In some situations, a metal chimney liner
might be needed to increase draft and reduce
condensation during burner operation, espe-cially
with large chimneys. Figure 31.
Annex E of NFPA 31 provides guidance for
the relining of masonry chimneys. Venting
tables are included that contain recommended
sizes for metal liners based on the firing rates
of the installed appliances.
These two tables below are provided for
units with 88% and 84% steady-state effi-ciency.
Additional tables for other steady-state
efficiencies are published in NFPA 31.
This table is for an appliance with a steady-state
efficiency of 88% with 12% CO2 and 300
degrees gross stack temperature.
24 National Oilheat Research Alliance
Figure 31
Height
ft.
Lateral
ft.
Liner
6 in.
Liner
5 in.
Liner
4 in.
10
4 0.5–1.0 0.4–0.65 0.25
10 0.4–0.75 NR NR
15
4 0.65–1.25 0.4–0.75 0.4
10 0.5–1.0 0.4–0.75 0
20
4 0.65–1.5 0.5–0.85 0.4
10 0.65–1.25 0.65–0.85 0.4–0.5
25
4 0.75–1.5 0.65–1.0 0.5
10 0.85–1.25 0.65–0.85 0.5
35
4 1.0–1.75 0.75–1.0 0.5
10 1.0–1.5 0.75–1.0 0.5
45
4 1.25–1.75 .85–1.0 0.65
10 1.25–1.75 0.85–1.0 0.65
The table above shows that for a 10' chimney
with a 10' flue connector, the maximum firing
rate is 0.75 gallons per hour with a 6" liner.
It also indicates that 4" and 5" liners are not
recommended with a 10' flue connector.
This table is for an appliance with a steady-state
efficiency of 84% with 12% CO2 and 440 degrees
gross stack temperature.
Height
ft.
Lateral
ft.
Liner
6 in.
Liner
5 in.
Liner
4 in.
10
4 0.4–1.5 0.25–0.85 0.25–0.5
10 0.4–1.25 0.25–0.85 0.25–0.4
15
4 0.5–1.7 0.4–1.0 0.25–0.65
10 0.65–1.5 0.4–1.0 0.4–0.5
20
4 0.65–1.75 0.5–1.25 0.4–0.65
10 0.65–1.75 0.5–1. 0.4–0.5
25
4 0.65–2.0 0.5–1.25 0.4–0.75
10 0.65–2.0 0.5–1.25 0.4–0.65
35
4 0.85–2.25 0.65–1.5 0.5–0.75
10 0.85–2.25 0.65–1.25 0.5–0.75
45
4 1.0–2.25 0.75–1.5 0.65–0.75
10 1.0–2.25 0.75–1.5 0.65–0.75
The table above shows that for a 10' chimney
with a 10' flue connector, the maximum firing rate
is 1.25 gallons per hour with a 6" liner, .85 gph
with a 5" liner and 0.4 gph with a 4" liner.
NR= Not recommended