[Indyman317]

Hello I'm new here and just looking for some advice on my furnaces behavior. My furnace just recently started acting funny about a week ago and it started with the furnace not being able to warm the house up to 60 degrees without acting like it was struggling and eventually shuting down at about 56 degrees and then it comes back on and runs for a minute and it shuts everything down besides the blower and it will stay running until the system cools down, I also get a code 33 before the system shuts completely down and loses total power. The power will shut down and will not come back on regardless of how many times I reset the furnace or how many times I try to reset the main breaker, it will eventually come back on by its self after an hour or two. I have changed the filter, i removed the entire blower and cleaned it, removed flame sensor and cleaned soot. Nothing I have done has fixed it and I could really use some guidance. Thank you all

 

[user_12345a]

How do you know it's losing power completely vs locking out? If a limit is opening, after a certain number of times, furnace will lock out to protect itself.

What's the full model number?

 

[Indyman317]

How do you know it's losing power completely vs locking out?

What's the full model number?

I just assume it loses complete power because the furnace power indicator light goes out. The model is 312aav/jav

 

[user_12345a]

So it's a limit switch error. You aren't manually resetting any safety, right?

Cycling on limit can go unnoticed unless it happens enough times during one cycle or it starts not resetting.

You can verify it's the high limit with a meter - do you have one and know how to use it?

Furnace overheating caused by low airflow or being over-fired.

Check your air filter and blower wheel for excessive dirt.

Make sure none of the vents are blocked.

If you have a digital stem thermometer, measure the temperature difference between supply and return near the furnace after 10 minutes of operation.

https://structuretech1.com/diy-furnace-test/

Edit: being a 2-stage furnace, needs to be checked on high fire. 16 minute delay or if u have a 2-stage stat (W2 terminal on board has wire connected), raise the temp a few degrees above the setting.

 

[Indyman317]

So it's a limit switch error. You aren't manually resetting any safety, right?

Cycling on limit can go unnoticed unless it happens enough times during one cycle or it starts not resetting.

You can verify it's the high limit with a meter - do you have one and know how to use it?

Furnace overheating caused by low airflow or being over-fired.

Check your air filter and blower wheel for excessive dirt.

Make sure none of the vents are blocked.

If you have a digital stem thermometer, measure the temperature difference between supply and return near the furnace after 10 minutes of operation.

https://structuretech1.com/diy-furnace-test/

Edit: being a 2-stage furnace, needs to be checked on high fire. 16 minute delay or if u have a 2-stage stat (W2 terminal on board has wire connected), raise the temp a few degrees above the setting.

The limit switch on my furnace dont appear to have anyway to reset it. It always resets by its self. I do have a multi meter and I'm kinda familiar with using it. I have a laser temperature tool as well. I changed my air filter to a less restrictive one and I also blew what little dust out of the blower. The furnace was pretty clean. I can do that multi meter test but it will have to be early tomorrow, I would do it now but due to Indiana's cold weather me and the wife had to stay in a cheap motel just to keep warm until I can fix this thing

 

[user_12345a]

Monitor voltage drop across the main high limit and see if it goes up to 24v and stays there until it starts working again.

There may be a limit on the exhaust - need to verify it's the main limit, not another one.

It's not the rollout - that's manual reset.

The blower should be running continuously with an open limit circuit.

Those laser thermometers are worthless for testing air temperature.

 

[Indyman317]

Monitor voltage drop across the main high limit and see if it goes up to 24v and stays there until it starts working again.

There may be a limit on the exhaust - need to verify it's the main limit, not another one.

It's not the rollout - that's manual reset.

The blower should be running continuously with an open limit circuit.

Those laser thermometers are worthless for testing air temperature.

I appreciate you writing back so quickly. I will get a reading on that limit switch tomorrow and report back here

 

[Indyman317]

Monitor voltage drop across the main high limit and see if it goes up to 24v and stays there until it starts working again.

There may be a limit on the exhaust - need to verify it's the main limit, not another one.

It's not the rollout - that's manual reset.

The blower should be running continuously with an open limit circuit.

Those laser thermometers are worthless for testing air temperature.

Ok so this morning I got to test it. I turned off the furnace and unhooked the wires from the limit switch and test on ohms and it got a reading of 01.5 to 02.0 it bounces. The limit switch had reset so everything is working for the momment. I had the multi meter set to 20 ohms.

 

[yuri]

Let us start in the beginning.

Does the furnace lose total power. Meaning there is no red led error code lights flashing or on at all?

If that is happening then your power supply to it may be failing ( you don't have 120 volts ).

That led lights should always be on showing normal operation or a fault code.

If your door switch on the fan door is weak and not closing it may not be getting power to the circuit board. That is rare but you could try bypass it temporarily if you can connect the wires.

Also check or replace the power switch if it is a simple light switch as they can fail with age. I get a few of those every winter.

Post a pic of the circuit board. There were a batch of them that had faulty solder connections on them at the back and it may be failing and showing no led lights.

 

[Indyman317]

Let us start in the beginning.

Does the furnace lose total power. Meaning there is no red led error code lights flashing or on at all?

If that is happening then your power supply to it may be failing ( you don't have 120 volts ).

That led lights should always be on showing normal operation or a fault code.

If your door switch on the fan door is weak and not closing it may not be getting power to the circuit board. That is rare but you could try bypass it temporarily if you can connect the wires.

Also check or replace the power switch if it is a simple light switch as they can fail with age. I get a few of those every winter.

Post a pic of the circuit board. There were a batch of them that had faulty solder connections on them at the back and it may be failing and showing no led lights.

The furnace does loose total power sometimes according to the indicator light, it usually will act up if I attempt to heat my house for long periods so for example I keep my furnace on 56 degrees but if I decide to heat the place up to 65 degrees the furnace will start blinking code 33 and it will kick the furnace off before it reaches actual set temperature and sometimes it will completely lose power but it will power itself up usually by its self randomly so it could be anywhere from 30 minutes to 2 hours. The furnace is clean and has a new filter. I have checked every capacitor on the board for bulging and everything looks good. It puts out good heat and the air intake really sucks good air. Could a new chimeny cap cause this?

 

[Indyman317]

The furnace maintains well as long as I keep it low like 56 degrees, if it's high 60s the furnace seems to have a struggle which possibly leads to the 30 minute to 2 hour furnace shut down

 

[yuri]

I suspect the extra long running time is building up excess heat in the heat exchanger and furnace cabinet. That can cause it to eventually trip the limit control.

Strange part is the loss of the LED lights which should never happen.

I suspect your circuit board is dying and having a intermittent problem. It may be due to heating due to the long run times as that seems to be the common factor.

That is a older furnace and the average life of some of those boards is 10-15 years. They can look normal but have intermittent problems.

Make sure you have at least 110 volts to the furnace. If lower then that can be a problem.

 

[Indyman317]

Just wanted to add that something strange happend early today. So the furnace completely lost power and the weird part is that it turned the power off to my living room as well. The living room and furnace are on separate 20 amp breakers. It's also weird because the two breakers were not tripped and no mater how many times I flipped them they still had no power but after about an hour the power restored it's self and the living room and furnace started to get power. Hmmm could it be a electrical box issue?

 

[beenthere]

Could be a problem with the main breaker, or at the transformer that feeds your house.

 

[user_12345a]

Issues with the neutral and cause odd problems.

 

[yuri]

Where I am all furnaces use 15 amp breakers and so do circuits for living rooms.

Only stoves, dryers, elec baseboard heaters and AC's use 20 or 30 ampers.

Check the wiring in the main panel including the neutrals to make sure all are tight.

Check the wiring to the furnace and make sure it is not aluminum wiring. If it is check all connections to make sure they are very tight. Aluminum is soft and notorious for loose corroded burning connections.

 

[Indyman317]

Where I am all furnaces use 15 amp breakers and so do circuits for living rooms.

Only stoves, dryers, elec baseboard heaters and AC's use 20 or 30 ampers.

Check the wiring in the main panel including the neutrals to make sure all are tight.

Check the wiring to the furnace and make sure it is not aluminum wiring. If it is check all connections to make sure they are very tight. Aluminum is soft and notorious for loose corroded burning connections.

So today I messed around with the furnace in an attempt to try and figure it out and I think I'm on to somthing. I ran the furnace from 55 degrees to 70 degrees and the furnace short cycled and cut off at 67 degrees followed with the blinking code 33, by the time the furnace had a chance to come back on I removed the air filter and then let the furnace continue heating the house up so I went ahead and turned it up to 75 degrees and it gradually heated the place up with no problems at all. So question that's on my mind is the furnace blower motor becoming so weak that it's no longer able to suck air through a merv 5 air filter anymore?

 

[user_12345a]

Is the blower wheel clean?

Do you have a/c? Coils can get dirty.

Capacitors can get weak and reduce motor rpms.

Gas valve regulators can drift causing over-firing or under-firing -> meter can be clocked to verify proper btu input.

Have you actually measured the temp rise across the furnace? - i think i mentioned it early in this thread.

 

[Indyman317]

Is the blower wheel clean?

Do you have a/c? Coils can get dirty.

Capacitors can get weak and reduce motor rpms.

Gas valve regulators can drift causing over-firing or under-firing -> meter can be clocked to verify proper btu input.

Have you actually measured the temp rise across the furnace? - i think i mentioned it early in this thread.

I blew dust out of the blower wheel when I removed it last week and it wasn't to dirty. I do have A/C coils but isn't that irrelevant to the actual furnace operation? What kind of tool can i buy to measure temp rise?

 

[user_12345a]

You need a digital thermometer with a stem you can insert into the duct.

It gets put into the return and into the supply, but can't be in line of sight with the heat exchanger.

All the air that the furnace moves goes through the indoor coil - if it's plugged up furnace can overheat.

If the capacitor is weak, the motor will take longer than normal to get up to speed.