I recently experienced a series of small explosions while my furnace was operating. After extensive troubleshooting, I discovered the rollout switch was tripped, after I reset it, I watched the furnace startup properly, but as soon as the blower came on, it had extensive blowback tripping the rollout switch again. I learned that most likely a damaged or cracked heat exchanger was the cause.

So I removed the exchanger and sure enough it had quite a few holes. I believe those small explosions caused the holes, not the type of failure you might see from 11years of operating.

I also found a crack in the 3rd hole from the left that the flame enters, shown in the pics.

I now have a new exchanger, but I'm afraid that what caused this type of failure may happen again, and I don't want to ruin the new part. Any ideas?

Usually you find those metal rings at the bottom of the unit and that is a dead give away to the problem. Many times it is not the small explosions that cause the ring failure but the ring failure that can cause the small explosions.

What you have is a heat related stress cracks that started at the weakest point of the heat exchanger along with an engineering problem.

Many of the cures are addressed in the install manual that comes with every furnace.

Maintain the burner by annual tune-up's designed to check that the gas pressure is not higher than posted on the info tag on the unit since this will prevent over firing and heat related problems.

The most important thing is air flow across the heat exchanger, Restricted return air duct or very restrictive air filters cause the heat exchanger to run at a higher temperature but usually below the temperature of the high temp limit control. These increases flex of the heat exchanger. In other words a unit with proper air flow will run at a lower normal operating temperature thus heat exchanger flex or expansion and contraction will be limited as compared to a unit that runs on the ragged edge of the high limit control. Remember your heat exchanger flexes every time the furnace turns on (heat up) and turns off (cool down). That is a lot of flexing over the life of the furnace.

This is just common sense as if you run a furnace temp of 140 degrees as compared to running a temp of 180 degrees there is bigger spread temperature. This bigger spread adds to the thermal expansion and contraction (during the cool down phase) which leads to quicker heat exchanger wear and eventual failure.

During many years of heat exchanger failure diagnostics usually performed for various manufactures we discovered that many items can contribute to heat exchanger failure but design and air flow(Improper instillation) were usually the main culprit followed by air borne contaminants (Hair spray, laundry products etc.)

On a side note. I have a few very old furnaces that were made by Janitrol/Goodman corporation. Some of them are 30 years old and the heat exchangers are just fine. The reason they have lasted so long is that the furnaces have return air that is 25% larger than the factory recommends to further help with the flex problem. Most HVAC equipment is installed with a to small return thus helping to send the heat exchanger to a early grave.

Thanks for your timely response! The gas man did use a manometer type device to measure the incoming gas pressure. It sounded to me like it was above the maximum of 9"s WC. He said if my HVAC person wasn"t pleased with the pressure, they could add a regulator. I thought they could adjust the pressure that's why I called them in the first place. Will I be able to just use a little metal putty to fix that exterior crack on the hole where the flame enters the heat exchanger? Looking at the air returns, for what I know, the returns are all good, but I noticed where the duct attatches to the furnace housing, there is a hole cut out with a standard sized vent cover mounted over it. The moving door that is used to adjust incoming airflow through the vent, is missing. Judging by the dust and dirt that is accumulating over the vent's slots, there is a lot of air being drawn through that vent just prior to the filter. I'm logically thinking, this vent is defeating the amount of air designed or needed to keep the furnace running cooler. I'll post a pic tomorrow morning, as I'm at work this evening. Once again thank you for your help!

The manifold pressure is MAX at 3.5 so you better have the HVAC service person check that right away.

You better post some pictures on the putty thing since that would be verboten per code depending upon exactly what you are talking about.

The pic that has arrows pointing to the crack is a pic I took of the holes that the burner flames fire into the heat exchanger. I believe the hole with the crack is hole no# 3 from the left. There are 5 holes that the burner fires into. I read that in some states code allows that you can drill a small hole at the end of the crack to prevent it from spreading, and I thought about filling it in with metallic putty (the crack and the drill hole). I will upload a pic in the morning of the vent I spoke of that resides on the outside the return air duct not far from the filter. I guess if that's not possible, I'll have to order that part. It took 3 weeks for the heat exchanger to come in. Since this all happened, I have not been able to stay at my house. I've been using 3 liquid filled heaters to keep the house around 50 degrees, so I was hoping to install the heat exchanger and get the house heated again and get the gas pressure and airflow problems ironed out ASAP.

Cracks like that is metal fatigue it is best to replace that part also if possible

Ok, well I hope they have that part. Going back to your improper installation theory, here is a pic of the vent I've been describing on the return air side, looking through the vent you can see the filter, as there's no damper. Wouldn't this damperless vent effect the total volume of air designed by duct size length etc? Is this normal to place the vent there? If so< I'm going to remove it and cover it and seal it. Thanks again, for your time and expert advice!

Usually a grill like that is installed because

There is not enough return air for the system and someone thought that a small RA grill would help the situation.

The area being served by the furnace has no return air so they thought installing one would help the area.

Usually that is BS and does not work but it makes them feel better.

Without knowing the entire design of your system and not having the time for me to check your design I suggest leaving it alone since if you block it off the situation could get worse.
------------------------------------------------------------------------------------------------------------

The front plate is actually a flue restrictor and the cracks are from stress and over heating. Repairing it is futile since it has been stressed already and will just crack somewhere else again. It should be replaced.


I hope you followed the FACTORY recommended heat exchanger directions when you replace the parts. This is of up most importance since it involves very dangerous flue gas/byproducts of combustion that can and will kill you or any other member of your family including your pets. This type of work is usually performed by a skilled professional in order to prevent very costly mistakes.

MAKE SURE to use all new gaskets on all sealed areas and make sure they are installed properly. Many times the factory tells the service people to use a very high temperature RTV silicone base adhesive so that must be used NOT some cheap silicone normally used for caulking windows and doors since it WILL NOT WORK!!

When the work is done you need to have a sniff test and professional CO test performed to make sure the unit is working as designed. Unless you have 2000.00 worth of equipment and know how to use it you cannot perform this absolutely necessary testing.
So have it done by a professional.

As far as your duct system is concerned I would tell you to check it out or have it checked out to make sure it is sized properly for the size of your furnace. This type of work needs to be done ON SITE at the place this furnace is installed. If not you may end up with another failed heat exchanger.

If you have central air make sure the coil is cleaned before putting the unit back together. Many times a dirty A/C coil can cause over heating problems which in turn will cause problems with the furnace.

I believe all your questions have been answered to the best of my ability without actually seeing the situation personally. Good luck and always follow the factory directions and observe proper mechanical skill levels to obtain a properly done job.