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Thread: Evap coil efficency
06-13-2008, 08:34 PM #1
Evap coil efficency
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?
06-14-2008, 04:41 PM #2
Are you refering to vertical or horizontal slab coils.Contractor locator map
How many times must one fix something before it is fixed?
06-14-2008, 04:55 PM #3
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
06-14-2008, 08:36 PM #4
06-14-2008, 08:50 PM #5
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
06-15-2008, 12:26 PM #6
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
06-15-2008, 01:01 PM #7
in that case darrrel you would have to install the A-coil high enough above the HX to be equivalent to a 45 degree transition on centre. I do not think you would have to taper it, just get the height up.
You are SOL if the basement is a low ceiling.
Same house with the big oil furnace is probably fairly old, and the previous inhabitants lived for 40 years without AC. Maybe they are prime candidates for mini-splits or window shakers or a vertical air handler in the basement ducted in parallel with the big oil furnace, with a back draft damper
06-15-2008, 08:53 PM #8Professional Member*
- Join Date
- Jul 2006
trane and lennox still use slab coils