This is not normal behavior. Your furnace should be able to run for hours without high limit fault.

What type of filter are you using and is it clean? If you are using a restrictive filter it will reduce the air flow and cause an over heating situation. As an experiment you could try to run the furnace without the filter to see if that resolves it. If it does then the filter is either too small or too restrictive.

You could also have a exhaust blockage or the blower may have an issue. How old is this furnace and when was the last time it had preventative maintenance?

Did you pay any money of either of these "professionals" to diagnose the problem?

Thanks for your reply. Yes, they both charge a visitation fee, as they say it service call. I changed the filter, tried running without the filter, and with the door open, meaning, it can draw air unrestricted from the space around it. On the unit it says manufacture date is Jan 2012. I also feel that it is not normal for it to heat itself more than the switch lets it to. I also think it has been this way for years, and I only noticed it because I installed a new Smart Thermostat which gets its power from the unit's board. Thus I noticed the thermostat periodically turn off and restart as well.

Are you the original owner of the furnace? I am thinking this furnace was never set up properly when first installed.

It could be something simple like the blower speed setting is too low which would cause it to overheat. It could also be the position of the sensor, or the sensor itself.

You need to hire a competent and honest tech who knows how to diagnose the problem and fix it correctly.

No, I am not the original owner. When we got the house a year ago, I did notice that the furnace would as if take a break every so often. Then only after installing the new Thermostat I realized that it completely turning off, with only the fan blowing- then starting up again.

Fan switch is set to on?

Nope, tripping a high limit on a furnace is not normal. The furnace should never do that. The high limit on a furnace is there to prevent a fire... because if the furnace gets too hot - that is the next step.

There are a myriad of things that can cause this problem. Due to the nature of this it is best left to only professionals to correct the problem in person.

The pros you called thus far... were they associated with a home warranty by chance? (Just curious)

I know you're not in my area, because we are still in the 90's. (Katy, Texas area)

Thanks for your reply. No, I got in touch with them via Thumbtack, and they both had solid reviews. In Boston, so it will be getting cold soon. But I am not sure what else I can say to the second HVAC professional. He is convinced that because the system did everything right(shut everything down, but the fan), it is working as intended. In your opinion, what could I possibly ask him to check again, if he'd be willing to come again?

@klem1, what does it mean: Fan switch is set to on? where is this switch? The fan keeps going the whole time, if that is what you're asking.

Fan switch is on the thermostat. Usually set to "Auto". Can be switched to "On" and will run constantly. Make sure it's set to "Auto". If problem persists after this, get a different AC service - find one on hvactalk.com. As stated above, this is NOT normal and could be dangerous.

Ohh. My Eccobee has: Select the minimum fan run time per hour in Auto mode (min/hr). I've been having at 10min. This what you mean? But anyway, during the above problem when unit is heating, the fan is on constantly, even when the rest of the unit shuts itself down.

Ah, yes, Ecobee............ Do you have "fan control" by Ecobee or HVAC? Set "minimum fan run time" to 0. See if that fixes it (it wont effect the heating time).

In your opinion, what could I possibly ask him to check again, if he'd be willing to come again?

Ask to see that he's licensed to do the work you are paying him to do. If he doesn't have the experience to fix this problem without you tellling him what to do --- do you really want him working on the most expensive appliance in your home?

This isn't the type of problem you will diagnose properly from a forum board... and the risks are high if you get it wrong... because this device that is shutting the furnace off is a safety device.

I would not call back either of the two guys you called. They are either incapable of repairing your furnace or don't want to be bothered.

I suggest you call a local Carrier dealer before the cold weather sets in. No guarantee you will have success, but it is better than looking up someone in the internet and reading unverified reviews. Ask him to do a preventative maintenance. Describe the problem you are observing and ask him you would like it to be fixed.

Are all of the vents open?

Be careful by using reviews on line as a method to determine if someone is a pro or not. A large extensive use of friends and family can easily make an average unskilled person look like the best thing since sliced bread.

Pro is tossed around these days like confetti. Want to be a pro --- get friends and family to vouch for you or pay people to write nefarious reviews intended to deceive the clueless public.

Realize if 'someone' can do this, even larger companies could do this. It's very hard to get people to write a good review. Most people will only write a negative review when things don't go as planned. So realize an absence of 'ANY' reviews can often times be the better choice... especially if that company has been in business a good amount of time. (5 years or longer)

So I got some good advice, actually from a guy at Home Depot. He said to check for continuity on the 3 limit switches after the furnace turned off. I did that and found that the one going into the chamber, L180-20F HLS had indeed overheated as it showed an open line on my ohmmeter. I ordered another ($10 Amazon), installed and same thing. Next, as he suggested, I tried swapping wires on the board to increase the fan speed. Swapped blue(from heat to spare) with yellow(from spare to heat). After turning it on this way, it did seem to push more air(based on noise), and indeed, it no longer restarted. So what can I learn from this, besides that Home Depot really does have some true experts? Thank you.

Just because the high limit over heated and shut the furnace off doesn't not necessarily mean it's bad. That is what it's intended to do. If the new HL switch over heats it will do the same thing.

Furnaces in nearly every part of the country require people who offer these services to be 'LICENSED' and insured.

How much is your home worth? $100,000 - $200,000 - $300,000 - $400,000?

A HLS is there to protect your house against fire created in that 'hot box'.

Thank you. Yes, that is the case. I am saying that the issue must be elsewhere, and I am seeking advice for where to check next.

One thing you learned is that there are incompetent and dishonest HVAC service technicians.

Another lesson is if a limit switch trips, it is because a limit has been reached. It is rarely the fault of the switch itself.

The furnace was running too hot because the air flow was too low. You increased the air flow so that the heat limit is not reached. I would not suggest this fix if the furnace was running fine for several years and then developed this problem In your case it is possible the blower speed was never set up properly when it was installed.

A furnace that runs hot puts additional stress on the heat exchanger which may cause it to crack prematurely. It was good that you observed the problem and took the effort to fix it.

Thanks.. I am thinking to try this out tonight.

https://www.handymanhowto.com/how-to-clean-air-conditioner-evaporator-coils-part-3/

I assume there is an HVAC control unit with associated control logic in this system?

I had issues with a hydronic heat boiler that shut off when it hit the high limit, and then would never turn the burner back on again even if thermostat called for heat. After the tech went through all the troubleshooting, the final fix was to replace the controller board.

In my case all the sensors worked fine - but something "stuck" in the control unit when it hit that high limit. I found out later that the original Honeywell controller had such a bad failure record that they quit using in their new boilers.

Bruce

Thanks, so how did you go about finding the right board? And then what, it is just plug and play from there?

After running it with the evaporator-coils door open, it does not seem to shut off, and the coils themselves appear to be clean. This seems like the duct work is not big enough for the system at the current motor speed. So I can keep it running with increased blower speed, by swapping blue(from heat to spare) with yellow(from spare to heat). Is that damaging to anything, or for some reason a bad thing to be doing? I also head that it is possible to pug one of my 3 the burners? This would heat up the system more slowly.. I guess.

Firstly, you check out your sensor and search what your code is saying then just go to check pipe and pipe joints either damaged or not,if you got something so it's cool otherwise just hire the contractor. In North Carolina, USA i just hire only Comfort Shield HVAC Services.They are best for HVAC services.