The design of the oil furnace with its ultra large heat exchanger
coming to near the top of the furnace and the blower set to the side of
the furnace represents an engineering airflow problem. The blower set to
the side blows against the heat exchanger and the back of the furnace
which blocks directional airflow velocity thus generating high initial
velocity & static back pressures against the blower.

Most installers set the A-Coil directly on top of the furnace with no
transitions resulting in another restriction and more blocking of
directional velocity air flow and a huge leap in (ESP) static
pressures'. Thermo pride states that the E-Coil must be at least 3"
above the furnace. That might work for a small 1.5 or 2-ton A/C, but
what about required airflow for 3 to 5-ton A/C's?

In my opinion, these low boy oil furnaces should be designed with space
above the heat exchanger depending on the airflow requirements of the
air conditioning application size to be used. There should also be a
transition beginning at the top of the heat exchanger and transitioning
to the intake area of the evaporator coil. This would greatly reduce the
backpressure and improve airflow. The worst place to lose velocity and
generate static back-pressure is below the evaporator coil. Where it
needs the velocity and static pressures' is at the diffusers.

The low airflow probable cause is "an unbalanced airflow heatload
through the evaporator coil, along with "back pressure and extreme
turbulence," due to the evaporator coil being too close to the very
large oil furnace heat exchanger.

With the DX coil set perhaps illegally close to the heat exchanger thus
causing an airflow restriction and wicked turbulence problem, a few of
the coil's circuits may be unevenly heatloaded. Since the liquid
refrigerant is not completely evaporated it will cause the outlet line
that the TXV sensor bulb is on to be too cold and the TEV will shut-down
the refrigerant flow, which can greatly reduce the BTUH capacity of the
DX coil and the entire system. On piston refrigerant control systems, it
may flood back liquid, which could damage the compressor.
My scan of my Thermopride OL 11's - Blower Curve -
with a quarter HP 700-rpm blower motor & .5" ESP it won't support
1.5-ton of cooling airflow!
http://www.udarrell.com/Blower_Curve_Graph.tif

- Darrell - udarrell

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Air Conditioning's Affordable Path to the "Human Comfort Zone Goal"
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