How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Phil Your Own Question
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 5648
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
23932409
Type Your HVAC Question Here...
Phil is online now
A new question is answered every 9 seconds

I have a Lennox ML193UH. The furnace turns on and then off

Customer Question

I have a Lennox ML193UH. The furnace turns on and then off right away. The diagnostic light is steady red. We had a tech come out and he couldn't find anything wrong but it seems to turn on and off constantly.
JA: I'll do all I can to help, Lennox ML193UH. Tell me a bit more about what's going on so we can help you best.
Customer: The furnace turns on and then off right away. The diagnostic light is steady red. We had a tech come out and he couldn't find anything wrong but it seems to turn on and off constantly.
JA: Is there anything else the HVAC Technician should be aware of?
Customer: The Humidifier is only showing 26% but it is set at 5 so it should be higher.
JA: I'm sending you to a secure page on JustAnswer so you can place the $5 fully refundable deposit now. While you're filling out that form, I'll tell the HVAC Technician about your situation and connect you two.
Submitted: 8 months ago.
Category: HVAC
Expert:  Phil replied 8 months ago.

Hello, I need to know how many seconds the furnace is on before it goes off right away.

I need to know the approximate age of the furnace.

And I need to know if the furnace is heating the house or not.

I need all 3 bits of information to help diagnose the problem, we can go from there.

Customer: replied 8 months ago.
It goes off within a second or two. The furnace is a little over a year old. It seems to be heating the house. I think the problem is tied to the humidifier (Aprilaire 600c). We just turned the humidifier down to a setting of 2 vs. 5 and the furnace stopped turning on and off. I don't think the humidifier is working so I think I need to replace the solenoid.
Customer: replied 8 months ago.
Do you need any more information?
Expert:  Phil replied 8 months ago.

Hello again, the thermostat may be programmed incorrectly, for the wrong type of system compared to what you have, and the differential temperature set to the default of 0.5F when most systems work better at 1 to 1.5F temperature differential settings.

Looking into that range if issue will be step number 1, after that, it sounds like there might be wiring errors.

I need the brand and model number of the thermostat, it is stamped on the back after you gently pry the thermostat from its wall mount base.

We can go from there.

Customer: replied 8 months ago.
It's been working fine for a year since we moved into the house (newly built). I can check the differential temperature setting to see if that's at 0.5F. I really think it appears to be related to the humidifier because when we set it down to 2, the furnace stopped acting up. I don't think water is getting to the humidifier so I'm going to try replacing the solenoid and replacing the water panel.
Customer: replied 8 months ago.
Also, I isolated the thermostats by turning each off and leaving the other on and the furnace still acted up. Setting the humidifier down to below the current humidity in the house seemed to stop the furnace from cycling.
Expert:  Phil replied 8 months ago.

Hello again, a flaw has developed in the humidifier wiring, indeed if the water solenoid is going bad and sapping too much of the 24v power to the thermostats, it could cause the problem.

Let me know how it goes.

Related HVAC Questions