Typical draft overfire measurements* (0.02 inches H
2
O)
and flue draft measurements** (0.04 to 0.06 inches
H
2
O) insure that there is continuous negative pressure
in the combustion system. Excess draft will increase
your stack temperature and decrease your combustion
efficiency.
It is important to measure draft on the chimney side, well
d o w n s t ream from a draft divert e r. This measurement will
tell you if there is sufficient draft to move the flue gases
up the chimney and vent them to the outside air.
CA-9
Will frequent testing increase my fuel
savings?
The most basic and simplest way to save fuel is by
adjusting the combustion process to obtain optimum
combustion efficiency. Improving combustion efficiency
will ultimately save on fuel. Every situation is different,
but frequent testing to improve combustion efficiency
will maintain fuel efficiency.
Additionally, frequent testing will give you a “running
history” of the maximum operating capability of each
piece of combustion equipment that you test. By using
this history to establish a benchmark, you can maintain
your equipment at peak efficiency.
Although some combustion experts test on a weekly
basis, many other experts test only three or four times
per year. Annual testing is the minimum acceptable
level of testing that must be done on small furnaces.
CA-10 As my combustion efficiency improves, how
do I calculate my fuel savings?
First you have to determine the change in efficiency
with instrumentation. Then you can use chart #6 to cal-
culate your fuel savings.
CA-11 How much does temperature and CO
2
affect
efficiency?
A decrease in stack temperature of 40°F (20°C) will
result in increased combustion efficiency of 1-2%. An
increase of 1% in CO
2
will increase combustion effi-
ciency by 1/2% to 1%.
CA-12 What should my O
2
, CO
2
and CO be on the
common fuels?
Chart #7 (pg. 5) will give you a guideline for common
fuels and percentage of gas, but you should also check
the manufacturer’s recommendations for excess air.
Carbon Monoxide (CO) should always be minimized.
CA-13 How do I run a combustion test on a high
efficiency furnace?
Testing of a high efficiency furnace is not much diff e re n t
f rom the testing of a natural draft furnace. The biggest
d i ff e rences are the sampling points (see chart #7, pg. 5)
and the need to be able to calculate higher eff i c i e n c i e s :
up to 99.9%.
CA-14 Is is important to measure smoke in oil fired
furnaces?
Yes, because smoke is a primary sign of incomplete
combustion and fouling, it is very important. Using an
authentic smoke spot testing instrument will determine
oil burner condition and operating efficiency and safety
(see chart #8, pg. 5). However, smoke spot testing of
modern oil burning equipment may not indicate the
presence of CO. Because smoke and CO may coexist,
or exist independently, and both affect combustion effi-
ciency, both smoke and CO tests are necessary.
4
Chart 6
S AVI N G S F O R E V ERY $ 1 0 0 F U E L C O S T S BY I N CR E AS E O F CO M BU S T I O N E F F I C I E N C Y
Ass u mi ng co nst a nt r a di ati on a nd ot h er un a cco u n ted - for lo s ses
From an original
To an increased
efficiency of…
combustion efficiency of…
55%
60%
65%
70%
75%
80%
85%
90%
95%
50%
$9.10
$16.10
$23.10
$28.60
$33.30
$37.50
$41.20
$44.40
$47.40
55%
-
8.30
15.40
21.50
26.70
31.20
35.30
38.90
42.10
60%
-
-
7.70
14.30
20.00
25.00
29.40
33.30
37.80
65%
-
-
-
7.10
13.30
18.80
23.50
27.80
31.60
70%
-
-
-
-
6.70
12.50
17.60
22.20
26.30
75%
-
-
-
-
-
6.30
11.80
16.70
21.10
80%
-
-
-
-
-
-
5.90
11.10
15.80
85%
-
-
-
-
-
-
-
5.60
10.50
90%
-
-
-
-
-
-
-
-
5.30
* Always be sure to follow manufacturer ’s recommendations.
** 1 inch H
2
O = 2.5 hla
