Shore power runs out the moment you leave the dock, and running the engine just to stay warm burns fuel, wears the engine and fills the anchorage with noise. This 12V diesel boat heater draws its fuel from the tank you already have and its power from your house bank, so you get dry, warm cabin air at anchor with the engine off.
It is a marine version of our Apollo diesel air heater, supplied with a stainless steel exhaust hull fitting and a stainless steel L-bracket instead of the mild-steel floor mount that comes in a truck kit. Two sizes: 2.5KW (Apollo-MV1) for boats under about 34 ft, and 5KW (Apollo-MV2) for 40 ft and up.
Owners fit it as a sailboat heater, as a yacht heater on larger motor boats, and in trawlers, narrowboats and pilothouse cruisers. Both units run on 12 volt DC, and both are also available in 24V if that is what your boat is wired for.
As a boat cabin heater it does one job well: it moves dry warm air into the cabin through ducted outlets, overnight, with the engine off and nobody watching it. If you have been comparing marine cabin heaters and cannot see what separates them, the sizing section below is where the real difference sits.
What Size Diesel Boat Heater Do You Need — 2.5KW or 5KW?
Match the heater to the boat, not to your budget: under 34 ft take the 2.5KW, over 40 ft take the 5KW. The published marine rule of thumb is about 2KW for boats under 30 ft, 4KW for 30–40 ft and 5–6KW for 40–50 ft, and a second rule that matters just as much — roughly one air outlet per kilowatt.
That sizing rule is the whole reason we sell this 12V marine heater in two outputs rather than one. Most 12V boat heaters on the marketplaces come in a single 5KW size because it is cheaper to stock one part, and the buyer absorbs the mismatch.
| Boat length | Model | Air outlets | Typical fit |
|---|---|---|---|
| Under 34 ft (10 m) | 2.5KW | 2–3 | Sailboat saloon plus fore cabin, small trawler, day boat |
| 34–40 ft (10–12 m) | 2.5KW if well insulated 5KW if pilothouse or lots of glass |
3–4 | Cruising yacht, aft cabin layouts |
| 40 ft (12 m) and up | 5KW | 4–5 | Liveaboard, pilothouse, cold-climate cruising |
Why oversizing costs you more than undersizing
A 5KW heater in a 30 ft boat will spend most of the winter running at its minimum output, and that is the condition that cokes up a burner. The heater reaches your set temperature quickly, throttles down, and never gets hot enough for long enough to burn the carbon off its own combustion chamber.
Owners on the sailing forums keep arriving at the same conclusion from the other direction — that a heater you can run near full output most of the time is the one that stays clean. That is the honest argument for the 2.5KW on a small boat, and it is why we do not simply push everyone toward the bigger unit.
Go 5KW anyway if: you liveaboard through a genuinely cold winter, you have an uninsulated hull, or you have a pilothouse with a lot of glass. In that case give it at least four outlets so it is not fighting back pressure, and run it hard periodically.
If you want the longer version of this decision, our marine heater selection guide works through cabin volume and outlet placement, and the general diesel heater buying guide covers sizing for other vehicles.
What a Marine Kit Gives You That a Truck Kit Does Not
The single biggest source of failed boat installs is a truck-spec kit fitted to a boat. The heater itself is often fine — what is missing is everything around it, and boat owners find that out halfway through the job.
A truck or RV kit ships with a painted mild-steel floor mount, a plain exhaust tail pipe with no way to pass through a hull, and hardware chosen for a dry chassis rail. On a boat the mount corrodes, and there is no fitting to get the exhaust safely through the topsides. It is the same core heater — an RV diesel heater and a boat heater share a burner — but the boat heater kit has to solve a hull that a chassis does not have.
This marine diesel heater kit changes three parts and they are the three that matter:
- Stainless steel exhaust hull fitting (skin fitting) — a proper through-hull for the exhaust, so you are not improvising a hole in your topsides. This part alone often costs more than a budget heater.
- L-shaped stainless steel mounting bracket — bolts the heater down in a locker or lazarette without rusting into a stain down your bulkhead.
- 2 m (6.5 ft) of annealed copper fuel line — a truck kit gives you nylon only, and nylon is not fire-rated marine hose. Copper is the first material named in the US metallic fuel line rules.
Straight answer on the exhaust: our marine exhaust pipe is single-wall 304 stainless, not a double-lagged insulated pipe. The section closest to the heater runs hot and you should lag it yourself and keep it clear of anything that can burn. We would rather tell you that now than have you find out with a scorched locker.
Sealed Combustion Keeps the Exhaust Out of Your Cabin
Your cabin air never touches the flame or the exhaust. Fuel burns inside a sealed 2520 stainless steel chamber, combustion air is drawn from outside the cabin, and the exhaust leaves through its own pipe — the only thing that crosses into the boat is warm air blown over the outside of the heat exchanger.
This is the difference between a diesel air heater and an open-flame bulkhead stove or an unvented propane heater. There is a well-known thread on the sailing forums where an owner sat in his cabin all day feeling fine while his partner got a headache and nausea within minutes of coming below — the fumes were coming back in, and nobody could see it happening.
Sealed combustion removes that failure mode at the source. It does not remove your responsibility to fit a carbon monoxide alarm, and we will say that on every page we write: fit a CO alarm, and check its expiry date. Alarms have a service life and an expired one is decoration.
For the full venting rules, including intake clearance and exhaust routing on a boat, see are diesel heaters safe indoors.
How to Route the Exhaust on a Boat
Get two things right and the exhaust install is straightforward: loop it high before it exits, and put the outlet on the side of the boat rather than the transom. This is the step that separates a good boat heating install from one that swamps a heater in the first serious weather.
Height above the waterline
Run the pipe up into a swan neck and bring the hull fitting out at least 12 in (30 cm) above the waterline, with the fitting angled slightly down. The rise is what stops a following sea pushing water back down the pipe into the heater. This is the one dimension that is not negotiable — flooding the exhaust destroys the unit.
Side of the hull, not the transom
A transom outlet sits in the turbulence behind the boat, and lying head to wind that eddy can carry exhaust straight back into the cockpit and down the companionway. The aft quarter on either side clears the boat much better. Swept transoms cause a second problem — a raked transom catches rain and spray in the fitting.
Condensate
A diesel heater produces water in the exhaust, most of it in the first minutes of a cold start. Give the pipe a continuous downhill run to the hull fitting after the swan neck so the condensate drains overboard instead of pooling in a low spot and sitting against the pipe wall. Long runs benefit from a drain at the lowest point.
Our diesel heater exhaust pipe is 304 stainless with a 24mm connection and can be cut to the length your route needs.
Built for Salt Air — What Is Inside
The parts that decide whether a diesel heater survives its third winter are the combustion chamber, the heat exchanger and the glow pin. They are also the three parts you cannot see when you compare listings.
- 2520 stainless steel combustion chamber — holds its shape under repeated heat cycling instead of distorting, which is what eventually opens a path between exhaust and cabin air.
- Die-cast aluminium heat exchanger, 2.1 kg on the 5KW model — many budget exchangers we have weighed come in at 1.2–1.5 kg. That difference is wall thickness, and wall thickness is the barrier between the exhaust and the air you breathe.
- Ceramic glow pin — reliable ignition down to -40°F/-40°C, and less carbon than the old wire-element plugs. Replacements are stocked: diesel heater glow plug.
- Brushless fan motor — lower current draw off the house bank and no brushes to wear out.
- 304 stainless spiral exhaust — flexible enough to follow an awkward route through a locker, and it will not rust through the way a plain steel pipe does in salt air.
The 5KW unit also carries the NVH work we did after the 2.5KW — less noise and less vibration through the mounting bracket, which matters when the heater lives one bulkhead away from a berth.
Full Specifications — Apollo-MV1 and Apollo-MV2
| Specification | 2.5KW — Apollo-MV1 | 5KW — Apollo-MV2 |
|---|---|---|
| Heat output | 2,500 W · 8,500 BTU | 5,000 W · 16,500 BTU |
| Working voltage | 9V–16V DC (12V) · 18V–32V DC (24V) | 9V–16V DC (12V) · 18V–32V DC (24V) |
| Current draw | ~8A on startup · 1–2A running | ~8A on startup · 1–2A running |
| Fuel consumption | 0.14–0.25 L/hr (0.04–0.07 gal/hr) | 0.20–0.48 L/hr (0.05–0.13 gal/hr) |
| Fuel type | Diesel | Diesel |
| Cold start | Starts at -40°F/-40°C | Starts at -40°F/-40°C |
| Noise |
Dynamically balanced blower — see note below
|
Dynamically balanced blower — see note below
|
| Combustion chamber | 2520 stainless steel | 2520 stainless steel |
| Housing | Fire-resistant nylon | Fire-resistant nylon |
| Dimensions | 13 x 5.5 x 5.4 in (330 x 140 x 138 mm) | 15 x 5.5 x 5.4 in (380 x 140 x 138 mm) |
| Weight | Approx. 17 lb (7.7 kg) | 20.3 lb (10.6 kg) |
| Control | LCD controller + wireless remote | LCD controller + wireless remote |
On fuel: one US gallon runs the 2.5KW for roughly 15–27 hours and the 5KW for roughly 8–19 hours, depending on how hard it is working. A night at anchor is well under a gallon on either unit.
A note on noise
What Is in the Box
The complete 12 volt boat heater kit ships in one box, including the three marine parts a truck kit leaves you to source. You supply the tools and a tap into your diesel tank.
- 12V diesel air heater — 2.5KW (Apollo-MV1) or 5KW (Apollo-MV2) as selected
- Stainless steel exhaust hull fitting with gaskets, backing ring and stainless bolts
- L-shaped stainless steel mounting bracket
- 2 m (6.5 ft) annealed copper fuel line
- 304 stainless steel spiral exhaust pipe
- Combustion air intake hose
- Insulated warm air ducting and an outlet vent
- LCD controller with 7-day timer and onboard diagnostics
- Wireless remote
- Full wiring harness with connectors, fuses and ring terminals
- Fuel pump with rubber mount and bracket
- Inline fuel filter and nylon fuel hose
- Fuel tank standpipe with tank flange
- Heater mounting gasket plate
- Hose clamps, exhaust clamps, P-clips, screws, nuts and cable ties
- Installation manual
Planning more than one outlet? One heater has one air outlet, so the kit ships with one vent and one length of ducting. That suits a single-cabin install. If you are following the sizing table above and want three or four outlets, you need extra ducting, extra vents and a T-connector to split the flow — we stock all three. Email us your boat length and how many outlets you want and we will quote the exact parts with the heater.
Why the kit includes copper fuel line
Every diesel heater on the market ships with nylon fuel line, and nylon is not fire-rated marine hose. On a boat that matters more than it does in a van — a surveyor or an insurer can pick it up. So this kit includes 2 m (6.5 ft) of annealed copper tube instead of leaving you to source it.
Annealed copper is the first material named in the US metallic fuel line rules and it is the traditional marine answer. Run the copper between your tank and the heater, and keep the short nylon section at the heater end where the pump connector lives. If your run is longer than 2 m, add copper or A1-15 fire-resistant hose from any chandlery. ABYC A-7 is the standard that covers boat heating systems specifically.
One customer's marine install writeup on our blog shows the job done exactly this way.
How It Compares to Webasto, Eberspächer and Budget Kits
Against a Webasto Air Top or an Eberspächer Airtronic, this is the same category of product at a lower price, from the factory that builds it. Against a $150 marketplace kit, the difference is the marine hardware and the metal you cannot see.
Most diesel boat heaters on the market are the same basic architecture, so the comparison is not really about how they heat. It is about which marine heaters for boats arrive with the parts a hull needs, and who picks up the phone in February.
We are not going to quote you a budget heater's price, because the number is not the point. The point is what a cheap kit leaves you to solve: a corroding floor mount, no hull fitting, single-wall exhaust hardware chosen for a truck chassis, and no one to email when it throws a fault code at 2 am.
The 8KW listings deserve a specific warning. There is no 8KW unit in this size class — those are relabelled 5KW heaters, and you can spot them by body length. A genuine 5KW is about 15 in (38 cm) long.
When something does go wrong, our diesel heater error codes page decodes E01 through E19 with the fix for each, on new and older units.
What This Is Not — Three Things Worth Checking Before You Buy
Diesel boat heaters get shopped alongside three other products that solve different problems, and the wrong pick is an expensive return. Here is how to tell them apart.
It is not a boat space heater. Electric marine heaters plug into shore power and put out heat only while you are on the dock paying for the electricity. A diesel unit runs off the tank and the house bank, which is the whole point at anchor — but it needs an exhaust route through the hull, so the install is a real job rather than an afternoon.
It is not a marine water heater. This is an air heater: it blows warm air through ducts into the cabin. If you are looking for hot water at the taps, or a hydronic system feeding radiators and a calorifier, that is a coolant-based product and a different install. Ask us before ordering if you are not sure which one your boat needs.
It is not a portable boat heater. The heater is a fixed installation — bolted down, plumbed to the fuel tank, and vented through the hull. There is no version of this you drop on the sole for a weekend and take home afterwards.
If none of those disqualify it, this is the right category for you: fixed diesel heaters for boats, sized to the cabin, running unattended overnight on tank fuel.
Certifications, Warranty and Shipping
This heater is CE, RoHS and FCC certified, and every certificate number is published and verifiable on our certifications page — we list the issuing body and the reference so you can check it rather than take our word for it.
You are buying from the manufacturer. Vvkb has been building vehicle heaters since 1996, which is why we can tell you the wall thickness of the heat exchanger and the grade of the stainless in the combustion chamber.
- 12-month warranty — if a unit fails, we help you fix it or ship a replacement.
- Free shipping, dispatched within 3 days, tracking sent the day it leaves.
- Technical support that answers — the people who reply build these heaters.
Frequently Asked Questions
Will the exhaust hull fitting get hot enough to damage my fiberglass?
The hot section is the pipe close to the heater, not the fitting. Exhaust gas leaves the heater at around 400–570°F (200–300°C), but it sheds heat along the run, and owners who have put a thermocouple on the far end of a long pipe report wall temperatures far lower by the time it reaches the hull. Give the exhaust a decent length of run before the fitting, lag the hot section near the heater, and keep the run clear of anything combustible. If your route is short and direct, add length rather than accepting a hot fitting.
How high above the waterline does the exhaust outlet need to be?
At least 12 in (30 cm) above the waterline, reached by a swan neck loop that rises above the outlet before dropping to it. The loop is what matters — it means a wave slapping the fitting cannot run downhill into the heater. Angle the fitting slightly down so it drains and does not catch spray.
Should the exhaust exit through the transom or the side of the hull?
The side, on the aft quarter. A transom outlet sits in the eddy of turbulent air behind the boat, and when you lie head to wind that eddy carries exhaust forward into the cockpit and down the companionway. A raked transom adds a second problem, catching rain and spray in the fitting. Boaters who have moved an outlet from transom to quarter generally report the smell problem goes away.
Will exhaust condensate stain or damage my gelcoat?
Plan the install so condensate never runs down the topsides. Most of the water appears in the first minutes of a cold start, before the pipe is up to temperature, and the fix is drainage rather than chemistry: keep a continuous downhill run from the swan neck to the fitting so the water leaves through the pipe, and on a long run add a drain at the lowest point. If your fitting sits above a light-coloured hull, rinse it occasionally rather than letting soot build.
Is a 5KW diesel heater too big for a 30-foot boat?
Yes, in most cases — take the 2.5KW. A 5KW on a 30-footer will hit temperature fast and then run at minimum output all night, and long periods of low-output running are what coke up a burner. Two exceptions: an uninsulated hull in a genuinely cold climate, or a pilothouse with a lot of glass. If you do fit the 5KW on a smaller boat, give it four outlets and run it at full output regularly to burn off carbon.
Is it safe to sleep with a diesel boat heater running?
Yes, with a correctly installed sealed-combustion heater and a working CO alarm. Combustion air comes from outside the cabin and exhaust leaves through its own sealed path, so the air in the boat is only ever warmed, never burned. That safety depends entirely on the install — a leaking exhaust joint or an intake drawing from inside the cabin removes the protection. Fit a CO alarm regardless of what heater you own, and replace it when it expires.
Do I need to plug the exhaust when the boat is hauled out for the winter?
Yes, block the exhaust outlet and the combustion intake while the boat is ashore, otherwise damp sits in the pipework and corrodes the burner. The important part is making sure the bung comes out before the heater is ever lit again — tie a bright label to the controller, not to the bung. The same applies if you bung the outlet while sailing in heavy weather.
Fitting one yourself? Our how to install a diesel marine heater walkthrough covers mounting, exhaust routing and the fuel pickup step by step. For other Vvkb units, see all diesel heaters.
Watch a Full Marine Install Before You Start
The video walks through a marine install on a real boat - where the heater goes, how the exhaust and fuel line are run, and what the finished job looks like. Watch it before you order so you know what you are taking on. If you would rather read, our how to install a diesel marine heater guide covers the same ground.
