Astralpool / Hurlcon MX, HX, JX Bypass Kit
Astralpool / Hurlcon MX, HX, JX Bypass Kit is backordered and will ship as soon as it is back in stock.
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Don't Guess - Ask Us!
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Delivery and Shipping Info*
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Astralpool / Hurlcon MX, HX, JX Bypass Kit
Astralpool / Hurlcon MX, HX, JX Bypass Kit is backordered and will ship as soon as it is back in stock.
Astralpool
Replacement bypass assembly kit for selected AstralPool, Hurlcon and WWDG gas pool and spa heaters.
We keep these physically in stock for fast shipping or local pick-up, unlike most drop-shipping websites.
This bypass assembly is used inside the heater manifold/water header and helps regulate internal water flow through the heat exchanger. A worn, damaged or missing bypass assembly can affect heater performance, water flow, pressure switch operation and overall heater reliability.
- Common point of leaks in gas heaters.
- May also be the source of F2 errors in the heaters, in addition to the high-limit switches.
Suitable For
This bypass assembly suits the following AstralPool / Hurlcon heater models:
- Hurlcon / AstralPool HX 70
- Hurlcon / AstralPool HX 120
- Hurlcon / AstralPool HX 150
- Hurlcon / AstralPool MX 125
- Hurlcon / AstralPool MX 150
- Hurlcon / AstralPool MX 200
- Hurlcon / AstralPool MX 250
- Hurlcon / AstralPool MX 300
- Hurlcon / AstralPool MX 400
- Hurlcon / AstralPool JX 130
- Hurlcon / AstralPool JX 160
- WWDG XH 110
Included In This Kit
- Bypass spindle
- Bypass disc
- O-ring
- Two split pins
Part Numbers
- 76021
- 76040 x 2
Important Installation Notes
- Check the bypass spring condition while the heater is apart.
- If the spring is corroded, weak, broken or badly worn, replace it at the same time.
- If required, select the bypass spring from the Add-ons section on this listing.
- Use a suitable silicone lubricant on the O-ring during installation.
- Take care not to drop the bypass disc down into the pipework or heater manifold.
- Confirm correct orientation and movement of the bypass before reassembling the heater.
- Inspect plastic manifold for wear. Badly worn manifolds may leak.
When This Part May Be Needed
- Heater has poor water flow through the heat exchanger
- Spring has corroded.
- F2 Errors
- Bypass disc, spindle or pins are damaged or missing
- Old bypass components are brittle, worn or distorted
- The heater is being serviced and the bypass assembly is due for replacement
- There are flow, pressure or temperature issues related to the heater header assembly
General Information
Gas pool and spa heater parts should be inspected, diagnosed and installed by a suitably qualified and experienced person. Incorrect installation, poor water chemistry, blocked filters, restricted flow, incorrect plumbing or faulty safety components can all affect heater operation.
Please compare your existing bypass assembly, heater model and part numbers before ordering. Images and descriptions are provided to assist identification, but manufacturers may revise parts over time.
Manufacturer Catalogues, Diagrams & Part Numbers
Need exploded views, spare part numbers, product numbers, diagrams, or more product information? See the manufacturer catalogues: AstralPool Catalogue 2026 | Zodiac Catalogue 2026
In plumbing, named sizes are not the actual measurements. A pipe called “50mm” or “2"” does not measure 50 mm across. Instead, it follows nominal sizing, an internationally-standardised naming system.
Use the Outside Diameter (OD) to check your pipe against fittings. Below is a quick reference for common spa & pool pipe sizes:
| Imperial Name | Metric Name | Pipe Diameter (OD) |
|---|---|---|
| 1/2" | 15mm | ~21.2–21.5 mm |
| 3/4" | 20mm | ~26.6–26.9 mm |
| 1" | 25 mm | ~33.4–33.7 mm |
| 1.5" | 40 mm | ~48.1–48.4 mm |
| 2" | 50 mm | ~60.2–60.8 mm |
| 2.5" | 65 mm | ~75.2–75.5 mm |
These are nominal sizes and are designed to fit matching fittings — not to measure exactly what their name suggests. These are based on PVC Pressure Pipe AS/NZ 1477 Dimensions1.
- º Internal diameters vary slightly by manufacturer and class.
- 1 Not all pool/spa fittings match these dimensions exactly. Some do not follow Australian or international standards. Chinese spas may use proprietary fittings.
Understanding Plumbing Sizes & Nominal Measurements
Plumbing sizes can be confusing. PVC and many other pipe systems use nominal sizes—the size is a name, not the exact measured dimension. This convention is used across most pool and spa systems so that pipes and fittings remain compatible.
Why pipe sizes don’t match the measurements
The Nominal Pipe Size (NPS) system standardises pipe dimensions so that pipes, fittings, valves, and accessories interoperate. Plumbers work with these nominal names every day, but DIYers can be tripped up if they try to match by a tape-measure alone.
How to Measure Pipe Correctly
-
Pipe size is identified by:
- The pipe’s Outside Diameter (OD).
- The fitting’s Inside Diameter (ID) (socket opening).
- Pipe Class / Schedule (e.g., Class 9, 12, 18 or Sch 40) relates to wall thickness and therefore the Internal Diameter (ID) and pressure rating.
- The outside diameter of a fitting is not useful (wall thickness varies by manufacturer).
Hose & Tube Dimensions
Common flexible hose sizes used in pool/spa systems (approximate IDs).
| Imperial Name | Metric Name | Inside Diameter (ID) |
|---|---|---|
| 3/8" | 9–10 mm | ~9 mm |
| 3/4" | 19 mm | ~19 mm |
BSP – British Standard Pipe Thread Sizes
BSP thread names are also nominal. The thread OD will not equal the nominal name.
| BSP Thread Size Naming | Outside Diameter (mm / inch) | TPI |
|---|---|---|
| 1/16" BSP | 7.7 mm / 0.304" | 28 |
| 1/8" BSP | 9.7 mm / 0.383" | 28 |
| 1/4" BSP | 13.16 mm / 0.518" | 19 |
| 3/8" BSP | 16.66 mm / 0.656" | 19 |
| 1/2" BSP / 15 mm | 20.99 mm / 0.825" | 14 |
| 5/8" BSP | 22.99 mm / 0.902" | 14 |
| 3/4" BSP / 20 mm | 26.44 mm / 1.041" | 14 |
| 7/8" BSP | 30.20 mm / 1.189" | 14 |
| 1" BSP / 25 mm | 33.25 mm / 1.309" | 11 |
| 1-1/4" BSP | 41.91 mm / 1.650" | 11 |
| 1-1/2" BSP / 40 mm | 47.80 mm / 1.882" | 11 |
| 2" BSP / 50 mm | 59.61 mm / 2.347" | 11 |
| 2-1/4" BSP | 65.71 mm / 2.587" | 11 |
| 2-1/2" BSP | 75.18 mm / 2.960" | 11 |
| 3" BSP | 87.88 mm / 3.460" | 11 |
| 4" BSP | 113.03 mm / 4.450" | 11 |
| 5" BSP | 138.43 mm / 5.450" | 11 |
| 6" BSP | 163.83 mm / 6.450" | 11 |
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76021 x1 | 76040 x2