Quote Originally Posted by beenthere View Post
Udarrell is correct about the static pressure. Many oil furnace heat exchangers are so close to the top. That the coil does impede air flow.
Thanks for the support 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