Q&A: Which Heater Is More Efficient?

Question: Could you please address the fuel consumption of forced-air bus heaters vs. the Refleks?

Answer: An accurate comparison of fuel consumption and relative efficiency between the two technologies would require equipment and time we don’t really have at our disposal. However, since both systems burn with an essentially clear exhaust, indicating good efficiency, and diesel fuel has a fixed amount of available heat per unit of volume, I think we can assume that their base efficiency is very close to identical. But the air bus heaters lose a lot of heat through their ducting, as much as 20% in a cold bilge, I’m told, and in addition, more fuel must be burnt—unless the boat is fitted with a wind generator and the breeze is very steady—to replace the electricity used by the bus heaters. Bottom line, I would expect the Refleks to be a clear winner in overall efficiency.

See this post for a general comparison of the Refleks and Eberspacher heaters.

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Meet the Author


John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for twenty years, first in yachting magazines and, for the last 12 years, as co-editor/publisher of AAC.

24 comments… add one
  • Paul Dec 15, 2013, 11:48 pm

    Having had both heaters installed on our last boat, and spending two winters in Annapolis Md (not much of a winter compared to PEI) I can say that using the espar whilst under way and the Dickinson heater at anchor, works great. However, I may remind everyone that a bulkhead heater also loses a lot of heat out of the chimney.

    I’d have to give the Espar the nod as to which was more efficient. The Dickinson Alaska was rated for 16000 btu and the espar 11000 btu, but the espar definitely “felt” warmer. We had a circulating fan for the dickinson, but it never outperformed the espar.
    We already have one espar for the new boat, and will get a second espar for the second hull too. But if we didn’t have a Cat, we’d also have a dickinson if we had room for the chimney. There’s nothing better than cruising in a cold climate, when it’s cold, as long as I’m warm while doing it.

    Paul from PEI, Canada.

  • Ray Durkee Sep 21, 2014, 10:18 am

    I am not sure where one would use 75 Ah of electricity with a forced air heater—that sounds really high even for a Webasto or Espar????. I have used a Wallas 30 D for nearly 20 years (I am on my second unit) and the thing uses 10 amp draw at startup for a few minutes, but more like an amp or two once it is running—depending on the setting. I agree that these units are not maintenance free and that it takes some guts to do it yourself, but I have learned over time. Also, I realize that Wallas units are not marketed as heavily as others but I have had very good luck with mine and consider it one of the essentials of cruising life. I even used it during a couple years cruising in Central America—nothing like it to dry the boat and everything in it out. Absolutely essential now that my boat is in Maine in order to take advantage of the great Fall sailing. ScanMarine folks are supberb support folks. The Wallas unit is specifically made for boats and is made in Finland.

  • John Twaalfhoven Dec 16, 2014, 4:44 pm

    Hello heating experts. Two questions you might be able to help us with regarding a Refleks heating unit. We sailed a new boat from France to the Netherlands late this year. The Refleks comes with a pretty useless manual. I admit every idiot can operate it so a manual should not be needed. Nevertheless we had some issues. First question: instructions say to start the heater on pilot setting. There is no such marking, only high or low. One internet story-telling Skipper sets the pilot to high. Unusual, sounds like a choke on the lawn mower. Could this be correct? Second question: sudden flame out, lots of smoke in the cabin. The boat is known to have an airtight main compartment in case of an emergency. It actually has a full size sealed door. Could it be that openening the door (to the outside) draws air back through the chimney? As you can read, we are guessing…….
    Thanks for any advice.
    Save journeys into 2015

    • Eric Klem Dec 17, 2014, 9:57 am

      Hi John,

      I worked aboard a boat with a Refleks for a while and I will admit that I never read the manual. We found that our favorite way to light it was to turn the fuel on without lighting it until there was a very small puddle in the middle of the pot and then throw some toilet paper down there to wick up the fuel. We would light this and let it burn for a few minutes. When it was almost out, we would turn on the fuel to the highest setting and it would be lit. After a few minutes, we would dial it back to whatever setting we wanted. I suspect that there are probably better ways to do it than this but once we had something that was reliable and didn’t smoke out the cabin, we stuck with it.

      Having an airtight boat with a heater like this is definitely a problem on some level. Shutting the boat up could create draft issues and be dangerous to those inside. Suddenly opening a hatch would have an effect that is hard to predict as you will get a lot of air movement and the air will have varying concentrations of oxygen and combustion products.


  • Bill Attwood Dec 17, 2014, 5:04 am

    Hi John,
    The burner control on our Refleks is marked from 1 to 9, and we set it to 1 for lighting. We use a shot of meths to light the burner pot. I made the mistake of using filtered but 6 year old diesel oil at first, and that did lead to problems. Replacing it with new fresh diesel solved the problem, but not before we had to clean out the burner pot – a dirty job. If you put meths into the burner pot, does it light and burn satisfactorily? If it lights and then goes out, this is a sign that there isn´t enough oxygen = the burner pot holes need cleaning out. If the burner pot lights, but produces a lot of smoke and soot, this is a sign that there isn´t enough diesel flowing into the burner pot = clean out the regulator and injection “needle”. I think it unlikely that opening the door would lead to the problems of smoke that you describe, rather the opposite. At home we have a wood-burning stove which does excatly this when we switch on the extractor hood in the kitchen. We have the yacht-type chimney cap on our Refleks, with no additional chimney above deck. Refleks recommend an insulated chimeny pipe above deck for cold conditions, could this be the answer to your problem? If you really can´t solve the problem yourself, a phone call to Refleks in Denmark may be useful. They are very helpful.
    Good luck.

  • Wilson Fitt Dec 17, 2014, 10:38 am

    We have a diesel fired Dickenson stove, same principle as the Refleks, and once in awhile it would backdraft when the wind was forward of the beam, filling the cabin with smoke. I eventually concluded that the open companionway hatch with dodger over acted like an enormous extraction cowl vent lowering the air pressure in the cabin. The relatively small forward facing dorade vents could not compensate and the result was that the draft in the stove chimney reversed itself. I cured this (mostly) by bringing a 4″ round duct through a bulkhead from the aft end of the boat to a point just below the heater stove where its air intake is. I think it would be more effective if this air supply duct had a little in-line fan to provide positive pressure.

    • Hans Dec 18, 2014, 11:28 am

      Hi all,
      on my boat i have a Refleks 66MK – the smallest model they make – and i have the same problem with backdraft when the wind comes from port side. After trying various tricks the only cure i found for that is rigging a piece of tarp to shield the chimney cap. The Refleks folks in Denmark also didn’t have a simple solution, they recommended a second pipe parallel to the existing chimney for drawing air from outside and thus being independent of combustion air from inside the cabin. They sent some pictures of such an installation – slight resemblance with a nuclear power plant piping, so i didn’t go for it. Not a real problem once you know what to expect and what to do about it.
      As for lighting i turn the control to full and as soon as the first puddle of diesel appears i turn the control back to low and light the oil with a stove lighter, a waxlike piece that is made especially for the purpose. Doesn’t always work the first time so the trick with the toilet paper is sure worth trying. When the flame starts burning blue and the stove makes cozy bubbling noises i know that i’ve suceeded.
      I wouldn’t want to miss my Refleks for anything. Great stove ! Burns for days and weeks, consumes no electricity, keeps the water kettle hot and needs about 2,5 ltrs./24hrs at low setting.
      Question to Wilson: Isn’t the Dickinson normally fitted out with an electric fan to support draft in difficult conditions ?


      • Nick Kats Dec 19, 2014, 5:31 pm

        Hi Hans
        My experience is the same as yours. Same Refleks stove, chimney offset to port, dislikes wind from port, shielding with a piece of plywood solves that.
        Next to the Refleks I have a woodstove, chimney offset to right, this also has a problem with wind from starboard, etc.
        Changing the chimney caps dramatically improved both but did not fully solve the problem.

  • John Dec 18, 2014, 10:39 am

    Hi All,

    Thanks very much for fielding that one. As a long time Espar user I had no clue.

  • Svein Lamark Dec 23, 2014, 9:38 am

    The energy Conversion efficiency of a traditional pot burner like Refleks is very low, usually around 40% or less. It can be increased a little by adding a hot water system inside the burner pot. But still the combustion is incomplete and around 60% of the energy is going up the pipe as smoke, soot and pollution. This is a pollution machine contributing to global warming as well as making your sails black of soot. Pot burners like Refleks have been forbidden in the Norwegian fishing fleet at least for 30 years after many tragic accidents. The smoke can change direction and kill a crew. The length of the pipe is vital. It should be more than 2,5 m from the burner pot to the down draft cap. The little pressure difference in the air will then help. The pipe line above deck must be insulated and the best down draft cap is H-shaped. The pipe line under deck must have a protection grid to protect against burning bare hands. This protection grid must be positioned where it is natural to touch the pipe. Another danger with Refleks is that the burner pot is only made of 1-1,5 mm thin steel. The pot can fall down by corrosion and cause a large fire in the ship. There is always sulfur in diesel oil and if you get salt water down the pipe, this proses may go fast, only 6 months according to the original manual. The Danish fishermen in the 1960-ies used this oven a lot, but placed in a large tray of steel under the oven to prevent fire. But even installed this way it is dangerous. The picture in AAC of a Refleks in a Boreal yacht shows a wrong and dangerous installation: The protection grid is placed wrong and the burner pot is sitting directly on wood.
    Both Dickinson and Blake & Taylor have spare burner pots. By changing the pots regularly you reduce the risk of fire. Kabola has a pot burner in cast iron which burns cleaner and has a long life time.
    Diesel burners that use hot air to distribute the heat has an even worse energy conversion efficiency, somewhat above 30%. It takes a lot of diesel and electricity to heat a ship with this technology.
    Diesel heaters with hot water around the combustion chamber have a high energy combustion efficiency, often above 90%. They are also much safer and rarely produce fire in the ship. The best of the small “bus type” is Webasto that can live up to 15 000 H. But many of this “cheap” heater a constructed to last only 3000 hours. Many of the cast iron water heaters are constructed to last 30 years or more, like my favorite the Kabola. A water heater can also be connected to the warm water of the main engine and the generator engine. This way diesel can be saved a lot. In an Arctic expedition yacht this is a must, but you rarely see it. A warm water system is complicated to install and should be planned before building. It is difficult to do it later. I am sorry to read that the A40 will have no heating system. I think it is a pity because there are so few yachts with ice quality that persons will certainly by A40 as an Arctic expedition yacht. The A40 deserves a good heating system.

    • Eric Klem Dec 23, 2014, 6:22 pm

      Hi Svein,

      I just wanted to comment on the efficiency of different heaters. The good news for heaters is that we are trying to produce heat which is usually the exact opposite of what you are usually trying to do in energy conversion so they have an inherent advantage. Your major loss will be your exhaust loss. In a diesel engine, a very rough rule of thumb is 1/3 of your energy goes to shaft power, the radiator, and the exhaust each. Obviously heating is very different than an engine but you should have even lower stack losses. For comparison, the worst hydronic heaters for home heating are on the order of 80% efficient and the best are over 95%.

      While I have never seen true efficiency numbers on the Refleks or Espar, their literature gives some indication. Unfortunately, I don’t see a mention of the tests being done to something like an ASTM standard so they may not be totally comparable but they are still useful in a general sense. The Refleks 2000K has a published max output of 4200W (they don’t state what assumptions are made about coils unfortunately? and max fuel consumption of 0.54L/hr which works out to an efficiency of ~72%. The Espar Airtronic 4 is very similar in size and has a published fuel consumption of 0.51L/hr (I am ignoring the electrical draw which could be an additional 185W) at a max output of 4000W which works out to an efficiency of ~73%. I would consider these efficiencies to be very reasonable for the output of these units, if they were significantly more powerful, I would expect them to be better. While I don’t think it would be fair to draw firm conclusions from this data, I don’t think that it would be unreasonable to say that you could hit 70% efficiency with either the Espar of the Refleks under good conditions.


      • Laurent Dec 23, 2014, 10:32 pm

        High efficiency stove use mainly 3 kinds of tricks:
        – 1) elaborate pathway inside the stove, between the burner and the exhaust, to slow-down fumes and increase heat-exchanges.
        – 2) an air entry device after the stove exhaust at the start of chimney tube : the chimney ventilates the oven because it contains a column of hot air, which is lighter than surrounding air. This air tend to escape upward (Archimedes….), which create a draft of cold air in the stove burner. Point is that those system must work at fairly low temperature when the oven just started, which means that in most cases the air draft is quite excessive when the stove has reached normal temperature and this excessive draft takes way too much heat out of the stove in the open air.
        The idea of trying to regulate this by hand is not very good (risk of CO intoxication in case of very bad adjustment…). Fortunately there is a simple classic device with a balanced air door that can regulate this fairly efficiently. I don’t know if it works well with boats’ movements or list, but it should be quite OK at port or anchorage.
        – 3) an additional heat exchanger in the chimney tube, to take heating energy out of exhaust fumes and back inside the house or boat.

        -Point 1 is difficult to calculate and can not be improved on an old stove (might be dangerous..). Recent progress of high efficiency stove are supposed to happen mostly there. I am afraid that Refleks, Kabola etc…. don’t contain that kind of features today, but they might improve that in the future….
        -Point 2 is very easy to adapt and is standard on most high efficiency house stove. I have never seen such devices on boat stoves, perhaps there is an adjustment problem when boat is moving or listing which causes them too be considered as too dangerous in that case (CO…).
        -Point 3 is the latest trend in house stove efficiency, but existing products are probably too clumsy for boat use.

        I guess it might be interesting to look further at current boat stove offerings to find out which manufacturer did the best job and with what results, considering that this job should mostly consist in elaborate smoke pathway inside stove to slow-down exhaust without risking CO generation. Guess that newer models are supposed to be more elaborate than older ones because of higher customer requirements and better engineering tools. I have in my house a wood stove that has elaborate internal fumes path and that I saw recently advertised as one of the few that do respect French latest efficiency norms for wood ovens (they call it: “flame verte”…). Reading this add, it seemed obvious that the manufacturer was presenting his latest product. Point is that my oven is probably more than 30 years old.

        • Hans Dec 24, 2014, 5:56 am

          Hi Laurent,

          as for point 2, Refleks provides what they call a “flue damper” to take care of too much draft. http://www.toplicht.de/en/shop/ofen-herd-und-kocher/abgasfuehrung-und-zubehoer/refleks-abgasfuehrungen/abgasrohr-refleks-mit-zugregulierung
          Maybe i should add this device but never saw a need to do so.

          Merry Christmas to everybody !

          • Laurent Dec 24, 2014, 8:35 am

            Yes it is exactly that. On house stoves those devices are supposed to reduce chimney heat losses by 30%, which means substantial improvements in stove efficiency. I have never seen those devices installed on boat stoves and I guess they should also give substantial efficiency improvements. You should be careful using only well tested (perhaps stove manufacturer’s certified ?…) chimney equipment because inappropriate chimney systems might be dangerous (CO…).

  • Hans Dec 23, 2014, 2:29 pm

    Hi Svein,
    i wasn’t aware of the bad energy conversion factor in my Refleks stove and i’m not fully convinced that it is as bad as you state above. All i know is my experience with my own stove and that is just outstanding. My 66Mk was used by the former owner of my boat as a home heater in his small wooden house. After that it was transferred to the boat. That was 1994, 20 years back and the stove performed perfectly (apart from the sometimes annoying habit of being blown out with wind from port side). Its duty included a North Atlantic circle, a winter in the Baltic with 2 people living on the boat, voyages to Norway, the Shetlands, Iceland, Greenland and Newfoundland. So it saw a lot of use. I cleaned the burner pot once a year and once i sent the whole stove to Denmark for inspection and adjustment of the regulator. The burner pot was found to be in good enough condition to remain in service for more years to come. There is some soot to be found in the annual cleaning but no more than, say, a cup. Just a little smoke leaves the chimney cap – i use a Dickinson cap – while firing up and after the stove and the chimney pipe is warm there is no smoke to be seen at all. No soot whatsoever on deck, canvas or sail covers.
    Of course my stove is mounted in a stainless pan or tray. There is a heat sensitive fuel safety valve on the regulator (on the newer regulator model only) that shuts off the fuel supply at more than 73° centigrade. This stove has served me extremely well and i’m confident that it will do so in the future. But you are right about the installation in the Boreal: I’d never consider to mount the stove directly on a wooden deck.

    • Hans Dec 23, 2014, 3:04 pm

      Hi Svein,
      maybe i should have mentioned that i never use the stove when under way, i never even tried to do so. As a singlehander i really don’t need heating when sailing or motoring. If i ‘d have used the stove at sea – who knows , i may have experienced the smoke and soot problems that you mentioned.

  • Nick Kats Dec 26, 2014, 5:11 pm

    Hi Svein, low efficiency of energy conversion is unimportant in a small volume. Unless wintering over in high latitudes.
    Hi Hans, I’ve often sailed with the Refleks or the woodstove lit up. Its great to have a dry cozy interior. Is sailing with a heater on contraindicated? Why?
    Happy New Year, folks.

    • Hans Dec 27, 2014, 7:37 am

      Hi Nick,
      contraindicated it is certainly not. I just never saw the need to bother with the stove when under way. As a singlehander I’m either fully dressed or lying in my bunk – with a quick lookaround every 15 minutes – . Also I anticipated the problems mentioned above, perhaps wrongly so. When at sea the upper part of my stove’s chimney is stowed in a cockpit locker and is only screwed on when I’m going to use the stove. A watertight lid seals the opening. Another concern is list: Refleks states that their stove can be operated at a list of no more than 10°. Oil can spill out of the regulator when this value is exceeded, they say. The spillage woul be contained in the drip pan, but still… it smells and is nasty.

      Happy New Year !

  • Svein Lamark Dec 28, 2014, 9:52 am

    Hi guys, it has been minus 10-15 Celsius outside the last days. It makes sense to discuss energy efficiency. When using percentage it is always best when knowing what 100% is. In Norway the scientific tradition in this field is to use as 100 efficiency an electric heater inside water. The diesel burner has to be compared with this. By doing it this way Eric Klem, you get the bad result for some traditional burners. The leading university in this field, SINTEF, has published papers on this subject. Eric, I find your figures unrealistic and to high. However as Laurent writes, it is possible to extract energy from the pipeline. Many commercial ships do it. Even if you increase the efficiency of a pot burner up to 60-70%, it is a long way up to the 90-95% of a diesel water heater. I am sorry Hans, but a traditional pot burner will remain a dangerous pollution machine.
    The efficiency of a traditional wood burner is about 40%. In 1998 Norway forbid installation of old wood burners. Today they have to be of the modern two step technology that gives above 70% efficiency and much less pollution. Still there are a lot of old wood burners in use and they are a problem. In Oslo city many people have problems when breathing the bad air. This problem in a more general way is well known from big cities like Peking and Los Angeles. Refleks has a charming legal wood burner for small boats, the Captain Lange’s ship oven.
    A diesel hot water heater in combination with the hot water from main engine and generator engine is the most efficient system. However this will also give pollution. Norway has forbidden all type of diesel heaters to be installed in houses from 2020.This is because diesel burners produce CO2 and global warming.
    We should try to choose the best solution in our boats. I am a bit shocked that modern yacht constructors and yacht owners (like Nick Kats and also the A40 project) gives a shit in this and sticks to old pollution machines. Each of us has a responsibility to save nature as best we can.

    • Eric Klem Dec 28, 2014, 12:18 pm

      Hi Svein,

      The result that I got from my quick calculation is what I would call thermal efficiency. What this stands for is the total heat output divided by the total thermal energy available in the fuel so it is a pretty standard measure of efficiency and it is the definition used here in the US to compare all boilers, furnaces, etc. It is important to note that this thermal efficiency is an overall number and is a sum of all the losses including inefficiencies in combustion and heat transfer. As I noted in my reply, I used the manufacturer’s numbers and I did not see reference to something like an ASTM test so I don’t know if unrealistically idealized conditions were used for the testing but I would not expect the efficiency to be significantly different. If you have a link to a study where they found significantly lower efficiencies, that would be interesting to see.


  • Martin Wright Jan 18, 2015, 7:30 pm

    Thank you for an interesting thread of comments. I am preparing a Bowman 48 for a trip to high latitudes (Greenland) this summer, and have an Espar Aitronic D5 fitted. I have solved the problem of heat loss in the trunking by using the precut Thinsulate insulation that Eberspacher (as it is known here) sells. It is very effective indeed. The heat now gets through to all the cabins, and items stored in the bilge do not get too hot.

    I have also insulated most go the boat with either 25mm or 44mm thinsulate in the headlining and where possible against the hull topsides behind panels.

    It appears to be most effective, and I hope it proves to be sufficient!!

    I would certainly welcome any other advice regarding keeping warm and effectively in the Greenland summer.

    Many thanks,


    • John Jan 20, 2015, 12:08 pm

      Hi Martin,

      Thanks for a great tip, one I think I will try. How did you attach the insulation to the trunking? There seem to be several adhesives on offer.

      • Martin Jan 20, 2015, 5:37 pm


        There is no gluing involved. Ever spacer sell either pre-cutbThinsulate sleeves or wraps that you can wrap around existing pipes that secure with poppers down the long edge. The product is called “Maxitherm Duct Insulation”.

        Have a look at http://www.krueger.co.uk.

        I hope this helps!


        • John Jan 21, 2015, 11:32 am

          Hi Martin,

          That’s great, definitely the way to go, thanks for the tip and the link.

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