Carrier 50M56U-751 High Temp LImit switch

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Old 12-04-19, 07:01 PM
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Carrier 50M56U-751 High Temp LImit switch

I have a carrier single stage furnace that I replaced the control board in last year as I wouldn't fire up. It's worked fine until now. The error is gives is code 4 high temp limit switch. It tries to fire and the igniter glows but then shuts down. My question is I understand the high temp limit switch is a fan limit switch but is it an integral part of the control board on this unit? If so I'll need to replace the whole control board again.

Thanks guys!
 
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Old 12-04-19, 07:32 PM
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Also the fan is stuck on and wont stop running.
 
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Old 12-04-19, 07:34 PM
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It’s a separate part.
Is the burner firing or does it not make it past ignition trial?
 
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Old 12-04-19, 07:36 PM
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igniter heats up and then shuts off - the burners fire for a couple seconds and then cuts out.
 
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Old 12-04-19, 07:41 PM
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Sounds like the limit is stuck open. You can test it with your meter.
 
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Old 12-04-19, 07:49 PM
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ok will do. I imagine they cost less than the control board. Maybe I should just replace it anyway (if it is stuck it would surely stick again if I managed to get it working).
 
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Old 12-04-19, 07:53 PM
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how can i tell which one is the limit switch? from the pictures I've seen online a rollout switch looks very similar to a limit switch.
 
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Old 12-04-19, 09:13 PM
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Limit is in the discharge airstream above the heat exchanger.
The furnace isn’t supposed to cycle on the limit so I’m sure you’ll find the cause after you get the limit issue taken care of.
 
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Old 12-04-19, 09:20 PM
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thanks roughneck. do you mean i’ll find the cause of what stuck the limit switch open? i had this exact same problem last year and a guy who tested it told me it was the board and that the limit switch was fine. i replaced the board at that point and it worked fine for 10 months but its done it again. is it possible the limit switch is burning out my board?
 
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Old 12-04-19, 09:23 PM
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No, it’s possible there’s a reason the limit tripped or was tripping. They are meant to trip during an unsafe condition, not during normal operating conditions.
 
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Old 12-04-19, 09:33 PM
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got it. ill figure out limit switch and see what pops up next.
 
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Old 12-05-19, 07:51 PM
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update:

I had a furnace company come out today and they determined that is it the rollout switch causing the failure. They said it's probably due to too much static pressure which is caused by the inducer motor being near end of life (making noise) and needs replaced. They said the inducer motor is $1620!!! No way it's that much. I see inducer motors all over the web for $100. But I can't find a parts breakdown for this furnace to get the proper part number so I can order one. Can you help? It's a Carrier 58STA070-11108 furnace. resetting the rollout switches and leaving the lower door off the furnace is running. But if you put the lower door on the rollout switch trips.

I think it might be this one: 326628-761 but this is a complete assembly with fans etc and I just want the motor.

thanks.
 

Last edited by the_dude; 12-05-19 at 08:45 PM.
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Old 12-05-19, 09:13 PM
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Not so sure the draft inducer is the only problem.

However..... that motor should be available but can be difficult to get the old parts off the old motor.
Check the link. See if it's the one you need.
Draft Inducer Motor HC21ZE121
 
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Old 12-05-19, 09:23 PM
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thx Pete thats probably it. what else could it be? the rollout switch went again even with the door off the furnace and no filter in the furnace. so no heat again tonight
 
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Old 12-06-19, 07:03 AM
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Is it the rollout tripping?
That furnace is not on the lawsuit list for secondaries, but it could still have a heat exchanger problem.
 
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Old 12-06-19, 07:51 AM
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Yes the rollout is tripping. The inducer sounds unhealthy so I'm going to replace that today and cross my fingers. Might also add an additional cold air return to help with static pressure.
 
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Old 12-06-19, 08:01 AM
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The inducer can be ruled out with a manometer.
Constant tripping on the rollout is concerning to me. What is the gas pressure?
When it fires does the flame pattern change when the blower comes on?
 
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Old 12-06-19, 08:10 AM
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The furnace guy didnít do those type of measurements and unfortunately I donít have a manometer. I will look today though at the flame pattern to see if it changes. The inducer is making a low pitch humming/grinding noise when running so at any rate if needs to be changed. If that doesnít fix it Iím going to have to call another furnace company. I just donít want to get to the point where Iíve put so much money into it I would have been better off with replacing the furnace.
 
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Old 12-06-19, 06:08 PM
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A so called pro who doesn't have a manometer really has no business working on a furnace.

----------

A failing inducer alone won't normally cause flame rollout.

The pressure switch should not be staying closed when the inducer is running so slow the burner box is overheating.

It's possible that you have a bad secondary heat exchanger in addition to a failing inducer.

When the secondary heat exchanger gets blocked up, the pressure switch sees a normal or above normal pressure difference. It's measuring between the difference between the collector box behind the inducer and the burner box.

The negative pressure in the collector box increases as the secondary fails so the switch stays closed even though there isn't sufficient flow.

Replacing just the inducer assembly may temporarily stop the rollout tripping.

Changing it with the rollout tripping is a huge gamble because you could spend the money then be forced into spending more on heat exchanger replacement labour.

At a minimum the pressure differential should be measured* and the blower assembly pulled, secondary inspected before spending money on parts.

*If the differential is normal or above normal your secondary is shot.
 
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Old 12-06-19, 07:56 PM
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#s show it to be an 80% furnace.
 
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Old 12-06-19, 08:15 PM
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my bad.

yah, watch the flames as the burners come on.

the blower can be pulled to inspect the heat exchanger.
 
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