Isotemp Water Heater in a Camper Van (120V + Engine Coolant)

Material and Resources

Why the Isotemp

Why not the Bosch

Why people pick it:

• It costs about a quarter of the Isotemp.
• Works out of the box on 120V.

Why we didn’t pick it:

• No heat-exchanger coil for engine coolant. All of its hot water has to come from battery or shore power, forever.
• 1,440W / 12 amps (vs Isotemp 750W). Higher numbers usually win, but not here. The Bosch will put way more pressure on the electrical system when heating. Having a smaller element is beneficial here, as it leaves room for other appliances to work simultaneously (induction cooktop, etc.).
• No drain. Draining the heater’s tank is routine maintenance in a van, between trips or for winterizing.
• No mixing valve. The thermostat tops out around 65°C and the tank delivers what it holds: 9.5L stretches to maybe 15–18L at shower temperature. The Isotemp’s 15L stored hotter delivers roughly double that.
• Thin insulation. It’s designed to reheat constantly from a wall outlet, not to hold a charge of heat overnight in a cold van.
• Residential design in a moving vehicle. It’s built for under a sink in a house: glass-lined steel tank, no vibration rating, upright-only mounting. The Isotemp is built for boats, where constant motion is the baseline.

	Bosch Mini-Tank	Isotemp Spa
Engine-coolant heating	No	Yes
Electric element	1,440 W (12 A)	750 W (6.3 A)
Tank	2.5 gal (9.5L), glass-lined steel	15–25L, 316 stainless steel
Drain for winterizing	No	Yes
Thermostatic mixing valve	No	Yes (~2× effective capacity)
Insulation	Thin	Thick polyurethane
Mounting	Upright only	Horizontal or vertical
Built for	Under a sink in a house	Boats in constant motion

That said, many people have used the Bosch for a long time, and it works. If you did your homework and still opt for the Bosch, it’s all good.

How It Works

Inside the Isotemp there are two completely separate circuits. The tank holds your water. The engine coolant flows through a sealed heat-exchanger coil immersed in that water — heat crosses the coil wall, the fluids never do. Your water and the antifreeze never meet.

The 750W electric element sits directly in the tank, like a kettle. The two heat sources work independently, or at the same time.

On the coolant side, the loop is plumbed in series with the cab’s heater core, and the engine’s own water pump does the pushing — no extra pump needed. Once the engine is up to temperature, some of the ~90°C coolant that would normally shed its heat at the radiator takes a detour through the tank instead.

At the outlet, the thermostatic mixing valve blends in cold water, so your taps see a steady, safe temperature no matter how hot the tank is.

Choosing the Model

Isotemp offers three families that make sense in a van — and they all share the same DNA (316 stainless tank, 750W element, coolant coil, mixing valve), so everything in this guide applies to whichever you pick:

Capacity. Thanks to the mixing valve, stored liters aren’t delivered liters: 15L stored at 80°C delivers roughly 30–35L at shower temperature — call it a couple of water-wise showers plus a round of dishes. Going bigger costs three ways: cabinet space, weight (every liter is a kilogram, and the tank lives full), and energy — on the 750W element, heating 15L from cold takes about 1 kWh (~80 minutes), while 25L takes ~1.7 kWh (~2¼ hours). Off-grid, that’s a meaningful chunk of battery.

The dual-source logic favors small. The engine reheats the tank every time you drive — a small tank that’s always hot beats a big one you hesitate to heat. We’d size up only for 3+ people, or long stationary stints without shore power.

Our pick: the Spa 15. It fit our cabinet, it showers like a tank twice its size, and the engine brings it back to temperature in roughly 20–30 minutes of driving. So far, we haven’t wished it were bigger.

Mounting

Requirements

How to Mount

Plumbing

Requirements

How to Plumb

Electrical Install

Requirements

How to Wire

Just plug the Isotemp into a 120V outlet, and voilà!

Now, installing and wiring a 120V outlet is another story, and we tell the full story here:

Engine Coolant Install

Requirements

How to Connect to Engine Coolant

It’s extremely challenging to take clear photos in the engine bay and under the van… we took as many photos as we could, and let them speak for themselves!

Engine Bay

Front Axle

Mid Axle 😆

Rear Axle

Floor Pass-Through (to Water Cabinet)

Water Cabinet

Top-Up Coolant

• Close the cap (until hard stop). Start the engine and idle with cab heat on max temperature, fan on low. Full heat guarantees strong flow through the heater circuit your loop lives on.
• While it idles at temperature, inspect the loop. Hot and pressurized is when a weep shows itself.
• Turn off engine, let cool, and top up to MAX line. Close the cap. Again, probably still not much at this point…
• Drive around the block (short drive, 1 minute or so), re-check and top up. Still not much…
• Drive to a steep hill, up and down. Still a short drive. Keep an eye on the temp gauge. This helps air bubbles trapped at a high point move to the degas bottle. Driving up/down and getting the engine to higher temp is what really moves the coolant! Expect to add quite a bit more coolant at this step!
• Drive to a steep hill, again (short drive). We had to do this 2–3 times before the coolant level stabilized.
• Watch the temp gauge on the next drives, and carry coolant with you. Abnormal climb means airlock: shut down, cool, top up, resume.
• A few days later, re-torque every worm-gear clamp. Rubber takes a compression set after the first heat cycles. Keep coolant in the van, just in case.

Filling the Water Tank

Initial Heat-Up

120V

Heating a full 15 L tank is one steady pull: 726 W average for 1 h 21 min. Total: 980 Wh at the plug, ≈1100 Wh from the battery with the 12% inverter loss. In practice we do the initial heat-up on shore power before leaving the house, so the battery only pays for top-ups on the road.

Engine Coolant

coming soon!

Maintaining Temperature (No Hot Water Use)

120V Only

With nobody using hot water, the Isotemp fires a short reheat burst (≈800 W for 3–4 min) about every 55 minutes: every ~45 min at night, with longer gaps during the warm afternoon. That adds up to ≈1386 Wh per day at the plug. Keeping the tank hot costs ≈1552 Wh per day from the battery with the 12% inverter loss — more than heating the tank from cold once. Worth switching the heater off when you won’t need hot water for a while.

Engine Coolant Only

coming soon!

A Week of Real-World Use

120V Only

During a 7-day trip in August (two of us), we heated the tank on a simple routine (one session in the morning, one in the evening, ≈850 W while the element runs) rather than keeping it hot all day. That gave us enough hot water for dishes, plus showers on some days. Days without showers came in at 602–690 Wh at the plug; shower days ran 1035–1472 Wh. Average: ≈975 Wh per day, 6.8 kWh for the week. With the 12% inverter loss, that’s ≈1090 Wh per day from the battery — about 30% less than the ≈1552 Wh of keeping the tank on hot standby around the clock, with actual hot water use included this time.

Thermostatic Mixing Valve Setup

The Thermal Fuse

Manual Reset

The Isotemp Spa has overheat protection (a thermal cut-out, or thermal fuse) located under the front cover. If it trips, you can reset the fuse by pushing the red button on top of the thermostat.

Relocate the Thermal Fuse

If using the water heater on 120V only, you shouldn’t have to reset the fuse. But with engine coolant, the thermal fuse is known to trip quite often… So, just in case, we made the reset button accessible without having to remove the cover:

Draining the Tank / Winterizing

On second thought

We initially thought we’d use the Isotemp Spa only on 120V, since we have a huge battery bank, but it was the appliance that consumed the most power. Connecting the engine coolant is definitely worth it!