Well, the verdict is in. I called a Goodman authorized HVAC dealer/contractor (in Denver Metro), and their technician told me that the reason that return air temp must not be less than 55F is because of the possibility of excess condensation.
BTW, I first called the Goodman hotline, but they told me that, for liability reasons, they didn't provide tech support directly to individuals. However, they told me to call one of their authorized dealers with any questions. They gave me three names, and I called one of them.
Well how high is the air film resistances in forced convection?
You have a lot of radiant heat transfer initially on the inside of the HX, then perhaps in the tubular HXs , the tubes tend to taper down once past the line of sight of the flame to enhance convection by making the products of combustion more turbulent.
You then have one of your favourite issues, thermal capacitance, heat needed to get the HX hot enough in the first place to start driving heat into the air.
Old octopii are still in existance, utilizing this capacitance and some old heavy duty clamshells still around too. The forced air clam shells worked off of a fan/limit control, and would take some time before the fan would engage.
Newer, thin wall HX's the fans are timing on maybe in 30 seconds tops after ignition, and the heat stored in the HX s is gone in a short period of time as well. Not like a heat anticiaptor shuts the heat off
5 minutes before the home is up to temperature.
So, I seriously doubt you ever desiogned a forced air furnace in your life, so perhaps you could go stay at a Holiday Inn Express and then after a good night's sleep, you can derive non-intinsic PI yet one more time, and wow us with all the calculations involved in this simple heat transfer problem.
It will most likely end up like your futile attempts to model a cooling gain on a home as a simple "Conductance" based on indoor and outdoor temperatures, and be 'completely out to lunch', yet one more time.
Add them. So the "less efficient" heat exchanger would have a thermal resistance of 1/5+1/12K+1/5 = 0.4000833 h-F-ft^2/Btu vs the "more efficient" 1/5+1/24K+1/5 = 0.4000417 h-F-ft^2/Btu, with 0.01% less thermal resistance :-)
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