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!

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Comment by Curt Kinder on April 8, 2013 at 11:18pm

Is there any way to set preferences for this website's forums so that newest comments appear on the bottom of the page?

I can't get used to having to read from the bottom up, and the whole thing is a complete disaster when threads extend to multiple pages. It's to the point where the limitation imposed by this structure so drastically distracts from the flow of a thread that it is hardly worth responding!

Comment by Dennis Heidner on April 8, 2013 at 10:20pm

Adam,  I would down play the culpability of the consumer.  Very few would (or do) understand any of the ACCA sizing guidelines.  They are not likely to have heard of ASHRAE.   All they want is comfortable and low cost.  HVAC contractors know that.  They can do the comparisons, they should know the laws and they should be able to explain the choices available and their impacts.   They don't

I've seen furnaces that were upsized to a larger furnace -- when an older unit failed after 20 years.  There were duct problems,  in ground under slab and rotted out.  The upsized furnaced did indeed add more heat in the house, but it also brought in much more dirt as the larger blast of wind picked up the dirt.  This was an early 70's house with more insulation added into the attic and upgraded doors/windows.   The HVAC contractor didn't check the duct work or attempt to seal it.   

They were called back to fix the furnace multiple times -- it sounded like a jet engine in the house.   The fixed the air noise problem by downsizing - to the same furnace as before.

The better solution would have been to get the in-ground duct work fixed first. Re-line it if necessary.  Air seal the return attic duct work.  Insulate the foundation AND then calculate the HVAC requirements.

This is at in-laws house in the midwest -- after they mentioned the problems with the dust/dirt from the ducting,  I found a place that could re-line the ducting.  That helped.  They still need foundation insulation... but all of these could have been arranged by the HVAC contractor if they viewed the upgrade not as just a furnace/airconditioning unit.   They should have been able to refer them to a reputable list of contractors that could fix the ducts first and insulate the foundation.  They are in their mid 70's.  

I do not put any blame on them for following the suggestion or accepting the larger furnace initially.  They did what most reasonable and prudent consumers would do.  They trusted a reputable installer.

Comment by Bob Blanchette on April 8, 2013 at 9:37pm

Bradley, I've NEVER seen a furnace that cuts it close, much less undersized. It just doesn't happen in the real world. Furnaces are typically 2-3X the size needed to do the job. The fact a 125K furnace is only $200 more than a 45K furnace doesn't help. The customer suffers with COMFORT issues: hot then cold, then hot, the cycle goes on. Reducing cycle time is just a band aid.

Comment by Bradley Wiesneth on April 8, 2013 at 2:51pm

A properly sized furnace is one that is sized to efficiently handle the load needed to heat a home based on its location, insulation, etc.  Although a properly sized furnace will efficiently handle the heating load needed for 99% of the heating season, there are going to be those 2 or 3 days a year where the furnace has a hard time keeping up with the demand.  It's on these 2 or 3 days, where the customer calls up their HVAC installer in a panic, because they believe the furnace is not working properly.  Contractors will many times install oversized furnaces to compensate for these colder than normal days that seem to happen each year.  They don't get the phone calls and the customer is comfy cozy (although their wallet is crying!).  And it's easy to push an unnecessarily oversized furnace on a customer, because after all, everyone knows......BIGGER IS BETTER!!!  Instead of this approach, a good contractor will take the time to explain the reasons for properly sizing a furnace, and that the furnace will have to work hard on those few super cold days.  I think a customer who has been educated will be more willing to understand and accept those 2 or 3 days a winter if they are saving money and more efficiently heating their home in the long run. 

Comment by Adam Gloss on April 8, 2013 at 1:09pm

It isn't just the contractors who are at fault, it is consumers too - Caveat Emptor! Your story (one that is all too familiar to many GOOD contractors) started with a homeowner trying to save a few bucks by having a system installed illegally and improperly by a "friend in the business". Until consumers really understand that proper HVAC design and installation is actually AS or MORE IMPORTANT than the brand or model of the equipment itself, there will be problems like this in the marketplace.

Many consumers think that buying and installing an HVAC system, is like buying a refrigerator or a stove. They try to comparison shop by brand and price and think they are getting a deal when they find "quality" equipment at a bargain price. Manufacturers support this myth by touting the quality of their equipment (which is important), over the quality of the installer, which is also critical. Consumers are left with few tools to actually help them understand how to get what they pay for, and accept guesstimates based on inaccurate and out-of-date rules of thumb from contractors who don't know or don't care that they are not doing things right.

We work hard to educate consumers about the importance of good design and quality installation with information on our website (, and hand-outs from third-party authorities like ACCA, DOE and more. Still we find consumers who are not willing to give us the time we need in their home to do a proper analysis and develop a good design, or who aren't willing to pay for a quality installation. It is easier to believe the the 5 other guys who tells them it can be done cheap, and still be done right.

Until utility rebates are focused on installation quality as much as they are on lab-tested efficiency (knowing all the while that delivered efficiency is a completely different thing), until manufacturers work as hard to promote quality installers as they do their own brand name, and until consumers take the time to be educated, problems like  this will continue. ACCA is doing it's part with a new quality installation program ( Hopefully blogs and posts like this will help too!

Comment by Bob Blanchette on April 4, 2013 at 9:46pm

I've questioned coworkers on the 500sqft per ton rule. My logic is the rule was made in the 60's when houses were poorly insulated. Ask an HVAC contractor how much better homes are insulated to day vs 50 years ago, most will say twice as good or more. Ask them why aren't they installing units 1/2 the size in newer homes and they look at you like you are nuts. It's almost like the 500sqft per ton rule cannot be overcome with common sense...

Comment by Gustavo Melo on April 4, 2013 at 9:27pm

I would agree with #1 and #3 going through a contractor's mind, I'm not sure about number 2. Those contractors that care about the consumer's energy bill are likely using proper sizing methods though. Reality is, they probably make more money because of a higher close rate being able to bid a half ton size under the competition.

Comment by tedkidd on April 4, 2013 at 12:28pm

 It's almost as if HVAC contractors have no faith in modern insulating/sealing practices.

Bob, it's more an incentive thing.  What is the incentive for installing smaller?  To many it just looks like added risk and LESS money.  

I think this is what goes through the HVAC guy's mind:

  1. "I do not want this person calling on a 100f day complaining "my house is not cooling DOWN fast enough" - those calls do NOT have revenue attached to them, and they come when I'm at my busiest and can least afford them. 
  2. "I do not care about the energy.  NMEB - it's Not My Energy Bill.  Making it go DOWN is not something I'm getting rewarded for. "
  3. "Bigger is better in the consumers mind, and mine too.  Based upon my markup strategy of doubling equipment cost, I make an extra $100 for doing no additional work. " 

Comment by Curt Kinder on April 4, 2013 at 12:14pm

HVAC systems need to be sized (and duct systems designed) according to what a Manual J load calc calls for, not 500, 600, 750, or 1000 SF per ton.

Glad no one was hurt too badly.

Comment by Bob Blanchette on March 31, 2013 at 8:02pm

Hack Attack !! I hope they revoked Joe's license (if he had one to start with).Seriously though, I wish Contractors would stop oversizing furnaces. Why is it OK to put a 15KW electric heater in a house, but if it's gas a 75k or 100k BTU furnace gets installed? 20KW is the about practical limit for electric heat, yet a 100k or 120k furnace gets installed in the same application...

AC units need to be sized closer to 750-1000sqft per ton for construction newer than 2000 or so. Yet AC units are still sized 500sqft per ton just like in the 60's when homes weren't insulated. It's almost as if HVAC contractors have no faith in modern insulating/sealing practices.

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