply replied 2 years ago
I did that and there is no response. The LED continues to flash slow and steady. I can measure 24 VAC at the Heat input on the PCB. (White wire, center terminal) I have checked the High limit switches by continuity check from pins 11 to 14 of the 15 pin plug an also checked the rollout switch loop from pins 7 to 15. All are OK. If there is embedded error codes that have locked out the board I don't know how to clear them. FYI the unit model is G24M3X-75A-6. The existing board is a RAM-3MC5. It is obsolete and the upgrade kit is 19W94.
HVAC Technician: Rick, HVAC Supervisor replied 2 years ago

Could be a problem with the board. Since you should get a response with R & W jumped. If the inducer doesn't run either due to a bad motor or bad board then you should be seeing an error sequence on the LED. Sine you aren't it makes the board suspect.

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Customer reply replied 2 years ago
The fan will run in "Fan Only" . A/C works OK. But nothing in Heat even with the R/W jumped. The board upgrade is expensive and I didn't want to do this until all other possibilities were eliminated.
HVAC Technician: Rick, HVAC Supervisor replied 2 years ago

Nothing in your last post changes my opinion that this is a board problem. If I was on your job and determined what you have told me then I'd replace the board. You have 24 volts to the board and no response to a call for heat and no error message. That make the board the prime suspect.

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Customer reply replied 2 years ago
The best price I have found for an upgrade kit which I am told is a 19W94 is $299. I presume that if there was an lockout code in the processor that isn't cleared I would get an error code. Other similar boards I have looked at have an error code reset switch but I don't see one on this board.
HVAC Technician: Rick, HVAC Supervisor replied 2 years ago

That's a circuit breaker not a reset switch. You can reset by powering the unit down waiting a couple minutes then power it back up. If the board was functioning normally you should get an error displayed if you get no response to a call for heat.

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Customer reply replied 2 years ago
I tried that already, so I will go ahead and order the board upgrade kit
HVAC Technician: Rick, HVAC Supervisor replied 2 years ago

Ok

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