Wood-Stove Hot-Water system

Photos have been added o the pdf document

Photos have been added o the pdf document

Well done Mountain! I believe you are correct that this can be called a solar application.

Good benefit for minimum investment which is what everyone is looking for.

Waste heat that you have already worked for (or paid for) is captured and put to use rather than going up the chimney.

Between this and Mountain's solar thermal air heater he is saving a lot of wood cutting and splitting!

Russ

One comment about scavenging waste flue gas heat -- beware of flue gas condensation. Burning wood produces large amounts of moisture (hydrocarbons + O2 => CO2 + H2O ...) and you don't want to cool the flue gas below the dew point IN the flue gas stream. That's why super-efficient heaters and the like require double-walled flue pipes -- to prevent additional cooling and condensation.

Good point & well put.
I knew I didn't want to over-cool the flue, and thank-you for articulating exactly why.

There is a temperature sensor immediately adjacent to the flue exterior. Energy is only extracted when the flue is HOT. As soon as the fire burns down a bit, the circ-pump shuts off. Also, there is no point in running the pump if the temperature rise (supply vs return) is insignificant. Energy harvesting only happens at higher flue temperatures, when condensation is very unlikely.
The control points (pump-on & pump-off) were set empirically. If I notice any hint of condensation I will raise the set-point values.
M

Wow great idea whom ever came up with it!!
I'll have to suggest something to my friend who has a cabin and heats with a wood stove, it sure would save a few hundred over the winter months in electricity!

Great find.

Mountain has a good imagination and the skills/abilities to support it.

Over the past couple of years that I have been following him he has carried out several neat projects plus his full time job.

Well done once again Mountain!

Russ

whoah! this man is amazing.. I have never met a man whose able to make such technical and complicated problem easy to understand.

Fantastic

I just love this idea Mountain. Brilliant. I have been looking into installing a wrap around wood burner (wood burner with 3 water sides) to heat our DHW during the winter months to utilise the wasted heat from the fire and to augment the reduced water heating capacity of our solar hot water heating set up. This would have meant that I would be paying out 1000 - 1500€ for the wood burner alone when I have a perfectly functioning wood burner already installed (but without the water sides of course). Your flue pipe heat exchanger system would save me a lot of money and still deliver me the additional hot water.

The hot water storage tank (300litres) has a secondary heat exchanger coil already installed. Would it be feasible for me to link the flue pipe set up into that heat coil? If so, what safety features do you think I would need. In France the systems are "closed" or "pressurised" systems. Thanks in advance for any thoughts or guidance. Good luck. Paul.

I have been thinking about a redneck engineered take on the wood stove water heating. My stove is in what could be called a sun room off the middle of my single wide mobile home. I was pondering making a copper coil around the flue, run it through heater hose to a scavenged auto heater core in a corner of my living room. Build a little box around it with a PC cooling fan to move air through the core and a tiny pump to circulate the water. One power switch to turn them both on when there is a fire in the stove.

Do some calculations first.
To get much heat from the flue, you will have to reduce the temperature of the flue gas significantly, and that will affect both draft and creosote buildup even before you get the temperature low enough to worry about condensation. A characteristic of all condensing furnaces (or even a high efficiency one) is that the combustion air is driven by a fan rather than relying on convection.

If you can get a significant amount of heat from a copper coil and still keep the flue temperature high enough, you might be able to get an equal efficiency increase just by adjusting the draft in the stove.

It is a way to get the heat from the stove from one place to another in the house, but it would be easier to get the heat from the stove body than from the flue and to move air instead of a liquid. And finally, this cannot be a closed loop water system without very reliable overpressure and over temperature protection, similar to what you see in a hot water heater.

Within those constraints, engineer away! And also look at building a solar thermal panel system to provide some of the heat instead of working at getting more from the wood stove. The payback on solar thermal is very fast and unlike solar PV, solar thermal is very suitable for DIY once you understand how it works.

PS: I once rented a granny cottage (one room plus bath) whose heat was a radiator and fan connected to the hot water heater. Very much somebody else's DIY job, but it worked.

Excellent advice, thank you. My problem is I have a resistance furnace and the furnace is not in the main part of the home so I have been thinking of ways to move the heat from the stove from the sun/stove room inside. Thought an inexpensive (cheap) water circulator would be an efficient way of doing it. I was concerned about the heat building up pressure and also it has been tough to find something in the range of 1-2 gpm inline pumps.

I agree about the thermal panels. My first piddle around projects will probably be a solar batch water heater to preheat what goes to my domestic water heater and a solar air panel. Water is great location since heater is on south end of trailer. That being said, the long sides face E/W so getting good sun on an air panel will be a challenge. Thought about modifying the usual design and making it side intake/exhaust but haven't seen anyone mention a cobbled up system like that.

What about another crazy idea. Per your idea using the stove body as the hot air source, attach a rectangular metal air box to the stove and run dryer vent (metal within about 6 ft of the furnace and flexible after that to move the warm air to another location? Slap a boxer fan in the line somewhere. Yeah, that's the kind of ideas I get.

That could actually work. A little awkward running the flex duct through the house, and note that there is more air resistance in the accordion flex than in smooth duct, but worth some experiments!
Just make sure you do not enclose too much of the stove and cause local overheating if the fan is not running. Start small.

Grundfos alpha

More good advice, thanks.

LOL @ Rich, did you miss my many times saying I am cheap as can be?