Evap coil efficency

[nchvac]

I would like someone to explain to me how it is more efficient for a 13 x 24 x 21 evap coil to be more effcient than an old 22 x 24 slab coil. I understand surface area and how all of that works, but the fact is that you are taking a 22" hole and closing it down to a 13" hole. Not only does this not seem inefficient, it seems that you won't get enough air across your heat exchanger in a fossil fuel situation. I have to wonder why we did away with the slab coils when the 13 seer units came out. I think that we took a step in the wrong direction with the airflow in this senario. What do you all think?

[beenthere]

Are you refering to vertical or horizontal slab coils.

[Carnak]

sounds like a slab on an angle as opposed to an A-Coil

the A-Coil 2 x 13 x 24 =624 square inches

a 22x24 slab = 528 square inches

so possible the A-coil has more surface area, will run warmer

[nchvac]

sounds like a slab on an angle as opposed to an A-Coil

the A-Coil 2 x 13 x 24 =624 square inches

a 22x24 slab = 528 square inches

so possible the A-coil has more surface area, will run warmer

I get the surface area part. What I am refering to is choking down the unit because you blocked off about 192 square inches of honest duct size(straight, no leaning or nothing). We are chocking down the system to be more efficient. It doesn't seem right.

[Carnak]

it is just choked getting through the opening of the drain pan

Been the same thing for every furnace that has had AC added to it

[udarrell]

The design of the some OIL furnaces' with their ultra large heat exchanger coming to near the top of the furnace and the blower set to the side at the bottom of the furnace, can be an engineered 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, between the HT-EX, and more blocking of directional velocity airflow and a huge leap in (ESP) static pressure. Thermo pride states that the E-Coil must be at least 3" above the furnace. Three inches above furnace might work for a small 1.5 or 2-ton A/C, but what about a 3.5 or 4-Ton A/C's required airflow?

In my opinion, these Thermo Pride OL 11 Low Boy Oil Furnaces' should be designed with a transition-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 furnace and transitioning to the intake area of the evaporator coil.

This would reduce the back-pressure and improve airflow. The worst place to lose velocity and generate ESP back-pressure is below the evaporator coil. Where it needs the velocity and static pressure 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 high turbulence, a few of the A-coil's circuits may be unevenly heat-loaded. 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.

What is your experience with these types of oil furnaces? - udarrell