Hello November; Autumn Leaf Mould Energy Potential (Part 2 – Construction – Heating A Greenhouse with An External Compost Bed); Bat and Butterfly

Most of this post will be devoted to the detail of constructing ‘The Reactor’ – a compost bed designed to help to heat the greenhouse and keep it frost free over the winter. (Addendum: – There are a lot of posts now about how this idea has developed and its implementation. If you want to find the other posts quickly, then click here). But before that just a few images from the last fortnight to match those from October of special mists, clouds and light.

I’ve continued to trawl the internet to find examples of workable designs for a compost heap based system for heating a smallish greenhouse. Much of interest has been gleaned, but little concrete information beyond the many frustratingly inadequate You Tube postings of various trail blazers – usually lacking details or follow up on how well a system has worked. Even more frustrating when our satellite broadband insists on chopping any videos into 5 second bits.

So, here goes with an old tech blog record of my own attempts.

My initial research using leaf mould prepared as in my last post gave really useful temperature data which Fiona has very kindly converted into the graph below:

Essentially the 3 big bags I monitored were filled with pretty similar chopped leaf material, although the bags were filled at slightly different times. (Big bag 1 was started 3 days before I decided to check temperatures, and big bag 2 the day before the graph starts – the usual problem of poor design with amateur experiments). Leaf mould is pretty widely accessible and generally nutrient poor, but is still in percentage terms a relatively carbon (C) rich, but nitrogen (N) poor starting material. So, I opted to use the easiest measurable N source potentially available to all gardeners to try to correct this lack of nitrogen. The squeamish should skip the next bit.


The general consensus from many sources is that for a good composting process a ratio in the raw material mix of about 30:1 (C:N) will give best results. But how on earth do you know what actual mix you have? The best summaries I read on the factors involved, and the relative C:N ratios of various compostable materials, can be found by clicking here, and here.

But for my low N leaf mould, I added about 5 litres urine from a watering can onto the surface of filled bags 1 and 2 immediately after filling the bags with the mulched and pretty moist leaves. Bag 3 received no urine at all. And bag 1 received 3 additional urine ‘doses’ as shown on the graph. All bags were then covered with single flat layers of cardboard boxes. Using a watering can gets round the physical issues with even distribution over the surface, particularly if your joints are getting creaky, and is more discrete.

The temperatures are the maximum reading I could detect taken once a day around dusk, down the central air filled tube, and the external temperature is given for comparison. So, ignoring the usual scientific malarkey about statistical significance which would kill off any amateur observations, I now felt I had the perhaps unsurprising information:

  1. That nitrogen, at least as watered on urine, results in higher temperatures being reached than with ‘untreated’ leaf mould.
  2. That repeated applications seem to slightly extend the period of higher temperatures.
  3. That with any of these systems, potentially usable heat generation lasts only about 10 days.
  4. That by the end of this period, the volume of the leaf mould has settled by about a quarter. Although the form and appearance of the leaf mould has certainly not significantly changed from the starting appearance over this short time.
  5.  Leaf mould with no supplementary urine will probably retain its heat generation capabilities, albeit at a lower level, for longer than urine treated leaf mould.

The big bags at the end of the trial, showing how leaves had settled from the top. Impatience and the need to finish ‘The Reactor’ quickly precluded a longer experiment time span.

I was left concluding that since I didn’t fancy turning over a compost heap every few days, which will result in even higher temperatures and speedier compost formation, I needed to design a system which would:

  1. Allow simple topping up of the heap on a frequent basis to help to try to keep temperatures at a modest, though raised level.
  2. Expect to need to regularly add supplementary nitrogen as urine, and since we now have access to fresh poultry manure, this as well, to aid heat generation over the winter months. Fresh green material such as grass clippings, which are the other N rich compostable material normally widely available to many gardeners, are obviously less abundant during the autumnal and winter seasons.
  3. Ideally raise the heap to allow finished compost to fall from, or be raked out from, the heap’s base, and so allow the heap to be managed for several winter months without having to be completely dug out, and started again from scratch.

The long list of photos below is designed to illustrate how the eventual compost bed was constructed. As with most things we do, no formal plans were drawn up. (And it shows, I hear you sigh). If at any point anyone reading this would like further clarification of what we’ve done here, do feel free to add a question through the comments box below, and I’ll try to answer.

The only item bought in specifically for this construction was this 25 metre roll of 80 mm perforated land drain, of which about 19 metres has been used inside and outside the greenhouse.

I probably could have sourced un-perforated pipe. And indeed I should have done so. But I needed a few metres of land drain for another project, and reckoned that with some surplus cartons of Gripifill filler/bond from an internal insulation job, I could easily fill all the holes. Someone else can work out how just many holes that was for about 17 metres of used and hole filled pipe, with roughly 8 holes per linear cm. A huge false economy, and probably an inadequate seal of the whole pipe, which may be a fatal flaw with the system. Advice note number 1 – Buy un-perforated pipe!

The plan was to wrap the pipe around a form made from old sheep netting, in a tight spiral, tying it in with wire, and eventually filling the central space with recycled 2 litre water/pop bottles filled with water. The whole of this ‘Reactor’ would then be placed in the centre of the compost heap and the 2 pipe ends passed through holes cut in the greenhouse gable end. A fan would then be located in the greenhouse to draw air (hopefully heated by the compost) from the top of the spiral, with the lower spiral pipe sucking air from the greenhouse, to pass up through the reactor coil.

This drainage pipe has a fairly thin corrugated wall – in theory good for heat transfer, but has a mind of its own, so allow a big space, peg the sheep netting form to the ground before starting, use strong wire to tie it in (3 ties per layer), a digging bar or extra pair of hands to hold the spiral fairly tight, and allow a bit of extra pipe for feeding through the greenhouse wall, and general odging into place.

In the end, the spiral was upended for use, thus lifting it off the ground by a few inches.

How to raise the compost bed off the ground required some ferreting through the outbuildings. We suffer from an inability to discard stuff with any potential for future projects. Perhaps we are indeed compulsive hoarders? And so I came across a number of old angle iron bed frames, some with castors, and an extra bag of bed castors and fixing plates. Their moment had arrived.

With a bit of angle grinding I had a raised base to take the 5 grids we’d salvaged from a commercial fridge we’d once owned when we toyed with ‘Dine At Home’. This was a previous ‘Good Idea’ I’d had to keep Fiona out of mischief, years ago in Bristol. An upmarket freshly cooked 3 course meal from a seasonal changing menu, with wine, if required. Huge fun setting it up. A ridiculous amount of work trying to run it alongside a veterinary practice, and finally seen off by Black Wednesday in the early 1990’s, and the supermarkets starting to roll out exactly this sort of classy prepared food. So nearly 20 years later these grids had also found a niche use.

So this really is a compost BED to heat a greenhouse.

The metal sheets laid beneath the bed which will hopefully ease raking out fallen compost were inherited with our Bristol house bought in 1987, and haven’t really found a good use until now.

Advice note 2. Buy black pipe. It beats trying to paint it as dark and dew fell, with inappropriate, but to hand, black paint. The thought being that black walls would aid heat transfer to the circulating air.

After reading an interesting undergraduate thesis on the general concept of heating greenhouses with compost by Courtney Gilson from the University of Waterloo in Canada, I started to think a lot more about the need to insulate the heap to maximise any available heat generated, and avoid its loss to the wider world. The thesis explains how a compost heap with a footprint of about 16% of the greenhouse footprint should be adequate to heat the structure. Designing my heap, before I read any of this, to fit the available space between the greenhouse and woodstore, I noticed that my heap of about 1 metre x 1.5 metre x 1 metre compared with an inner greenhouse zone to be heated of about 3 metre x 2 metre x 1 metre  and so is well within this stated capacity calculation. Which cheered me up. For now.

To incorporate insulation, all of the sides, top and base were made using left over black DPC thickness plastic sheet, and surplus 70 mm thickness Celotex sheeting. Some of the panels have simple wooden frames made from old timber floorboards. The 2 litre plastic pop/water bottles are chosen to act as an additional means of holding heat as a thermal store inside the reactor core, to help to smooth out any temperature fluctuations when the fan is switched on.

The final element of the design was incorporating a series of plastic 22 mm water pipe with a few drilled holes added to each as breather tubes wired into the grids. These may allow a little more air into the centre of the compost bed, and also allow a little bit of wriggle movement to encourage compost settlement from top to bottom, and create some vertical air channels through the compost material. Oxygen is by definition vital for aerobic composting which tends to be quicker, generate more heat, and ensure better compost than anaerobic methods where inadequate oxygen is present throughout the compost.

Achieving aerobic conditions are even more critical on a small scale heap like the one constructed here. But I should here provide a link to a real pioneering Frenchman, Jean Pain, who in the 1960’s/70’s produced 4 litres of 60 degree C water per minute for up to 18 months from a huge composting brushwood pile, as well as methane from anaerobic composting at the centre of the pile, stored in large tyre inner tubes to fuel his cooker and a light van. (Don’t even think about it, Julian).

So, after about 10 days of working on the construction when it wasn’t raining it was completed. It would have been impossible without a lot of help from Fiona in initially levelling the site.

Prising a pickaxe from her is tricky these days.

And then coping with me deciding to dig out the base even deeper, half way through the project, to allow insulation to be added at the base as well.

And finally agreeing to cut and fix the plastic to the end and top frames. Guess who wraps all our Christmas presents?

This is one of the top panels, where I opted to use 2 layers (140 mm of Celotex insulation), hence the extra depth. The panels are a little awkward to lift because of dimensions, rather than weight, so I later added a couple of wire strap handles to make lifting them on and off easier. So I’d now got to a point, where I needed to fill up the bed.

And after laying the wet cardboard sheets from the top of the big bags as a base over the grids.

I used as source material most of the 3 big bags of leaf mould from the above trial which had obviously already started their  composting process, clearly not optimum material, but I figured there was still a lot of decaying to take place, alternating with material from a couple of big bags which had poultry house waste – droppings and paper/hay, alternating with layers of leaves or wood waste.

A couple of hours shovelling and carrying tubs of material before the fading light called a halt nearly finished the job, and me, but the following day, in spite of a howling gale I couldn’t resist lifting the lid for a temperature reading. Only 2.5 degrees C.

This is an earlier photo, with the lower panels are now added as well….

Oh dear. Patience was clearly going to be required. Gloom settled with the lowering clouds….

And I suspect that whilst a silly amount of hard work has gone into this, and others may judge an equally bizarre or extravagant use of materials has got me to here, the real challenge of understanding how to operate this system if it works at all, and its limitations, will play out over a much longer time frame. And whether in due course more exotic Welsh fruit can be grown to rival the Welsh nectarines we enjoyed this summer. (Click here) .

Welsh apples, Costa Rican pineapple, Spanish oranges.

I’ll add an interim progress report next time.

As light relief to all these endeavours, two wildlife related stories should be noted from the last fortnight.

Firstly, as I checked my last post for errors just before lights out, Fiona shouted from the bed behind me ” Oh, B***** !” This was sufficiently out of character to get me instantly swivelling round on the computer chair to see what she’d forgotten to do.

But the look on her face as she pulled the quilt above her rang alarm bells just before she called out “There’s a bat in the bedroom!”. She’d been alerted to it by the whoosh of air as it flew low over her face. Past similar experiences had me stooping low to the bedroom door, and waiting until it flew out onto the landing before slamming the door closed. As usually happens after these occasional close encounters, in the morning it was nowhere to be found and hasn’t revisited us. Yet.

The second episode was a strange noise at supper time, apparently behind the freezer and reminiscent of the scratching sounds of birds or rodents in the roof space, fiddling or chewing through the celotex foam insulation. Worried that a rodent was about to chomp through the freezer cable (years ago a mouse committed a similar kamikaze mistake with our mains supply, causing a momentary power dip, before dropping at our feet), we moved appliances out trying to find the culprit. And then spotted that the sound was in fact being produced by a temporarily awakened Small Tortoiseshell butterfly which had become trapped between two very thin walled willow bowls, which I’d turned years before. Its flapping wings were generating the strange, chewing like sounds.

And on that note of mistaken origins, I place below an interesting image taken outside last week. It has been tweaked a bit, but anyone brave enough to attempt to identify the 3 key components of the scene will encourage me that folk still read my musings.

Finally, by chance after writing in my last blog post about energy efficiencies which have followed from installing our own PV panels 28 months ago, I came across some old electricity bills.

Our annual bill before installation was 11,900 KW Hours (KWH) – we do have electric night storage heaters as well as wood burning stoves.

20 months later the usage had fallen to 6,900 KWH annually.

Last quarter, it had fallen to 3,900 KWH annually.

Now what’s remarkable here is that we only generate about 3300 KWH annually ourselves. So the extra savings we’ve achieved have come about entirely as a result of getting more savvy about our energy consumption. As you can see, this saving amounts to much more than we generate. I can’t think of a better example of the benefits of encouraging micro-electricity generation schemes, whatever one thinks about global warming and the mantra of carbon reduction targets.

And this on the day after the local Planning Committee rejected the initial planning application to build 21 huge 140 metre tall wind turbines on nearby Mynydd Llanllwni, much to the relief of most local residents. The opening salvo in a long struggle, no doubt.


15 thoughts on “Hello November; Autumn Leaf Mould Energy Potential (Part 2 – Construction – Heating A Greenhouse with An External Compost Bed); Bat and Butterfly

  1. Oh my goodness…what a great deal of hard work you two have put into keeping your greenhouse warm…all good reading and do hope it all works out successfully! Love all the sky photos too. Hope you are planning to put your feet up for a few days at Christmas…one always needs to have a rest so that time is allowed for thinking through future projects and knowing you two there’ll be plenty!

    • Hello Marianne,
      Thanks for the comment, and yes its a bit sad really, isn’t it? But I’m just fascinated by the possibility of it working,and what sort of temperatures might be achievable – preferably without burning the greenhouse down,( apparently dry heaps can spontaneously ignite.. though unlikely here!) and its potential use as a heat source in other areas….. We do plan a few days REST sometime, but before that there’s the issue of Christmas dinner which is troubling me, and a painting Fiona’s had requested….. Hope that you haven’t been washed away today with all the wind and rain,
      Best wishes
      Julian and Fiona

  2. Hi Grumpy Hobbit,

    I was about to pour scorn, as is my way, over your incredibly ambitious and hugely energetic attempts to heat your greenhouse, by asking whether you had taken into account the number of kWh you and especially her nibs had spent in preparing the thing and whether if you had spent the same amount of energy walking in circles in the greenhouse you might have achieved even more spectacular temperature rises, when I suddenly underwent a sort of Damascene conversion. It was the annualised usage of electricity after the PV installation wot did it! That and the fact that in my idle state, I awoke at 4am this morning and watched on the flat screen (one can’t call it the box anymore) a documentary about Bradley Wiggins’ “Year in Yellow”, and fascinating it was too.

    Your usage statistics caused me to have an embarrassed look at our own and I dug out an email from our energy supplier which told me that what with gas and electricity, last year we used 28,936 kWh of energy in our little Surrey home, which means that at your latest consumption, our usage would have powered a full 7.4 hobbit homes! Now that is a vindication of your efforts if not a condemnation of our own (except that the vultures who wheel and ascend on the heat thermals above our house would feel bereft if we suddenly became hobbits!).

    While watching the Wiggins thingy, I spied a PowerPoint chart in the background, showing the “power curve” of his energy output. With the magic of the PVR (personal video recorders: hobbits may be ignorant of the term), I was able to freeze the action and look carefully at the graph, which showed that Wiggins regularly achieves an output of around 1kW at peak, reducing to much lower levels downhill, but just prior to the “Tour de France” his average output in training was 400W.

    Now then, Mr Wiggins completed his Gold Medal winning Time Trial in the Olympic Games at an average speed of 51.2 km/h. Assuming he achieved this at a power output of 0.4kW, it follows that a single kWh of Mr Wiggins’ effort would drive him 128 km. It also means that if Mr Wiggins was to expend the power output that we consume in our home per year, he would travel 3,703,808 km. The circumference of the Earth at the Equator is 40,075km, which means that Mr Wiggins would ride around the Earth a total of 92.4 times in a single year! (10,000km/day)!

    Perhaps we should turn the dial down a bit!

  3. Dear KTB,
    As always I value your comments (and frequently humorous and entirely valid criticisms). I seem to remember expending a huge amount of steam over the carbon input of mega Wind Farm projects versus the projected carbon saving over their lifetime usage. So I’m a valid sitting duck on what energy we’ve put into thus little scheme (physical, material and mental)
    BUT!!!…. This time your observations and analysis using Mr.Wiggin’s spectacular energy output data excel all your previous comments, so without further ado I shall nominate this submission for a new feature of this blog. Namely COMMENT OF THE YEAR.
    Quite what reward the eventual winner of this award (and which I should point out still has nearly 6 weeks for further nominations to be made and considered) is still to be determined. But in the light of your energy consumption figures, perhaps a Good Energy Bag, or even better a personalised sign for the corrigendus kitchen along the lines of….”Just Do It. Down, Off or Out”. More thought is needed for this.
    In the meantime, and not to detract from your statistical comparison with the Hobbit home, I should point out that you do of course fail to take into account the forest of trees which annually fall to fuel the hobbit stoves in addition to our electricity, so perhaps you don’t lag us by quite such a mile….. more like a kilometre? BW, Julian

  4. Kept since 1987!!!!
    Julian, when you have finally used up all your ‘squirrelling stuff’ what are you going to do with your time?
    In the interests of having ‘some’ free time, if you are to save yourselves the hassle of changing all the leaf litter every 10 days or so, I think your suggestion was to just add some more litter + pee every few days. But it slightly worries me that the ‘old’ litter will just clog up the system and act as insulation keeping all cold. If you remove your blue pipe system to enable you to stir the mix on, say, day 5 and day 10, allowing fine stuff to fall through the grate, then top up (on day 10) with more matter(litter + pee + fowl foul) then it might save you ‘some’ free time….?

  5. Oh, and I shall look forward to seeing the graph of composter temp v internal greenhouse temp v outside temp. Do your thermometers read max and min? It would be helpful to find out those too. Though the time or even time lag between them would be interesting too…..

    • Hello Rinka,
      As always thanks for the comments.Plenty more years worth of squirreled material I think, so no worries on that front….ever wondered why guests never get a tour of the outbuildings? I’m certainly going to take some more temp. readings, and your suggestion of max/min is a really good one…I shall have to get a suitable thermometer asap. As to how to work the system, and tweak it, if its anything like my breadmaking or using our ESSE woodburning stove, I’m still learning how to optimise results after 30 and 9 years respectively…so don’t expect too much too soon! Still the tedious bit of construction is at last now nearly complete, with today a trial fan switch on, so I should have at least something to report next time….either upbeat or depressing!

  6. I am glad you are pursuing this idea. I have a feeling that most inventors “waste” a huge amount of time and materials developing prototypes for inventions, most of which amount to nothing. However, I am thankful that they have the creativity, curiosity, and attention span to persevere. You remind me of residents of Cliff Island in Maine—everything is saved because you might need it some day and you can’t just run to the store, which is three hours round trip by ferry. It is amazing the simplicity of life that not being able to run to the store engenders. I have no idea what’s in the photo—metal/plastic fencing, some kind of fungus, and Christmas tinsel. I don’t really think that’s it but I wanted you to know that I was reading.

    • Hello Carolyn,
      Thanks for the comment. ‘Inventor’ is a bit grand, but time wasting is very apt…..But interestingly you touch on an issue that I raised with Fiona just this morning – in a very real sense living a ‘simple’ existence actually is much more complex and demanding than most folk would imagine, and practical problems (e.g. rain erosion on our track, blocked spring water supply, mastering a woodburning stove) demand analysis and often trial and error solutions, because you’re on your own….you can’t just turn on a switch, or pick up the phone for a tradesperson to call. For now I do enjoy these simple challenges….
      Re the image, thanks for having a go at working it out.It was a bit unfair – even Fiona who saw the original coudn’t work it out, so I’ll include the original image in my next blog, but for now thanks for reading and such a perceptive and interesting comment, BW, Julian

  7. Julian…So very glad to hear that the windmills won’t be turning nearby any time soon. The concepts being worked out in the attempt to heat your greenhouse are interesting…Good luck with that and , keep us posted! Jenny (from Oregon!)

    • Hello Jenny,
      Great to hear from you and thanks for the comment. I hope you’re well. I guess that winter has arrived with you now. I wonder if you’be heard of the Rocky Mountain Permaculture Institute. I have yet to mention it on the blog, but it looks like they’ve done some pioneering work using moved air (though greenhouse/solar heated) to grow exotic stuff through the winter at 7,000 feet up. A link is ………Jerome’s talk on Rocky Mountain Permaculture Institute » Colorado …
      http://www.coloradopermaculture.com/colorado-permaculture…/img_0753…Colorado Permaculture. Education ~ networking ~ resources ~ activism ~ fun. Gallery » Jerome’s talk on Rocky Mountain Permaculture Institute …,
      Best wishes

      • Hi Julian,

        Just a few random thoughts on your project.

        1. Have you looked at the mechanics of industrial compost making and if any spin offs which could be modified to suit your application. There is a small setup just outside Aberystwyth with rows of composting material.

        2. Assuming one heap at a time would do the job I keep going back to the new zealand system with 2 bins.
        3. heat rises so if the bins were very well insulated could you harvest the heat from the top of the bin using a movable extraction fan and when the heap is exhausted move the fan across to other bin whilst that one is emptied.- only need one lid with fan mounted in it. and bins could be much larger/ taller.
        4. The ingredient mix is probably fairly critical within a little so large quantitiesof c and n materials required coupled with shredding.

        5. Is there a limit on how fine the material is shredded ie if too fine then air (oxygen) is impeded and anaerobic conditions set up.

        6.Is chicken manure,animal manure as efficient as urine? I seem to remember digging out cow sheds with a tractor when the manure was 3 ft deep so must have been anaerobic but it would steam for ages when heaped up outside.

        7. Should the heap be mechanically areated ie blow ambient air in at bottom and pipe the resulting warm air into green house.

        8. Is the warm air being put directly into the stone heat sump or does it just go into green house. Should the warm air be channeled into the soil ie warm the soil first.

        9. Presumably the air temp changes rapidly with the ambient air temp but how quickly does the soil warm up/cool down would a constant soil temp of say 3 degrees above the outside soil temp be advantageous.


      • Hello Dave
        Thanks for the comment and many ideas. I reckon there’s a huge number of thoughts here which are ideally suited to your trialling in your polytunnel area, …….But more specifically:
        1: One of the issues I’m going to address is volume of material need to keep the compost fires burning, I don’t think I’ll ever make it into in the industrial league, and I really want to try to show something might work elsewhere in a similar garden setting.
        2: I’d be worried re outside moveable electrics on top of the bins, given the climate and my poor grasp of issues. Plus I suspect you’d be into a more expensive fan type.
        4, and 5: Whilst the mix is obviously important, and degree of shredding, I reckon once you get the basics you can have a go. Again, if I can show the size I’m working with , and material mix, people could replicate or tweak as necessary.
        6: Don’t know. Since I wanted to get something out of it, I’ve opted , in true permaculture fashion , to use what’s to hand. If it works to any degree, then again its up to folk with deeper pockets or more resources perhaps to run trials on all these other permutations…I reckon it’s beyond this poor hobbit’s ( or more imortantly Mrs.Hobbit’s) limits of patience!
        7: Could be good, but I’d be worried about the potentially noxious gases which might be generated from the compost. Hence my decision to go the nearly sealed route (i.e. it would have been sealed. if I hadn’t used filled-hole perforated drainage pipe!) with an external compost bed.
        8: Why didn’t you tell me this 3 years ago!! Before I’d put in the heat store channel? Again, probably would have been a good idea, but wouldn’t you need a beefier fan to get the air out into the soil? And I’m trying this system with permanent cool winter requirement stone fruit trees, round the perimeter with inner pots for growing vegetables, so don’t know how you’d get the air into the veg. pots – to allow changing vegetable growing compost without disturbing tree roots too much.
        9: At this stage I’ve ditched any preconceptions of what I might achieve. At least last night with a minus 10 deg. C beside the greenhouse), it was still frost free in the inner zone, so not a complete fiasco!)

        As before Dave, you’ve provided lots of interesting ideas here….Watch the next blog for a bit more detail on the specifics so far of what I’ve found,

  8. Have the results from the 2012 “reactor” project been posted anywhere? thnx, brad – kelowna, b.c. canada

    • Hello Brad, Thanks for the comment. Not sure what you mean…in other words I’ve written everything up that I’ve done on this topic on this website/blog, over about a 2 year period but it hasn’t been posted/ published anywhere else. Hope this helps,
      best wishes

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