Guide to Replacing Older SDHV Air Handlers for Unico HVAC Systems

1. Unico, Inc. / 7401 Alabama / St. Louis, MO 63111 / Phone: 800-527-0896 / Fax: 314-457-9000 / www.unicosystem.com
UNICO’S GUIDE TO REPLACING OLDER SMALL DUCT HIGH VELOCITY AIR HANDLERS
NOTE: When replacing both the indoor blower and the outdoor condensing unit you MUST check the matchup rating on www.ahridirectory.org
Equipment size. The modern AHUs are not exact replacements; they are better. They are more efficient, quieter, and, for most applications, have more static pressure than the older unit. In addition, the dimensions, although similar, are slightly different. Use the following table to determine the closest match.
Existing Unit
Replace With
Tonnage
Blower
Coil
Unico
HP / VP 2436
2
M2430BL1-EC2
MC2430*
2.5
M2430BL1-EC2
MC2430*
MB2436H
3
M3642BL1-EC2
MC3642*
HP / VP 4260
3.5
M3642BL1-EC2
MC3642*
4
M3642BL1-EC2
MC3642*
MB4260
5
M4860BL1-EC2
MC4860*
Spacepak
ESP1824
1.5
M2430BL1-EC2
MC2430*
2
M2430BL1-EC2
MC2430*
ESP3036
2.5
M2430BL1-EC2
MC2430*
3
M3642BL1-EC2
MC3642*
ESP4260
3
M3642BL1-EC2
MC3642*
4
M3642BL1-EC2
MC3642*
5
M4860BL1-EC2
MC4860*
* Can use either "C" or "H" coil

2. Common Plenum Configurations:
Shotgun Pattern. For the 4860/3642 units, use 10-inch insulated metal or fiberglass duct for the first 30 per-cent
of the outlets coming off the 10-inch plenum; then reduce to 9-inch if desired. For the 2430/1218 units,
use 9-inch insulated metal or fiberglass duct for the first 40 percent of the outlets coming off the 9-inch ple-num;
then reduce to 7-inch if desired.
60 % 40 %
9-INCH DIA.
7-INCH DIA.
IL00124.CNV IL00126.CNV
30% MAX.
Airflow - The older systems were designed for 200 CFM
per nominal ton and often the actual airflow was 180
CFM per nominal ton. The modern units are designed
for 250 CFM per nominal ton with a minimum of 200
CFM for acceptable performance. It is very important to
maintain the minimum airflow. It is absolutely required
if the outdoor unit is being replaced.
Plenum - Most of the older systems use 7 inch round
plenum. The modern units often use slightly larger duct
sizes. Therefore, you will need to adapt to the existing
duct using reducer fittings. Be sure to seal the ducts and
insulate. It may be necessary to upsize the plenum in
some applications. Primarily this will entail using a
larger, 9 or 10 inch, duct up to a tee; then downsizing
to the 7 inch. This is usually seen in horseshoe or
perimeter loop configurations.
Common Recomendations
For the 4860/3642 - Replacing the duct work be-tween
the AHU and the first fitting (in most cases it is
a Tee) needs to be changed to a 10-inch I.D. insulated
metal plenum for a minimum of 2 feet.
For the 2430/1218 - Replacing the duct work be-tween
the AHU and the first fitting (in most cases it is
a Tee) needs to be changed to a 9-inch I.D. insulated
metal plenum for a minimum of 2 feet.
24-inch (610 mm) Rule - Use at least 24-inch [610
mm] of straight plenum before any fitting, such as an
elbow, tee, or takeoff. Electric duct heaters require 48
inches [1.2 m]. Avoid elbows directly off units.
Outlets - The older systems usually were designed with 4
to 5 outlets per ton. The modern systems require 6 to 8
outlets per ton. This does not mean that outlets automati-cally
need to be added. Often the modern blower will
have enough reserve to compensate for the fewer out-lets.
However, the increased airflow may create objec-tionable
noise. If this occurs, or the required airflow is
not achieved, then you will need to add additional
outlets.
Space Takeoffs Evenly - Maintain distance between
takeoffs as evenly as possible. Space the takeoffs at
least 6-inch [152 mm] apart and 12-inch [305 mm]
from end cap.
Return Duct - Typically no changes are required. Use the
following table to determine if the return should be up-sized.
(Downsizing is never required.)
Check Airflow - The EC motor makes it easy to check the
airflow. Set the airflow capacity switch (high/low) to the
correct airflow per the following table
Note: be sure that
the model switch is
set to the correct
model size.
*These are default
settings. These can
be customized if you
use a laptop and the
ECMconfig software.
Move the Aux (or Check Airflow) switch to the ON (or
CHECK) position.
Count the flashes. Each flash is equal to 100 CFM. If you
see a half flash at the end, add another 50 CFM.
IF THE AIRFLOW IS LESS THAN 200 CFM PER NOMINAL
TON (BASED ON THE OUTDOOR UNIT SIZE), THEN YOU
WILL NEED TO MODIFY THE DUCT SYSTEM. Either in-crease
the plenum size or add outlets, or both. Consult
with customer service if you need assistance.
IF YOU EXPERIENCE NOISE ISSUES BECAUSE THE NEW
BLOWER IS DELIVERING MORE AIR THAN BEFORE, YOU
WILL NEED TO ADD OUTLETS. If the unit is delivering
more air than the minimum you can reduce the airflow
using the laptop. Consult customer service if you need
assistance.
Airflow in CFM Return Size
400 12”
600 14”
800 16”
1000 18”
1300 20”
Nom Ton Switch Setting Airflow*
1.5 M2430 (low) 500
2 M2430 (low) 500
2.5 M2430 (high) 625
3 M3642 (low) 750
3.5 M3642 (high) 925
4 M3642 (high) 925
4 M4860 (low) 1000
5 M4860 (high) 1250