Line D *Size air supply* Combustion Air Supply Systems & Sizing

Question 1

Why do air requirements differ for appliances with and without draft control?

Answer 1

Units with draft control require dilution air

Question 2

An atmospheric gas appliance has an input of 390 Mbh and is equipped with a draft hood. It is to be installed in a building that complies with code clause 8.2.1. A or B. What size duct is required to provide air to this appliance?

Answer 2

9”

Question 3

A furnace room complies with code clause 8.2.1. (a) or (b). It contains a 260 Mbh furnace without draft control and a 120 Mbh hot water heater with a draft hood.

Size the combustion air duct based on a length of 21’

Answer 3

7”

Question 4

A boiler room contains the following five pieces of equipment:

• one 3500 Mbh boiler with draft control
• two 450 Mbh hot water heaters with draft hoods
• two 750 Mbh duct heaters without draft control.

Calculate the required free area of the combustion air duct.

Answer 4

436 in2

Question 5

The following gas appliances are installed in a mechanical room:

• one 1400 Mbh steam boiler with a barometric damper
• one 550 Mbh hot water heater with draft hood
• two 600 Mbh direct fired make-up air units

Select a square grill for the combustion air opening in the boiler room wall that will meet the minimum requirements of the B149.1 gas code (grills rated at 67% free area).

Answer 5

18” x 18” (314.507 in2)

Question 6

The air supply to a 20,000 Mbh boiler, using a force draft burner, is provided by mechanical means through a horizontal duct 42 feet in length. What is the quantity of air required to be supplied to the boiler?

Answer 6

600,000 ft3 (30 ft3 for every 1,000 btu/h)

Question 7

A 2,752 Mbh forced draft natural gas boiler is operating in a mechanical room, excess air set at 40%. What is the required free area for the combustion air duct?

Answer 7

92 in2

Question 8

A structure is equipped with a boiler rated at 5,300 MBH and a hot water tank rated at 400,000 Btu/h. Both pieces of equipment are equipped with draft controls and the air supply is ducted from outdoors. What is the size of the free ventilation air opening?

Answer 8

47.9 in2

Question 9

A structure is equipped with a boiler rated at 750 kW and a barometric draft control. What is the free area of the combustion air supply opening in mm?

Answer 9

162,750 mm2

Question 10

A structure is equipped with appliances rated at 1,900 MBH. None of the appliances is equipped with draft control devices and the air supply is ducted from outdoors. What is the free area of the ventilation air opening?

Answer 10

10 in2

Question 11

A structure has a mechanical room into which air must be ducted from outdoors. It contains the following appliances:

• One boiler with no draft control rated at 500 kW
• One boiler with draft control rated at 4,000 MBH
• One duct heater with no draft control rated at 125 kW
• One hot water heater with a draft hood rated at 800,000 Btu/h

What is the free area of the combustion air supply opening?

Answer 11

485.37 in2

Question 12

A mechanical room is equipped with gas appliances with a total input of 3,200 Mbh. All appliances are equipped with draft control devices. The louvre specified for the combustion air opening has a free area of 70%. What is the size of the opening required to fit the louvre?

Answer 12

428.58 in2

Question 13

What is the minimum opening size on a screen used on a combustion air-supply opening?

Answer 13

1/4”

Question 14

On a combustion air louvre with dampers the dampers must be controlled in what way?

Answer 14

Automatically

Question 15

What interlock is required for an automatically adjustable combustion air louvre?

Answer 15

Normally Open contact interlocked with burner control

Question 16

What two fan styles are commonly used for mechanical air supply systems?

Answer 16

Axial and Centrifugal

Question 17

When air is provided by mechanical means what type of switch must be used?

Answer 17

The code does not specify the type. Air proving switch (sail or differential)

Question 18

What three factors would lead you to want to pre heat combustion air?

Answer 18

Prevent the freezing of pipes, prevent thermal shock of the boiler, maintain appliance efficiency

Question 19

What do ICI Appliance manufacturers do for high altitude installations? (above 4500 feet)

Answer 19

Increase blower motor horse power

Question 20

What is the relationship between static pressure and air flow?

Answer 20

As static pressure goes up air flow goes down

Question 21

What three factors have the greatest affect the static pressure in ducting?

Answer 21

Cross sectional area, length, number of fittings

Question 22

What is the static pressure ranges of the following combustion supply-air ducting systems?

a. Low Pressure:

b. Medium Pressure:

c. High Pressure:

Answer 22

a. less that 3” w.c.
b. 3” to 6” w.c.
c. 6” to 10” w.c.

Question 23

A structure has a mechanical room into which air must be ducted from outdoors. It contains the following appliances:

• One boiler rated at 3,000 MBH, equipped with a barometric damper
• Two duct heaters without draft controls rated at 950,000 Btu/h each
• Two hot water tanks rated at 250,000 Btu/h each. Both tanks are equipped with draft hoods

What is the free area of the combustion air supply opening?

Answer 23

385 in2