D20
FAN COIL UNITS
6-6-07
Electric Heating Coils • Application Guidelines
Discharge Air Temperature
When considering the capacity and airflow for the heater,
discharge air temperature can be an important factor.
Rooms use different types of diffusers, and they are
intended to perform different functions. Slots that blend the
air at the glass and set up air curtains within the room, must
be able to blow the air very low in the room. Hot air will be
too buoyant to be effective in this case. Discharge air
temperatures for this application should be in the 85 – 90°F
(29 – 32°C) maximum range.
Diffusers in the center of the room blend their discharge air
as it crosses the ceiling. Discharge air temperatures in this
application can be as high as 105°F (41°C) and still be
effective. However, if the return air grilles are in the
discharge air pattern, the warm air will be returned to the
plenum before it heats the room. Again, the air temperature
needs to be blended down to an acceptable temperature
that can be forced down into the occupied space by the
time the air gets to the walls. Discharging warm air into the
room at temperatures above 105°F (41°C) usually will set
up stratification layers and will not keep the occupants
warm if there is a ceiling return because only the top 12" –
24" (300 – 600 mm) of the room will be heated.
The maximum approved discharge air temperature for any
Engineered Comfort Fan Coil Units with supplemental heat
is 120°F (49°C). No heater should be applied to exceed this
temperature.
Electric Heater Selection
To properly select an electric heater, three things must be
determined: the heat requirement for the room, the entering
air temperature and the desired discharge air temperature.
The heat requirement for the room is the sum of the heat
loss calculation and the amount of heat required to raise
the entering air temperature to the desired room
temperature. Usually, the second item is small compared to
the first for fan coil units in a return air plenum. MBH can be
converted to kW by using the chart or by calculation. There
are 3413 BTUʼs in 1 kW. If using the chart, find the MBH on
the left scale, then move horizontally to the right and read
kW.
Next, the desired discharge air temperature should be
ascertained. This will depend on the type of diffusers that
are in the room.
The desired heating airflow for the room can then be
calculated using the following equation:
cfm = kW x 3160
ΔT (Discharge air temp – Inlet air temp.) °F
Assuming 70°F (21°C) supply air temperature to the heater,
the room airflow can be selected directly from the chart.
Start at the left at the design kW. Move horizontally to the
desired discharge air temperature. Then, move vertically
down to the cfm at the bottom of the chart.
The kW can be selected directly from the chart. Start at the
bottom with the design cfm into the room. Move vertically
up to the line that represents the desired discharge air
temperature. Then, move left to the kW.
The discharge air temperature can also be selected directly
from the chart. Start at the bottom with the design cfm into
the room. Move to the left side of the chart and find the
design kW. Move horizontally and vertically into the chart
until the lines intersect. The intersection will be the desired
discharge air temperature. Interpolation between the
curves is linear.
Heater Selection Chart
0 500 1000 1500 2000 2500 3000 3500 4000
AIRFLOW, CFM
50
45
40
35
30
25
20
15
10
5
0
kW
110°F
105°F
100°F
95°F
90°F
85°F
80°F
Assuming 70°F inlet air temperature at heater.
Diagonal lines are constant output temperature.
115°F
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
MBH
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