THB & TLB Oil Furnaces – Furnace Manual
8670-000-005/1010
3Connect supply and return ducts
Duct sizing
Determine air flow CFM
The temperature rise through the furnace must not exceed 85oF and
should be at least 55oF for comfort. When calculating air flow,
assume a temperature rise of 70oF.
The sensible heat temperature change for cooling would be
approximately 27-30oF. Actual temperature change will be
approximately 18-21oF due to humidity of the air.
To calculate the sensible heat temperature change (ΔT), you can
use the formula:
ΔT = BTU/h/(1.1 x CFM) Eq. 3-1
To calculate air flow when you know temperature change (ΔT), you
can use:
CFM = BTU/h/(1.1 x ΔT) Eq. 3-2
You can estimate air flow using the following rules of thumb:
Heating: 14 CFM per 1,000 BTU/h output Eq. 3-3
Cooling: 400 CFM per ton air conditioning Eq. 3-4
Determine the required air flow 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 air flow rate of 1,200 CFM?
Use Equation 3-1 since you know CFM and BTU/h:
ΔT = 100,000/(1.1 x 1200) = 76oF
•The temperature rise would be 76oF.
•If the air enters the furnace at 70oF, it would leave the
furnace at 70oF + 76oF = 146oF.
2. What would the air flow be to obtain a 70oF rise through a
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 air flow would have to be 1,558 CFM to obtain a
temperature rise of 70oF.
3. Estimate the required air flow for a 75,000 BTU/h output furnace
installed with a 2-ton air conditioning evaporator coil.
Heating mode air flow (use Equation 3-3):
CFM = 75 x 14 = 1,050 CFM
Cooling mode air flow (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 9.
4. Estimate the required air flow for the same furnace installed with
a 4-ton air conditioning evaporator coil.
Heating mode air flow is still 1,050 CFM.
Cooling mode air flow (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.
Always check the size of existing ducts, particularly
if you are adding air conditioning. The air pressure
loss through the cooling evaporator coil reduces
available air flow. If the ducts are too small as well,
the system may not work satisfactorily on either
heating or cooling.
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 duct board, 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 inch water
column per 100 feet equivalent length of duct.
Use Table 3 below to size or check sizing of takeoffs to supply
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 inch water column.
Install a return air filter, sized per specifications on 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 takeoffs
TAKE-OFF SIZE
(Inches) SUPPLY RETURN
5 Round 60 45
6 Round 100 75
7 Round 140 110
8 Round 210 160
3 ¼ x 8 Stack 70 55
3 ¼ x 10 Stack 100 75
3 ¼ x 14 Stack 140 110
2 ¼ x 12 Stack 70 55
2 ¼ x 14 Stack 90 70
6 Round 55 40
8 Round 120 90
10 Round 200 160
12 Round 320 250
14 Round 480 375
16 Round 660 530
18 Round 880 680
20 Round 1200 900
CFM
Sheet metal or ductboard
Flexible duct (keep bends to minimum)
