Astralpool / Rolachem H Block Roller - RC8, RC9, RO9, RP9 Dosing Pumps - 521015
Astralpool / Rolachem H Block Roller - RC8, RC9, RO9, RP9 Dosing Pumps - 521015 is temporarily out of stock and will ship as soon as it's available.
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Don't Guess - Ask Us!
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Astralpool / Rolachem H Block Roller - RC8, RC9, RO9, RP9 Dosing Pu...
Astralpool / Rolachem H Block Roller - RC8, RC9, RO9, RP9 Dosing Pumps - 521015 is temporarily out of stock and will ship as soon as it's available.
Astralpool / Rolachem H Block Roller - RC8, RC9, RO9, RP9 Dosing Pumps - 521015
- We keep these physically in stock for fast shipping or local pick-up, unlike most drop-shipping websites.
- Shipped from VIC, NSW, WA, QLD
Genuine Astralpool H block, part 521015, the roller carrier inside the pump head of a Rolachem peristaltic dosing pump. Astralpool lists it simply as H BLOCK in its Rolachem spare parts pages.
The H block is the rotating part that carries the rollers around the inside of the pump head. As it turns, the rollers squeeze the tube against the housing and push chemical along it. Nothing else in the pump touches the chemical, which is why the roller block and the squeeze tube do all the wearing.
When the H Block Is the Problem
Astralpool's own Rolachem troubleshooting lists a broken roller or H block as a cause of a pump that is not dosing properly, alongside a cracked pump head, an empty chemical drum and a squeeze tube that has come off the barb.
Signs the H block has failed:
- The motor runs but no chemical moves along the tube.
- A roller is missing, seized, or no longer turns freely on its pin.
- The block is cracked, or an arm has snapped.
- Dosing has become erratic or the output has dropped well below the set rate.
If the motor does not turn at all, the fault is the gearbox and motor or the board, not the H block.
Replace the Squeeze Tube at the Same Time
A squeeze tube and a roller block wear together, and a worn tube is the far more common failure of the two. If the H block has broken up, inspect the tube before refitting, because debris and a damaged tube will take out the new block as well. The chlorine and acid tubes are different parts, so order the one that matches the pump you are working on.
Acid and chlorine should never share a tube or a pump head.
Specifications
- Part number: 521015
- Astralpool description: H BLOCK
- Function: roller carrier inside the peristaltic pump head
- Listed for: Rolachem RC8, RC9, RO9 and RP9 dosing pumps
- Quantity required: 1 per pump head on RC9, RO9 and RP9, 2 on the RC8
- Brand: Astralpool (Fluidra)
Related Rolachem Dosing Parts
- Chlorine Squeeze Tube, Rolachem - 520113
- Acid Squeeze Tube, Rolachem - 520112
- Pump Head Face Plates, Pair - 521010
- Squeeze Tube Hose Clamp - 520115
- Chlorine / Acid Check Valve, 6mm - 520118
- Chlorine & Acid Supply Tube, 5 to 6mm, per metre
- Acid / Chlorine Drum Venting Kit - 78366
- Mixing Cell Complete, Body with Unions - 520139
- pH Probe, eQuilibrium / Rolachem - 525147
- pH 7.5 Calibration Solution
- ORP Calibration Solution 700mV - 600117
Frequently Asked Questions
What does the H block do?
It is the rotating roller carrier inside the pump head. The rollers it carries squeeze the tube against the housing to move chemical along it, which is how a peristaltic pump works.
My Rolachem motor runs but nothing is being dosed. Is it the H block?
It can be. Astralpool lists a broken roller or H block as a cause, along with a cracked pump head, a squeeze tube that has come off the barb, and an empty chemical drum. Check the tube first, since it is the more common failure.
How many do I need?
Astralpool's spare parts lists show one per pump head for the RC9, RO9 and RP9, and two for the RC8.
Should I replace the squeeze tube as well?
Usually yes. The tube and roller block wear together, and fitting a new block against a worn or damaged tube will shorten its life.
Does it suit a Viron eQuilibrium?
The matching pump head face plates, 521010, are listed for Viron eQuilibrium and EQ-Series as well as Rolachem. Astralpool's own catalogue lists the 521015 H block under Rolachem only, so send us a photo of your pump head and we will confirm before you order.
Manufacturer Catalogues, Diagrams & Part Numbers: Need exploded views, spare part numbers, product numbers, diagrams, or more product information? See the manufacturer catalogues below.
Important Before Ordering: Please ensure you order correctly. Confirm this product is suitable before ordering. Use the catalogues above, seek professional assistance, or contact us with photos of your existing part, model details, serial numbers, and any labels.
As with all products, these must be diagnosed and fitted by a qualified and experienced person.
To the layperson, plumbing sizes and their names can be confusing due to the nominal sizes and naming conventions used.
This is not something we made up. Plumbers and manufacturers have used nominal (meaning ‘by name only’) sizing for decades.
Simply put, in plumbing, named sizes are often not the actual measurements of the pipe, fitting, or part. A PVC pipe called “50mm” or “2"” does not measure 50 mm anywhere. Instead, it follows nominal sizing, an internationally-standardised naming system.
Almost all fittings we use in the pool & spa industry use “pressure PVC” pipe & fittings. Do not confuse with “DWV” or other pipe & fittings and their names and sizes.
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 |
To reiterate, these are nominal sizes and are designed to fit matching fittings — not to measure 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 and other plumbing areas 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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521015
