Furnace blows semi warm air for 3 minutes, gets lukewarm, shuts off

[DropShotter]

We just had our AC completely cleaned and serviced this last summer. Coils, ducts, etc. The other day I woke up to find that my house was 61 degrees even though the thermostat was set to 71. It has never had a problem keeping our small house warm until now.

Its always set to auto and 71 degrees. To make sure everything worked I turned AC on, dropped it to 60, AC worked fine. Turned it back on to heater, upped it to 70. Heater came on, started blowing warm air, then I noticed after about 3 minutes the air got luke warm (didnt feel that hot to begin with though) and then the unit shut down. It seems that it keeps doing this.

I went up in the attic (I have no idea what I am doing) and looked inside the furnace, had my wife turn it on, could see the fire ignited inside. No idea what else to look for or what to do. Took some pics to see if anyone could figure out what kind of furnace it is and where to start.


p.s. From our master bedroom we started noticing a "buzzing" noise from the vent whenever the furnace would turn on. not sure what that is either

Pics:

 

[Mabus77]

Sounds to me like your high limit safety switch is tripping. The cause could be a very dirty air filter or airflow obstruction. The blower motor is good because you tested it with the AC running so that is not your problem. Check the air filter and your airflow, if both of those things are good then it's probably your high temperature safety limit switch that is bad.

 

[Tim The Tool Man]

Sounds to me like your high limit safety switch is tripping. ... it's probably your high temperature safety limit switch that is bad.

^ Probably this. Your high temperature safety limit switch is located near where your first picture has an arrow with the label "sensor." It is not the thermocouple it is just a small plastic thing (see pic0. This is maybe a $10 to $15 dollar item but you will probably have to go through a HVAC guy to get one. Maybe Grainger has one you can purchase as a cash sale???

You can test it by pulling the sensor and tying the two wires together. if the system works, the switch went bad.

 

[philjafo]

It's probably not your high temp limit, never never never bypass any safety features. It's rare for that switch to actually "go bad" usually the switch is doing its job and shutting the furnace down to prevent further damage or a dangerous condition. You admit in your post you have no idea what your doing or even looking at, from your description of the problem its probably not a simple bad switch or completely failed part. Your best bet is to call an HVAC company.

 

[philjafo]

Just looked back at the picture says right on the sticker in bold letters DO NOT bypass the switch

 

[zmaxmotorsports]

If it was going out on the limit switch youre blower would stay running.
My guess would be the flame sensor is dirty and the furnace is lighting for just a second but shutting the gas off since its not picking up the flame through flame sensor.

 

[55bigblockcid]

Here`s where I would be looking first.If you clean it use a file.^^^^^

 

[Pumpman1968]

It's probably not your high temp limit, never never never bypass any safety features. It's rare for that switch to actually "go bad" usually the switch is doing its job and shutting the furnace down to prevent further damage or a dangerous condition. You admit in your post you have no idea what your doing or even looking at, from your description of the problem its probably not a simple bad switch or completely failed part. Your best bet is to call an HVAC company.

Words of wisdom.

 

[Tim The Tool Man]

Just looked back at the picture says right on the sticker in bold letters DO NOT bypass the switch

I said to do it as a test. I did not mean to permanently bypass it. Just a temporary diagnosis. Sorry for the confusion.

...And they do go bad, we replaced 3 of them this season alone for various clients.

 

[Mabus77]

If it was going out on the limit switch youre blower would stay running.
My guess would be the flame sensor is dirty and the furnace is lighting for just a second but shutting the gas off since its not picking up the flame through flame sensor.

If it was going off on the flame sensor then the furnace would eventually lock out. It only lets it go off on that sensor so many time and then it locks out the whole ignition operation and you have to cycle the power back on.

 

[Mabus77]

Like somebody stated earlier, if you are not sure with what you are doing then call a professional, that is your best bet. It's very difficult even for a professional to troubleshoot this on a forum.

 

[philjafo]

Tim I figured thats what you were getting at, jumper it out as a test, I have seen them permanently bypassed though and for someone that admitted he has no clue what he's looking at I just wanted it to be clear that it's no ok to bypass thes things. As for the switches going bad your right it does happen, usually a result of excessive cycling on limit. Meaning its a symptom of a bigger problem and replacing the switch is only a temporary fix. As for the OP's problem, he hasn't given enough information for a proper diagnosis, and he could screw around with it for a few days guessing and throwing parts at it being cold the whole time. Or he could just call someone that knows what they are doing and he will probably have heat that day. Sure it cost more but is it worth it to be cold for a few days just to save a few bucks.

 

[zmaxmotorsports]

If it was going off on the flame sensor then the furnace would eventually lock out. It only lets it go off on that sensor so many time and then it locks out the whole ignition operation and you have to cycle the power back on.

Ive seen them do that for quite a while before locking out

 

[Mabus77]

Good input from everyone here, but as stated earlier, we do not have enough information to diagnose this problem. It could be many too many different things .


When in doubt, contract out.

 

[DropShotter]

thanks for the replies. I am first going to try and clean the flame sensor. What are the recommendations of cleaning it? or how to?

Also, I had forgotten about my air filter for about 6 months but then replaced it. Shortly after is when furnace started acting up. So is it a possibility that it just got too dirty (the sensor) since I hadnt changed the air filter?

 

[zmaxmotorsports]

Just use a scuff pad like those green things for scrubbing dishes,go with the rod not across it.
God I should be writing **** scripts!

 

[DropShotter]

Ok, looking at the instructions on the panel really doesnt help as it doesnt look much like the inside of the service box. I cant seem to find the sensor, but I am guessing that it is a metal bar that goes horizontal and then ascends vertically behind the box into an area that I can not see or get to.??????

 

[DropShotter]

Im trying to find the exact model my furnace is but not sure where to look? I found the flame sensor, cleaned it and am running the heater now to see if that did the trick. if it doesnt and I have to replace a few parts, any idea where to get them?

 

[DropShotter]

ok, set the thermo to 73, its 63 in here right now. Heater ran for about 4 minutes again, shut off....

so should I consider replacing the flame sensor since cleaning it didnt seem to work?

 

[DropShotter]

also, it seems like at about the 4 minute mark the air its pumping out begins to cool before everything shuts off


EDIT: got the model. Ordered new flame sensor. Anything else I can try if this isnt the case?

EDIT2: now the system keeps turning on but only for a minute or two and the air is luke warm or slightly warmer...

 

[Tim The Tool Man]

Did you test the flame sensor before you ordered a new one? You don't want to just throw parts at it. As was stated above, it might be the flame sensor... See above for ways to test it.

 

[DropShotter]

Did you test the flame sensor before you ordered a new one? You don't want to just throw parts at it. As was stated above, it might be the flame sensor... See above for ways to test it.

Ya i was a little confused. Pulling the sensor and tying what two wires together? The sensor wires or the limit switch two red wires? Do I just take the two red wires off of the limit switch, splice them together and then pull the flame sensor and try running the system?

 

[philjafo]

Replacing the flame sensor will not fix your issue. This sounds more like an airflow problem and can't be properly diagnosed over a msg board. I highly recommend you have a reputable HVAC company come out to look at your problem. It's cheaper to pay someone who knows what they are doing then it is to randomly throw parts at it.

 

[DropShotter]

Replacing the flame sensor will not fix your issue. This sounds more like an airflow problem and can't be properly diagnosed over a msg board. I highly recommend you have a reputable HVAC company come out to look at your problem. It's cheaper to pay someone who knows what they are doing then it is to randomly throw parts at it.

Like stated before, we just had a guy come out and paid 500 dollars for him to do a complete detail, cleaning/maintenance on the whole unit. Cleaned all the coils, ducts, etc.

After that we had a lot more airflow and it was working fine. Just as the AC is working fine now. Just not the furnace.

Call it dumb but I like learning how the things I own operate and function. I dont believe in immediately just calling an expert and throwing money away if it is something I can do myself. Everyone has to learn somewhere.

the flame sensor is a ten dollar part. Not worried if that doesnt fix the problem.

 

[enrare]

Agree with posts on having HVAC company come look at your problem. I speak from experience of throwing parts at it, still not fixing the problem and having to pay an after-hours service call fee since I spent all day trying to fix it myself. I my case it turned out to be a dirty flame sensor that needed a quick cleaning with some emery-cloth. I literally had back in my house within 10 minutes of the tech setting foot in the door. Cost for all this: $300 for the after-hours service call plus close to $200 of parts I had "thrown at it". Having a working furnace: priceless since by the time I got heat back the temp in my house was 40 deg and temps at night where dipping from single digits to sub-zero.

 

[DropShotter]

Agree with posts on having HVAC company come look at your problem. I speak from experience of throwing parts at it, still not fixing the problem and having to pay an after-hours service call fee since I spent all day trying to fix it myself. I my case it turned out to be a dirty flame sensor that needed a quick cleaning with some emery-cloth. I literally had back in my house within 10 minutes of the tech setting foot in the door. Cost for all this: $300 for the after-hours service call plus close to $200 of parts I had "thrown at it". Having a working furnace: priceless since by the time I got heat back the temp in my house was 40 deg and temps at night where dipping from single digits to sub-zero.

If i didnt live in Southern CA I would be more worried but the coldest it gets is about 60 in the house and winter is pretty much over here.

I just want to figure out exactly how to bypass the limit switch to see if that does anything different so I can rule some things out

 

[DropShotter]

Ok well I think I may have just figured it out and I dont know why I didnt consider this. I noticed it started acting up right after I changed the filter. I just took the filter off to see if it would change anything and now its running perfectly fine, raising the house ten degrees in the normal ammount of time.

why on earth would the filter cause this? Its the same kind of filter that I have always used? The only thing I can think of is that I have to cut my filters since the housing is a weird size: 20X12X1 so I buy a 24x12x1 and cut four inches off since NO stores around me carry that size

Any suggestions how to get it running with the filter in place?


EDIT: I bumped the thermo up to 80 and the house got up to 75 and then it shut off. So maybe I didnt solve the problem....

 

[zmaxmotorsports]

Ill be back in the monterrey area sometime in june to do a heat pump install for a friend of mine ,but till then you can call me tomorrow during the day and I can walk you through some of the basic operation cycles of the furnace over the phone to give you an idea what to look for.
That would be much easier than trying to look at pictures and talk you through the process.
Jim four zero 2-321-four zero zero 3
If I don't answer leave message and Ill call ya back.

 

[Ric in Richmond]

I have a unit that measures pressure differential in the intake and exhaust plumbing and shut down just like you are describing if they aren't correct.

Mine was a hose that had cracked leading to the pressure sensor thereby throwing off the measurements.

I made sure all the hoses were connected correctly and crack free and no issues.

Also made sure any condensate lines were clear.

 

[robin1731]

Ok well I think I may have just figured it out and I dont know why I didnt consider this. I noticed it started acting up right after I changed the filter. I just took the filter off to see if it would change anything and now its running perfectly fine, raising the house ten degrees in the normal ammount of time.

why on earth would the filter cause this? Its the same kind of filter that I have always used? The only thing I can think of is that I have to cut my filters since the housing is a weird size: 20X12X1 so I buy a 24x12x1 and cut four inches off since NO stores around me carry that size

Any suggestions how to get it running with the filter in place?


EDIT: I bumped the thermo up to 80 and the house got up to 75 and then it shut off. So maybe I didnt solve the problem....

This was actually the firstthing I thought when you described the problem. Did you have the filter facing the right direction? They do have to go in a certain way. My wife changed ours out one time while I was gone. Similar problems to yours. Filter was in backwards.

 

[DropShotter]

Ill be back in the monterrey area sometime in june to do a heat pump install for a friend of mine ,but till then you can call me tomorrow during the day and I can walk you through some of the basic operation cycles of the furnace over the phone to give you an idea what to look for.
That would be much easier than trying to look at pictures and talk you through the process.
Jim four zero 2-321-four zero zero 3
If I don't answer leave message and Ill call ya back.

im not used to random forum goers being so nice.... thanks for the hand. Ill give you a ring when I get a chance

 

[kbs2244]

Has anybody suggested checking for a clogged exhaust?

I have had identical problems.
To fix, I have taken squirrel bodies out of chimney pipes.
They sit on it to stay warm, get sleepy and fall in.

 

[DropShotter]

Has anybody suggested checking for a clogged exhaust?

I have had identical problems.
To fix, I have taken squirrel bodies out of chimney pipes.
They sit on it to stay warm, get sleepy and fall in.

where is the exhaust at?

 

[kbs2244]

Depending on how old it is it is either a chimney going up through the roof or a white plastic pipe going through the wall.
Either way, if it is clogged in any way your overheat sensors will shut you down after a few minuets.

I have seen squirrels and birds fall down chimney all the way to the top of the furnace, but you have to check at any bends or reducers along the way.
The most common thing I have seen with the newer white plastic pipe design is spider webs.
Once a small bird nest. Maybe a humming bird?

 

[oatie]

Replacing the thermocouple aka flame sensor worked for me. My symptoms were that the flame would shut off after only about 30 seconds after firing up. After replacing this part about 3 times I have learned to just clean them with steel wool before the start of heating season as well as about half way through. No more issues. I was told not to use sandpaper as the sand residue will get cooked on the sensor. Good luck.

 

[DropShotter]

Got my flame sensor, will be putting that in tomorrow. But I noticed something. The air flow in one of the bedrooms is extremely light compared to the rest of the rooms. its trickling out air while the other vents in the house have a good push. Could this be my problem and the reason why the system works better when the air filter is off? Im trying to figure out how to check if there is a clog going into that room and maybe the system pressure is off because of this clog... would any of that make sense?

Or does each vent have a its own separate blower and maybe on of them is going out?

 

[Strouty]

I had a similar issue recently and it was my flame sensor. I have an oil furnace, but the function is the same. Let us know if it worked.

 

[6t7gto]

As far as low air output from one of the supplies.
We were playing ping pong in the basement and my grandson hit the ball on top of the supply trunk. My son retrieved the ball and noticed the S-clip was not properly installed. Leaving a sizable gap on top of the trunk.
I reinstalled the S-clip to eliminate the gap.
My house was built in 1979 and we have lived here 20 years.
Evidently had been that way since day one.
Check all the duct work and make sure none has seperated.

David

 

[danski0224]

Like stated before, we just had a guy come out and paid 500 dollars for him to do a complete detail, cleaning/maintenance on the whole unit. Cleaned all the coils, ducts, etc.

After that we had a lot more airflow and it was working fine. Just as the AC is working fine now. Just not the furnace.

Obviously, they missed something

Maybe they messed with the blower speed taps to make you think the duct cleaning worked wonders...

Was the AC, fan AND furnace tested after all this work?

Did they check the static pressure readings at the furnace and ductwork? How do they/you know that there is enough airflow?

Judging by the rusty burner pictures that you posted earlier, I'd wager that this problem has been there for a long time and the equipment is either way oversized for your home or it has always run on the limit switch vs the thermostat.

Ok well I think I may have just figured it out and I dont know why I didnt consider this. I noticed it started acting up right after I changed the filter. I just took the filter off to see if it would change anything and now its running perfectly fine, raising the house ten degrees in the normal ammount of time.

why on earth would the filter cause this? Its the same kind of filter that I have always used? The only thing I can think of is that I have to cut my filters since the housing is a weird size: 20X12X1 so I buy a 24x12x1 and cut four inches off since NO stores around me carry that size

Any suggestions how to get it running with the filter in place?


EDIT: I bumped the thermo up to 80 and the house got up to 75 and then it shut off. So maybe I didnt solve the problem....

Odds are that a 20 x 12 x 1 opening, much less an opening with a filter installed, doesn't allow enough airflow. It is worse if you are using ANY TYPE of pleated air filter.

I have also heard stories of coils becoming full of debris after a "duct cleaning" and blocking airflow.

Prior posts mentioning blocked flue intakes or exhausts are not likely because of the safety devices in place. If the flue is blocked enough, the equipment will not operate at all. It will not start then run for a bit and then shut off- the exception would be a rare oddball intermittent issue.

You, or a contractor, will not get to the root of your problem without airflow diagnostics.

Replacing the limit switch may provide the illusion of a repair, because these items fail over time if the system is using the limit switch as the thermostat.

 

[DropShotter]

replaced the flame sensor. Ran for about 8 minutes, raised the house 3 degrees, then lukewarm air started coming out, unit shut off. Have it set to 73, stopped at 68. So, not the flame sensor. Not worried about the ten dollar waste of money though. at least I can rule that out.

Checked the duct the leads to the room not getting a strong current of air, everything seems to be fine but I have no way of checking inside without ripping it open to see if its clogged...

also the coils are cleaned. that was part of the service call we did, the coils had a tremendous ammount of black stuff caked all over them, once they cleaned the coils off is when our airflow was much better. And yes, the heater and air worked perfectly fine for a long time after the call. The heater only started acting up a couple weeks ago RIGHT after I had put in a new filter (though the one I replaced was in there for 6 months and extremely layered in dust).

EDIT: After about 5 minutes after it shut off, it turned back on, blew only luke warm air for 1 minute, then shut off again....

EDIT2: Now it keeps doing this repeatedly...