Nothing says winter has arrived like a failed heat exchanger…

Nothing says winter has arrived like a failed heat exchanger…

In the next few weeks a lot of you will be knee deep in failed gas heat exchangers; it’s our industry’s way of letting you know winter is here.

From residential furnaces to commercial rooftop units and hanging unit heaters; non are immune to the infamous failed heat exchanger.

Some will rust through, others will crack, and some will be packed with soot.

Some of you will replace the victim (yes, I said victim) and move on to the next job, never giving the components early demise a second thought.

You’ll blame the failures on ‘thin metal’, ‘ foreign materials’ or ‘bad design’.

Unfortunately heat exchangers are like everything else in our industry; few die of natural cause, most are murdered.

At the top of our Heat Exchanger Serial-Killer list is Mr. “Bigger is Better”

Most folks size a rooftop unit’s cooling capacity based on the building’s cooling load…novel idea.

But, when it comes to sizing the heat they get all kinds of scared and opt for the biggest heat section available.

Rooftop sales person: “What size heat section do you need?”

Answer: “I don’t wanna have any problems come winter, better make it high heat”

Wrong, wrong, wrong.  Oh, did I mention WRONG!

There are usually three or four heating capacities for every rooftop unit cooling size.

Example; a typical 10 ton unit is offered with 144, 192, or 280 MBH heating capacities.

Think about it, the high heat option is over a quarter million BTUs while the low heat is barely more than a typical residential furnace.

I know what you’re thinking; ‘so what, who cares?’

You care. Here’s why…

A unit with oversized heat will warm the space and shut off in no time, much quicker than if it was properly matched to the heat loss of the building.

Again; ‘So what, who cares?’

Question for you:

What drips from your car’s exhaust pipe on a cold day? Answer: Water. Here’s why;

The exhaust system is cold and below the exhaust gasses dew point causing water to condense inside the exhaust pipes. (that’s why there’s water dripping out.)

After a while the exhaust pipes warms up, above the dew point of the exhaust gas, and the dripping stops.

Here’s the problem- if you only drive the car a short while then shut it off, the exhaust system never gets hot enough to boil off the water that collected inside.

What happens when you mix water with flue gas? It makes acid. What does acid do to the steel exhaust system? It causes it to corrode (rust)

Guess what, it’s the same for a gas furnace heat exchanger. If the heat run time is too short, water vapor forms in the heat exchanger, mixes with flue gas, forms acid, and the heat exchanger rusts out.

How do you prevent this from happening? Size the heat or the building heat loss. This increases run time between starts, reducing condensation build up.

Over the next few days (weeks?) I’ll be dropping some more articles on the topic.

Stay tuned and become enlightened.

Together we can stop the senseless killing of innocent heat exchangers.


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Patrick is Zen HVAC’s diagnostic and training guy. Patrick started in the trade the day he left technical school and never looked back. He's served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training. His moto: If I can understand it, anybody can. Patrick uses the Zen common sense approach to teach Patrick’s Likes- His Wife, kids and dog. Old pickup trucks. Hiking. The industrial Revolution. Patrick’s Dislikes- Taking work too seriously. Anything unintuitive. Emoticons :( Patrick’s Favorite famous person- Theodore Roosevelt “I am only an average man, But I work harder at it than the average man" Famous Patrick Quote- “Well, that was stupid of me”

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