Wood fired, passive, gravity-fed hot water system for tiny house

This could get pretty in depth!.. I am building a tiny house on wheels in the country about 8'x21' with a loft space. I recently just purchased a small wood stove with a back boiler for heating water. The stove is The Hobbit from Salamander Stoves. It measures about 18"x12"x11" so it is definitely a little guy. I am hoping to use the stove off-grid through thermosiphoning. I have two ideas so far.. one : to have around a 30 gallon hot water heater in the loft space which will cycle through the stove and back up to water heater via convection and gravity feed to the shower and sink almost directly below the tank..problem with this set up is I am unsure how to heat the water in the summer.. maybe I could have a separate outdoor system with connections to indoor tank in summer?. My other idea was to make a "bread box" batch heater which would be situated in the rafters of the house. This would use solar as well as the wood stove. It could be a built as an insulated box with triple paned glass exposing the storage tank (painted flat back) to solar radiation for simple heating in the summer..however I am worried this might be susceptible to freezing since I live in Idaho.

. Both of these systems give me a few questions.. how to supply them with water? How to mix the heated water with cold..if so where would the best position for the cold tankbe ? Is there enough pressure to run through sink faucets without a pump?

Any thoughts or suggestions on this kind of set up would be very appreciated..and I hope this isn't too complex of a subject to try and discuss without visual aids. However, I can draw a picture of my thoughts if this may help. Thank you!!

Hi Westin,

Ben Falk has a wood-stove heating setup in his house, you can see a diagram at the 42 minute mark in this video:

Of your two systems I would be inclined towards the first, which is much like Ben's system. A separate system for summer, like a solar thermal panel or a batch box, that heats the same tank, would be simple in most ways. The only fiddly part about that is location, since if you want to rely on thermosiphoning to circulate the heating water, the heating location can't be at the top of the loop.

While your bread-box heater would eliminate that issue, you'd then have that freezing concern. Maybe someone from a similar climate can tell you if that is well-founded, but since I don't know I'm going to assume it is. I suppose you could close it off with external insulation in winter, but I prefer the idea of heating sources separate from the tank, which can then individually drained and put offline as required.

As to your specific questions:
-You need a thermostatic mixing valve to mix the hot and cold water. The only way around this is to heat the water to a comfortable temperature, then shut off the heating, and use the water directly. I don't think this is a good plan, myself.
-If you want to get away with only gravity feed, you'd need the cold tank to also be up high. If it is above the hot tank, it can refill the hot tank via gravity, and you only need to fill the cold tank manually.
-I do not know how much pressure you need for the thermostatic mixing valve to work, something to check.
-For pressure, one foot of height gets you about .43 PSI. Standard household pressure is apparently 50-70ish, so you will have extremely low pressure by house standards. It should run out of a tap anyhow; for a showerhead you'd probably need to look for something like one that is meant for a camping shower.
-To refill those tanks, you could rely on the water pressure of a hose, and put a filling port at a usable location on the outside; you'd need to make sure you have some way to prevent overflow, ie a shutoff at the tank where you can monitor it while filling, or an overflow line... I would be keen on the option for manual filling just in case, but that would definitely be suboptimal.

The alternative to all that gravity feed fiddling would be to run a pump and an accumulator to provide pressurized cold water, which would in turn pressurize the hot water, and give you some semblance of normal water pressure. To do so with minimal power demands, you can look at RV/marine pumps and accumulator tanks meant for this purpose. There's a diagram of this for an RV: http://www.expeditionportal.com/forum/threads/138258-Water-System

The catch there is this assumes a single pass through a heat-exchanger(back boiler) is enough to heat the water, which probably isn't true, so you'd need to add a hot tank to the diagram.

Thanks Dillon for all the useful information! I really want to read Ben Falk's book now! I too was leaning towards the first system idea. I think it would be best to keep them separate and drain the summer system for the winter. I like the idea of the cold water up high and gravity feeding the storage tank I should consider this further. It would be convenient to use mains water to fill the tank but not have to rely upon it entirely. I will keep researching this system for ideas and hopefully this will all come together. Thanks again!

Do you plan to ever move it? Or locate in the shade? The issue with solar water and houses on wheels is you always have to park a certain direction. Solar electric is light and rugged enough to make portable, solar water is a lot more cumbersome.

Small pumps that run on solar or battery are pretty cheap, or you can use a hand pump to a small pressure tank if it is a backup to mains.

There are a few mixing valves made to run on low water pressure, Dale and I talked about them in a solar water heater thread a while ago. He looked up the brand and model, I can't remember it (lucky I even remember talking about it).

If you do pressurize the system, make sure to use good relief valves to prevent boom squish. One close to the hot side of the water jacket and one on the top of the tank minimum.

Thanks R Scott, what kind of hand pump would you suggest? Here is a pretty interesting video of a set-up a guy uses in his tiny house. He uses a bilge pump but I am not sure exactly how it is utilized. I have tried to contact him with not much luck. https://m.youtube.com/watch?v=T3_Ro9vk-QY

I am beginning to think the best option is getting a solar pump to relay that water up to the storage tank.. looks like I have some more research to do! The woodstove water heater seems like such a simple set up but trying to incorporate it in such a small space has got me riddled! The house will be parked East/ West.. I should have excellent solar access to the south. I will have it positioned on a plateau where I have access to the sun all day except in the late afternoon I start to get shade from a nearby tree. (As seen in photo below)

Well, prices have changed a LOT since I looked a few years ago. Solar is definitely cheaper than manual now for pressure.

For simple transfer, a hand bilge pump like for a sailboat will work. Probably the smallest one you can find with enough lift head. Under $100, can fit a tiny house. Cheapest manual pressure pump I could find was $300 and that one isn't great. Doesn't fit in small spaces either. Lehmans has a couple to look at.

Any rv pump will work for solar/battery, you just need to choose the one for the flow rate you want and get good ratings for durability and efficiency.

Woodstoves in tiny houses are a huge challenge to begin with, adding water is a definite complication! I have a water jacket in my big house and it can be a pain to work on as the plumbing gets cramped really fast. And most of it needs to be copper, you can't use pex at those temperatures and pressure. Unpressurized definitely simplifies the hot water side.

Thanks for the suggestions! I will make sure to share with everyone what I discover along the way! In the meantime, I thought I would share a few useful links I have discovered for others interested in these hot water systems. Feel free to share your resources too!

http://sustainablepreparedness.com/index.php/blog/hot-water-for-free-from-the-wood-cook-stove

https://www.survivalmonkey.com/threads/domestic-hot-water-from-a-woodstove.32038/

http://www.green-trust.org/2003/fireandwater.htm

Great thread. Here are some more useful links to boilers that are made in America. http://www.royallboiler.com/
http://www.outdoorwoodfurnaceboiler.com/
http://wood-furnaces.net/indoor-boilers.aspx

Also, here is the new EPA ruling on boilers updated on 2-4-15:
http://www.wood-heating-solutions.com/2015-epa-law/

Thanks for sharing the information recently released from the EPA. The Hobbit Stove I purchased (yet to receive) is now available in North America the only version they ship here is the Hobbit SE which is specific to "smoke control zones" so I would imagine this stove falls under the category of new, efficient stoves. It is suppose to burn clean and can use coal,pellets, and wood. I have heard there is a problem with creosote build up in boiler systems..particularly with ones that involve a coil around the flue. I don't think this one will accumulate too much junk but if it does it should be an easy clean due to it size. I also spoke with Salamander Stoves (The Hobbit) manufacturer today and they told me the back boiler may be removed, the holes blanked-off and rear brick placed back in. I was worried that it might be a permanent accessory (must pay extra for back boiler) and I would always have to use the stove with water flowing through..but this is not the case, fortunately..(Do not burn back boilers without adequate amount of water in tank and lines)! I also mentioned I planned on using the stove with a 20-30 gallon tank. He told me he suggested using it with a tank half that size to get the water REALLY piping hot. I think I will still go with a tank this size so the water may circulate for a longer time at a cooler temperature..(any thoughts on this?) Anyways, I thought I would share some of this information with you all as I found it useful. You may read more on the Hobbit here: http://salamanderstoves.com/the-hobbit-stove/
I hope this stove will work out for my ogg-frid.. lol*off-grid (had to leave that typo)..tiny house. I will give a thorough review of it in the future. Hope this helps!

Hi Westin, how is your wood stove hot water system working? I'm looking to build this next summer and would be really interested in the details of your setup. I live in Portland Oregon so was thinking solar power for the summer and wood stove for a nice winter combination. If you have time I'd love to hear how you worked everything out?
Best,
Teresa

Hi Teresa! Unfortunately, I haven't got the system dialed in nor do I have my woodstove installed. I think I will end up removing the back boiler and blanking off the holes in the stove and just use it as normal stove until I can get the plumbing feature figured out. I am pretty sure I will speak with Fred from fredstinyhouses.com.au for help designing the system. He offers consultation at a price of 50 an hour, I believe. If you look at his youtube video you will see his has a pretty killer system in his tiny house! I'd strongly suggest you check it out! Also, if you care to see the progress Ihave made on my tiny house you can check me out on instagram under westintromburg I will certainly be posting pictures and talking about the installation process of the stove and water system on there in the future. Also, if you get any good leads on devising A system such as this please me know as well! As I will certainly do for you ! Good luck!

Ughh, well now I'm going to have to sign back up for instagram! 😄 I'm taking a course right now in designing my tiny house from Lina Menard of Niche consulting, anything I figure out for the water system I'll definitely send on to you. Thank you for the link to Fred's sight.
Best,
T

Hello everyone (if anyone is out there haha) I don't know if this forum is very active anymore but I think I have decided to use a pump to relay the water into the hot water storage tank. I did a preliminary drawing to see if anyone had any thoughts, tips, or suggestions for this particular set up or sees any potential flaws. Keep in mind I am not a plumber. I will have help from someone who is more experienced in the matter. I have done quite a bit of research and am aware of the particular sizes and materials needed for this set-up. I am more curious about whether or not the pump will have enough pressure to bring the water into the hot water tank. Which will be located fairly close to the woodstove and about 4 feet off the ground.. the wood stove will be about 12-18 inches from the ground. I am also curious about bypasses, pumps, TMV valves or any other features which could help make this more of an efficient system.

Notes on the drawing: This is an overhead view of how the system will run through the tiny house. The top of the drawing runs along the right side of the house with most of the lines running through the cabinetry. It then run along the front of the house (still in cabinetry) before entering the shower room (at the left of the house)where I was hoping the lines would rise up to the ceiling and while running through the room and exit through the wall. It would then enter the storage tank located in a closet near the woodstove. I hope this helps. I would love to hear your thoughts! If you have any questions please let me know! Thank you all for your time .

Hello tiny house wannabes! Westin posted a simple question which requires a complex answer.

Hope these may be useful: http://www.resilience.org/stories/2016-03-18/ultimate-tiny-house-design
which describes two gravity feed thermosiphoning systems, one system which uses separate tanks (one for hot water to the taps and the other for thermal mass heating) and the other system shares one common hot water tank.

the above is a follow up article to
http://www.resilience.org/stories/2016-02-16/plumbing-the-tiny-house
which describes the manner by which water is pumped from the rainwater tank to the header tank and thereafter distributed to the taps and hot water cylinder.

the reason the blueprints don't match between the two articles is because this is a design still in evolution.

Does anyone know of someone who can help build a Masonry Heater with a bake oven and cook stove or also a Rocket Mass Water Heater or some type of heating system that I can cook with that is very efficient and doesn't produce creosote or smoke and something that can also be used with wood burning for water heating for a small house on the Western side of Virginia or the Eastern side of West Virginia? Willing to pay someone and help out with the work just need someone who knows what they are doing to guide me through it or can give free living arrangements in exchange for help, etc. Thanks. Contact Tara: 540-474-3146 or mustobeyacts2_38@yahoo.com

I don't think I would pressurize the hot tank. You can pressurize the hot lines with a 12v rv pump outside of the tank easily enough without having to risk the epic explosion that could follow a relief valve's failure.

So far the system will have a 50 gallon storage tank with an RV pump and pressure tank. This will supply the cold water lines as well as fill the 30 gallon hot water tank in the closet above and next to the wood stove (which it is plumbed to). The hot water tank will not be pressurized as it will have a Temperature pressure relief valve in the event it gets too hot. However, it will pressurize the hot water system by convection. There will also be a Pressure (not temp) relief valve and both the cold in and hot out pipes entering and exiting the stove. I hope this helps!

thought about this set up and decided a small thermosyphon tank for use when you do not want the heat inside of your thow.  just keep it stowed and empty when not in use.. only fill when you need it.  that way it never have a legionares problem and no over heating in your thow.. though you could also use it as back up heat in the winter time