Isotherm Isotemp 20 Liter SPA Water Heater Stainless Tank (5.3 USG)

A Higher Standard of Marine Hot Water

Most production boats leave the factory fitted with aluminium-tank water heaters that are adequate but not built for the long haul. The Isotemp SPA line was developed specifically to address that gap, offering boat builders and owners a more durable, corrosion-resistant alternative at an optimised cost-to-quality ratio. The inner tank is fabricated from AISI 316 stainless steel, the same grade used throughout the Isotemp range of stainless water heaters, and the outer casing is moulded from robust grey polypropylene that stands up to the damp, salt-laden environment of a marine bilge or locker.

The SPA 20L holds 20 litres (5.3 US gallons) of hot water, making it a practical choice for couples or small crews on sailboats, powerboats, and trawlers where space and electrical capacity are at a premium.

Thermostatic Mixing Valve: More Hot Water, Safer Delivery

Every Isotemp SPA water heater ships with a thermostatic mixing valve as standard equipment. The valve blends stored hot water with cold water before it reaches the tap, regulating delivery temperature to prevent accidental scalding. Because the tank can be set to a higher storage temperature without the risk of burning anyone at the fixture, the effective volume of usable hot water is extended well beyond the physical capacity of the tank. This is a meaningful advantage on a 20-litre unit, where every litre counts.

The mixing valve is not an optional add-on; it is integrated into the standard configuration of every SPA heater, reflecting the product line's commitment to safety and usability from the moment it is installed.

Dual-Thermostat Safety System

All Isotemp water heaters are equipped with two independent thermostats. The primary temperature thermostat maintains the water at the set storage temperature during normal operation. A separate safety thermostat is designed to trip and cut power to the heating element in the event the tank is run dry, protecting the element from damage and preventing a potential hazard. Together, these two controls provide a straightforward layer of protection that requires no additional wiring or configuration from the installer.

Efficient Heating from Engine or Shore Power

The SPA 20L is equipped with a front-mounted heat exchanger featuring corrugated, extended-surface coils that maximise heat transfer from the vessel's engine cooling circuit. When the engine is running, waste heat from the coolant loop is captured and transferred to the tank water efficiently, making use of energy that would otherwise be lost.

When the engine is off, the 115V AC immersion heating element takes over. Rated at 750 watts, it is deliberately sized to avoid overloading small generators or tripping dock power breakers, a common frustration with higher-wattage elements. The immersion element is positioned to heat water at the bottom of the tank, promoting natural convection and even heating throughout the vessel.

The extra-thick insulation surrounding the stainless tank minimises heat loss during periods of inactivity, keeping water at a usable temperature for up to 24 hours after the heat source is removed.

Flexible Installation for Horizontal or Vertical Mounting

Space is always at a premium aboard a boat, and the SPA 20L is designed to accommodate that reality. The unit can be mounted in either a horizontal or vertical orientation, giving the installer the flexibility to fit it into whatever locker, bilge compartment, or engine room space is available. Aluminium mounting brackets are included and are adjustable to suit both deck and bulkhead installations.

Water connections are 1/2 inch, a standard size that simplifies integration with existing plumbing. The electrical connection is plug-and-play, reducing installation time and the potential for wiring errors.

Certifications, Warranty, and What Is Included

All Isotemp products are CE certified and manufactured in ISO certified facilities, reflecting the quality controls applied throughout the production process. The SPA 20L comes complete with a safety relief valve and the thermostatic mixing valve as standard inclusions.

The warranty coverage is among the strongest in the category:

• 5-year limited warranty covering the tank against leakage due to manufacturing defect or corrosion
• 2-year warranty on all other parts
• Includes safety relief valve and thermostatic mixing valve as standard
• CE certified and manufactured in ISO certified facilities

Specifications

• Tank Capacity: 20 litres (5.3 USG)
• Inner Tank Material: AISI 316 stainless steel
• Outer Casing Material: High-quality polypropylene
• Mounting Bracket Material: Aluminium
• Heating Element Voltage: 115V AC
• Heating Element Power: 750 W
• Immersion Element Material: Copper covered by nickel
• Tank Diameter x Length: 310 mm x 550 mm (±5 mm)
• Weight: 8.3 kg
• Rated Pressure: 6 bar
• Water Connections: 1/2 inch
• Heat Exchanger Position: Front
• Mounting Orientation: Horizontal or vertical
• Certifications: CE certified; manufactured in ISO certified facilities
• Tank Warranty: 5-year limited (leakage due to manufacturing defect or corrosion)
• Parts Warranty: 2-year on all other parts
• MFG Part Number: 6P2023SPA0003

Key Features 

• AISI 316 stainless steel inner tank: resists corrosion far longer than aluminium alternatives commonly found on production boats
• Standard thermostatic mixing valve: prevents scalding at the tap while extending the effective volume of usable hot water
• Dual-thermostat protection: separate temperature and safety thermostats guard against overheating and dry-run element damage
• Front-mounted corrugated heat exchanger coil: captures engine waste heat efficiently to heat water while underway at no additional electrical cost
• 750W, 115V AC immersion element: heats water when the engine is off without overloading generators or tripping dock power breakers
• Extra-thick polypropylene insulation: keeps water at a usable temperature for up to 24 hours after the heat source is removed
• Horizontal or vertical mounting: adapts to tight or awkward installation spaces aboard a wide range of vessels
• Adjustable aluminium mounting brackets: support both deck and bulkhead installations for a secure, custom fit
• Plug-and-play electrical connection: simplifies installation and reduces wiring complexity
• CE certified and ISO facility manufactured: independently verified quality standards for marine use
• 5-year tank warranty: long-term coverage against leakage from manufacturing defect or corrosion provides lasting peace of mind