Specifications

38
One-sided replaces for solid fuel
In a one-sided replace a total front velocity of 36-
48 FPM with a mean chimney temperature of min.
300°F should provide a good combustion and a well-
working replace.
Example: Fireplace height is 24”, width is 36” and the
frontal velocity is estimated at 48 FPM. The ow into
the replace is:
The density expansion factor is 1.6 at a 300°F
temperature rise, so the ue gas volume is:
In other words, a wood-red replace with a 2’x3’
opening produces 460 ACFM of ue gas at an
average ue gas temperature of 300°F, while it only
consumes 288 SCFM from the room.
One-sided replaces for gas
In a one-sided replace a total front velocity of 18-
36 FPM with a mean chimney temperature of min.
200°F should provide a good combustion and a well-
working replace.
Example: Fireplace height is 24”, width is 36” and the
frontal velocity is estimated at 36 FPM. The ow into
the replace is:
The density expansion factor is 1.4 at a 200°F
temperature rise, so the ue gas volume is:
In other words, a gas-red replace with a 2’x3’
opening produces 302 ACFM of ue gas at an
average ue gas temperature of 200°F, while it only
consumes 216 SCFM from the room.
Open replaces without a smoke chamber
If the replace does not have a smoke chamber, or if
the smoke chamber is very small, it is wise to use a
higher frontal inlet velocity – preferably in the range
of 36-48 FPM.
Freestanding replaces
This type of replace requires a high frontal
inlet velocity in order to capture and contain the
products of combustion. For design purposes it is
recommended to use a velocity of 36-48 FPM. This
is in line with the code requirements for commercial
kitchen hoods.
Fireplaces with natural draft chimneys follow the
same gravity uid law as gas vents and thermal ow
ventilation systems.
To a certain degree, mass ow of hot ue gases
through a vertical pipe is a function of the heat
release, the chimney area, height, and the ow
resistance (system pressure loss coefcient).
According to ASHRAE, standard sizing of chimneys
must include an estimate of the ue gas volume and
the available draft. Available draft is the difference
between the natural draft and the system pressure
loss. The available draft must be able to overcome
the system pressure loss for the system to work.
There are limitations to this approach. A lot can
be determined and explained via calculations, but
some factors must be determined by using common
sense and experience. The location of a building, the
presence of cross winds, eddies etc. are examples
of such factors. The factors can be expressed as
pressure losses, but it is not easy to put a value to
them.
The ue gas volume depends on the fuel burned
and the amount of air used for the combustion.
The general method used to determine the ue
gas volume is to base the calculation on the air
requirements of the combustion.
The air requirement is found by determining the area
of the air inlet and multiplying it with the frontal inlet
velocity. As the air expands when it is warmed up in
the re, the actual ow in the chimney depends on
the temperature in the chimney.
A 300° F mean gas temperature rise above ambient
(usually 60°F) is used as a guideline, and with this
temperature rise, the air will expand by almost 60%.
Airow at 60°F is usually expressed as “standard
cubic feet per minute” or SCFM. If the air is
heated and expansion is accounted for it is usually
expressed as “actual cubic feet per minute” or ACFM.
The following examples will show how the ue gas
volume can be determined for different applications.
Estimating Flow
The combustion air requirements can be determined
using this formula:
Appendix A: Design Theory
Q
t
= q x A
inlet
x V
inlet
144
Q
t
= Flow at t °s (CFM, or cubic feet per minute)
A
inlet
= Area of opening (square inches)
V
inlet
= Frontal inlet velocity (FPM, or feet per minute)
q = density expansion factor (unitless)
Q
60
= 24x36 x 48 CFM = 288 SCFM
144
Q
300
= 288 SCFM x 1.6 = 460 ACFM
Q
60
= 24x36 x 36 FPM = 216 SCFM
144
Q
200
= 216 SCFM x 1.4 = 302 ACFM