Simple Wood-Fired Water Heater Is Surprisingly Effective

These days, humans have gotten all fancy-schmancy with their gas and electric water heaters. Heck, some are even using heat pumps to do the work as efficiently as possible. [HowToLou] got back to basics instead, with his simple wood-fired water heater design.

The design is straightforward, featuring 100ft of quarter-inch copper tubing wrapped directly around a steel barrel. Room-temperature water is fed into the tubing via a garden hose, and comes out much hotter, thanks to a fire burning away in the barrel stove of [Lou’s] own construction.

For an input water temperature of 41 F, the output reaches 105 F at a flow rate of 0.67 gallons per minute. By [Lou]’s calculations, that’s a heat transfer to the water of roughly 21,000 BTU per hour. [Lou] achieved this with just $55 worth of copper tubing, and he notes that simply doubling up the tubing would increase the heat transfer to the water even further.

If you’re looking for a hot shower from your outdoor wood stove, a build like this might be just the ticket. With the stove burning hot and your hose as a water supply, you could experience the joy of the hot water while you’re standing in the snow outside. We’ve seen [Lou]’s work before, too. Video after the break.

55 thoughts on “Simple Wood-Fired Water Heater Is Surprisingly Effective

  1. I tried that with a couple wraps of 1/2 inch tubing around a steel drum full of solar heated water, and it only lasted ten years until it rotted pinholes in the copper. It was OK while it lasted, because it was wrapped with insulation to stop heat loss to the outside air.

    1. I wonder how much the copper is necessary if you’re using solar heated water. At lower temperatures like that, you might be able to get away with a longer-lasting material, though you might lose some heat transfer efficiency. Still, a whole bunch of plastic tubing might work just as well and last longer.

    1. Although, with storage and controls that would add up to plenty of water for many purposes. However, unless the firebox is burning for some other purpose it may not make sense.

      I once added a small coil to a wood-fired heater, to supply domestic hot water. It turned out to be not workable because unless water usage was higher than normal for the household the storage tank would end up overheating. Also, at times when lots of hot water was needed the small coil couldn’t provide it quickly.

      1. Normally it is used for area heating with hot water as a byproduct. You have a circulating water loop heating an area and a liquid to liquid heat exchanger for domestic hot water usage. Temperature in the loops are thermostatically controlled pumps and draft controls. You make the fire hotter and colder by allowing a less combustion air and by circulating faster or slower.

        1. Understood. The system I described was rudimentary, a coil-around-stack of “wood stove” heater (not boiler) through which DHW circulated directly. Since the DHW was secondary to firing of the heater it was difficult to strike a balance, as described. I considered various exchanger and control schemes but decided that the juice wasn’t worth the squeeze. “Buying” DHW isn’t that big of a deal so an alternative system’s build has to consider that too.

          FWIW I switched to stand-alone wood fired heater for DHW. It’s a very simple system with almost no controls. With each build the design became less complex. Basically the quantity of fuel loaded into the firebox is the primary control. After a bit of trial the quantity of wood needed for a desired heat became easy to estimate.

  2. Well… meanwhile the output of the wood fire is probably north of 500,000 BTU.

    Heating with wood is tricky, especially heating water because of the low temperature of the water. The heat exchanger just condenses creosote like crazy. There have been a long string of horrible water heating wood stoves that are basically tar and smoke machines.

    The trick is to completely and I do mean completely combust the wood before running it into a heat exchanger, and with wood that’s non trivial, involving preheating air, secondary air injection, sometime catalytic elements that can get poisoned by burning certain inks on magazine papers…

    I can speak on this subject with some authority, given the incredible number of logs that I’ve split for many a cold winter in Maine dealing with one of these monsters. This is an area that’s ripe for development. There’s a lot of energy in wood but nobody’s figured out a very great way to get it out because the fuel has traditionally been so cheap that nobody gave a damn.

    1. Modern wood burning water heaters use a batch burning principle where you load the wood into an insulated box where it is deliberately burned with so little air that it turns into wood gas. The wood gas is then burned in a separate fire box that also heats the insulated box above it. This lets you burn the wood gas with enough secondary air to burn off tar and particles, and the clean flue gases go up into the actual water boiler. It’s a forced air system though, so you need some electricity to run the fan – it doesn’t work by draft alone.

      The advantage is that by moderating the gasification rate, the wood can be burned incredibly slowly. A batch of wood can go for 12 hours or more, so the heat output rate is better suited for heating your house.

      1. There’s a nut job in Australia that’s putting old turbo chargers on his burn barrels and getting impressive results. It’s kind of terrifying to watch actually.

        In one video they’re actually able to generate 30 lbs of boost and in another I think the burn barrel finally explodes, or disintegrates due to metal fatigue.

        Turbo chargers might be an answer to the efficiency issue.

  3. He doesn’t state how much wood he needs to burn to produce that measly 5.7 kW. I’ll wager he’s getting less than 5% thermal efficiency, and feeding that thing more than 10 kg of wood per hour. (a cord every 4 days or so)

    Oddly enough, my whole house took an average of about 6 kW to heat in January: if I were to try to heat my house with this I would have burnt 7 or 8 cords last month (real 128 cubic foot cords mind you, not those fancy city “face cords”)

    It’s funny: I know three separate families who supply all their hot water and/or heat their houses with a very similar system, except the whole upper barrel is the heat exchanger: not remotely like that nod to a barely-there heat transfer device in that video. They can be remarkably efficient if decently engineered.

  4. It is too bad the Yukon stoves are not legal anymore in quite a few regions. Basically anyplace where a wood stove has to be “certified”. They are a great way to heat a big shop or shed or garage.

    1. I tried to find what is the metric equivalent for a cord of wood, and I found this:

      The difficulty is that the same amount of wood takes a different volume depending on how it is cut and stacked, because of the voids in between. One can have a “thrown stere”, “stacked stere”, “solid stere”… each containing a different amount. In other words, whether you use stere or cord, it’s an arbitrary amount of wood. Of course you could also measure wood by the ton, but then the water content varies…

      1. I’m not the expert but I cut, split, hauled and stacked 21 tons of scrap wood the tree crew left in our town one winter of highschool. We sent it up to my grandfather. No cord is the same, but any cord you buy is way short of a ‘true’ cord. Most of this has to do with the dimensions of a truck bed and how it stacks up.

        I felt all that work was worthwhile when he showed me his gas bill for January with multiple -10 F days it was $11.00 USD and he just payed electric to keep the water pump running.

        It takes a lot of planning (you cut three years ahead before you burn) and knowledge to do that and his boiler from the 70s is not allowed in new builds here (you have to know what you’re doing when you dump water through the pipes or else water can flash boil and rapidly expand from the boiler).

    2. A “BTU” is apparently 1055Joule, and that would make 21000BTU/hour 21000*1055/3600/1000 = 6.1kW A “cord” of wood is apparently 3.456 cubic meters

      The “heating value” of firewood varies a lot, with a ballpark value of 6GJ per M^3. Because it’s not even mentioned how much wood is used to warm up that bit of water no calculation can be made. But it does have all the signs of being very inefficient.

      1. It is inefficient unless you have a lot of property with dead standing wood in which case it becomes very efficient for us. We also have a lot of logging in the area and limbs and pretty much free so it becomes a contest of propane at acouple dollors a gallon and free wood.

        1. Yes they are. The US customary units are convenience units that are defined by metric base units, exactly like a liter of water or an hour of time is. Every metric country has units like these.

          If you wanted to use purely SI units, you should also be measuring your drink in cubic meters and counting your time in kiloseconds.

    1. It’s ironic how we pollute the air so much, causing so many very real problems, but the air we actually breathe is probably cleaner than most of history, just because we usually don’t burn anything outside of carefully engineered furnaces.

      1. Not quite. Because there are so many more people, the level of pollution has gone up regardless since the industrial revolution started. The entire world population was less than half a billion. If you cut emissions by 90% but increase consumption by a factor of 16 you end up with 60% more.

        Of course, modern people aren’t happy with the living standards of a 17th century peasant, so we also consume more energy per person and end up polluting way way more.

        1. Here is a fact about polution of burning wood. If you collect the wood you can use it to heat your homes, if you dont collect it, it will inevitably build up fuel in the forest and then create forest fires. Nature is going to get rid of that fuel if we don’t. In fact when we use it in a furnace applicatin it polutes less because the combustion is more controlled.

          It is a continuous carbon cycle for all plants that will occur whether or not we capture the energy from it or not.

    2. That report is pretty irrelevant because it doesnt include the entire fuel supply chain. You also have to remember that dead trees laying in forest produce greenhouse gases during their decomposition so it is a matter of making use of that energy as opposed to capturing it. It is shocking how many people dont realize that trees die and drop all the time. Here in Northern Wisconsin there are millions of dead oak trees due to oak blight. So many that my uncle has heated his home for ten years now without dropping a single live tree.

      1. Also wanted to comment that wood fired boilers are not at all new or problematic technology. Been running one to heat a home of many years with little maintenance other than checking water loop chemistry and checking that the flue stays clean (depends on wood species and fire temeratures mainly). In this case it is pretty much shop heat with a little hot water as a byproduct.

  5. Thanks for sharing the build.

    A concern would be that 1/4″ tube could quickly “scale” with mineral deposits.

    Another concern would be if there’s any chance of freezing the tube will burst. There’s no simple way to drain such a coil.

    A pipe loop located directly in the firebox would make a much more effective water heater.

    By the way, I toyed with several different designs of wood-fired water heater and finally settled on pipe located at the inside top of the firebox. The current design is at about five years of service, working like a champ.

    Best luck to all with your builds.

    1. Compressed air should be all you need to clear it of enough water to not have to worry about it freezing. It’s how homes are winterized if they are going to be left without heat.

      1. Understood, the theory of a compressed air blow-out. However, in practice with a coiled length of small diameter some moisture stays behind and then this slowly collects at the low points; especially so with vertical coils that create a trap at each coil. The only workable method seems to be to pump in antifreeze.

        With an installation that’s used often but still has to be protected from freezing between firings anything other than a simple easy gravity drain is a pain.

        1. Compressed air blow out works just fine in practical application. Any remaining moisture in the coil can freeze but because it does not fill the coil there is room for it to do so with no harm.

          1. I have to disagree that it’s practical to use compressed air blow-out when it leaves residual water to collect and freeze.

            First, partially drained tubes absolutely can freeze and rupture. Next, a coil with random “plugs” of ice becomes unusable until thawed.

            Example, a wash-down hose stored coiled on a reel. If the hose holds “plugs” of ice at its low spots then it will not allow flow and is not ready for use until first thawed. Not practical at all.

  6. I grew up with water heated via wood stove. Drill holes opposite each other, feed 1″ copper pipe through, connect with elbows, direct into electric hot water heater (we had two). The electrics are nice as a) insulated storage, and 2) when the wood stove is out (summer), they’ll kick in.

  7. – It’s not inefficient if it’s also your room heater.

    – Remembers old apartment that had a furnace/water heater combo unit. Remembers having to crank up the room heat just to get a not-cold shower.

    – Nevermind

      1. Only if you’re selling things that you don’t measure in metric units. You’re perfectly at liberty to sell stuff by the wimblegroat, as long as you state clearly that one wimblegroat weighs 248.7g.

        You only run into trouble if you insist on using the old family set of wimbles that nobody’s weighed since 1736, and where strangely, you can get 16 or 17 wimblegroats to the wimblethicket, depending on which combination you use and the prevailing wind.

    1. Yes, one thing is to also include unscientific units in the text if that’s what’s used in the original, but to publish something here on hackaday for an international audience without using metric units is shameful.

      1. +1, but let’s raise the expectations bar for American audiences ALSO?
        This is, arguably, a science/STEM website.

        It’s not hard to offer metric, followed by the system of our former oppressors.

        Fun fact: the first use of metric currency was the USA, 1790-something. We beat the French.

  8. I would suggest looking at any commercial hot water water heating system to see how to construct one. Whether the fuel is coal, wood or gas does not matter that much. Just about every large building uses hydronic heat running to coils inside air handlers. No need to re-invent ancient technology here.

  9. Just a comment. If you are running potable water through the pipes/tubing, much better to use Propylene glycol in a heat exchanger loop. This is non-poisonous and is even used as a food additive. I have one in the bottom of my electric hot water tank so no worries if I have a leak in my drain-back solar system. Its been running for 34 years now. I also heat with wood but never had the need to add wood heating to my water system. Home built, of course including the differential control.

    1. Earle Rich, Are you suggesting PG is a better at heat exchange than, for example, water? Or safer than “rank” circulating water in case of cross contamination? Or something else?

      1. PG is used in most home and RV furnace systems because the temperatures can go down to way below zero in case of a power failure. We recently saw -19F and many homes and businesses had some serious damage because of failed plumbing. My shop and tub room has plastic tubing in the floors but the furnace circulates PG for those as well as the rest of the baseboard heat exchangers. PG isn’t as good as water for its heat capacity but the safety aspect makes it a good choice.

        1. Thanks for the detailed reply. Since I’ve recently been firing the rudimentary outdoor -located water heater in sub-freezing weather, the details of an isolated loop system have been on my mind. Suddenly things get a bit more complex, still, may be worth it.

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