BK's Compost Water Heater

Discussion in 'Designing, building, making and powering your life' started by Brian Knight, Jan 26, 2015.

  1. Brian Knight

    Brian Knight Junior Member

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    nspired by the warmth of rotting leaves, groundwork of Jean Pain and others, allow me to present my compost water heater. To me, this strategy embodies the idea of permaculture. Heating water represents a huge portion of most people's energy and environmental footprint. Many of us also have a need for compost in these volumes. Why not use it to cut our energy costs and the resulting hidden costs of heating our water.

    One of the many nice things about this strategy is that you typically dont need a new water heater. Compost water heating in this example is really more of a pre-heater. Water coming out of the ground is typically around 40-55F. Then a water heater, usually powered by burning fossil fuels obtained through mining or fracking brings the temperature all the way up to 110-125F. Thats a big jump, requiring enormous amounts of BTUs or energy.

    Of course you dont need a "backup" water heater and can just use water raw from the compost pile. In the summer, I will probably attempt this as I dont take hot showers when its hot out. Most of the time though, I treat myself to indulgent showers. Very long, hot and wasteful. Its my important "hot shower yoga" session and I dont plan on adjusting my lifestyle anytime soon.

    Four months into building my first pile, Iam still getting 68F water from the pile. In power outages (on city or gravity supply) or more "roughing it" situations, this water is going to feel pretty good compared to more available sources. Iam hoping its stays in this range for a full 6 months, when I will change out the pile. I got 100 degree water the first month, 90 degree the second and 75 the third so we seem to be leveling out a bit on month #4, but who knows maybe it drops below 60 soon, still a big improvement to the 42 I just measured coming out of the ground.

    I may be creating a compost water heating website, more detailed blogs and a Youtube video on the tear down and re-build as I lost the first version with my gopro to the bottom of the Gauley river. Woops, leashes are important after all.. Anyways check back for that and I will jump into the construction and some details with the pictures..

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    Most sites and situations are not as good as mine. I compost in my backyard that is downhill from my drive/delivery spot, behind my walkout basement which contains the existing water heater. The garden and orchard is also downhill from there. This project was delayed forever with my CMU basement exterior insulation project and finish grading improvements. Sorry neighbors of Montford!

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    Here's the finished pile. The diameter is 6' using 6' tall fencing, and its probably closer to 4.5'-5' tall. I put the it against the foundation wall opposite the water heater in the basement, the closer the better. This also should keep tubes safer from freezing from low maintenance or eliminates underground tubing which would need insulation.

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    The foundation is some scrap roofing underlayment on top of 2" XPS, which is also what I used in two layers to wrap my CMU basement prior to finish stucco. The giant, silver-leaf maple has been a tenacious predator of my compost piles despite the plastic I always put on the ground to protect it. Anywhere the pile spills over the top of the plastic, the maple roots quickly invade.

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    I attached the fence to the ~1" thick stucco with AL termination bar and tapcon fasteners. Because of the slope away from the house, there is lots of weight tugging on this attachment but I was still careful with stomping the pile, trying to force out the sides instead of the downhill opening for a better circle and less weight pulling at the wall attachment. So far so good..

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    Compost against a home shouldnt be taken lightly. Rodents love the warmth + earthworms and would gladly follow the tubing into the home if you let them. Behind the blue XPS foam is another layer of expanded metal lath, well detailed against the stucco and tubing to discourage tunneling critters. Dont let these penetrations leak air or water either. Seal all penetrations up as tight as possible. Ignore the white pex tube in the middle, I used it to house a temperature probe which is completely unnecessary. Its best to measure the actual water temperature.

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    Since the bottom of compost piles turn to cold soup pretty quick, I built the pile up about 18"-2' before beginning to bury the tubing. This photo also shows how much coffee I was adding to my carbon source of neighborhood leaves. Iam not a fan of turning compost and with this type of compost water heater, forget about it. I prefer to pile in about 2-3' of leaves, wet them down, stomp them to around 4-8" and pour on the coffee grounds. Tease in the grounds by hand trying to eliminate clumps and distribute the grounds as best as possible in that layer. Slightly wet it again and repeat. Lots of potential material variation here and Iam beginning to think C:N ratios barely matter. Iam getting hot piles with leaves only and very hot piles with coffee only. I think what matters most is compaction and moisture.

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    I thought I could lay the coil without using joints/fittings outside whatsoever. I quickly realized cutting it would work best. Of many nice things about PEX, couplings are cheap and easy. This pic also shows my "hanging spool" technique. Lucky for Brian, that pesky silver leaf maple had a tree branch right above the pile. From my roof, I rigged a pully, rope and piece of plywood turning a tough part of the job into sweet cake. 500' of 3/4" pex was my selection for the tubing and its a pretty heavy deal. Hanging the spool allowed plenty of headroom for working the layers and pulling out slack I needed between layers raising the spool up as the pile grew in height. If I didnt have a tree branch, I would probably rig up a frame or scaffolding considering the setup was so useful to the operation.

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    Between each described layer, I would pull out 4-6 loops off the coil and create this flower shape, first securing with sod staples and later using old straw twine to tie stubborn, tighter inner layers of the spool to the fence. I tried to keep the tube about 8-12" away from the exterior of the pile.
     
  2. Brian Knight

    Brian Knight Junior Member

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    [​IMG]

    Here we are on the other side of the wall. I added an expansion tank, as I kind of think my plumbing system needed more capacity anyway. The hose bibb at the bottom is crucial for when its time to re-spool and build a new pile. As heavy as the spool gets dry, its actually much heavier with water in it! Who would have thought.. Also notice there are no 90 degree fittings which I avoided to decrease any pressure losses. Those metal "bends" are very handy for tightening up the pex turn without adding a fitting.

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    The tank is about 6' from the exterior wall and here the tubing travels overhead. Ball valves are important to avoid in-line pressure losses and these are used to isolate the compost water heater side for maintenance. The hose bibb at the top is needed to allow air in for proper draining at the bottom but also for measuring the water temperature on the way to the backup water heater.

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    Here is the topside of my mtn-top removal mined coal combustion powered water heater, pre-compost. The gray polybutyl pipe is from the ground as it supplies the dip tube or bottom of the water heater. I dialed back the lower heating element all the way, ensuring savings yet have not noticed any decrease in quantity or temperature of hot water. Notice the typical 90 degree fitting.

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    Here she is after. The water takes a detour through a tee and returns with another. The ball valve between the two is in the down/stopped position. When its time for maintenance, its opened up while the other two previous are closed. By changing that one fitting to a "metal bend" sweep, I was able to keep total 90 degree turns (the tees) to only 1 extra. While I can notice the pressure difference filling the tub, there is no noticeable difference at sinks or shower. I plan on adding pipe insulation to the returning "warm" leg from the compost pile.

    So I got some pre-compost measurements of my water heater energy usage, roughly 6 months from Feb to Sept. While the measured average was around 131 KWH per month, I think that's too low for a year round average as it included all of the summer but not all the winter. I would expect my real average to be closer to 160 if I had included the other months where I typically see much higher usage.

    Feb: 160 KWhrs
    Mar: 151
    April: 190
    May: 164
    June: 107
    July: 74
    August: 89
    Sept: 113

    These measurements, pre-compost were "summer weighted" for an average of 131. Again, guessing 160 is more accurate for year round use.

    Here's what Ive measured for KWH so far after compost water pre-heater installation:
    Oct/(sept): 65
    Nov: 126
    Dec: 109
    Jan/(Dec) 108

    This averages to 101 KWH per month (measured at middle of month) so the compost water heater appears to have lowered my mtn-top removal mined coal combustion costs by ~25% for the unweighted average or ~40% for a weighted more likely average. This will certainly drop the next two months but should be made up for in the summer when I plan on turning the water heater off and bypassing it. Iam scared I might have to build a 3rd pile for the year to preserve the lifestyle thing but Iam also thinking of attempting the next pile as leaves-only to even out and extend the warm temperatures.

    I plan on installing a low-flow shower fixture, which is definitely the cheapest way to savings in the hot water arena. I like my current rain-head fixture but apparently some of the newer designs do a pretty good job with lower flows. This is lower hanging fruit and would love to get opinions on high performance models.

    Iam also planning a DIY version of a waste heat recovery device, wrapping my existing cast iron plumbing stack with pex to recapture the heat. Commercial versions seem to achieve the same efficiencies Ive measured with this compost heater.

    I often run a dehumidifier in my basement so replacing my current water heater with a heat pump water heater tends to make a lot of sense as they also help dehumidify the air. Heat pump water heaters are one of the most efficient ways to heat water and should pair nicely with the pre-heated water from the compost pile.

    Out of time but will return with costs and questions..
     
    Maia sue likes this.
  3. 9anda1f

    9anda1f Administrator Staff Member

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    Excellent write-up and description of your system BK!

    Question: Do you turn your pile periodically?
     
  4. Brian Knight

    Brian Knight Junior Member

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    Thank you! Ive been building my piles this way for years and never been much of a turner. At most, I will unpile it completely into a new pile and rarely do that although the re-turned 1x pile seems to be a better product for compost tea.

    I take the extra time upfront in trying to thoroughly mix the Carbon (leaves) with the Nitrogen (coffee grounds) at the time of building the pile. Occasionally when harvesting the compost I come across pockets of dry leaves. The better I wet it out during construction, the less dry, un-composted pockets of leaves I find. As mentioned, Iam pretty confident that a leaf-only pile, well moistened and compacted, will produce plenty of measurable benefits in tempering ground temperature water before it goes into the backup/main water heater.

    One of my questions/goals was how big of pile does one need to do this? Most of the other versions Ive seen out there seem to be much bigger. Iam sure there is a balance between size of pile and time of usable heat before needing to rebuild. Who knows, maybe one could get very close to the same output of a 4' pile as a 10' pile. Iam not sure of the relationships between C:N ratios, size of pile and time of usable heat. It will also be interesting to explore other compost materials and tubing length/diameter storage capacity too.

    There isnt much of a drop in temp as you go through a long shower. Iam thinking 300' may be fine or who knows.. maybe 100' isnt all that much different.. I chose 500' because the ~8 gallons seemed like a good amount of storage and heat transfer area.

    Unlike many solar water systems, compost water heating seems pretty affordable. Tubing + Plumbing parts are all that you need.
    Here's what I paid in US dollars:
    500' 3/4" PEX: $180 Chosen for its compatibility, reliability, affordability and labor of associated plumbing parts.
    2G Expansion Tank: 40 Technically needed one anyway and its helping reduce stress on entire home's plumbing system.
    Fittings and valves: 80
    Total: $300
     
  5. 9anda1f

    9anda1f Administrator Staff Member

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    One of my favorite tutorials on C:N is from a (semi) local university: https://whatcom.wsu.edu/ag/compost/fundamentals/needs_carbon_nitrogen.htm

    Cornell also provides a composting with this rather esoteric chart of materials and their C:N ratios (good for a grin if nothing else!): https://compost.css.cornell.edu/OnFarmHandbook/apa.taba1.html
    The Cornell handbook is very detailed reading for composting specifics.

    There are few good ways to provide heat ... most entail burning of stuff. Any progress we can collectively make towards capturing solar heat is highly worthwhile. Good luck on your experiments! Keep us updated on your progress.
     
  6. Brian Knight

    Brian Knight Junior Member

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    Absolutely agree with implementing passive solar design when possible. I also think that too many people rely on burning stuff for heat. Even when burning stuff efficiently, its a crude form of heat and one that introduces serious risk of fire and air quality concerns. Indoor biomass (wood) combustion is one of the most overlooked form of indoor air pollution.

    Ive always been a fan of solar thermal, the familiarity and experience with which have led to my comfort of building a home plumbing integrated compost water heater. Still solar thermal, especially in regions with freezing temps, is generally expensive, complicated and trouble prone.

    Great compost links. Dont think Ive seen the WSU ones before. Of particular interest is this typical compost graph:

    [​IMG]

    Some might be fine with rebuilding every month or two for the highest temps but I would rather get 4-6 months out of a compost water heater pile if possible even if it means lower temperatures. Would love to see this graph with a further timeline if anyone knows of any. Or a graph with higher Carbon ratios.

    I would rather have the 60:1 scenario where the temps dont get as high and last longer. Ideally, I think an 80-90F temp may be a good target for longest lasting usable heat, at least with my tubing heat exchange capacity. Depending on the material, high temps arent as important for killing off the nasties of concern. 80:1 might be the ideal ratio for a long lasting warm pile and noticed that "straw-general", a widely available resource, according to Cornell is in this range.

    I noticed Cornell and WSU give similar ratios for the C:N ratios of various leaves. However, WSU gives "oak leaves" a value of 26:1. Most of my leaves are indeed oak and am skeptical that oak leaves have a similar ratio to coffee grounds or other high nitrogen sources but maybe that explains why my leaf-only pile is measuring 100F. Anyone have insight here? Are oak leaves really that similar in Nitrogen content to coffee grounds or manure?
     
  7. sweetpea

    sweetpea Junior Member

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    You like to shower in 68 Farenheit? 30 degrees below body temp? Eeeeeeeeeeeeek! :)
     
  8. Brian Knight

    Brian Knight Junior Member

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    Not this time of year! The compost water heater is really more of a pre-heater but depending on the pile variables, its capable of doing most of the work. While hot piles do more work at first, they drop off quickly. I think less hot piles offer the most potential to those who dont want to re-build every two months but still very early in figuring out the details.

    What I refer to as "Mtn-top removal mined coal combustion" water heater is a typical tank style electric. Its a Rheem marathon model which is one of the most efficient of this style but my favorite part is the plastic tank has no anode rod to replace. Anode rod replacement maintenance, or lack of it is the main reason water heater tanks fail.

    Heat pump water heaters are where its at though. They are the most efficient "conventional" water heater available IMO and from a cost-effective standpoint, often a better value than solar thermal if PV is in the mix. I plan on replacing my perfectly good marathon someday for a heat pump water heater, partly as an attempt to get off of my extra basement de-humidification needs.

    Most electric tank styles have two elements that have power running to them. I turned the lower one all the way down, which almost guarantees energy savings but at the risk of having less hot water. As mentioned, that 68 degrees still seems to be enough for the less energized lower element and fully energized upper element to keep up with my demand and needs.

    My upper element is set to give me water out of the tank/tap at 110F. Most of my showers are around 20 minutes long and the last 10-15 minutes are ALL hot water with the cold turned completely off. Thats ridiculously indulgent, the guilt of which has lead me down this path.

    Most of Asheville's electricity is supplied by a coal-fired power plant and most of the coal comes from the mountain top removal mines of our neighbors to the west. Our energy provider Duke Power has also had some devastating coal ash spills lately and they are being buttheads about improving these waste dumps. The society at large is also being buttheads, like taking 20 minute hot showers! Of course the mtn top mines and coal spills only scratch the surface of what all this coal burning is doing to the environment.

    68 may feel cool, but its going to be heaven compared to the 42 I measured coming out of the ground right now. Mainly, it represents a huge boost in temperature that my coal burning costs dont have to account for to deliver the hot water needs at my usual 110F.
     
  9. Brian Knight

    Brian Knight Junior Member

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    Here's the illustration Ive been working on to better explain the gist of the system/strategy. Is it easy to understand? Looking for help on thoughts for improvement..



    [​IMG][/URL][/IMG]
     
  10. sweetpea

    sweetpea Junior Member

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    Brian, oh, I see, you've got it heating a room and preheating the tank. How are you fixed for solar exposure? I've stripped an old water heater, painted it flat black, installed it in an insulated box, covered it with an old shower door and it gets water up to 140 on a sunny day, which is mostly spring, summer, fall for us, but in our drought it's been in the winter as well. Have you seen these videos? They are calling them batch heaters.

    https://youtu.be/VZxBQRyfKzo

    https://youtu.be/F6BvsxFPN2o

    Kyle also has a 4-month update.
     
  11. Brian Knight

    Brian Knight Junior Member

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    Iam not heating space with compost but will be looking for that opportunity in the future. This would also be a good fit for greenhouses, especially below the planting beds for warmer roots.

    Ive got pretty good solar access except for the big maple tree, but could work around it. I also have some used solar thermal panels that I got for free from an abandoned system in our neighborhood about 7 years ago. There are many abandoned solar thermal systems in Western North Carolina and I think it hints to the problem of solar thermal in general. They involve alot of plumbing, parts and labor and inexperienced installations can lead to lots of trouble. Martin Holladay's infamous Solar thermal is dead article is an interesting read for those exploring solar thermal. Dont necessarily agree with all of his conclusions but he makes a valid argument and do agree with most of it.

    Batch heaters tend to be the most affordable and DIY friendly, but they arent the best fit for climates with freezing weather. They work well as pre-heaters, like compost, but the much higher temperatures can often present problems. It corrodes metal plumbing parts quicker and usually warrants copper as the hot leg to the water heater, which can be a big cost for both materials and labor. Sometimes the high temps might require heat dumps or tempering valves.

    Compost heating can work in all climates, regardless of sun, shade and orientation. I think a compost pile is much more attractive than a batch heater and of course solar water heating doesnt give you compost for the garden.

    Iam still a fan of solar thermal and will probably do a system some day on my own future new home. Iam tentavly planning on using one of my used panels to heat the slab of my carport/addition in the same way I have the compost space heating illustration, to thermosiphon without pumps and controllers. I think there is lots of opportunity for both solar thermal and compost out there. One could probably get great performance by using a batch heater most of the year and then switching to compost for the winter months.
     
  12. Brian Knight

    Brian Knight Junior Member

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    Just put up this compost water heating post over at our green building blog collective. I cleaned up this version with updates. Interested readers can keep questions and comments here.
     
  13. 9anda1f

    9anda1f Administrator Staff Member

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    Excellent article on your work Brian! I especially like the radiant floor information. Do you see any possibility of perhaps installing such a system in one of your commercial projects?
     
  14. Brian Knight

    Brian Knight Junior Member

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    Thank you Bill! It's been an extremely rewarding project. I really don't see installing them in any of our houses unfortunately. If a client seems committed for the long term maintenance involved then absolutely. It's a pretty small investment in time and materials with the right site, so even if they didn't maintain it, no big deal. The tubing is the main investment and it's easily used for other things.

    We have installed solar thermal collectors on several homes and while they require less maintenance, I fear they may be abandoned from lack of care. It's a bigger problem when the system is so much more expensive and the "backup" water heater is designed to have solar input. Some owners will be great at maintaining it and others will not. One system we installed had a major malfunction only 2 years in when the water heater developed a leak. Luckily, it was still under warranty but was a major inconvenience to repair and labor was not covered. Solar thermal water is often too damn hot, which leads to a lot of it's problems.

    I think you're asking about the radiant floor application of compost water heating and do have a decent opportunity to try it on my future renovation. I have a carport with a slab and the plan is to enclose it, pour a new slab on top of insulation and radiant tubing. I had been planning on hooking the tubing to a thermosiphoning, near-vertical mounted solar thermal collector but using compost instead will allow me to eliminate heat exchangers and most copper parts.

    It will take a new project to accurately measure the impact of compost water heating for radiant floors. Hope to try it someday and I think it's possible to build a home to be heated in our climate entirely on passive solar and compost. It's summer here now and the all-coffee pile is sending 100F (38C) water to my water heater. The pile was built 9 months ago! (built late fall, didn't heat up until spring) If I can get the timing and insulation right, one well built pile could possibly handle the heating loads of a small, ultra efficient home.

    Used coffee grounds seems to be the ultimate compost water heater ingredient and coffee shops are sending tons to the landfill every day.
     

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