Caleffi 108866A iStop™ 1" press union antifreeze valve

Automatic, mechanical freeze protection for water-filled outdoor heat-pump circuits, in a 1" press-union configuration.

As air-to-water heat pumps become increasingly popular for residential and commercial hydronic heating and cooling, protecting outdoor hydronic circuits from freezing is an important part of system design. The Caleffi 108866A iStop® Antifreeze Valve is designed specifically for this application, providing automatic mechanical protection for water-filled hydronic circuits connected to air-to-water heat pumps.

The 108866A features a 1-inch press connection with union, a brass body, and a maximum working pressure of 150 psi. Caleffi specifies it for air-to-water hydronic heat pump applications.

What Is the Caleffi 108866A iStop® Antifreeze Valve?

The Caleffi 108866A iStop® is an automatic antifreeze valve designed to help protect hydronic heat-pump circuits from freeze damage when water is used as the heat-transfer fluid.

The valve monitors the temperature of the water in the circuit. When the water temperature falls to approximately 37°F (3°C), the internal temperature-sensitive mechanism opens and allows water to discharge from the system. Removing water from the exposed outdoor section helps reduce the risk of ice formation and potential damage to the heat pump and hydronic components.

Technical Specifications

Designed for Air-to-Water Heat Pumps

The 108 series is intended for air-to-water hydronic heat pump applications, including systems where outdoor piping and components can be exposed to freezing ambient conditions.

Caleffi's North American documentation specifies that iStop valves should be installed in pairs, with one valve in the supply line and one in the return line to and from the outdoor heat-pump unit. The system is intended for installations using 100% water as the heat-transfer fluid, rather than glycol. Caleffi also recommends confirming acceptance with the heat-pump manufacturer where warranty requirements apply.

How the iStop® Antifreeze Valve Works

The operating principle is straightforward:

• Under normal operating conditions, the valve remains closed.
• The internal sensor monitors the hydronic fluid temperature.
• When the water temperature reaches approximately 37°F (3°C), the valve opens.
• Water is discharged from the circuit.
• This helps prevent freezing and ice formation in vulnerable outdoor sections.
• As the temperature rises, the valve closes again according to its operating mechanism.

This makes iStop® a mechanical freeze-protection device rather than an electrically powered safety component. Caleffi describes the valve as protecting against freeze damage in situations such as a power interruption combined with below-freezing outdoor temperatures.

Key Features of the Caleffi 108866A

Mechanical Freeze-Protection Operation

The valve responds directly to the water temperature, allowing the system to discharge water when the temperature reaches the specified threshold instead of relying solely on an electrical heating element or control signal.

1" Press Union Connection

The 108866A is specifically configured with a 1-inch press union connection, making it suitable for installations where press-connection piping is preferred.

Brass Construction, 150 psi Rating

The valve body is brass, with a 150 psi maximum working pressure and an ambient temperature range of approximately -22°F to 140°F.

Part of the Series 108 Family

Caleffi offers the iStop® series in multiple connection configurations, including 1-inch NPT female union and 1-inch sweat union versions, so contractors can select the valve appropriate for the piping system.

Why Freeze Protection Matters for Hydronic Heat Pumps

Outdoor hydronic heat-pump piping can be exposed to temperatures below freezing. When water freezes, its expansion can create significant stress inside heat exchangers, piping, valves and other components.

Water Temperature Drops → iStop® Opens at ~37°F → Water Discharges → Freezing Prevented

A freeze-protection strategy is therefore an important consideration for systems using water as the circulating medium.

Caleffi 108866A vs. Other iStop® Configurations

Caleffi offers the same iStop® product family in multiple connection styles, so it is important to select the correct part number based on the project's piping requirements.

Typical Applications

Air-to-Water Heat Pump Systems

The primary application is protecting water-filled hydronic circuits connected to outdoor air-to-water heat pumps.

Monobloc Heat Pumps

Caleffi documentation identifies the iStop® series for monobloc heat-pump systems, where water may circulate directly through outdoor sections of the heat-pump circuit.

Hydronic Heating and Cooling

The valve can be used in appropriate water-based hydronic applications where outdoor portions of the circuit require freeze protection.

Residential and Commercial Installations

The compact valve design and press-union configuration can be incorporated into new construction as well as suitable hydronic retrofit projects.

Important Installation Considerations

• Install in pairs: one valve on the supply line and one on the return line to and from the outdoor unit.
• Plan the discharge: account for proper drainage so discharged water does not create an additional hazard around the equipment.
• Water only: Caleffi specifically notes the antifreeze valve is not used on systems containing glycol.
• Check the heat-pump manufacturer: review installation and warranty requirements before installation.

Water vs. Glycol Systems

Why Consider the Caleffi iStop®?

The iStop® valve provides a dedicated mechanical approach to protecting outdoor hydronic heat-pump circuits from freezing. Its temperature-responsive operation, brass construction, 150-psi pressure rating and availability in multiple connection configurations make it a practical component for properly designed air-to-water heat-pump systems.

For contractors and HVAC professionals, selecting the correct connection configuration is particularly important. The 108866A is the 1-inch press-union version of the iStop® product family.

Why Buy from KPaul Industrial?

KPaul Industrial supplies HVAC, hydronic heating, heat-pump and industrial components for contractors, commercial facilities, government customers and maintenance professionals. When sourcing the Caleffi 108866A, customers can benefit from:

• Competitive pricing on genuine Caleffi products
• Assistance identifying the correct Series 108 connection
• Support with hydronic and heat-pump component sourcing
• Support for commercial, government and residential projects
• Experienced industrial customer service

Contact KPaul Industrial for Caleffi 108866A pricing, availability and sourcing assistance.

Final Thoughts

The Caleffi 108866A iStop® is a 1-inch press union, brass antifreeze valve that opens at approximately 37°F to discharge water from outdoor hydronic heat-pump circuits. Installed in pairs on the supply and return lines and used in water-based systems, it provides mechanical freeze protection that does not depend on electrical controls.

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