Heat Exchanger Failure #2 – Bigger Ain’t Better

biggerEveryone sizes a rooftop unit based on the building’s cooling load, but when it comes to the heating side they always say-

“I don’t wanna have problems come winter, make it high heat”
Rooftop units are sized by cooling capacity, and for each cooling capacity there are three to four heating capacities. Example; a typical 10 ton rooftop unit is offered with 144 MBH, 192 MBH, or 280 MBH heating capacities.
Think about it, the high heat option is over a quarter million MBH while the low heat is barely more than a residential furnace.

I know what you’re thinking, ‘so what?’


If the heat section is oversized, the unit will warm the space and shut off much quicker than if it was properly matched to the load of the building. The result is short cycling; short run times, and short off times.
Again, so what?
Have you ever noticed what comes out of the exhaust pipe of your car when you first start it on a cold day?

Water.
For the first few minutes the exhaust gas temperature is below dew point. After the exhaust warms up it stops producing water vapor. 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 the rooftop unit’s heat exchanger; if the heat run time is too short, water vapor mixes with flue gas and forms acid. The acid causes the heat exchanger to rot out.
How do you prevent this from happening? Size
The units heating capacity to the building heat loss.
Check back soon for our next installment

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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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