Flow switches for boiler systems are used to detect adequate water flow and to start or stop equipment like burners or alarms. For buyers, the goal is to match reliability, material compatibility, and electrical ratings to the boiler setup. This guide highlights five practical options and explains who each is best for, with clear, decision-driven comparisons to help you choose the right model.
| Product | Best For | Key Benefit |
|---|---|---|
| McDonnell & Miller FS4-3 General Purpose Liquid Flow Switch | Small to mid-size boilers needing simple start/stop control | Durable general-purpose design for basic signaling |
| Mcdonnell & Miller, FS8-W, Flow Switch | Compact installations requiring 1″ NPT | Reliable signaling with compact footprint |
| HFS-25 1″/dn25 Water Flow Switch SPDT | Dynamic fluid monitoring with SPDT control | Vane-actuated with robust electrical contacts |
| McDonnell & Miller FS251 General Purpose Flow Switch | Replacement for FS4-3 in older systems | Electronic component with standard dimensions |
| Lochinvar Air Pressure Switch for EnergyRite | Specialized air pressure signaling for energy systems | Pairs with Lochinvar energy heaters |
Note: Each option has distinct electrical and environmental specifications. Review compatibility with your boiler, wiring, and the liquids in use to avoid issues such as non‑corrosive liquid compatibility or improper voltage ratings.
McDonnell & Miller FS4-3 General Purpose Flow Switch
The FS4-3 is designed to start or stop electrically operated equipment such as signal lights, alarms, motors, automatic burners, and metering devices. It uses 1″ NPT threads and handles a temperature range from 32 degrees F to 300 degrees F. Best for basic signaling needs in small to mid-size boiler applications. A limitation is that detailed electrical ratings beyond basic signaling are not specified here, so verify compatibility with your control circuit before installation.
Mcdonnell & Miller FS8-W Flow Switch
The FS8-W provides a 1″ NEMA-compliant flow switch profile, suitable for compact installations. It is best for boiler or burner setups requiring a straightforward flow signal in a narrow footprint. The listed data focuses on package dimensions, which helps with fitting in tight panels. A caution is that more electrical characteristics would be needed to confirm field compatibility in high-amp lines.
HFS-25 1″/dn25 Water Flow Switch SPDT
This flow switch uses SPDT contacts with 6-380V ratings and supports a maximum working pressure of 1.0 MPa. It is designed to measure the dynamic state of fluids, including water, ethylene glycol, and other non-corrosive liquids to copper. It’s best for more demanding electrical signaling and higher pressure environments. A potential limitation is its operating temperature range, which should be checked to ensure compatibility with your system’s liquid temperature range.
McDonnell & Miller FS251 General Purpose Flow Switch
The FS251 is marketed as a general-purpose electronic flow switch, intended as a replacement for FS4-3 in some applications. It carries standard dimensions and is suitable for typical boiler control circuits. Best for systems needing a straightforward electronic switch with familiar form factors. A limitation is the lack of detailed electrical specifications in this listing, which should be verified against the existing control panel requirements.
Lochinvar Air Pressure Switch for EnergyRite
This air pressure switch is designed for EnergyRite systems and works in conjunction with Lochinvar residential pool heaters. It’s best for specialized signaling in energy-efficient or pool-heating setups. The main caveat is that it is tailored to Lochinvar ecosystems; compatibility with non-Lochinvar boilers should be confirmed before purchase.
Buying Guide
Key considerations when selecting a boiler flow switch include:
- Flow range and setpoint: Ensure the switch responds at the flow rate your boiler requires to prevent dry firing or improper burner operation.
- Electrical rating: Match coil voltage and contact type (SPDT, NO/NC) with the control circuit to avoid miswiring or signal loss.
- Pipe size and mounting: Confirm 1″ NPT or DN25 (as listed) and compatible mounting configurations for your existing piping.
- Liquid compatibility: Some switches are specified for water or non-corrosive liquids; ensure the liquid in your system won’t degrade components.
- Environment and temperature range: Check ambient and liquid temperature specs to avoid premature failure.
- Specialized signaling needs: If you require airflow or specific energy-system integration, some models are designed for those purposes only.
Frequently Asked Questions
- What is a flow switch used for in a boiler system?
It detects adequate water flow to start or stop equipment and can trigger alarms or safety shutdowns.
- What does SPDT mean in a flow switch?
SPDT stands for single-pole double-throw, allowing the switch to energize one circuit while breaking another.
- Which size is most common for residential boilers?
1 inch (1″ NPT) is a common size, but always confirm with your system’s piping and valve connections.
- Can a flow switch be used with glycol or other non-water liquids?
Only if the liquid is non-corrosive to copper or the switch’s material is rated for that liquid; always check compatibility data.
- Do I need an air pressure switch for a boiler?
Air pressure switches are used in some energy systems or pool heaters; confirm your boiler’s control needs before selecting one.