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First we can determine what size motor and what blower RPM has been set by the factory based on the initial static pressure numbers. By looking on the 15” Blower Curve, we pick the intersection of

0.125” (from the left axis) and 4000 CFM (from the bottom axis). This point is just below the 700 RPM line indicating a blower speed of approximately

690 RPM. This point is also to the left of the 1.5

horsepower line, indicating a 1.5 horsepower motor was probably supplied. Next, we can determine the new blower RPM and what motor will be needed. By looking at the 15” Blower Curve again, pick the intersection of the 0.25” static pressure at 4000 CFM’s. This point is halfway between the 700 and 800 RPM curves, indicating that approximately 750 RPM’s will be required from the blower. The correct motor for this situation would NOT be the 1.5 horsepower, because the point is to the right of the 1.5

horsepower curve. The correct choice would be a

2 horsepower (it is to the left of the 2 horsepower line).

This is important because the incoming gas pressure is often measured when the heater is not running.

Even though the line may appear to have the appropriate pressure, the pressure will drop when the heater is operating. If the gas supply system is sized properly, the upstream regulator will compensate for the drop, and increase the pressure to the heater.

Another important part of heater performance relates to the heating value of the fuel. Natural gas has

Methane as its main constituent, while LP consists of

Propane. The same heater is capable of burning both of these fuels, however, the volume required for LP is roughly half the volume required for natural gas for the same BTU’s. This is because natural gas (-380,000

BTU/lbmol) has a heating value that is approximately half the heating value of Propane (-955,000



Gas Flow

Another critical element in understanding heater operation is gas flow. Gas pressure changes based on flow rates. A simple example of this is shown in example 3, where a portion of the plumbing is smaller in the middle. This could represent a device in the line such as a gas valve.

Note in the first illustration, with no gas flow, there is equal pressure in both sections of the pipe (P1 and P2). In the second illustration, where the gas is flowing, the second section of pipe has much less pressure than the first (P2 is less than P1).

The direct-fired heater is most easily understood when broken down into smaller individual systems.

There are two main systems, a make-up air fan and a heater. The make-up air fan consists of a heavy-duty blower and motor. The heater may be further broken down into two control systems, the Flame Safety

Control (FSC) and the Modulating Gas System

(MGS). The burner mixes air with the gas (Natural or

Propane) which heats the air.

Example 3

Illustration 1: No gas flow. P1 = P2

Illustration 2: Gas flow from left to right. P1 > P2


Fireye Flame Safety Control

The first system to understand is the Flame

Safety Control. The FSC is there only to monitor the flame NOT to control temperature. The FSC uses an ultraviolet (UV) sensor mounted on top of the burner assembly to view the flame in the burner. The FSC is also wired into an airflow switch, which tells it whether there is proper airflow through the unit (not just any airflow, but proper airflow). The FSC controls the opening of the redundant solenoid gas valves and the operation of the spark ignitor to initiate a pilot flame upon start-up. When there is a call for heat, the OPR

CTRL light will turn on indicating that the unit has power. Next, the airflow light will come on if there is proper airflow through the unit. Third, the unit will pause ten (10) seconds to purge any gasses or combustible vapors before attempting flame ignition. Then, there is a Pilot Trial for Ignition

(PTFI) and the PTFI light comes on. During PTFI, the FSC opens the redundant gas valves and allows gas to bypass to the modulating valve (part of the Maxitrol system). The Maxitrol valve is not yet energized, so there is a minimal amount of gas passing through it (called “low fire”). At the same moment, the spark igniter is started, causing the spark plug in the burner to ignite the gas. This results in a low fire or pilot flame. When the UV sensor detects the flame it turns on the flame light, turns off the PTFI light, and powers the modulating gas system. This is the normal operating mode.

The FSC continues to monitor the flame and airflow.

The airflow switch is a single pole double throw

(one common contact, one normally open contact, and one normally closed contact) switch that is

“switched” by air pressure. There are two opposing airflow tubes in the heater, located near the burner and profile plate assembly (profile plates surround the burner and channel air into the burner section). With the differential pressure created in the airflow switch by suction on one tube and velocity pressure on the other, the switch will change state, indicating airflow. In the case of clogged filters or a blocked intake, a differential pressure is not achieved, not allowing the airflow switch to close. With high static pressure or lack of blower movement, no suction is placed on the rearward-facing port and a differential pressure is again not achieved. The airflow switch may need to be adjusted for different pressures that occur at different CFM’s.

The other system, the Maxitrol modulating gas

system, consists of a temperature selector dial, a discharge air sensor, an amplifier, and a modulating gas valve. The two types of Maxitrol systems found on these units are the Maxitrol 14 and the Maxitrol 44.

The Maxitrol 14 utilizes a discharge air sensor and modulates the discharge air to the selected temperature on the temperature selector dial. The

Maxitrol 44 utilizes a room temperature sensor as well as a discharge air sensor in order to control the room temperature. The modulating gas valve controls the amount of gas to the burner based on the temperature rise needed. When the modulating gas valve is all the way open and achieving the maximum BTU’s and temperature rise of the unit, it is called “high fire”.

Maxitrol 14 Amplifier

One back up safety device is the high temperature

limit switch. This switch is a mechanical thermostat that measures the temperature inside the unit directly above the burner. If the factory set temperature is exceeded, it will shut down the power to the FSC. This requires a manual reset. The pre-set temperature that the factory uses is based on the temperature above the burner when the outlet temperature is 185 ° F. The settings are as follows:

Airflow Switch With Cover Removed


Honeywell High Temperature Limit

#1 Size

#2 Size

#3 Size

#4 Size

#5 Size

160 ° F

150 ° F

225 ° F

245 ° F

150 ° F




Heaters set for less than 700,000 BTU’s use a combination gas valve. The combination gas valve has redundant solenoids and a regulator built into one body. Larger heaters use a regulator and two separate solenoid valves. The redundant gas valves shut gas flow off to the burner in case of a malfunction, no call for heat, or power outage. They are normally closed, and are energized by the Fireye

FSC. The regulator controls the gas pressure going into the gas train from the supply line. The “high fire” setting for the heater unit (maximum temperature rise) is adjusted by the regulator before the two redundant solenoid valves or on the combination gas valve (whichever the case may be).

The following is a summary of a normal start for a direct fired gas heater:

• With the blower already running

ü The summer-winter switch is set to winter


ü The outside air temperature falls below the setting of the outside air override

• The FSC is energized and the following occurs:

ü Indicates that it has power by turning on the “Opr Ctrl” light

ü Verifies airflow and turns on the “Airflow” light

ü Pauses 10 seconds to purge the air in the heater

ü Begins Pilot Trial for Ignition and turns on the “PTFI” light

ü The redundant gas solenoid valves are opened, the Honeywell spark igniter begins sparking and the UV sensor watches for flame initiation

ü When flame is established, the “PTFI” light turns off and the “Flame” light turns on

ü The Maxitrol system is powered and the unit begins heating

ü The FSC monitors the flame while the Maxitrol system adjusts to the selected temperature

• The Maxitrol system checks the discharge air temperature (and the room temperature for the Maxitrol 44) and regulates the gas going to the burner to satisfy the temperature setting.

• The last light on the Fireye is the “Alarm” light. This will turn on when the Fireye determines an unsafe condition, and won’t allow the unit to recycle for heat until it has been properly reset. Anytime the Fireye flame safety has gone into “Alarm” mode, the problem must be diagnosed and corrected to avoid future lockouts after resetting. To begin troubleshooting, or to reset the Fireye, the following procedure must be used (just pushing the reset button does NOT reset the unit)

• With the power to the unit

ü Push the reset button on the Fireye

ü Turn power off (the 5 Amp circuit breaker is convenient for this)

ü Turn power back on and allow the heater to start up



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