Pentair 42001-0061S Air Flow Switch
The Pentair Master temp air flow switch (AFS) senses the pressure drop across the air metering orifice. ...

Used on the following models:MSTRTMP175LPMSTRTMP175NAMSTRTMP200LPMSTRTMP200NAMSTRTMP250LPMSTRTMP250NASta-Rite Max-E-Therm 400 LPSta-Rite Max-E-Therm 400 NASR400LPSR400NASta-Rite Max-E-Therm 200 LPSta-Rite Max-E-Therm 200 NASta-Rite Max-E-Therm 333 LPSta-Rite Max-E-Therm 333 NAPentair MasterTemp 400 LPPentair MasterTemp 400 NASR200LPSR200NASR333LPSR333NAPentair MasterTemp 200 LPPentair MasterTemp 200 NAPentair MasterTemp 250 LPPentair MasterTemp 250 NAPentair MasterTemp 300 LPPentair MasterTemp 300 NAMSTRTMP300LPMSTRTMP300NAMSTRTMP400LPMSTRTMP400NAPentair MasterTemp 175 LPPentair MasterTemp 175 NA

The Pentair Master temp air flow switch (AFS) senses the pressure drop across the air metering orifice. As soon as there is sufficient air flow,the AFS closes, closing the circuit to the hot surface igniter (HSI), which ignites the fuel mixture. On a call for heat, theblower and HSI are energized. In about 20 seconds, the gas valve opens and ignition occurs. The HSI then switches to asensing mode and monitors the flame.
The air flow switch, is a safety device used to insure that the
combustion air blower (fan) is operating and has been designed to monitor the
vacuum (negative) pressure within the blower housing. The air flow switch is
factory set and is connected upstream of the ignition module. The ignition module
does not operate unless the air pressure switch and all safety switches are
closed.

The heater requires a gas supply of not less than 4" (10.2cm) wc and not more than 14" (35.6cm) wc. Gas supply
pressures outside of this range may result in improper burner operation. A minimum flowing or dynamic inlet pressure
(while the heater is running) of 4" (10.2cm) wc is required to maintain input rating with no more than a 2” pressure drop
between static and dynamic. The gas supply must be installed in accordance with the National Fuel Gas Code,
ANSIZ223.1, or standard CSA B149.1, Natural Gas and Propane Installation Codes, as applicable and all applicable
local codes. Install a manual shut-off valve and a sediment trap and union located outside the heater panels, see Figure 6.
Do not use a restrictive gas cock.