Specifications

Instruction 0011-9026Page 14
Proper O
2
content for any fuel fi red is the lowest O
2
value that will ensure complete,
clean combustion with adequate safety margin for variations in fuel, draft, atmospher-
ic conditions, and mechanical wear. Consult manufacturer of heating equipment or
local authorities for specifi c recommendations. Figure 15 illustrates the relationship
between CO
2
, excess air and oxygen for gas and oil.
For guidance, it should be added that modern boilers are capable of 80% or greater
combustion effi ciency. Rough guidelines for setting conventional power burners are
10-15% excess air for natural gas and 15-20% excess air for oil, with little or no smoke
and carbon monoxide formation.
5.5.4 Measuring O
2
in Addition to CO
2
Figure 16 illustrates why it is good practice to measure oxygen, in addition to carbon
dioxide, when fi ring gaseous fuels. Note that the same CO
2
reading can be obtained
on both sides of the stoichiometric (or perfect) mixture. Carbon dioxide alone cannot
defi ne proper excess air operation for gaseous fuels.
A check must also be made for the presence of oxygen, which confi rms operation with
excess air. Gaseous fuels should also be checked for presence of carbon monoxide (CO)
in the fl ue gases and adjusted for either “no CO” or to conform to applicable safety
regulations.
5.5.5 Where is Percent CO
2
or O
2
Measured? (Refer to Figure 17)
For residential and most commercial and industrial combustion units, percent CO
2
or
O
2
is measured by analyzing gas sample obtained through a 1/4-inch diameter hole lo-
cated between the fl ue outlet (breeching) and any mechanical opening in the fl ue such
as the barometric draft regulator or draft diverter. Best practice is to locate sampling
hole at least six inches upstream from such fl ue openings. The metal FYRITE Sam-
pling Tube should extend at least 2-1/2 inches into the fl ue gases.
For larger installations, consult manufacturer of combustion equip ment for special
instructions regarding sampling point, special sampling tube required, or advice on
averaging readings.
Residential gas furnaces with built-in draft diverter (gas designed) will require the
FYRITE metal Sampling Tube to be inserted as illustrated in Figure 17 to avoid dilu-
tion of fl ue gas sample.
Additional information on residential furnace combustion testing may be obtained
from the following Bacharach publication:
Bulletin 4097 - Technical Combustion Brochure
5.6 Background Gases Affecting FYRITE Readings