How to Recondition a Diesel Pot-style Cabin Heater

In order to cruise year-round in the Northwest, cabin heat is essential. On cold, wet days you don’t want to be shivering at anchor while the temperature drops to 35 F. So around October, repairing the Sig Marine diesel heater that came with our boat became a #1 priority.

Working on a diesel heater was scary to me though – these heaters involve setting a fire inside your boat! Fire is usually the last thing you want on a boat. But, as I learned more about it I gained confidence. It was a bigger project than expected though.

On a club meetup at Port Orchard last month with the Puget Sound Cruising Club, we were on 3 or 4 other boats in 30 degree weather that had nice, calm burning diesel heaters keeping their boat nice and toasty. This was a stark contrast to our heater which had flared up into an angry inferno a couple weeks ago. Getting the fuel and air mixture right on a heater that hasn’t been used in a number of years is more difficult than it seems.

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Fuel Type

The first thing to decide is the fuel type. You basically have three choices: #2 diesel (what your diesel engine runs on), #1 diesel (aka stove oil), or kerosene. Some people install the heater fed directly from the engine’s diesel tank, or from a smaller day tank that is fed from the larger tank – in that case, the choice is already made for you.

Since we already had a system installed, we knew we had a dedicated fuel tank in the stern area which gravity fed to the cabin heater about 15 feet forward. I decided to use kerosene because I heard it is cleaner and less smelly than diesel. Actually picked up a kerosene substitute, Klean Heat, from Home Depot, because kerosene was fairly difficult to find (Home Depot may sell it, but the employees didn’t seem to know where it was).

Kerosene has a lower viscosity than diesel, meaning it flows faster through the metering valve. The Sig Marine manual says it’ll mean a 50% faster flow rate than #2 diesel. #1 diesel, aka stove oil, is slightly thicker but less so than #2 diesel, which is what you put in your engine. #1 diesel has a 30% faster flow rate than #2 diesel.

You can get valves calibrated for kerosene or #1 diesel, but unless you specifically requested that, most valves are made for #2 diesel.

The fuel valve - fuel comes in through the inlet at the bottom left, and out through the rubber tube at the bottom.

The fuel valve – fuel comes in through the inlet at the bottom left, and out through the rubber tube at the bottom.

Exhaust Hood / Conduit

As mentioned in a previous post, rebuilding the deck exhaust cap was one of the things we had to do to get the heater working. The deck cap had been mounted on some kind of wood that easily rotted, and it was falling apart and leaking water into the cabin and deck. Eliminating leaks in an opening this large isn’t easy, but at least we know the deck penetrations are sealed (epoxy potted) now. The finished teak cap plate

Fuel Line Issues

The first problem with a cabin heater that hasn’t been used in 4+ years is that the fuel line is probably clogged. It was. We had some fine adventures with trying to unclog the fuel line while having kerosene drip – or spray – into the bilge, and onto the floor and hands.

If you have trouble clearing the clog you can try sticking some old electrical wire through it or a coat hanger.  Or blow through it with you mouth, while trying not to swallow any kerosene.

You also want to make sure the fuel hose from the heater valve to the heater pot is clear, so we opened that up and blew through it.

I bought a Dickinson fuel filter off of Amazon. [LINK] It was actually $30 or so cheaper there than direct.

I bought a Dickinson fuel filter off of Amazon. It was actually $30 or so cheaper there than direct.

Lighting the Burner

The instructions recommend making a wick out of a piece of paper towel and dropping it in to start the starter fuel. This is harder than I expected, and it often takes 10 minutes or more to start the burner.

The first time consuming part is getting the starter fuel into the pot – you need about 2 tablespoons. Opening the valve to setting 1 like recommended it’ll take 6 minutes or more just for that part of the process. Sometimes there’s an air bubble in the fuel line so you need to tap on it or tap the valve to get that cleared. Usually I’ll turn the valve up to 3 or 4 to get it started.

Then once I dropped the flame in, it often got extinguished by the air flow as it dropped or the kerosene itself extinguished it. Kerosene (and diesel) really aren’t that easy to light, especially when the fuel is cold. It needs a big enough flame on it for a long enough time to heat it up a bit and start combustion.

With practice I found a better wick shape. When I made a thin candle-like wick, the paper ended up burning slowly – like a candle wick – with the kerosene soaking providing some longevity to the candle wick (it took close to 5 minutes to burn out) but not enough to light the kerosene.

But a wick with a largish unrolled section in the middle, and twisted ends (like a cigarette with the middle unwrapped) worked better. The twisted ends burn slowly and allow me to light the end and drop it in before it starts to burn my fingers, and when the flame gets to the middle section it flares up into a larger flame that lights the kerosene.

The wick on the left didn't work as well as the one on the right. The one on the right is easy to drop in yet burns a larger flame when it gets to the middle.

The wick on the left didn’t work as well as the one on the right. The one on the right is easy to drop in yet burns a larger flame when it gets to the middle.

I’m sure there are other ways to do this. Perhaps I could just use one of the long-necked butane lighters we use to light the propane stove.

Controlling the Burn

Getting the flames to burn right is a delicate balance of fuel control and air control. The first few times we ran the burner, the flames were much too high and resulted in carbon buildup. Having a raging inferno isn’t very safe either, and creates too much heat. After some adjustments to the metering valve to decrease fuel flow, it got better, but we still had issues with the flames growing much too high after 30 minutes or so. Most likely the fuel warmed up and the flow rate increased.

This is what carbon buildup looks like from the flames burning too rich (for only 15 minutes - imagine what 4 hours would do) - it left a black soot layer which is hard to cleanup

This is what carbon buildup looks like from the flames burning too rich (for only 15 minutes – imagine what 4 hours would do) – it left a black soot layer which is hard to cleanup

The meter has always been on setting 1 (lowest fuel flow rate) and increasing air flow didn’t help – that just caused the flames to jump around even more, making agitated rushing air sounds (kind of hard to describe). It’s possible the flue was downdrafting, which can happen when it’s windy out or air pressure in the cabin drops below the outside air pressure (if you don’t have adequate ventilation into the boat). We have excellent ventilation however, and it was only slightly windy out (and we have the “H” style cap, which is the more wind resistant design).

Here we had a pretty good fuel / air mixture going

Here we had a pretty good fuel / air mixture going

Here’s a video of a pretty well controlled flame we had going for about half an hour:

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10 thoughts on “How to Recondition a Diesel Pot-style Cabin Heater

  1. Steve Hulsizer

    Use a large squirt bottle with painters alcohol. Prime the heater with the alcohol with the fuel valve open, lighting it with a BBQ lighter. By the time the alcohol burns off, the diesel fuel will be burning nicely. No ash, no mess. Use about 1/2 ounce per start.

    I recommend using a shutoff valve at the tank. Even if the valve on the float tank appears to shut off the fuel, sailing on a starboard tack (aft facing heater) will lift the float and allow the fuel to overflow the float tank. The float tank vent is the stem to the control valve. Rather messy, and a possible fire if you are using while sailing. These tanks are really meant to be mounted fore and aft, but who has the bulkhead space for that. We close the shutoff valve and allow the float tank to burn out.

    Additionally, we use a second air pipe, the same size as the exhaust, for incoming air. It should be mounted on deck as close to the exhaust as possible so they see the same air flow across the deck.

    Steve

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    1. Dave

      Try long tweezers (cheap set Harbor Freight) for dropping priming wick to avoid scorched fingers. Try lab wash bottle (cheap at Amazon or lab supply shop) for well controlled priming fluid. Head lamp to light the priming target squirt. I run #2 diesel and use kero to prime. Have metered settings and start protocol almost down to a fire andd forget procedure….but….it is an.open flame in a boat and I never trust it completely. Presently sitting in front of well behaved Dickinson Antarctic off Whidbey Island….comforting.

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  2. Rob

    Try gelled sterno for a starter. They sell it to use in the sterno burners used in buffets. Easy to use, and avoids the drips liquid alcohol can cause.

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  3. Jay N

    What we have learned over the many years of various sorts of Dickinson and Olympic heaters/stoves: 1) It’s easy to over fuel the pot when the heater is cold. We turn the carb on FULL, and watch with a flashlight until we can see the pooled fuel. Then, the carb is turned OFF. 2) We use kerosene starter for burning diesel. As others have indicated, a couple of squirts from a ketchup bottle works just right. 3) We use tissue paper, as paper towels tend to leave too much ash. A couple of twists like you do seems to work fine. 4) After lighting the paper, it’s inserted into the fuel in the pot. 5) It takes about 15-20 minutes for the fuel in the pot to be consumed, as soon as it starts to die off, the carb is then turned to the desired setting. At this point, the pot is hot enough to efficiently burn the new fuel.

    This procedure minimizes that chance of too much fuel in the pot when cold, and that minimizes the possibility that there will be too much fuel when the heater finally gets going.

    I must point out that there is obviously a lot of variety in stove make and installation. Some years ago, we had a heater that absolutely needed a combustion fan to make it work. There is good ventilation for the present heater, and other than some sensitivity to back pressure with high winds and an open door, it is very stable.

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    1. Patrick Post author

      We used Steve’s suggestion this weekend, of lighting the kerosene with denatured alcohol, and that worked great – going to use that everytime from now on. Now I don’t need to deal with a paper towel or tissue wick and can easily light the fuel with the superheater (metal thing with legs) and burner ring (metal thing that goes on top of the superheater) already in the pot.

      The latest valve calibration I did also seems to have fixed our metering rate – it was pretty stable in a 2.5 – 3.5 setting on the valve.

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  4. Wayne Gilham

    I have same Sig heater (built by factory with mixing-valve on side, for my mounting to fore’n’aft bulkhead)…. I prime it with KleanHeat odorless kerosene alternative (avail in gallon jugs at Home Depot or Lowes), avoids smoking on startup, doesn’t smell bad if spilled, dispensed easily: pour 1oz via bottle-type identical to the BioBor “squeeze-an-ounce” into the upper bottle-chamber… (not sure I’d be comfortable using an empty BioBor bottle, unless REALLY well cleaned… no idea whether even a tiny bit of this stuff burning would be toxic) here’s a source for new ones, apparently will sell one! http://tinyurl.com/jnto72t

    Wayne Gilham

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  5. Andy Cross

    The one thing that our Sig 180 struggles with is soot/carbon build-up. We’re running #2 diesel through the unit and I’m pretty sure we have the air/fuel mixture down, but it still builds up inside the unit and then kicks out lots of little soot-balls all over the deck. Any thoughts?

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    1. Patrick Post author

      Well, you do run your heater *a lot*. I’m not sure what might help, perhaps #1 diesel or kerosene would be cleaner but then you’d have to deal with recalibrating the valve, which is not fun.

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