Connect Supply And Return Ducts - Temp-O-Matic TMP-105 Manual

TMP
– Furnace Manual
Oil Furnaces
3

Connect supply and return ducts

Duct sizing
Determine airflow CFM
The temperature rise through the furnace must not exceed 85
o
should be at least 55 F for comfort. When calculating airflow,
assume a temperature rise of 70
The noticeable temperature
o
approximately 27-30 F. Actual
o
approximately 18-21 F due to humidity of the air.
To calculate the noticeable heat temperature change (ΔT), you can
use the formula:
ΔT = BTU/h/(1.1 x CFM)
To calculate airflow when you know temperature change (ΔT), you
can use:
CFM = BTU/h/(1.1 x ΔT)
You can estimate airflow using the following rules of thumb:
Heating: 14 CFM per 1,000 BTU/h output
Cooling: 400 CFM per ton air conditioning
Determine the required airflow based on whichever is larger –
heating mode or air conditioning mode.
Examples:
1. What would the temperature rise be for a 100,000 BTU/h output
furnace with an airflow rate of 1200 CFM?
Use Equation 3-1 since you know CFM and BTU/h:
ΔT = 100,000/(1.1 x 1200) = 76
•
The temperature rise would be 76
•
If the air enters the furnace at 70
o o
furnace at 70 F + 76
2. What would the airflow be to obtain a 70
120,000 BTU/h output furnace?
Use equation 3-2 since you know ΔT and BTU/h:
CFM = 120,000/(1.1 x 70) = 1,558 CFM
•
The airflow would have to be 1,558 CFM to obtain a
temperature rise of 70
3. Estimate the required airflow for a 75,000 BTU/h output furnace
installed with a 2-ton air conditioning evaporator coil.
Heating mode airflow (use Equation 3-3):
CFM = 75 x 14 = 1,050 CFM
Cooling mode airflow (use Equation 3-4):
CFM = 2 x 400 = 800 CFM
•
The larger number is 1,050 CFM (heating), so the duct
system should be sized for 1,050 CFM.
•
The supply duct would need to be 16" round or a rectangular
equivalent such as 8" x 25" or 12" x 16", using Table 4,
page 8.
4. Estimate the required airflow for the same furnace installed with
a 4-ton air conditioning evaporator coil.
Heating mode airflow is still 1,050 CFM.
Cooling mode airflow (use Equation 3-4):
CFM = 4 x 400 = 1,600 CFM
•
The larger number is 1,600 CFM (cooling), so the duct
system should be sized for 1,600 CFM.
•
The supply duct would need to be 18" round or a rectangular
equivalent such as 8" x 36" or 12" x 21", using Table 4,
page 9.
8
o
F.
change for cooling would
temperature change
Eq. 3-1
Eq. 3-2
o
F
o
F.
o
F, it would leave the
o
F = 146 F.
o
F rise through a
o
F.
Always check the size of existing ducts, particularly
if you are adding air conditioning. The air pressure
o
F and
loss through the cooling evaporator coil reduces
available airflow. If the ducts are too small as well,
the system may not work satisfactorily on either
heating or cooling.
be
will be
Determine duct dimensions
Table 4, page 9, and Table 5, page 10, provide typical round and
rectangular duct sizes for rectangular and flat oval galvanized ducts.
Do not apply these tables to size ductwork if the total equivalent
length of the duct exceeds approximately 100 feet. For longer
systems or for ductboard, fiberglass-lined or flexible duct sizing, use
the ACCA Manual D or the ACCA duct sizing slide rule. These
tables are based on pressure loss of approximately 0.10" water
column per 100 feet equivalent length of duct.
Eq. 3-3
Use Table 3 below to size or check sizing of take-offs to supply
Eq. 3-4
registers or return grills.
Verify the size and type of registers, diffusers and grills from the
manufacturer's ratings. Do not exceed the recommended flow rate.
The pressure drop allowance for each should not exceed
approximately 0.05" water column.
Install a return air filter, sized per specifications in Section 12.
Use only a return air filter mounted to the furnace. Do not add
additional filters unless the duct system is carefully sized to allow for
the additional pressure drop.
Table 3 Suggested maximum flow to runouts
TAKE-OFF SIZE
(Inches)
Sheet metal or ductboard
5 Round
6 Round
7 Round
8 Round
3 ¼ x 8 Stack
3 ¼ x 10 Stack
3 ¼ x 14 Stack
2 ¼ x 12 Stack
2 ¼ x 14 Stack
Flexible duct (keep bends to minimum)
6 Round
8 Round
10 Round
12 Round
14 Round
16 Round
18 Round
20 Round
CFM
SUPPLY RETURN
60 45
100 75
140 110
210 160
70 55
100 75
140 110
70 55
90 70
55 40
120 90
200 160
320 250
480 375
660 530
880 680
1200 900
670-000-004/1010