Further Clarification Concerning Airflow Variables...
Thanks for the support beenthere.
Originally Posted by beenthere
I need to explain further differences between most Oil furnaces & gas furnace applications.
First, the Oil HT/EX goes all the way up from the burner chamber therefore the blower is off to the side blowing directly at the side of the HUGE HT/EX.
The 80% gas furnace's blower blows up through the HT/EX'ers & there is airspace room between them.
If you were to put a 5-Ton coil within 3" of the two setups, which one would cause the most restriction of a 2000-cfm of airflow?
If you were to place the coil directly on top of each one, the Oil HT/EX would block off perhaps all the flow, the gas EX would allow air to flow between the HT/EX'ers.
You can get easily fooled when checking static pressures to determine airflow on an oil or gas furnace that has the blower wheel blades full & the E-Coil partially blocked with lint; because checking above the coil, on an upflow system, will show a low static. Looking at the chart you'll think it's delivering plenty of airflow; an airflow check will reveal that error.
Here in the cold north country I had a contractor tell me that a1.5-Ton system should have a 1.5-Ton evaporator.
The reality is that the system we were discussing had a 140,000-BTUH input & 112,000-output which called for around 1200-cfm of airflow in heating mode.
The 2-Ton coil could go to 900-cfm, but it really needed a 3-Ton coil rated at 1200-cfm at 0.50" ESP.
In the cold climates it is important to FIRST Weatherize to reduce furnace sizing & airflow requirements. Then make the airflow system as efficient as possible so you can work with a proper condenser & coil sizing setup.
Why the oversized A/C system with extremely low airflow doesn't cool many contractors will simply say you need a larger A/C...
Well, there is more, that I've never found in the best text books, but I need to quit typing. These airflow realities are extremely important, but I never see or hear anything on them, in textbooks, only in rare mfg'ers' manuals & no videos - illustrate those critical airflow design variation situations.
The old blower wheels would simply unload if the ESP static went much above 0.50", newer blowers are engineered to handle a higher ESP without unloading.
If you pulled those old blowers out of the furnace, as I did, & used a ply-board to change the resistance - using an amp-probe; when you moved the board up away enough, the blower would move too much air & the motor would overload & bog-down.
Moving the board closer would unload the blower wheel & amp-draw would move lower.
Got to quit, but never underestimate the airflow variables...
Skippedover was just having some fun with me, which I appreciate. - Darrell