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JBM. Lennox G26. Furnaces runs normal for approx 30 secs…

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Hi JBM. Lennox G26. Furnaces...
Hi JBM. Lennox G26. Furnaces runs normal for approx 30 secs then blower motor stops. Furnace continues to run until limit switch tripped. Throws "Primary/secondary limit switch open" code. I cleaned the flame sensor and installed new limit switch so far.
Submitted: 8 months ago.Category: HVAC
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Answered in 38 minutes by:
12/16/2017
HVAC Technician: HVAC Guru, HVAC TECH replied 8 months ago
HVAC Guru
HVAC Guru, HVAC TECH
Category: HVAC
Satisfied Customers: 18,751
Experience: 20yrs in the HVAC industry with certificates and degrees.
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One moment please..Note it takes time to type and add links so please be patient. Also the site will send you a phone call offer, that is not me.

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HVAC Technician: HVAC Guru, HVAC TECH replied 8 months ago

Can you see what limit is opening?

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Customer reply replied 8 months ago
I'm not sure how to tell which limit it is. I replaced the temperature limit switch. At first I couldn't get the blower motor to turn on at all, then after reading that it might be the vacuum switch, I blew into one of the vacuum tubes and that seemed to get the furnace running for 30 seconds.
HVAC Technician: HVAC Guru, HVAC TECH replied 8 months ago

This is likely a thermal rollout issue. There are high limit safety thermostats that are around the burner housing area that can trip open if they fail or the unit is overheating. Some of these have reset buttons on them that you can manually press down to reset while others do not. The ones that do not either are one time limits or are limits that are suppose to manually reset when cooled down. Now that does not always happen when they go bad so you will have to check them for continuity with an ohm meter across the two leads going to them. If you do not pwn a ohm meter you can physically bypass each rollout to make sure it is good by connecting the two wires to each rollout sensor together to see if the unit will then operate. Now once you have found the bad one you can change it out or reset it and then you should check your furnace filter for blockages and make sure you main furnace blower is running to speed. These two things can cause a unit to overheat and trip one of these rollout sensors. Mosts units have 3 of these devices and here is a link to what they can look like on your unit.

CLICK HERE

CLICK HERE

CLICK HERE

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Customer reply replied 8 months ago
I have the first two limit switches. The one with the reset button doesn't push in or pull out so not sure if it is tripped. I did bypass it but furnace still doesn't stay running.
What would happen if the vacuum switch was bad? Would furnace run at all?
Customer reply replied 8 months ago
How do I check if the motor is running to speed? It seems to be running normally.
HVAC Technician: HVAC Guru, HVAC TECH replied 8 months ago

If the pressure switch opened nothing would run. Can you upload a picture of the wiring diagram on your unit here for me to check out?

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HVAC Technician: HVAC Guru, HVAC TECH replied 8 months ago

Hi there,
I was just touching base with you because I have not heard back from you and the question is still open and unrated, I am not credited anything until a good star rating is supplied I would really appreciate it if you could do that thanks!

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