Wood Heat & Hot Water: The flue-mounted water heater

** NOTICE **
At least one aspect of the following post differs from that which we implemented
in the end-design and/or the actual construction of our Tiny House.
For the most accurate and up-to-date information please refer to our eBook.

I have a neat story to tell about the tiny wood stove we purchased to go in our tiny house and the system we have been devising to heat our living space and make hot water at the same time.  It’s a tale that relies on characters from four countries and three separate continents, the internet, and the generosity of someone we’ve never met.  It is rooted in innovation and DIY ingenuity, and it just might produce all of the heat and hot water that we’ll need for free. 

This is part 2 of 2 posts focusing on Wood Heat & Hot Water.Image: Dan

Using your wood stove to make hot water seems like a simple enough idea, right?

Well, amazingly, there’s not much out there about doing just that, and what is available comes mostly from the UK in the form of wood stoves and optional externally mounted boilers (none of which are available for sale or export to the USA), or a couple of universally panned, but domestically produced, internally mounted boiler loops.  The latter are bad news since they destroy the efficiency of the burn in the chamber by robbing it of BTU’s to heat water.  There is one guy here who, fed up with the lack of options, just went ahead and built his own.  But in terms of manufactured options, it seems that only the folks in the UK have it figured out.  It’s just too bad that the only way you can get a hold of anything they produce is to sail over there yourself and pick it up in person.  For those of us without either the inspiration or the means to mount an Atlantic crossing, I found an alternative.

“Axeman Fire Flue.” 

That was the internet handle that caught my eye while searching online, in vain, for information about using a wood stove to produce domestic hot water.  Intrigued, I clicked on the site, and so began the kind of relationship likely only possible since the advent of the internet.  Who knew such a helpful and knowledgeable person could exist in text only?  Since the chance encounter, Tim, the Australian designer of the “Made in Australia” Fire Flue, has been instrumental in helping us achieve our goal of heating water with our wood stove.  Now, before anyone offers the obvious – it’s true that anyone can heat hot water by filling up a metal container and placing on top of a wood stove – what the inventor of the Fire Flue has done, is take that principle a step further.  And what we’ve done, with his expert guidance, is apply his basic principle to a tiny off-grid house on wheels.

Visitors to Tim’s site will see his rudimentary sketch detailing use of his Fire Flue, essentially a flue-mounted hot-water heater, in a conventional domestic hot-water system.  Before continuing, I’ll point out that a flue-mounted boiler like the Fire Flue does not suffer from the same issue as the internally installed versions I mentioned before.  Since the combustion process takes place inside the stove and the Fire Flue is apart from that process, nothing is lost in terms of efficiency.  In other words, if your stove is rated to burn at 89% efficiency, this level of efficiency will be unaffected by an installed Fire Flue and your stove will burn just as hot and put out just as much heat as without a Fire Flue.  In a flue mounted boiler, the energy used to heat the water is stolen from the waste exhaust gas.  This means that the only real effect is to reduce the stack temperature (the temperature measured in the flue) in the same way new high-efficiency furnaces wring the last few BTU’s out of exhaust gases using a secondary heat exchanger.  They do this so well that when you place your hand over the exhaust of one of these furnaces there is no threat of being burned.  The air being emitted is barely warm to the touch.  I suppose that our flue pipe’s ability to draft could be slightly affected, although I hardly think it will be an issue in such a scaled down setup such as ours, with only about twelve feet of flue, and no elbows impeding a straight-shot out of the chimney.

So, back to our installation.  Note – I put together the full color diagram included in this post to to aid in the visualization of our system.  Since we don’t have the room for even a conventional small storage hot-water heater (even the small ones sold in North America are big and round and ugly) and since we will be employing a low pressure water system like those found on board boats and RV’s not hooked up to city mains, Tim from Fire Flue suggested we find someone locally to manufacturer a storage container, in this case rectangular and fabricated from stainless steel, to serve as our hot water storage tank (sort of taking the place of a conventional hot water tank in the drawing on his site).  In order for the Fire Flue to work, it relies upon gravity and convection (hot water rises, cooler water falls) so the hot-water storage tank must be positioned above the elevation of the Fire Flue.  We plan to mount ours on the wall above and slightly next to the stove.  Since it will be stainless steel, it will compliment the natural wood we plan to use on the interior walls and the stainless steel heat shielding that will accompany the stove.

He then suggested that we install a small 110V or 220V electric element to serve as a standby source of energy to make hot water for the occasions we are not using the wood stove or the sun is not out to power our planned homemade solar hot water system (which will be something like this, or maybe this).  We may or may not utilize a thermostat tied to the element, depending instead on a spring timer.  Once we get to know how long it will take to heat a full tank of water we can just crank the timer to, say twenty minutes, wait until it expires, then take our shower(s) or wash dishes.  We’ll see.  And lest anyone thinks we forgot, the design of the hot-water storage tank will provide for at least one over-pressure release valve, just like a conventional tank, to prevent catastrophic blow-out should we over-do it as we’re getting to know the ins-and-outs of our system.

I should have mentioned that unlike a normal hot water tank, which has a completely regulated fuel source (electricity or natural gas or propane) and a thermostat to automatically turn the fuel source on and off as necessary, there will be a little more finesse required to operate our manual wood-fired system.  Simply put, there is no good way to turn on or off a Fire Flue since there is no good way to turn on or off a wood stove.  Luckily, our wood stove has two dampers to adjust the burn inside.  One is the little sliding air adjuster on the front of our stove and the other will be a flue-mounted damper which we’ll install on the top of the Fire Flue.  Tim suggested he provide for a few extra inches of pipe on the exit-end of the Fire Flue to allow us to install our damper there, rather then in the first section of flue pipe downstream of the boiler.  This will be a more sturdy arrangement and will allow us to better meet the intent of the instructions found on Navigator Stove Work’s site to “…install a flue mounted damper 14” above the top surface of the stove.”  Utilizing those two dampers, we will have a great deal of control over the functioning and output of the Fire Flue.  If we need hot water fast, or the water in the tank is really cold, then we’ll open everything up to increase the Fire Flue’s output.  If we don’t, we’ll crank one or both dampers down to reduce the output of the stove and thus the Fire Flue.  In addition, we plan to utilize a second smaller tank, this one made out of plastic and positioned near the floor, or possibly even located under the trailer in an insulated drawer, with a three way valve (to isolate it if we don’t need it) and a drain (to empty it of water if necessary), giving us even more control over the Fire Flue.

Another advantage to our system is that it should work to mitigate what is a known problem in tiny houses, namely that they suffer from low thermal mass.  This means that because there is so little “stuff” comprising the interior of a tiny house, any little thing like opening the door to the outside on a cold day causes the interior temperature to rapidly drop and then makes it tough to heat everything back up again.  Cast iron holds heat very well (just think how long it takes a cast iron skillet to cool down relative to a stainless steel one) and having twenty or more gallons of hot or warm water in un-insulated storage tanks will add decently to the house’s thermal mass.  The tanks should act as radiators so when we crank the stove down at night before going to bed, the water in the tanks will continue to add warmth to our interior, as they naturally cool down.  At least that’s the idea.

Read our previous post, which discusses the issue of back draft and indoor air quality, and stay tuned for our installation notes and ultimately for our actual comments on the use of this system.

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