3
3. Appliances Located in Confined Spaces. 
a. All air from inside the building: Provide two
permanent openings communicating directly with 
an additional room. If all air for combustion and 
ventilation is to come from within the building: 
two openings, one near the ceiling and one near 
the floor of the boiler room shall be provided 
with the minimum free area of each opening 
equal to 140 sq. in. per gallon of oil burned.
b. If all air for combustion and ventilation is to
come from outside the building: two openings, 
one near the ceiling and one near the floor of the 
boiler room shall be provided with the minimum 
free area of each opening equal to 35 sq. in. per 
gallon of oil burned.
If ducts are used to convey the air, areas of 35 
sq. in. per gallon of oil burned for vertical ducts 
or 70 sq. in. per gallon of oil burned for 
horizontal ducts are to be provided. Ducts shall 
have the same area as the free area of the 
openings to which they are connected.
4. Special Conditions: If the boiler is located in areas
with exhaust fans, direct-fired water heaters or 
fireplaces can create conditions for unsatisfactory 
combustion or venting, special provisions must be 
made.
5. Specially Engineered Installations: The size of the
combustion air openings in this section may not 
apply to specially engineered systems. These systems 
are to be designed to ensure adequate supply of air 
for combustion and ventilation.
C. CHIMNEY / VENT AND DRAFT CONTROL
1. Draft Requirement – Minimum draft required in the
vent system is -0.03" to -0.03" W.C. depending on 
boiler model, see Table 7.1 (Section 7). This draft is 
necessary to provide the necessary draft over fire of 
-0.01" to -0.02" W.C. See discussion in paragraph 5 
below regarding draft.
2. A barometric draft control is recommended for
regulation of overfire draft. Follow manufacturer's 
instructions to locate and adjust the control.
3. Inspect the existing chimney or vent system. Make 
sure it is in good condition. Inspect chimney liner 
and repair or replace if necessary.
4. The vent system and installation must be in 
accordance with the current edition of the American 
National Standard ANSI/NFPA 211, "Chimneys, 
Fireplaces, Vents, and Solid Fuel Burning 
Appliances," or applicable provisions of the local 
building codes. Typical minimum chimney size is 
8" x 8", 15 feet high, unless otherwise allowed by 
code. If the vent system is not sized properly, the 
burner may not operate properly. This can cause 
poor combustion, sooting and odors to occur.
5. Background Information Regarding Draft: 
Modern boilers operate with higher efficiencies than
older boilers. Smaller flueways, as well as bars, pins 
and fins are designed into modern boilers to transfer 
as much heat as possible from the hot gases to the 
water or steam and prevent heat loss up the 
chimney. However, these design features result in 
higher pressure, or draft loss, in the boiler.
This draft loss must be taken into account when 
installing an oil boiler into a new or old chimney. 
New chimneys are less likely to have poor draft. 
However, they must have sufficient draft to support 
combustion. A -0.06" draft is desirable and preferred. 
Older chimneys may require a replacement liner to 
have them perform well enough to support 
combustion.
For Example:
The above readings are "cold" readings (before the 
boiler and chimney are heated up).
Note also that the higher the firing rate on a unit 
which has multiple firing rates, the higher the draft 
required. For example, increasing the firing rate 1/4 
gallon may increase the draft loss in a unit by 
approximately +0.01".
PREINSTALLATION
Old
Installation
New
Installation
Comments
Chimney
Draft
-0.04" -0.04"
No change, but older 
chimneys (especially unlined 
ones) have leaks which 
reduce draft.
Boiler
Design
Pressure
Drop
+0.01" +0.01"
Required for mandated 
efficiency increases.
Draft
Over Fire
-0.03" -0.00"
The old installation would 
have had a higher 
temperature in the chimney 
[as high as 800 degrees vs. 
400 degrees F], which would 
increase the draft.
Do not install this appliance where corrosive 
materials, such as ammonia, chlorine, water softener 
salt, etc. are stored.
CAUTION
An oil-fired unit shall be connected to a flue having 
sufficient draft at all times, to assure safe proper 
operation of the unit.
CAUTION