HEAT PUMP & MULTI-CIRCUIT SYSTEM COMPONENT — HYDRAULIC SEPARATOR WITH AIR & DIRT SEPARATION
The Spirotech SpiroCross AX125J is the Rp 1¼" variant of
the AX-J hydraulic separator series, designed for larger heat pump installations, apartment building
shared systems, and commercial multi-circuit HVAC applications where the AX100J's
2.0 m³/h capacity is insufficient. With a maximum flow rate of 3.6 m³/h,
it covers the primary circuit demands of most mid-range air or ground-source heat pumps up to
approximately 20–25 kW at typical Δt values. Like the AX100J, it combines
hydraulic balancing, microbubble air deaeration, and neodymium magnetic dirt separation
in a single brass unit with an identical footprint — no additional floor space
required when upgrading from a 1" to a 1¼" installation.
3-in-1
Hydraulic + Air + Dirt
INSTALLATION — ORIENTATION AND PIPEWORK
As with the AX100J, the AX125J must be mounted in horizontal pipework with the integral air vent on top. No directional arrow — either connection serves as inlet or outlet. Connect the higher-temperature circuit to the upper connections. Allow at least 100mm straight pipe before and after. At 6.9 kg dry weight, ensure pipework support is independently bracketed — do not rely on the pipe joints alone to carry the unit weight.
AX100J vs AX125J — SIZE SELECTION
Both models share identical external dimensions (H 462mm, L 236mm, D 84mm). The only hydraulic difference is the connection size and flow capacity: AX100J handles up to 2.0 m³/h on Rp 1" pipework; AX125J handles up to 3.6 m³/h on Rp 1¼". Select by matching the nominal size of your primary circuit pipework. If the system operates near the upper flow limit, choose the larger AX125J to avoid exceeding the 1 m/s velocity limit that degrades hydraulic separation efficiency.
IDENTICAL FOOTPRINT — LARGER CAPACITY
Because both AX100J and AX125J share the same physical envelope, it is straightforward to specify the AX125J in any installation where future heat pump upsizing is a possibility. The additional 0.4 kg weight difference is the only physical change; pipework adaptors from Rp 1¼" to smaller pipe sizes are readily available if needed in the short term.
Higher-Flow Hydraulic Separation
At 3.6 m³/h, the AX125J covers the primary circuit flow of most residential and light-commercial heat pumps. The SpiroTube ensures minimal fluid mixing between primary and secondary loops, maintaining the temperature differential that drives heat pump efficiency — a critical factor on systems where secondary zone valves open and close frequently.
Same Neodymium Magnet Specification
The AX125J uses the same neodymium magnet specification as the AX100J — a full-width collector that captures magnetite in the 1.3-litre internal volume. On a larger system with more pipework surface area to corrode, maintaining this collection capacity is more important, not less. Draining the collection chamber is completed in seconds while the system runs.
Viton-Sealed Valve Construction
The integral air vent uses a Viton-sealed float valve — a material chosen for long-term compatibility with both water and glycol solutions across the full 0–110 °C operating range. This is the same specification as the AX100J and is a direct contributor to the 20-year SpiroLIFE warranty backing.
Suitable for New Build and Retrofit
The AX125J is designed for new build, refurbishment, and retrofit applications. Rp 1¼" BSP threading is standard in commercial plant rooms and larger residential boiler houses. Spirotech recommends a full system power-flush and water quality check before fitting to ensure the separator operates in clean system water from day one.
Technical Specification — SpiroCross AX125J (Rp 1¼")
| Connection & Hydraulics |
|---|
| Article Number |
AX125J |
| Product Type |
SpiroCross AX-J Hydraulic Separator — Rp 1¼" threaded fitting |
| Connection |
Rp 1¼" BSP internal thread |
| Max Flow Rate |
3.6 m³/h |
| Max Flow Velocity |
1.0 m/s |
| Operating Limits |
|---|
| Max Operating Pressure |
10 bar (bar-g) |
| Max Temperature |
110 °C |
| Medium |
Water / glycol (max 50% v/v) |
| Design Standard |
EN 13445 |
| CE Marking |
2014/68/EU (Pressure Equipment Directive) |
| Construction Materials |
|---|
| Body |
Brass |
| Connections |
Brass |
| Fittings |
Brass |
| Deaeration Cap |
Brass |
| Valve Sealing |
Viton |
| Float |
Polypropylene (PP) |
| Drain Valve |
Brass |
| Spirotube Tube |
S235 JRG2 (structural steel) |
| Spirotube Wire |
Copper |
| Spirotube Solder |
Tin (SnCu3) |
| Magnet |
Neodymium |
| Physical Dimensions |
|---|
| Total Height (H) |
462 mm |
| Lower Height (h) |
135 mm |
| Connection Height (h1) |
144 mm |
| Upper Height (h2) |
183 mm |
| Length (L) |
236 mm |
| Body Diameter (D) |
84 mm |
| Width (b) |
53 mm |
| Min clearance above (x) |
>100 mm |
| Min clearance below (y) |
>50 mm |
| Air Vent Port (e) |
G 3/4" |
| Drain Port (e2) |
R 1/2" |
| Internal Volume |
1.3 litres |
| Dry Weight |
6.9 kg |
| vs AX100J — Key Differences |
|---|
| Connection |
Rp 1¼" (AX100J: Rp 1") |
| Max Flow |
3.6 m³/h (AX100J: 2.0 m³/h) |
| Dry Weight |
6.9 kg (AX100J: 6.5 kg) |
| All Other Dimensions |
Identical to AX100J |
| Testing & Warranty |
|---|
| Leakage Test |
Yes — test medium: air |
| Design & Manufacturing |
2014/68/EU compliant |
| Warranty |
20 years — SpiroLIFE |
Frequently Asked Questions
What heat pump sizes is the AX125J designed for?
At 3.6 m³/h maximum flow, the AX125J typically suits heat pump primary circuits in the 12–25 kW output range depending on the primary circuit Δt. At a heat pump primary Δt of 5 K, 3.6 m³/h equates to approximately 20–21 kW thermal. Always confirm the primary circuit design flow rate with the heat pump manufacturer's hydraulic specification before selecting between AX100J and AX125J.
Why do the AX100J and AX125J have the same external dimensions?
Spirotech deliberately designed the AX-J range with a common footprint so that engineers can upgrade from Rp 1" to Rp 1¼" connection without redesigning the plantroom layout. The only physical differences are the connection thread size and a 0.4 kg weight increase — the body, mounting points, and service clearances are identical.
Can I use the AX125J on a system with three or more secondary circuits?
Yes — the low-loss header function of the SpiroCross is not limited to two circuits. The primary circuit connects to the upper two ports (flow and return) and the secondary manifold circuit connects to the lower two ports. Multiple zone pumps on the secondary side are individually hydraulically isolated from the primary by the SpiroTube, regardless of how many zones are active.
Does the AX125J require a separate expansion vessel connection?
Yes. As with all low-loss header installations, the expansion vessel should be connected on the suction side of the secondary circuit pump. Spirotech's installation diagram for the AX-J series shows this connection explicitly. The AX125J body itself does not have a dedicated expansion port — the vessel connects to the closest pipework tapping in the secondary circuit.
Is the AX125J compatible with R290 (propane) heat pumps?
The AX125J operates on the water/glycol side of the heat pump system — it does not contact the refrigerant circuit. R290 heat pumps use the same hydraulic primary circuit specification as other refrigerant types. The AX125J is compatible with any heat pump where the primary circuit water conditions fall within 10 bar, 110 °C, and 50% glycol.
How does the AX125J differ from a standard plumbing low-loss header?
A standard low-loss header provides hydraulic separation only. The AX125J additionally provides active microbubble air deaeration via the factory-fitted automatic air vent, and neodymium magnetic dirt separation via the integral collection chamber. This three-in-one functionality eliminates the need for separate SpiroVent and SpiroTrap units on the primary/secondary connection, reducing installation time, leak points, and equipment cost.
