Caleffi 108869A iStop 1" Sweet union antifreeze valve

Automatic, mechanical freeze protection for water-filled outdoor heat-pump circuits.

Protecting outdoor hydronic heat-pump circuits from freezing is an important consideration when a system uses water as the heat-transfer fluid. The Caleffi 108869A iStop® Antifreeze Valve is designed for this purpose, providing automatic mechanical freeze protection for air-to-water hydronic heat pump applications.

The 108869A features a 1-inch sweat union connection, brass construction, and a 150 psi maximum operating pressure. Caleffi identifies it as part of its Series 108 iStop® antifreeze valve family.

What Is the Caleffi 108869A iStop® Antifreeze Valve?

The Caleffi 108869A is an automatic antifreeze valve designed to protect water-filled outdoor sections of hydronic heat-pump systems.

When the water temperature reaches approximately 37°F (3°C), the valve opens and allows a controlled amount of water to discharge. This helps keep water moving through vulnerable sections of the system and reduces the risk of freezing and damage to heat-pump components. Caleffi's North American documentation states that the 108 series is intended for systems using 100% water as the heat-transfer fluid.

Technical Specifications

Designed for Air-to-Water Heat Pumps

The iStop® 108 series is specifically designed for air-to-water hydronic heat pump applications. Caleffi states that the valves are suitable for monobloc and hydro-split air-to-water heat-pump systems in hydronic applications.

The manufacturer's North American documentation also specifies that the valves should be installed in pairs, with one valve on the supply and one on the return line to and from the outdoor unit. This configuration helps protect the outdoor heat-pump circuit regardless of which section of the system cools most quickly.

How Does the iStop® Work?

The operating principle is mechanical and temperature-responsive:

• During normal operation, the valve remains closed.
• The valve monitors the temperature of the water in the circuit.
• When the water reaches approximately 37°F (3°C), the antifreeze valve opens.
• A small amount of water is discharged from the outdoor circuit.
• Fresh, warmer water can then enter the circuit, helping prevent freezing.
• As warmer water reaches the valve, the sensor closes the discharge path again.

Caleffi notes that the 108 series is not intended to completely drain the system. Its purpose is to maintain water movement and prevent the outdoor circuit from reaching freezing conditions.

Key Features of the Caleffi 108869A

Mechanical, Temperature-Responsive Operation

Rather than relying entirely on electrical controls or a backup power source, the valve responds directly to the water temperature in the hydronic circuit. This makes it particularly relevant for systems where maintaining freeze protection during a power interruption is a design consideration.

1" Sweat Union Connection

One of the main identifying features of the 108869A is its 1-inch sweat union connection. This configuration is intended for copper piping installations where a sweat/solder connection is preferred.

Brass Construction, 150 psi Rating

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

Monobloc and Hydro-Split Compatibility

Caleffi identifies the Series 108 as suitable for monobloc and hydro-split hydronic heat-pump applications, installed as part of a supply-and-return freeze-protection arrangement.

Why Freeze Protection Is Important

Outdoor heat-pump piping can be exposed to freezing temperatures. If water becomes trapped and freezes, expansion can place stress on heat exchangers, piping, valves and other components.

Water Temperature Drops → iStop® Opens at ~37°F → Warmer Water Enters → Valve Closes

The Caleffi iStop® provides a dedicated mechanical response to this risk, keeping water moving through the outdoor circuit before it can reach freezing conditions.

Series 108 Connection Options

Caleffi offers the Series 108 in different connection configurations, allowing contractors to select the appropriate version for the piping system.

Typical Applications

Air-to-Water Heat Pumps

The primary application for the 108869A is freeze protection for water-filled hydronic circuits connected to outdoor air-to-water heat pumps.

Monobloc Heat Pumps

The valve is particularly relevant to monobloc-style systems where hydronic water piping extends directly to the outdoor heat-pump unit.

Hydronic Heating and Cooling

The valve can be incorporated into suitable water-based heating and cooling circuits where outdoor piping requires freeze protection.

Residential and Commercial HVAC

The 1-inch sweat-union configuration can be useful for residential and commercial installations using copper piping and conventional sweat connections.

Important Installation Considerations

• Install in pairs: one valve on the supply line and one on the return line, as close to the outdoor heat pump as practical.
• Plan the discharge: arrange the system so discharged water can safely drain away from the equipment and surrounding area.
• Do not insulate the valve: it must remain a point where water can cool quickly enough for the sensor to respond. Insulating the vacuum-breaker portion could interfere with its operation.
• Water only: the North American Caleffi catalog specifies the Series 108 for 100% water systems; it is not used on systems with glycol.
• Check the heat-pump manufacturer: confirm compatibility and warranty conditions before installation.

Water vs. Glycol Systems

Why Choose the Caleffi 108869A?

The 108869A combines a temperature-responsive antifreeze function with a standard 1-inch sweat-union connection, making it a practical option for copper-pipe hydronic installations. For HVAC contractors, mechanical engineers and facility teams, it provides a dedicated method of protecting outdoor water circuits from freezing while fitting into a properly designed air-to-water heat-pump system.

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 108869A, 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 108869A pricing, availability and sourcing assistance.

Final Thoughts

The Caleffi 108869A iStop® is a 1-inch sweat union, brass antifreeze valve that opens at approximately 37°F to keep water moving through outdoor hydronic heat-pump circuits. Installed in pairs on the supply and return lines, left uninsulated, and used in 100% water systems, it provides mechanical freeze protection that does not depend on electrical controls.

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