What Are Volt-Free Relay Contacts?

You are standing in front of a new boiler or heat pump, wiring centre in hand. The appliance manual insists on a "dry contact" or "volt-free" connection. If you get this wrong and accidentally send 230V mains power down an Extra Low Voltage (ELV) control line, you have a major problem.

Mistaking a switched-live connection for a volt-free connection will instantly fry the boiler's or Heat Pump’s Printed Circuit Board (PCB). That is a £300+ mistake that voids the manufacturer's warranty, leaves the customer without heating, and wastes hours of your day resolving the fallout.

Every Whisper Controls wiring centre is designed with dedicated volt-free relay contacts for boiler and heat pump connections. This guarantees complete isolation between your control circuits and the appliance's power supply, ensuring zero risk of accidental mains voltage crossing into sensitive appliance inputs.

Here is exactly how they work, why they are required, and how to wire them compliantly under BS 7671.

Understanding Volt-Free (Potential-Free) Contacts

A volt-free contact—also known as a dry relay contact—is a switching device that contains no internal power source.

• It does not generate or supply electricity.
• It operates exactly like a standard light switch, simply opening and closing to allow or block the appliance's own control current.
• The wiring centre sends the command (closing the circuit); the boiler or heat pump provides its own power to read that command.

Why We Use Volt-Free Connections

1. Safety & Isolation: They physically separate the wiring centre's electronics from the appliance's power supply, eliminating cross-voltage contamination between Extra Low Voltage (ELV) control circuits and 230V Low Voltage (LV) mains.

2. Universal Compatibility: Whether you are switching a 24V boiler, a heat pump, or a hybrid system, a volt-free contact adapts to the appliance's specifications.

3. BS 7671 Compliance: Industry best practice and UK wiring regulations demand strict isolation between discrete control circuits and power circuits.

How to Wire a Boiler or Heat Pump to Whisper Controls

Follow these steps to ensure a safe, compliant connection:

Step 1: Identify the Appliance Control Input Check the boiler or heat pump manual for the ELV control input. These are typically labelled "call for heat," "heating demand," or "enable."

Step 2: Wire to the Wiring Centre Using an appropriately sized control cable (e.g., 2-core 1.0mm²), connect:

• Conductor A to the Common (C) terminal on the wiring centre's volt-free relay.
• Conductor B to the Normally Open (NO) terminal on the relay.

Note: Because this is a dry mechanical switch, the relay itself has no polarity.

Step 3: Route Cables Compliantly (BS 7671 Band I & Band II) Under BS 7671, Band I (Extra Low Voltage / ELV) and Band II (230V mains) circuits must be strictly segregated to prevent interference and electrical faults. If they must share a containment route, all cables must be insulated to the highest voltage present (typically 230V).

Frequently Asked Questions

"Is turning off the wiring centre spur enough to make it safe?"
No. Because the volt-free relay brings an independent feed from the boiler into the wiring centre, the enclosure now contains two separate supplies. Isolating the wiring centre does not isolate the volt-free terminals; they will remain live from the boiler. Under BS 7671 (Regulation 514.11), you must apply a warning label to the wiring centre stating: "Warning: Dual Supply. Isolate both the heating control system and the boiler before removing cover."

"Does the wiring centre power the boiler?"
No. The wiring centre is solely a switch. The boiler or heat pump must be powered via its own independent fused spur.

"Do I ever need an external relay?"
You only need to introduce a secondary external relay or contactor if:

1. The appliance's control current exceeds the wiring centre's maximum amperage rating.

2. You are switching circuits across different RCDs or phases, which requires physical separation to prevent nuisance tripping or phase-to-phase faults.

Need technical guidance on wiring a specific appliance? Download the specific wiring centre installation guide or contact the Whisper Controls support team on 01993 640073.