Combustion Air Supply And Ventilation; Outdoor Unvented Operation - Modine Manufacturing MRL Series Installation And Service Manual

3.0 Installation •

Combustion Air Supply and Ventilation

Combustion Air Supply and Ventilation
The upper levels of the space to be heated are required to be properly ventilated to supply combustion air to the
heaters and to sufficiently dilute the products of combustion. It is also important to keep the flue discharge area
clear of gas piping and electrical wiring.
Provisions must be made to provide sufficient fresh air intake area and exhaust air outlet area. Natural or
mechanical means shall be provided to supply and exhaust at least 4 CFM/1,000 BTU/h of total gas input of
heaters installed. Exhaust openings for the removal of flue products must be above the level of the heater(s).
Where insufficient air movement exists, induced air displacement is required. A balanced system is essential to
avoid negative building pressure which causes excessive infiltration, unfavorable drafts, and affects combustion
efficiency.
Air displacement may be accomplished by either gravity or mechanical means. Mechanical exhausters are
preferred and typically mounted at high points on the roof over where stagnant air accumulates inside. For a flat
roof, considerations of prevailing winds, high and low pressure areas, and distribution of air movement must be
taken into consideration when locating exhausters.
Best air distribution is accomplished by using a number of small exhausters versus one large exhauster. Provide
a minimum of one square inch of inlet area per 1,000 BTU/h for combustion air supply. Inlet opening in the
building should be well distributed, located high on the wall, and should direct incoming air upward to dilute
products of combustion while preventing drafts at lower levels. Inlets are typically 1 to 3 square feet.
In certain applications, local codes may require that mechanical exhaust systems be interlocked with the
heaters to enable both to function simultaneously or allow control of exhausters with a ceiling mounted
humidistat.

Outdoor Unvented Operation

This appliance must be used outdoors only! Use of this heater indoors may result in
property damage, serious injury, or death.
This is an unvented, outdoor appliance. If the use of wind barriers is required, consider the
following:
• Where wind barriers are used, natural or mechanical means must be provided to supply and
exhaust a minimum of 4 CFM/1,000 BTU/h input of installed heaters.
NOTE: Gravity or mechanical means may be used to accomplish the air displacement. Local codes
may require that the mechanical exhaust system be interlocked with the electrical supply line to the
heaters, enabling both to function simultaneously.
• When the space requires mechanical ventilation, the exhaust openings for removing the flue
products must be located above the level of the heater(s).
Figure 3.13 • Minimum End Clearances
12 in. Min.
All heaters
• Outdoor Unvented Operation
WARNING
!
18
MRL Series
12 in. Min.
Unvented
Heaters