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Community and Q&A

Compost for Water Heating

boxfactory | Posted in General Questions on

I’m wondering if anyone here has tried, or makes regular use of compost to heat their home, or water.

https://m.youtube.com/watch?v=zbArnw2Tfu0

I have read many discussions about how a wood burning fireplace is not the best choice for a small home that is air sealed, and well insulated. However, I would still like to have some independence from the grid in the event that the power goes out in winter, and this seems like a reasonable alternative to stacking firewood in the warmer months.

It occurs that the compost pile could heat –

– pex that then is connected  to a radiant floor
– pex that is assists a heat pump hot water heater
– pvc that is connected to the ERV / HRV so as to act as a seasonal earth tube.  Admittedly this is an idea that I recently began considering, so please don’t judge me too harshly. I have read about air quality issues with permanent earth tubes, but being seasonal, it occurs that the setup could be disinfected at the end of the winter.

All this being said, I thought I’d see if anyone out there builds large piles of wood chips regularly for their home, and if so, what their experience has been like.

Thanks!
Hope all is well,
Ben

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Replies

  1. Expert Member
    NICK KEENAN | | #1

    I won't say it's impossible, but I see two problems: rate and temperature.

    To a first approximation, composting is very slow burning. So to produce enough heat to sustain composting and have enough surplus to remove enough to meet your hot water needs, you're going to need a really big pile. At the same time, if you remove too much heat the temperature will drop, which causes the composting to slow and you get a tailspin.

    I'd think you'd get best heat transfer from pex tubing going through the pile. Compost needs to be rotated periodically and you'd have to come up with a way of doing that without damaging the tubing.

  2. boxfactory | | #2

    These are all good points.

    I’m the first to admit that it looks very appealing to extract energy from a readily available material that would otherwise go to waste. If everything works out, a couple days of labor could net a winter (or so) of extra heat. But I sure would feel frustrated if the project didn’t work.

    1. GBA Editor
      Martin Holladay | | #3

      "A couple of days of labor?" That's wildly optimistic.

      An active compost pile is really hot for a very short amount of time. Then the carbon/nitrogen ratio starts to change, the nutrients get consumed, and/or the moisture content become sub-optimal, and the compost pile cools. It can be re-started by adding nitrogen, water, and by turning everything to aerate the pile -- but that's a tremendous amount of work, every 10 days or so.

      And in Vermont, composting stops dead when cold weather arrives.

  3. brendanalbano | | #4

    The problems with a wood burning stove in a small, air-sealed, and well-insulated home are that you need to supply makeup air for the stove, and that it is easy to overheat the home right?

    Since you're mainly looking for energy independence during a power outage, both of these issues can be easily solved by having a window near the stove that you can open. Gets you makeup air, and if you accidentally make it too toasty, you can vent a little. Sure, it's not maximally efficient, but if this isn't your everyday source of heat that's probably not a big deal. If you want to be fancier, you can duct makeup air to the stove.

    And as long as you have some firewood, the stove will always be ready for you to build a fire if the power goes out. That sounds much more appealing than having to maintain a gigantic compost pile all year just in case you have an outage!

  4. Expert Member
    BILL WICHERS | | #5

    I've heard of compost piles being used to help heat greenhouses in the winter, but the compost piles are massive -- usually as large or larger than the greenhouse itself. They don't get you up to comfy temperatures, either -- target temperatures for cold weather greenhouses are usually around 50F or so on the high end. The idea is to keep it warm enough to support slow growth of cold tolerant plants, not to keep things at "comfy for people" temperatures, so I think it's overly optimistic to expect to heat your home for comfort using compost. I can tell you that a few days of labor is not going to get far here. Composting on a large scale is a LOT of work, especially if you don't have at least some small scale earth moving equipment!

    Earth tubes are similar in terms of the temperatures they can achieve. There are people that use them to keep their cold weather greenhouses above freezing in the winter, sometimes all the way up to 50F or so. That's about where they top out though. You could potentially use them to help a heat pump to run more efficiently, but as many threads on GBA have discussed, ground source heat pumps -- which is what this would effectively be -- are rarely economical compared to less expensive and easier to maintain air source heat pumps.

    I think for home heating, a heat pump is your best option if you don't want to go old school with a wood stove. Wood stoves have the advantage of needing no power at all, so that's a big plus if you want backup heat when the power is out. If you want something you can automate, then a generator running a more conventional heating system (a heat pump, or some kind of combustion furnace or boiler) is probably a more practical option.

    Bill

  5. nynick | | #6

    I've been heating my home with old school "air tight" wood stoves for over 40 years. Think Jotul and Lange. We love the feel but my house is far from tight so I don't need to crack any windows. I've become quite proficient at getting and storing wood for the winter but at my age it's starting to become a pain bring it in and continually keep the home fires burning.. They do however come in handy during the power outages, yet we still need a generator for hot water and refrigeration.
    When we renovate this next home there will be no wood burning stove. Like electric cars, technology has moved on.

  6. boxfactory | | #7

    I have also seen examples of greenhouses being kept at mild temperatures through the winter. My thought was that if a space defined by a thin layer of plastic could be warmed by a small amount - what would happen in a superinsulated space?

    Though I’ll be the first to admit that I have been known to underestimate the amount of work that goes into a project. However, my background in manual labor gives me a degree of optimism about moving 30 odd yards of wood chips. That being said, someone with more land than I, and a tractor, would probably have a better chance of long term success.

    NYNick, in your renovation, you mention not having a wood stove, are you planning on a different secondary heat source, or are you building an envelope that will keep you warm until the power comes back on?

    1. nynick | | #8

      Current thinking is to redo the entire envelope as tight as possible and with new insulation. We even have a well known energy consultant involved for a total evaluation of the plans and recommendations. It's a 150 year old house so it's a big job.. Nothing has really been determined quite yet as we have a lot of balls in the air.

      But I'm thinking we'll end up with ducted heat pump(s) for the HVAC and a heat pump hot water heater. I'll wait and see if a back up heat and hot water system is deemed necessary for very low outside temperatures by the consultant.

      I already have a gas powered generator for power outages that I'll make sure the house is wired for. Those amps are easily managed by circuit breakers.

      We also have a fireplace that is impossible to remove but easy to seal. If push comes to shove I could always fire that baby up in a pinch, but I doubt I'll ever need it.

  7. boxfactory | | #9

    I figured that after all the great advice, I should let you all know that my girlfriend is not disappointed that you all gave me a number of logical reasons to not heat our future home with compost. Turns out she was not enthusiastic about the idea of yearly shoveling dozens of tons of moist wood chips.

    Thanks again,
    Ben

    1. Expert Member
      MALCOLM TAYLOR | | #10

      Ben,

      I guess you haven't told her about your plan for the micro fission reactor in the tool shed yet?

  8. boxfactory | | #11

    I hadn’t gotten around to mentioning that either.... was going to be a surprise. Though I suppose this is one of the advantages of planning before breaking ground, working through the details before things get stressful.

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