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We've been having some issues with our furnace lately.  It was a brutally cold month of February and so when the furnace would lock itself out for three hours, we would notice right away, especially when it seemed to happen most often during the morning, when it was warming the house up as we have it set to a lower temperature for the overnight hours.

The Diagnoses

We have an appliance repair plan through the energy company.    I called them to come out. They took a look and found two problems:

  • Build-up on the flame sensor – The reason our furnace was locking out is because after the burners ignite, there is a sensor that verifies that they have in fact done so, and are not instead just pouring out gas into the basement.  If the sensor does not detect flame, it locks out the furnace and tries again (or as we found out, turning the furnace off and on resets as well).  Over time, the sensor gets build-up and just needs to be scraped.
  • Cracked heat exchanger – As I learned, the basic way that the furnace builds is that the burners ignite and the gases from that are vented outside through the stack pipe that goes up and out through the roof.  This is all done within a chamber.  The actual heating is done by air flowing over that chamber, which obviously gets really, really hot.  You can't have the airs mixing, because the burners generate carbon monoxide gas, which is lethal in large enough doses, and that's what is vented outside.  A cracked heat exchanger breaches the chamber that contains the gases, and opens the possibility of carbon monoxide being drawn in with the other air flowing through.  So, it's a pretty bad situation.

Just How Bad Is A Cracked Heat Exchanger

As noted above, a cracked heat exchanger allows the possibility of carbon monoxide to enter the airflow of your home.  This is bad. Carbon monoxide is lethal in large doses, and if ingested, it makes you sleepy and unresponsive.  Entire families have died because they get sleepy, go to bed, and simply never wake up. This is bad.

This is dangerous enough that a repair service contracted through the gas company is required to tag the furnace and shut it off and close the valve leading to the furnace.

So Does Our Cracked Heat Exchanger Mean We Have No Heat?

Simply put.  No.

The repair firm is not allowed to leave the house with the furnace operating.  However, the man who did this was up front. He said that the crack he felt was minimal and in his estimation was not putting any carbon monoxide in the house.  He did indicate that a functioning carbon monoxide detector would catch any problems if they existed based on the current condition.  In severe cases he has been known to disable the furnace completely by disconnecting the gas line and/or removing wiring from the control board.  He pointed out that he had not done so with our furnace.

There was a little bit of reading between the lines, if you couldn't tell.  He was doing what he needed to in order to make sure he was fulfilling his contract with the gas company, but at the same time making sure we knew that I did not need to immediately subject my family to a house with no heat.

But, as he pointed out, cracks do not get smaller.  They get bigger.

What Caused Our Cracked Heat Exchanger

In talking with both companies that I spoke with, they both agreed that the following things likely contributed:

  • Age – Even the mb-2015-03-thermbest furnace will likely experience a crack at some point. Ours happened fairly early in the process which was likely in part due to the next two items.
  • Model – The model of our furnace is highly susceptible to these.
  • Sizing – The repair person that came to our house felt that our furnace was actually oversized.  Our house is one of the smaller floor plans in the sub (ours is around 2,200 sq. ft. while some homes go up to 2,700).  It's common to see builders put the same model in all homes so that they can buy in bulk and get the best pricing.  You'd think that the bigger the better, but with furnaces, that's actually not the case.  A furnace that's oversized is problematic for two main reasons: First, it cycles on and off more quickly, which I've noticed ours does.  Ideally, a furnace should run for a longer period, cycle off and stay off for a while.  When a furnace heats a house quickly, it will shut off faster.  This leads to more frequent cycles, which wears the parts down faster.  Second, our house likely has less ducts leading out.  If there is excess heat that is not blown through the house, it retains heat in the furnace itself.  What happens to items that are overheated?  They get brittle…and they crack.

Options For Our Cracked Heat Exchanger

Option 1: Repair

The furnace we have is 17 years old.  The heat exchanger has a 20 year warranty for parts, which would cost about $1,000.  However, it does not cover labor.  The labor costs to replace this is $500.  Basically, the entire furnace has to be dismantled piece by piece and then put back together.

Pro: It's the cheapest option.
Con: The furnace is of the age where it's likely that other problems will come about.  I would hate to drop $500 and then have something else happen in a few months and have to pay that amount plus the new setup.

Option 2: Replace

The furnace could be replaced.  However, furnaces are very expensive.  Based on the age of the setup, I would likely get the air conditioning compressor replaced as well.  This would cost at least $7,000.  Yikes!

Pros: This would fix the problem. We'd also get devices that would have higher efficiency, which would reduce our utility bills over time.  The utility company offers rebates that would offset about 10% of the costs.
Con: It's seven-freaking-thousand dollars that I do not have simply lying around.

Option 3: Do Nothing And Save Up

As he mentioned, and confirmed by another company with whom I've dealt and I trust very much, the problem isn't severe.  The one person I talked to said that after 15 years of age, the number of cracked heat exchangers starts rising a few percent a year.  And, while there are deaths from carbon monoxide, there's so few that it's evident that a cracked heat exchanger is a problem that can be monitored, at least in the short term.

Pro: It gives us time to shop around, make up our minds, and determine the best course of action.
Cons: The price could go up even further.  The dealer has said that the costs of air conditioning compressors have gone up on average a few hundred dollars per year because of the cost of copper, of which there is a lot.  So, waiting could cost us more.  Also, because the furnace is tagged and, in the eyes of the utility company, should not be operational, any other problems that exist will not be covered in the present condition.  Also, our current savings rate would leave us short for a couple of years.  Who knows how long we have before the cracking gets worse and forces the issue?

For the immediate time, we are going to look at the third option in hopes that we at least make it through the winter.

I just wish that they weren't so gosh darn expensive!

Readers, have you had to replace any part of your HVAC system recently?  Any words of wisdom are always welcome in the comments below.