Maxitrol M420B-1/2 Valve

The Maxitrol M420B-1/2 is a professional-grade modulating gas valve engineered for atmospheric burners and indirect-fired heating systems. Designed specifically for integration with the Maxitrol Selectra Series 94 and A200 electronic modulation systems, this 1/2" NPT valve provides precise, infinite flame adjustment to maintain tight temperature tolerances. The "B" designation indicates that both sides are fitted for low fire adjustment, allowing technicians to calibrate the minimum firing rate efficiently from either side of the manifold. With a maximum 1/2 PSI inlet pressure rating, this component is a staple for contractors servicing commercial make-up air units and space heating equipment.

Built for durability in high-demand environments, the Maxitrol M420B-1/2 ensures smooth gas flow modulation without the "hunting" often associated with inferior controls. It functions as a dedicated modulator, meaning a separate upstream pressure regulator must be utilized to set the high-fire manifold pressure. Whether you are performing a Maxitrol M420B-1/2 replacement or optimizing a 1/2 inch NPT gas line for an atmospheric burner system, this valve delivers the repeatable performance and low fire adjustment accuracy required for industrial-spec HVACR applications.

• Precise Modulation: Provides continuous, infinite flame adjustment when paired with Selectra Series 94 amplifiers.
• Dual-Side Access: The "B" suffix signifies both sides fitted for low fire adjustment, simplifying setup in tight mechanical rooms.
• Atmospheric Burner Compatibility: Optimized for 1/2" NPT configurations with a maximum 0.5 PSI inlet pressure limit.
• Reduced Hysteresis: High-quality internal components ensure consistent response to DC voltage signals from signal conditioners.
• NPT Threading: Standard 1/2-inch connections ensure a secure, leak-free fit for existing gas manifolds.

What is the maximum inlet pressure for the Maxitrol M420B-1/2?
This valve is rated for a maximum inlet pressure of 1/2 PSI (approximately 14 inches w.c.). Exceeding this limit can damage the internal diaphragm and affect modulation accuracy.

How do I adjust the low fire setting on this model?
The M420B features adjustment points on both sides. To set the minimum rate, disconnect one wire from the modulating gas valve terminal and turn the bypass screw until the desired manifold pressure or flame appearance is achieved.

Can the M420B-1/2 also regulate gas pressure?
No, the M-series valves are modulators only. You must install a separate gas pressure regulator upstream to control the high-fire manifold pressure.

Disclaimer: While we strive for 100% accuracy, product data and specifications are subject to change. Please contact our team to confirm compatibility or technical details before purchasing.