WARNING - Oil Furnaces & A/C Lack of Airflow Problems!

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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