Tecmark Hydroquip Adjustable Pressure Switch - Universal - 1/8" NPT - 3902 3903
Tecmark Hydroquip Adjustable Pressure Switch - Universal - 1/8" NPT - 3902 3903 is backordered and will ship as soon as it is back in stock.
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Tecmark / Hydroquip Spa Heater Pressure Switch replaces 3903-DF, 3902, 3903, 34-0178 1 Amp SPST for spa equipment and heaters. This is a very common pressure switch used by Balboa, Gecko, Davey, Spa-Quip, Spa Builders, HydroQuip, Pentair and others. Usually found near the heater assembly. They were originally branded as Tecmark, but may also be labelled Hydroquip now.
Details:
- 1-5 psi 7-35kpa adjustable SPST
- Found in a large number of spa and pool heaters
- On/Off device that controls heater operation
- Utilises a diaphragm to sense water pressure. This closes a switch.
- 1/8" NPT Thread rated to 1A. Fits most spa electric heaters.
- Has two Spade connectors
- 3902 and 3903 are interchangeable in spa applications.
Variant and Other Information:
- 3902 1A, 1-5PSI Adjustable, 1/8" NPT
- 3903 1A, 1A, 1-5PSI Fixed by glue @ 1.5PSI, but Adjustable, 1/8" NPT w/o Screws
- WWDG is rebranded 390x 3A version.
- -DF has terminal screws in addition to spade connectors.
- -BP is a 3A variant. Some were actually still 1A though. Go figure!
- 99% of spa applications do not require adjustment from factory.
- These have been rebranded by some other companies over the years.
Basic Installation Instructions:
(To be performed by an Electrically-Qualified Person)
1. Preparation:
- Turn off power at the mains before beginning installation.
- Apply thread tape or sealant to the switch threads.
- Hand-tighten the switch until snug—do not use tools to avoid over-tightening.
2. Initial Testing & Adjustment:
- Using a multimeter, check the switch’s status before turning the power on:
- It should read Open-circuit (no electrical continuity) when the system is off and no pumps are running.
- If the switch is Closed-circuit (showing continuity), adjust the screw on the switch until it reads Open-circuit.
- Turn the switch slightly further in to prevent excessive sensitivity.
3. Power-On Testing:
- Turn the power on and check the switch function:
- The switch should be Closed-circuit when the pump is running.
- It should return to Open-circuit when the pump turns off.
- Repeat the test to confirm proper operation.
4. Note:
- Dirty or clogged filters can restrict water flow, causing the switch to open unexpectedly. Ensure filters are clean for proper function.
Part Numbers & Descriptions This Replaces:
3902 - 1Amp, 1-5psi Adjustable, 1/8" NPT 3903 - 1Amp, 1-5psi Adjustable, 1/8" NPT3903-BP 3Amp, PSI Factory Set Non-Adjustable, 1/8" NPT 3903-DF 1Amp, 1.5psi fixed, 1/8" NPT, MH26414, 3903-EECS, A2F / 0517, 34-0178, 3900, 3901, 3903, 3904, 3906, 3909, PRS-1100, PRS-1101, M3218, 71076, 26775, 26776, 26778, R-113, R-130, R-132B, S-1921173328, B600025, 600025,
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" | 25 mm | ~33.4–33.7 mm |
| 1.5" | 40 mm | ~48.1–48.4 mm |
| 2" | 50 mm | ~60.2–60.5 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.
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).
PVC Pressure Pipe AS/NZ 1477 Dimensions1:
Common nominal sizes with typical outside diameters and indicative inside diameters by class.
| Imperial Name | Metric Name | Outside Diameter (OD) | Class 9 IDº | Class 12 IDº | Class 18 IDº |
|---|---|---|---|---|---|
| 1/2" | 15 mm | ~21.2–21.5 mm | 18.3 mm | 17.8 mm | |
| 3/4" | 20 mm | ~26.6–26.9 mm | 23.7 mm | 22.4 mm | |
| 1" | 25 mm | ~33.4–33.7 mm | 30.5 mm | 29.8 mm | 28.1 mm |
| 32 mm | ~42.1–42.4 mm | 38.5 mm | 37.5 mm | 35.4 mm | |
| 1.5" | 40 mm | ~48.1–48.4 mm | 45.2 mm | 44.1 mm | 42.8 mm |
| 2" | 50 mm | ~60.2–60.5 mm | 55.2 mm | 53.7 mm | 50.5 mm |
| 65 mm | ~75.2–75.5 mm | 68.9 mm | 67.0 mm | 63.2 mm | |
| 2.5" | ~73 mm |
- 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.
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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