12KW Diesel Water Heater 12V — Apollo-C3 Coolant Heater for Trucks, RVs, Caravans and Boats

One Heater, Two Jobs: Engine Preheat and Cabin Heat

The Apollo-C3 is a 12KW diesel water heater that warms your engine coolant and your cab at the same time, with the engine switched off. It burns diesel from the tank you already have and circulates hot coolant through the engine block and your existing heater core.

You get a warm cab before you climb in, and an engine that turns over like it is a mild autumn morning. No idling, no extension cord, no shore power.

The economics are simple. A heavy diesel engine burns 0.8–1.0 gallons per hour just sitting there making heat. A fuel-fired coolant heater does the same job on a small fraction of that, because it heats the coolant loop instead of spinning an entire engine.

There is a legal side too. More than 30 US states cap how long you may idle, with fines running from $100 to $25,000. Cold-weather exemptions usually apply only when you have no other way to make heat — so the heater is what keeps you both warm and compliant.

Truck, bus, coach, boat, car, caravan or generator set: if it runs a large diesel with a coolant loop, the Apollo-C3 installs the same way.

What 12KW Actually Buys You

12KW is roughly 40,900 BTU per hour delivered into a coolant loop. That is truck, bus, boat and generator territory, not van territory.

One thing to sort out before you buy: search results for a 12 kW diesel heater mix two completely different machines. A 12KW water heater and a 12KW air heater do not do the same job.

If you are shopping for a 12KW diesel heater to blow hot air straight into a living space, this is not that product — look at a diesel air heater instead. This one plumbs into a coolant circuit.

That extra step is what lets one burner preheat your engine, defrost your windshield through the vents you already have, and warm the cab. You will also see this class of unit sold as a liquid parking heater or a night heater — same machine, different name.

Is the Apollo-C3 the Right Size for Your Vehicle?

Buy the C3 if you are heating a large diesel engine or a big coolant loop. Buy something smaller if you are heating a van or a camper.

Oversizing a coolant heater is a real problem, not a theoretical one. A heater with more output than the loop can absorb reaches its setpoint fast, shuts down, and restarts — over and over. That short-cycling is hard on the glow pin and encourages carbon buildup in the burner.

Not sure which side of that line you fall on? Send us your engine displacement and coolant capacity at info@vvkb.com and we will tell you honestly whether the C3 or the 5KW C1 is the better fit. We would rather talk you down a size than take a return.

Specifications

Fuel Supply: The One Thing That Causes Most Failures

Do not tap the heater into your vehicle's existing fuel line. Fit a dedicated standpipe into the tank. This single decision separates the installs that run for years from the ones that fight air locks every cold morning.

The reason is the metering pump. It moves a very small amount of fuel per stroke, and it cannot clear an air bubble the way an engine lift pump can. Tapped into a return line or a shared feed, it loses prime, fails to light, and locks out after three attempts.

A standpipe drops straight into the tank with its own pickup, so the heater always draws clean fuel from its own point. Fitting one takes an afternoon and a careful hole.

• Fuel standpipe — the part that prevents the problem above
• Metering fuel pump — spare, 12V and 24V
• Kyocera ceramic glow plug — the wear part on any diesel heater

Two more fuel rules worth following. Switch the heater off before you refuel, and after you change to winter or arctic diesel, run it about 15 minutes so the line fills with the new fuel.

Plumbing the Coolant Loop

Plumb the C3 across your coolant loop, not in series with a single hose. The most common first-draft mistake is drawing it inline in a way that starves the circuit.

Three rules that decide whether your install self-bleeds or gives you an air pocket:

• Route hoses upward toward the heater so trapped air can rise out on its own.
• Never point the coolant inlet and outlet fittings downward. Air collects there and stays.
• Keep the coolant line to a rear-mounted heater below the highest point of the loop, or it will hold a bubble you cannot chase out.

Fit the heater ahead of your heat exchanger inlet where you can. Start the first run with the coolant below 86°F (30°C).

Setpoint tip: holding the loop around 167°F (75°C) keeps the engine thermostat closed, so your heat stays in the block and cab instead of going out through the radiator.

Exhaust and Air Intake

The exhaust must terminate outside the vehicle, pointing down or up to 20° rearward, and never into the direction of travel. Keep the outlet at least 0.8 in (20 mm) away from brake lines, wiring and anything else heat-sensitive.

Run the exhaust muffler at least 7.9 in (200 mm) from the heater body. It is not optional equipment.

Buying a Chinese-Built Coolant Heater Without Getting Burned

Yes, the internals of this class of heater are broadly similar across brands. What is not similar is who is still there in year three. We would rather say that plainly than pretend otherwise.

Vvkb has been in business since 1996 and building diesel heaters in China since 2003. We watched this category go from a handful of workshops to hundreds of competing suppliers, and we know exactly which parts get swapped for cheaper ones when someone needs to hit a price.

What you are actually paying for here:

• Parts you can still buy in three years — glow plugs, fuel pumps, mufflers and standpipes are stocked and listed, not discontinued.
• A 1-year warranty you can extend, handled by the people who built the unit rather than a reseller.
• Stock in four regions — US, Canada, Europe and Australia — so it ships without a six-week wait.
• Real answers by email, including the ones that talk you out of this model and into a smaller one.

Maintenance That Actually Matters

Run the heater once a month, even in July. A burner that sits unused all summer is the single most common reason a unit will not light in November.

Beyond that, the list is short. Check the exhaust outlet is clear before the first cold snap. Replace the fuel filter on your normal service interval. Inspect the glow pin if start times get longer than usual.

Fit a carbon monoxide alarm in any space where people sleep. The C3 keeps combustion sealed and vents exhaust outside the vehicle, and a working CO alarm is still non-negotiable. Clamps, seals and correct routing reduce risk — they do not replace a detector.

Frequently Asked Questions

Can I tap the heater into the existing fuel line on my truck?

You can, and it is the number one source of problems. The metering pump on any diesel heater moves a tiny volume per stroke and cannot push past an air bubble. Tapped into a return line or shared feed, it loses prime, fails to ignite, and locks out after three attempts. Fit a dedicated standpipe into the tank instead. It gives the heater its own pickup point and clean fuel every start.

Is 12KW too much heater for a van or camper?

Almost certainly yes. The Apollo-C3 is sized for heavy trucks, buses, marine engines and generator sets. On a small coolant loop it hits setpoint quickly, shuts off, and restarts repeatedly. That short-cycling is hard on the glow pin and builds carbon in the burner. For a campervan, caravan, motorhome or pickup, the 5KW Apollo-C1 is the correct choice and costs less.

Can I use the Apollo-C3 to heat shower water?

Not directly, and never by running potable water through it. People searching for a diesel hot water heater often land here, so it is worth being clear. The Apollo-C3 heats a closed coolant loop and holds it at a setpoint. It is not a tankless water heater and does not modulate for shower temperature. If you want domestic hot water, run the coolant loop through a calorifier or a marine water heater with a built-in coolant heat exchanger, and take the hot water from that tank.

How much current does it draw from my battery?

Most current is drawn in the first two to three minutes, while the glow pin is energized. Once combustion is stable the glow pin switches off and draw falls sharply, leaving the circulation pump and combustion fan running. If your only power source is a single starter battery and you park for short periods, add a dedicated battery or plan to drive afterward. Cold weather increases startup demand.

Do I need a carbon monoxide alarm?

Yes. Fit one in any space where people sleep. The combustion chamber is sealed and exhaust vents outside the vehicle, so cabin air and exhaust never mix in a correct installation. A CO alarm is what tells you when an installation stops being correct, such as a cracked pipe, a loose clamp, or a blocked outlet. Exhaust clamps and seals are not a substitute for a detector.

Will it still start after sitting all summer?

It will if you run it monthly. Ten minutes once a month keeps the fuel line wet, clears the burner, and confirms the glow pin still works while it is still warm enough to fix. Units that sit from March to November are the ones that fail on the first cold morning.

Can I install it myself?

Yes, if you are comfortable cutting into a coolant circuit and drilling a fuel tank. Budget a full day. The three jobs that decide whether it works are the standpipe, plumbing across the loop rather than in series, and an exhaust that exits outside pointing down or rearward. If any of those make you uneasy, have a diesel shop do the install.

What warranty comes with it, and can I get parts later?

One year as standard, extendable, handled directly by the people who built it. Glow plugs, metering fuel pumps, exhaust mufflers and standpipes are all stocked as separate items, so a wear part does not turn into a dead heater. Units ship from our US, Canada, Europe or Australia warehouse.