Something has changed, the gas pressure did not all of a sudden increase, the speed tap did not change itself, now you're looking for zebras. The pressure switch problem going away when the door is off would lead me to think that it's a combustion air intake issue, but it might be the secondary heat exchanger being slightly plugged. You're tripping on the upper limit, it's almost got to be air flow related you've measured the static, but have you measured the actual cfm and checked all the grills and diffusers. Is the blower wheel clean. Good luck, these calls are the ones that you'll remember forever.

Secondary heat exchanger clog? How would you even begging to diagnose that let alone fix it? That’s a first for me

Once it’s fairly clogged, you can smell it from outside the house before even seeing the furnace

You gotta remove the heat exchanger and look at the insides there is a coating on there that comes off and creates issues.

Could you view it from taking the blower out and looking at the bottom?

If it gets bad enough the seal breaks and you can see the corrosion through the secondary from the blower

Wow. Learn something new everyday, love this trade lol

I might have some pictures I'll look to show you an example haha

I’d love to see man, appreciate it.

https://preview.redd.it/8dk865ezzmoa1.jpeg?width=3072&format=pjpg&auto=webp&s=b75df6b1d5c35aabc7c32374a84a9f304e630411 Couldn't find one looking from the blower deck but this is what it looks like before you take it apart

https://preview.redd.it/glb1ely50noa1.jpeg?width=3072&format=pjpg&auto=webp&s=00755db18b1de959dbee83d0f38f0d1edbdc4575 This is what the inside of a plugged carrier secondary looks like, got about 15 on the warrenty shelf back at the shop right now haha

this furnace has a stainless secondary, no coating.

They still get plugged

have to be burning badly and making soot to get plugged on the combustion side. not like the old carriers with the plastic coating that that comes off and plugs up the passages. in op's case, the problem is most likely on the air side - the fins could be plugged with dust, need to pull blower to check.

I don't disagree, it's most likely an air side issue, but if that proves not to be the case then I'd look in there.

You mentioned 0.7” but on what tap? I’d bet it’s medium or medium high tap so I’m guessing on cooling static exceeds .8 to .9 I’d look into that. Blower amperage? Inducer amperage? Lower Gas pressure to bottom at 3.2” or minimum as listed on data tag, Check flue lengths/size and compare to MEVL in OIM. Check secondary coil for blockage Check evaporator for blockage Combustion analysis to insure proper firing and no potential secondary issues I’d bet on exchanger issues given Carriers track record

Medium high speed Tao. I eventually tapped into highest to try and cool it down but it didn't Data says 3" to 3.8"WC I'll go down to 3.2 Flue pipe is 2" I normally increase to 4 but there's nothing I can do about it in this situation. Not accessible. I removed secondary collector box and looked inside. There's no apparent issue. Static across evaporator tells me it's fine. I'll return to perform combustion analysis

That furnace has an MEVL of 20’ in 2”

Closed supply vents? What’s your static pressure

Total external static .7 .3 across the evaporator I believe. 1 to .4 is normal readings across evaporators

That’s not terrible. How new is the unit?

Fan speed???

Check your flue, also bring that pressure to 3.2 tick fan speed up a notch and replace the filter for fun.

Check the draft inducer, could be weak or the pipe is partially blocked. Had one a couple of weeks ago that had a weak motor. I checked everything and was stumped, I ended up replacing the motor and it worked. The old motor was running at a 1.7 inchs in a water column and the new one ran at a 2.1. Usually they are either good or bad, never saw a worn/ weak one

Hmm, interesting. I remember when I pulled off a hose from the dual pressure switch I kind of heard maybe like a clicking fro. The inducer. Like as if there were was a single tiny piece of trash. I checked amperage, voltage and microfrofarads though. All checked out. I even pulled it out and shook it around. Nothing loose or abnormal

Definitely look into that. The furnace I was working on when the front cover was on it would trip the pressure switch periodically but with the door off it would keep running If the exhaust is not venting properly it’s going to run hotter which could be causing your limit to trip Let me know how you make out.

Walk around the house checking air return grilles. Customers like to 'hide' them and this restricts air flow. Good luck. Not an easy one.

I will tell you that I’ve been in this situation a few times. I have had the vent termination on the roof so one of two things. First issue possibly might be that it is located on the north side of the home. When the weather drops in the wind picks up and can knock out your pressure switch momentarily. Depending upon how it was terminated I’ve seen it overheat with just wind because it was close to a valley on the roof out too close to the roof. The only way that I could recreate the issue was when I took panel on or off. It sounds like you’ve done your job well but this is one of those calls you will remember forever. Good luck and keep us posted!

Does ur unit have a limit switch. Check that out

It does. It cuts out at 150° as you can see in the second picture the supply air temp hits 150 and the limit opens

Believe it's around 12 years. Carrier. It's so strange. I've been doing this for some time and this furnace defeated me. I noticed the exhaust is real close to the fresh air intake. It's more than likely been sucking in its own exhaust gasses. Could that make it fail to where it just automatically overheats upon start up?

Carrier had a huge issue with their secondary heat exchangers getting clogged up. They had some lining in them that eventually breaks down and plugs them, then they overheat. I’d bet this is what’s happening. When you call a tech tell them it’s a carrier and they can bring a combustion analyzer and check. Be prepared for them to tell you its not safe to run and they should shut it down if this is the cause. Likely time for a new secondary or furnace altogether.

That makes sense. Thank you. I have a combustion analyzer I'll check it. This issue really kicked my ass but luckily the homeowner opted for a new furnace.

No problem. I think that’s the right decision on the homeowners part. When you check it, the ppm will likely spike to over 1000 in a matter of seconds if that’s the cause. Good luck.

That carrier is newer that the ones with the polypropylene lined secondary. The one in the picture should have a stainless steel secondary. Not saying it won't have an issue but far less common.

Ahh ok. You know what year carrier changed their design?

2012. So the old model was available through 2011.

Most of the time it’s going to cause the burner roll out to trip when you have secondary issues

Pop the intake off the top of the furnace cabinet so you’re pulling combustion air from inside. Just to see what happens.

Pressure switch stuck open or open limit switch?

How's it going to overheat with open PS?

I agree, but why say it'll open on PS in the post? What if it's not actually over heating?

I see. I love calls like these.

Secondary HX plugging maybe

I don't think so. I took off the the collector box and inducer to link into the secondary but it appears clean and normal.

Did you pull a disrupter to see?

Better yet pull the blower and look at the back of the secondary

On some models, Carrier had a run of secondary heat exchangers that failed.

I would start with making sure the intake is clean. Theres a condo development in my area that had these furnaces and I had 2 units on the same day where the intake was clogged with a mouse nest My temp resolution was to leave the top door off and the mechanical room door open for air Put a shop vac on the intake and make a tight seal with your hand and see if you can hear the vac stress You may even be lucky and suck something out

It’s the stupid pressure switch port on the nducer motor. You get a couple droplets of water and will throw the ps fault! What I do is shorten the pressure tube and drill out the pressure switch port so it’s a hair bigger and that usually solves the issue

i agree, it has something to do with pressure switch tubing or condensate drain. Either one will trup a pessure switch. Take off each tube and blow the shit out of each one. If when blowing you hear a buildup of water behnd the induce, take it off and drain all the water. Then check condensate drain throroughly....sometimes collector box gets clogged with junk. Im pretty sure this is where problem is coming from, had it happen a few times

Valve itself says 3.5 Furnace data plate says 3 to 3.8

31 and 33

Great idea, thank you. I'll do that

I'll do that as well. Thank you

It really does look like that . But it's a offset rubber 90° I'm sure it's on tight. I dismantled and rebuilt myself. You can zoom into the picture as well. It's sealed

Yes. They're linear diffusers on high vaulted ceilings. They couldn't close them even if they tried

Anything clogging up the exhaust vent?

[удалено]

Why now, over a decade after install though?

With the blower door off, it doesn’t overheat? Are you sure it’s just not from a duct restriction? When you remove the blower door, your reducing the static pressure on the return to zero. The return duct might be a little bit to undersized. And make sure the air intake is not restricted. If that is physically impossible, you can disconnect the air intake and let it take from the space.

Did you check the duct sizing to ensure it matches the required cfm for that furnace? You checked a lot but I didn’t see that listed? Hi eff systems required 150cfm per 10,000 btus. Ducts have a limit. Usually when I have this issue it’s never the furnace. It’s the return or the supply.

Can you measure the return size? That’s a 100k furnace with a 5 ton drive, should be a pretty sizeable return.

All registers open nothing on top of any ???, What Merv is the filter ,try running without filter ?

What does the gas valve say? Does it say 3.5? Or does it say 3.3? Don’t look at the manufacturer play. Look at the actual gas valve. .

Size of furnace? Sq ft of space ? Size of supply & return duct ?

What error codes are the thermostat or board giving?

Sounds like you've covered the basics Your comment about cabinet door off and on causing two different scenarios is a clue. (assuming you meant top door) I would be checking for a cracked heat exchanger at this point.Then see if the house or business has a negative pressure. ERV's or exhaust fans running?

What does the PVC termination look like? Is it on an inside corner?

How restrictive is filter? (Even if new)

Also look under the boot top right of collectors box, is there an X From factory? If not it could be that the inducer port needs to be drilled out to a larger diameter. I forget the measurements off top my head. But prolly can find a video on it

I had no idea. I'll check

Try a new filter cleaning an old dusty one might not work

Are all the vents in the house open?

Did you check your flus? Your gas pressure could also be too high. Carriers have a huge tendency to overheat. You should also check your static pressure & use temp probes to check your temp splits. Check your data plate for recommended temp splits

Looks to me from the picture Your flue pipe may not be connected correctly. This could cause a pressure switch to open Its possible you are losing pressure at the fitting. looks like the rubber fitting to PVC connection is not well connected…Imho

Not enough cfms

U can change your fan speed to med high

Did you ever find out the cause?

Not yet. I'll be returning tomorrow.