Why do we have Sizing Limits?

  There are not as many contractors out there as you would expect that are current with the most recent, and always updating, residential building code.  Particularly when it comes to the sizing limitations for HVAC equipment, most contractors consider this minimum requirement by law nothing more than a nuisance.  There is nothing more frustrating to see in a quality control role than a system that was sold, sized, and installed prior to the start of the system design process.  Completing a load calculation after the unit is installed defeats the entire purpose and renders the process useless.  Just because you have been installing systems for decades does not mean they are sized correctly, or operating efficiently when attempting to provide comfort for your customer.  But what happens when the process is ignored?  What happened before we had a code requirement?
  There once was a pretty successful homeowner that wanted to update his furnace and install an air-conditioner, but couldn't quite afford the most efficient systems.  So, to save a few hundred dollars, he talked a friend of his from a local HVAC distributor to install a system under the moon light.  The equipment was of a high quality, not the old "builder grade" everyone refers to.  The old system was always running during the winter months, burning through gas and was never comfortable in the home.  So, Joe we will call him, installed the largest residential furnace in stock.   No way this thing will run constantly, the homeowner will definitely be able to hear it cycle on and off!  Oh, and since there is already ductwork we'll add a/c to the system, all he needed to know was how many square feet the home was!

Genius!  An adjustable limit switch, bi-metal disc type!

  Well, lucky for Joe the system was installed in early Spring.  Unfortunately, Joe needed to return that first year because the damn condenser came with a bad compressor - the thing died after only a couple of months!  The replacement compressor didn't do much better, and Joe started wondering what kind of equipment were we selling to these contractors if he was seeing so many problems on just this one!.  Later that year, early in the heating season, Joe kept getting nuisance no heat calls because the burners kept shutting down and the blower remained on circulating cold air throughout the home.  Joe replaced the high limit switch twice and became convinced that the darn engineers must have got the temperature wrong when specifying that thing.  How can it shut off the burners at 160F, the heat exchanger was at least 240F+!  Joe thought he was brilliant when he came up with the idea to install a limit switch that was adjustable, the bi-metal disc type.  "This will keep those burners on," Joe thought to himself, "and I can finally get some sleep this weekend!"  Famous last words for Joe's blossoming side business.

What is left of the R-22 Evaporator

That night, when the heat exchanger reached unheard of temperatures, the evaporator's condensate pan melted onto the hot furnace and caught fire.  Temperatures reached a point that melted soldered copper joints in the evaporator coil, and released R-22 into the duct system.  Burning the refrigerant, which already displaces oxygen, phosphine gas quickly started to spread throughout the home.  Luckily, the homeowner was able to get his two children, ages two and four, out of the home with zero visibility.  A couple of weeks later, the homeowner's lungs finally cleared up, but it was months before the damages were fixed to a point that they could move back in.

  Unfortunately, the above story is true in most regards - maybe a couple of blemishes on how the "technician" reached his "genius" moment.  There is a moral to be learned here, properly size your equipment for the home!  Don't think you are doing anyone "a solid" by installing the next size up.  If you are not familiar with local building codes, lets just say it is your license and your livelihood on the line, never mind the homeowner's too!  Find out Manual S sizing limitations here.
  Special thanks to Aaron Lawrence, Lawrence Air Systems, Barrington, RI for sharing this unfortunate story and pictures.  I hope I did some justice by getting most of the facts correct.  Aaron was able to work wonders for this homeowner that now has a properly sized, efficient system providing the most comfort to the family - what they thought they were getting the first time!
  If you have a story you would like to share, but can't find the words, please send me some details and pictures (if you have them).  If we can avoid just one more situation like this than we have done our industry some justice!