RoboTeq are you saying this explanation of AFUE/bonnet capacity is incorrect?
If bonnet capacity is the maximum that the furnace can handle and not the actual output, how would bonnet capacity ever be exceeded without over firing the furnace?
There are many inaccuracies in that persons misunderstanding of furnaces.
Do you really think that the old natural draft furnaces with 6-8 inch flues and mammoth dilution air openings were operating at 80% efficiency just because they showed a bonett capacity that was 80% of their input rating? Why did the HVAC industry ever bother to develope induced draft furnaces if that were the case?
I ask because I know contractors in NC, GA and TX that don't seem to have the issues you post about.
Makes sense on the 6" flues and the gobs of additional dilution of air can no way be as efficient as a induced draft furnace that has controlled air volume, even steady state. The flue gases still have to be at a certain temperature to prevent condensation/rust out. I do find it odd that manufacturers listed a bonnet capacity without a means to change what the actual output of the furnace is. I don't see any way to increase the bonnet output without overfiring the furnace or doing other modifications. Not sure what the point of even listing bonnet capacity is since if it cannot ever be achieved.
For load calculations, rough guides like "Heat is 2X AC" you posted earlier tend to be the rule rather than the exception. For cooling Contractors typically use 500sqft per ton without regard to house construction, insulation, or even age. Only older homes (pre-1970) really need 500sqft per ton of cooling, new construction can get away with as little as a ton per 1,000sqft per ton if it's built right. A full load calculation is almost like a unicorn in our area. Correctly designed ductwork may possibly be even more rare, we see a LOT of ductwork "wall of shame".
Since your area does have average temperatures dropping below freezing with cooling averages during July and August in the low 90's, there is not that much difference between where you are and where I am.
Since you insist on continuing to admonish HVAC contractors on this site, at least have the courtesy to tell us how you come to your conclusions that so many of us don't know what we are doing. Hopefully, you can do that without directing us to another DIY home improvement site as your source of information.
I do believe there is plenty of quality work being done by contractors who take pride in their work (see wall of pride forum). However I see a lot more work that qualifies for "wall of shame". Unfortunately the lowest bidders get too many of the jobs and the work is marginal at best. Apartments and rent houses tend to be the worst around here. I don't have anything against the HVAC profession or I wouldn't have chose it as a way to make a living. I just a have a pet peeve with work not being done right.
Furnaces around here are constantly oversized, but most customers are used to it and few complaints are generated from the uneven temperatures and higher utility bills. Customers only complain when the furnace starts kicking of on the high limit switch and it shuts off power to the thermostat (if it's hardwired, on battery stats they normally don't notice). Normally this is due to marginal ductwork, sometimes due to dirty or overly restrictive air filters.
I'm surprised to see that your high temperatures in PA are close to ours in summer, and your winter temps are also similar. I would have thought it would be much colder that far north. I don't know where you are in Eastern PA, but years ago I lived in Carlisle PA and remember having to wear a coat to my senior graduation in 1989. We had no AC installed our house and many neighbors didn't either. You don't find many houses in Oklahoma without at least a window AC. While in PA I also remember the lake freezing so solid you could walk on the ice, something that's never happened in Oklahoma in the 10 years I've lived here. Carlisle also plowed ALL the roads, not just the highways and snow routes. Perhaps global warming has made both climates hotter over the past 20 years, but part of me thinks global warming is government propaganda. We did however have 18 days in a row over 100 degrees last summer, and going into attics really sucked those days. Here's a link showing how hot it got the past 2 summers, how does eastern PA compare?
How are furnaces and AC's sized around here? They SHOULD be sized using a proper load calculation program, unfortunately it doesn't happen that way in most cases. Typically AC is sized 500sqft per ton, then a furnace that has enough blower capacity is selected. To get a certain blower size for the AC means buying a certain minimum BTU capacity for heat. Then there is the issue of what the supply house has in stock, normally it's the high BTU for a given blower capacity that gets installed. You can buy a 50k furnace with a 1200CFM blower, but a 75K with a 1200CFM blower may get installed instead because that's what the supply house has in stock/on sale at that time. Doesn't matter that the house only needs 40K of heat according to a load calculation. Doesn't matter that the ductwork can only deliver 800CFM of air and the furnace will cycle on limit come winter.
I'm not sure if you see that many poorly done systems up north, I sure hope not. I'm sure you have had to go behind at least a few installs done wrong, shaking your head of why it was done this way. Hopefully you're pleased when you run into an exceptional job well done and everything is easy to get to and service. As we have all said to customers, it's the INSTALL that matters more than the brand of equipment :)
In my area, usually the furnace can be down sized, so the old duct system is more then adequate for the new furnace. pretty much the same for the A/C. Old A/C might be 3.5 ot 4 tons, and the house only needs a 3 ton, and the duct work is ok for a 3 ton, or only takes the adding of a return, or a minor alteration. So if the same size equipment would be put back in, all they would have is a new same size system that isn't working any better then the old system did.
That said. there are those small homes that have a 60,000 BU 80%er and a 1.5 ton A/C where the home owner says it heats fine and it cools fine. I don't bother with a load calc on them, I put the same size back in.