Why a Safety Switch Should Not Be Field-Adjustable

What the furnace OEM needed

An HVAC OEM developing gas-fired heating systems needed a fixed setpoint pressure switch. Notably this is not a comfort component.

It is a proof-of-airflow device. Specifically, it confirms the exhaust blower is running before the control board may open the gas valve and ignite the burner.

Two set points, two models

• First, one switch closing at 40 Pa and opening at 20 Pa.
• Next, a larger unit closing at 75 Pa and opening at 45 Pa.
• Meanwhile a compact footprint of 70.1 by 79.1 mm, 42.4mm deep.
• Additionally, compatibility with 125/250 VAC line voltage and 24 VDC control circuits.
• Finally, -40C to +120C ambient, with a diaphragm rated beyond 150C.

That diaphragm temperature reveals where it mounts. Anything asked to survive above 150C sits near the heat exchanger or the flue. There, a backdraft of hot combustion gas can reach it.

Choosing a fixed setpoint pressure switch

This was a sourcing and validation problem rather than a fan design. Even so, one decision matters more than the rest.

The choice that defines the safety case

Certainly a single field-adjustable switch would cover both set points. It is cheaper to stock, and installers like them.

However, we rejected it. The reason is the whole point of the component.

An adjustable switch can be set wrong. On a proof-of-airflow interlock, that means the gas valve may open while the exhaust blower moves too little air. In short, it is the exact failure the switch exists to prevent.

Consequently two separate fixed setpoint pressure switch variants were made, each calibrated at the factory. Nothing on site can change them.

Why not design one from scratch

However, a new switch means a development cycle and a certification program. On a safety component, neither is quick.

Instead we worked from a proven platform and calibrated it. The diaphragm is a gas-phase silicone meeting the 150C requirement. Meanwhile the internal micro switch options already carry UL, EMEC and CQC certification.

That last part matters commercially. Furthermore certified internals shorten the customer own compliance path rather than starting it from zero.

What it cost

Stock complexity. In practice the customer now carries two part numbers where one adjustable switch would have done.

That is the trade, and on a gas appliance interlock it is the right one. By contrast, on a non-safety application an adjustable switch is more practical, and we would recommend it.

Technical Specifications

Two set points appear below, one per model. Each is calibrated at the factory and cannot be altered on site.

Technical Documentation

The final production units for Wind pressure switch-04 and Wind pressure switch-05, ready for integration.

Wind pressure switch-04 Final Drawing (PDF)
The final engineering drawing for the Wind pressure switch-04, confirming dimensions and the 40 Pa / 20 Pa set points for design engineers.

Wind pressure switch-05 Final Drawing (PDF)
The final engineering drawing for the Wind pressure switch-05, detailing the 75 Pa / 45 Pa set points for higher-capacity HVAC systems.

An image of the approved technical schematic for the 40/20 Pa model, used by quality control and receiving departments.

An image of the approved technical schematic for the 75/45 Pa model, ensuring correct part identification on the assembly line.

If you are specifying a safety interlock

A fixed setpoint pressure switch is specified differently from a performance component.

• Prefer factory calibration on anything safety-rated. A fixed setpoint pressure switch cannot be misadjusted during installation or service. That removes a human error path entirely, which is worth more than the stocking convenience it costs.
• Give both set points, not just the trip point. A differential switch has a closing pressure and an opening pressure, and the gap between them is what stops it chattering near the threshold. Specifying only one leaves the other to chance.
• State the temperature the diaphragm will actually see. Ambient is not the same as what reaches a switch mounted near a flue. A backdraft of combustion gas is a short, hot event, and the diaphragm is what fails first if it was rated on room temperature.
• Ask whether the internals are already certified. A micro switch carrying UL or equivalent approval shortens your own certification. Starting from an uncertified component adds a testing program you may not have scheduled.

See our gas heater blower range, the accessories section, or HVAC systems. Appliance safety follows IEC 60335-1.

Questions about pressure switches

Why not use one adjustable switch for both models?

Because it can be set wrong, and on a proof-of-airflow interlock that failure permits ignition without adequate exhaust flow. A fixed setpoint pressure switch removes the adjustment entirely, so installation and service cannot introduce the fault. The extra part number is a small price for closing that path.

What does the gap between the ON and OFF pressures do?

It provides hysteresis. Without it, a switch near its threshold opens and closes repeatedly as pressure fluctuates, cycling the burner. So the 40/20 Pa and 75/45 Pa pairs are chosen to make the switch settle rather than chatter.

Why does the diaphragm need to exceed 150C?

Because it can meet hot flue gas during a backdraft, even though the surrounding ambient is far lower. The diaphragm is the thinnest, most temperature-sensitive part in the assembly, so it sets the real limit rather than the housing does.

Can the same switch work on 24 VDC and 250 VAC?

Yes, within the micro switch contact rating. Modern furnace control boards use low-voltage logic, while older designs switch line voltage directly. A switch rated for both lets one part serve across a product range rather than needing two variants.