Balboa 58mm OD "40mm" Spa Heater Gasket O-Ring Seal - Pair - GS100 / GS200 / VS100

Balboa 58mm OD "40mm" Spa Heater Gasket O-Ring — Pair

A pair of heater union gaskets for the 1.5" heater tubes fitted to compact Balboa and Gecko packs — most commonly the GS100, GS200 and VS100, which use a 10" heater rather than the full-size 15" x 2" flo-thru. If the old gasket measures 58mm across the outside and roughly 4mm thick, this is the correct seal.

• Physically Stocked, unlike most 'drop-shipping' websites.
• Shipped from VIC, NSW, WA, QLD

Note on thickness: this gasket is approximately 4mm overall including the lip. Several websites list it at 6mm, which is the specification for the larger 74mm "50mm" gasket, not this one. If you have been sent a 6mm gasket for a 1.5" heater, it is the wrong part.

Will this fit my spa? The 2-Inch Union Rule, in reverse

Fitment is decided by the heater tube, not the control pack. Check the union rather than the badge on the box:

1. Switch the spa off at the isolator.
2. Find the heater tube — the stainless cylinder the water flows through, with a union nut at each end.
3. Undo one union nut and slide it back. The old gasket sits inside the union face.
4. Measure its outside diameter. 58mm means this product. 74mm means you need the 74mm "50mm" pair.

Worked example: a Balboa GS100 in a small two-seat spa, heater tube stamped 10" and plumbed in 1.5". The old gasket pulls out at 57.4mm OD and 3.6mm thick after years of compression set, against a nominal 58mm and 4mm. That spa takes this pair. The owner's larger SpaPro X1 has a 15" x 2" tube and takes the 74mm pair instead — same brand of pack, different heater, different gasket.

74mm or 58mm? Choose in ten seconds

If you cannot get the old one out to measure, measure across the face of the union nut where the gasket seats instead. Do not guess from the pack model alone — Balboa fitted both heater sizes across overlapping years.

Which systems use the 1.5" heater

• Balboa GS100 and GS200 — entry-level Global Series packs, one pump plus ozone and light.
• Balboa VS100 — the compact Value Series pack, supplied with a 1.5" x 10" heater.
• Gecko and AEware compact packs plumbed in 1.5", including some Microspa and SSPA variants.
• Various small Chinese-import and portable spas built around a 1.5" heater canister.

Everything else in the Balboa range — the BP, VS, GS, GL, EL, Signature/SCS and legacy metal-box families, along with SpaPro and WWDG control systems — uses the 2" flo-thru heater and takes the 74mm gasket. When in doubt, measure.

Why the name never matches the measurement

Pool and spa plumbing uses nominal sizing: the size is a name, not a dimension. That is why a "50mm" gasket measures 74mm and a "40mm" gasket measures 58mm. As a reference across spa and pool systems:

• 50mm / 2" pipe = approx. 60-61mm outside diameter
• 40mm / 1.5" pipe = approx. 48mm outside diameter
• 25mm / 1" pipe = approx. 33-34mm outside diameter

The heater union gasket seals on the union face, which sits outside the pipe bore, so it is always larger again than the pipe OD. Measure the old gasket, never the pipe.

Measurements

• Outside diameter: 58mm (approx.)
• Inside diameter: 41mm (approx.)
• Total thickness: 4mm including the lip
• One side has a slightly rounded edge, the other a thicker square edge
• Supplied as a matched pair — one for each end of the heater tube

Symptoms of a failed heater gasket

A heater union rarely fails as an obvious leak. Look for a damp patch or mineral staining directly under the heater tube, white crusting on the union nut, corrosion creeping onto the heater terminals, or an RCD that trips intermittently once the heater has been running. Any of those means the gasket has taken a compression set and is weeping under thermal cycling.

When to replace

Replace both as a pair every few years, and always after a chemical imbalance event or whenever the heater tube comes out. These seal on the square edge, so hand-tight plus about a quarter turn on the union nut is enough — over-tightening splits the union collar and turns a cheap job into a heater replacement.

Replaces / may be stamped as

92155, B-92155SPA, B92155SPA.

Questions we get asked

Why does a "40mm" gasket measure 58mm?
40mm is a nominal pipe name, not a measurement. A 40mm (1.5") pipe is about 48mm outside diameter, and the union face the gasket seals against is larger again.

Is it 4mm or 6mm thick?
4mm including the lip. The 6mm figure quoted on some sites belongs to the larger 74mm "50mm" gasket.

Which side faces the heater tube?
The thicker square edge faces the heater tube flange; the rounded edge sits against the union tailpiece.

I have a GS501z or a BP200 — is this the right one?
No. Those use the 2" flo-thru heater and take the 74mm "50mm" pair. This 58mm gasket is for the compact 1.5" heaters on GS100, GS200 and VS100 packs.

Do I need two?
Yes. A heater tube has a union at each end and both should be replaced together, which is why this is sold as a pair.

Common search terms

o-ring, oring, Balboa 40mm heater gasket, 58mm heater gasket, GS100 heater seal, GS200 heater gasket, VS100 heater gasket, 1.5 inch heater union seal, Gecko heater gasket, spa heater o-ring, Balboa 92155, B-92155SPA, small spa heater gasket.