Don McLean

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since Dec 30, 2011
A few years as a hydrogeologist looking at contaminated land / groundwater; retrained as a domestic plumber, specialising in solar hot water systems. Some experience with solar pv, wind, thermal mass stoves. Now earning periodic incomes from domestic energy advise, plumbing, permaculture teaching and tutoring. Just set to start leaning how to rejuvinate an old coppice woodland in the south of England, wood bodging, building a woodland-based income and home...
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Recent posts by Don McLean

Hi Carola, no, no experience of heating a hot tub this way.
I guess if you set a bath in (or built a large compost heap around) a tub of some sort, & heavily insulated the top of the tub (to prevent heat loss), water in the tub would heat up over time, ultimately to whatever the temperature of the compost heap was.
A typical bath is aroudn 100 litres or so of water, a significant volume to heat, i.e. it would heat relatively slowly. The original show compst heap appears to have heated much more than this over its 2 month life though, so it should be possible. I guess you may need something in the water to prevent hot-water loving things growing in the lush-temperature water as it heated up. Epsom Salts maybe???
I've friends who removed the spiral copper heat exchanger from inside a typical (UK) copper hot water cylinder, and cobbled this (somehow; I can ask them if you are interested) to a tub of some description to make a hot tub. They just filled the empty centre of the copper coil with wood, lit it and the hot water naturally gravity circulated (thermosyphoned) to the top of the tub; with a lower, return water connection in the tub (possibly via the plg hole) for the (cooler) water to circulate back to the coil to be re-heated by the fire. Appropriate relative levels are ctircal for an efficient thermosyphoning system.
The simpler solution is a (heavy guage) tin bath directly over a fire. There was an article about this type somewhere on the Permaculture UK site recently (2011, via a Facebook notification), though all I could see just now was:
Cheers, Don
9 years ago
OK, thanks for clarifying Paul.
9 years ago
Carola, it normally takes a few days, maybe a week for a compost heap to heat up (maximum is around 70 degrees C, don't know what that is in F, sorry, I'm from Europe, 50 or less is more usual, due to poor mixing of air and materials).
You need a "reasonable" volume of material for good composting, probably at least 100 litres, more is better, all placed together pretty much in one go.
The number of showers you could power then depends on the mass of material in the compost heap and how quickly the heat transfers to the water pipe running through the compost pile.
You would want a reasonable length of pipe, so as to amass a “sensible” volume of water for each shower.
I had this discussion with a friend recently and calculated from Paul's video:
The coiled pipe is given as 100' long x 0.5” diameter. Assuming my maths is ok (a significant assumption!) = just under 4 litres, say 4l.
Assuming a toasty compost pile, 60C, = 4l of water at 60C.
He said he also had a cold feed to the shower. In summer, the temp of the “cold” would probably be well over 10C (assume 10).
Diluting the hot and cold together say gives 8l of 35C water; you could have more cooler water, or less hotter.
This is adequate for a short warm shower (i.e. ok for many “green” folk, but probably not for “normals”).

(You might get more hot water per shower: the compost heap has a large mass, which would be transferring its heat to the new, now cooler water in the coiled pipe in it, as the initially hot water is drawn off. I have no idea how to calculate this though. I think that in reality, the shower would cool quickly and dramatically. The system would then need to be left to re-heat the now ambient temperature water in the coiled pipe.)

He ways it delivered about 500 showers. Assuming these are all short (i.e. only use the 4 l of hot water, the compost pile has heated around 2000l of water over 2 months, an output of around 2000l x 1000g x 60C = 120,000,000 J*, around 33kW.
*(It takes 4.186 Joules to heat one gram of water by 1-degree Celsius).

Note that as available oxygen in the heap is used up, composting will slow, so it would be sensible to include some means of aerating the pile (though this could also promote heat loss).

Bill, if a “compost” heap is generating ammonia, then the composting process is turning anaerobic – it needs more oxygen (air) for "proper" composting to continue.
“True” composting is aerobic (i.e. with oxygen), which generates mostly heat (484 to 674 kcal per gram of glucose) and CO2.
Anaerobic decomposition generates far less heat (about 26 kcal of potential energy per gram), methane and CO2 (in varying proportions, but typically more methane than CO2).

CO2 concentration is generally a limiting factor for plants, so elevated concentrations in a greenhouse would be beneficial (to the plants).
You might want to air the greenhouse before entering, or monitor concentrations though!
9 years ago
Not sure that's resolved - would be really nice to!

A quick search I did seems to support it IS better to turn heating off when you don't want it, & on when you do, (AND HIGHLY recommends very programmable heating programmers, many time periods at different temperatures etc). Because, as others have pointed out here, the higher the temperature inside, the greater the heat loss to the outside.
For example:
Dear Umbra,
Many of my friends are environmentally minded and do lots of things to try to have a smaller carbon footprint. Yet when I tell people I turn my heat down when I leave the house even for an hour or two, and that I turn it down to 50 at night, they say, "I thought it takes more energy to reheat the house than to keep it at a constant temperature." Please clarify. If it is better to turn the heat down, then there is a LOT of room for education on this topic, as even many people in the environmental community are confused.

Let's say a typical, nice heat setting is 68 Fahrenheit while at home, 58 at night or while away. (Your 50 is probably lower than most people will try, but bravo to you.) The heater will save fuel as it falls to 58, and expend about the same amount of fuel as it rises back to 68. Therefore, these two transitional phases cancel each other out. And while the heater is set at 58, for as long as it is set at 58, it is merrily saving fuel (aka energy, money, and the planet). Because as we all know, it takes more heat to keep a house at 68 than at 58. Overall, then, fuel is saved.
To bulk up this answer a little bit and remind us all that insulation and sealing the house are important aspects of keeping ourselves, but not the planet, warm, let's discuss air-movement dynamics briefly. Remember, air lives a life of heat equality. Hot air wants to rush out and share the heat with nearby cold air until all air is the same temperature. This happens, as fervid readers may recall, through the processes of convection, radiation, and conduction. The stack effect is an example of convection: Hot air in a building not only rises, but is of higher pressure. As it rises, it pushes against any cracks in the ceiling or roof, escapes, and leaves a low-pressure area at the bottom of the house. The cold air rushes in to the low-pressure area, and must in turn be heated.
Our heaters are fighting an incessant battle on our behalf, warming all the new air. If we are not there to be warmed, or are sleeping under a cozy duvet, we can turn down the thermostat. Programmable thermostats are very helpful and quite cheap.
I repeat: Reheating uses less energy than keeping it hot while you're gone. No organization -- reputable or disreputable -- disagrees with this advice. To quote the EERE, "This misconception has been dispelled by years of research and numerous studies."

One link quoted supporting this view in one the replies might have clinched it
“If you want to really nerd out over this here is most recent, over-instrumented research: ...”
but frustratingly, won't work for me!

ALSO: extracts from the EERE advice:
“A common misconception associated with thermostats is that a furnace works harder than normal to warm the space back to a comfortable temperature after the thermostat has been set back, resulting in little or no savings. In fact, as soon as your house drops below its normal temperature, it will lose energy to the surrounding environment more slowly. The lower the interior temperature, the slower the heat loss. So the longer your house remains at the lower temperature, the more energy you save, because your house has lost less energy than it would have at the higher temperature. The same concept applies to raising your thermostat setting in the summer; a higher interior temperature will slow the flow of heat into your house, saving energy on air conditioning.”

[Unfortunately I am then lost, when EERE advise states:
Limitations For Homes With Heat Pumps, Electric Resistance Heating, Steam Heat, And Radiant Floor Heating
Programmable thermostats are generally not recommended for heat pumps. In its cooling mode, a heat pump operates like an air conditioner, so turning up the thermostat (either manually or with a programmable thermostat) will save energy and money. But when a heat pump is in its heating mode, setting back its thermostat can cause the unit to operate inefficiently, thereby cancelling out any savings achieved by lowering the temperature setting. Maintaining a moderate setting is the most cost-effective practice. Recently, however, some companies have begun selling specially designed programmable thermostats for heat pumps, which make setting back the thermostat cost effective. These thermostats typically use special algorithms to minimize the use of backup electric resistance heat systems.]

ALSO: advice from the UK Government's Energy Savings Trust:
“Programmer or time control
This will automatically switch your heating off when you’re not at home, or when you can do without it, such as when you’re in bed.
Programmers allow you to set ‘on’ and ‘off’ time periods. Most models will let you set the central heating and domestic hot water to go on and off at different times. There may also be manual overrides. Check that the timer on the programmer is correct before you set your programmes. You may also need to adjust it when the clocks change.
Choose a cold evening and time how long it takes for your house to warm up from cold to a comfortable temperature – this is the warm-up time. Then turn the heating off completely and time how long it takes for the house to start to get uncomfortably cold – this is the cool-down time.
You can now set your timers including the warm up and cool down time. So, for example, you can make sure that the heating goes on with a warm-up time before you wake up and turns off before you leave the house. If you insulate your home, it will warm up more quickly and cool down more slowly, so you’ll save money on heating.”

Please note however:
the EST advise says allow for the warm-up and cool down times.
Clearly, if your house / heating system is such that the warm-up period is longer than a period you leave then return to your house, then you can't turn the heating off (and expect the house to return to a comfortable temperature when you want it to). You'd need to turn the thermostat down to whatever the heating system can recover from, when you leave.
If this is the case, then we are dealing with “extreme” cold and / or relatively poor house construction (or possibly high heat expectation).

I think Paul's point:
“And .... I stand by my point that it takes much more energy to warm a home 15 degrees than 2 degrees.”
is certainly true, but the logic wrong, since you've been paying (using energy) to keep your home that extra 13 degrees warmer.

(I am a home energy advisor in the UK, so have this discussion quite often) I do meet some people who claim they have tested this and found that they use less energy keeping their house on a mid-temp “tick over” while they are out, then boosting it up when they are in, rather than letting the house go “cold” (I am assuming not freezing).
I can only accept what they say, though it does not fit with what I can justify to myself, nor what “official” guidance seems to be, both in the UK and US.

Other considerations:
Paul is no doubt aware of Ianto Evans' thermal benches / beds and the like – heating furniture (rather than the surrounding air) seems a more sensible way to go, ESPECIALLY when burning wood, because the most efficient burn is a very hot burn, so the deal there is to store and moderate the heat (which massive furniture allows).
Mass furniture is unlikely to be acceptable in most modern (i.e. light-weight homes, where I suspect load-bearing capacity is a limiting factor)

Obviously we don't want our homes to freeze (burst water pipes). Modern heating programmers should include a frost setting, which will prevent indoor freezing - clearly sensible.

And yes (of course):
it is far better to only heat those areas of the house where you are (i.e. not “spare / guest bedrooms...”),
and, more extreme, but I think entirely logical, Paul's personal heating (rather than space heating), e.g. as simple as a blanket on a couch, as per your great “how I saved 87%..." article

Most of us in Europe have got used to very cheap heating, mainly from natural gas which is now getting less cheap, so I think the days of central heating (i.e., in my opinion heating under-used areas of a house) are numbered. In my childhood (I'm approaching 50), my family, who I wouldn't describe as poor, had just one heated room (in addition to the kitchen, which was warm while cooking was happening). I htink this was normal.
So in winter, we were mostly in the (one) heated room. I didn't spend too much time in my bedroom in deep winter, which could get ice on (the inside of) the windows.
Bodies are likely to radiate around 100watts / person, so a few people in one room begins to make a significant heating impact.
9 years ago
If you are building up against the hillside agree with need for drainage and damp-proof membrane.
I wouldn’t leave a gap, as nature will try to fill it over time, and you'll probably get damp.
An ideal solution would be to build up a drainage layer, by lining with a silt-filtering “geomembrane” (otherwise the drainage gravel will silt up over time), porous drain at the bottom, gravel to the top; insulation layer; building your inner wall against that. So, you'd have several layers to build up at the same time, from the outside in:
silt-filtering wrap + gravel
damp-proof membrane
wall (tyres, or other mass structure)

This construction is therefore a multi-barrier approach – trying to intercept and direct water away from the building AND installing a barrier against the damp. A bit of a pain, but if you are using your building much, you really don't want it to be damp.

Damp-proof membrane can be pretty inconvenient as its is usually difficult to work with (big, heavy gauge plastic sheet) and you don't want to damage it while you are building each layer up and gradually unwrapping it. Have tape to fix tears...

Earth-filled tyres are a useful thermal mass building block, and turn a fairly tricky-to-sensibly-dispose of waste product into something useful (a building block).
I find filling them very hard work. Other people have told me its about technique rather than simply strength, but I don't seem to have picked that up.
You could build a mass wall using rammed earth in formers, wacked in layers. Can be quicker (e.g. I think its quicker & less exhausting to use a manual wacker plate, on earth layers, than a sledge hammer in individual tyres – I've personally tried the tyres but not wacking a rammed earth wall, so that's just my guess).

As you probably realise, Ben Law's house is timber frame with straw-bale infill. This is a different principle to the earthship principle: insulated building walls vs: mass walls. I believe Ben's walls are lime rendered; his roof has a large over-hang to keep water off.

What is your building orientation / aspect? - could you be on for an “earthship-type” passive solar heating, heat storage in a massive back wall design? Check: (its called “groundhouse as it is not a “Mike Reynolds-approved earthship design)
Mike-Reynolds “approved” earthship design (in France):
the earth-ship originator (Mike Reynolds):

I don't think the Americans usually use damp-proof membranes, as their original structure were above ground (with an earth bank at the back, in relatively dry climates).

The Brighton and Brittany buildings are both in temperate / wet climates, so both had complete damp-proof membrane wraps, heavy-guage polythene, which completely lies under the floor, up the back wall, to the roof (which is at ground level at the back), all in one sheet.
Daren & Adi have produced a brief book:
(Yes, there is a review by me, for a permaculture magazine - its a nice, quick, book). There is a lot of detail tucked away on their website though + videos, so tuck in! (I'm credited somewhere for the solar “plumping”, which sounds rather nice, but is supposed to be “plumbing”. I did the underfloor heating too, which we thought would be sensible, since were not sure how the passive nature of the structure would perform in that climate.

The Brighton Earthship & Brittany Groundhouse also have insulative “break” layers at the back, so that you have a mass wall on the inside and are not trying to heat up the whole hillside behind. The material used was rigid sheets made from spun recycled glass, “foamglass”. High compressive strength.

In a drier climate, southern Spain,, I think Dave & Laura didn't use a damp-proof membrane. Time will tell!

The “render” used on the tyres, all over in fact, is adobe (i.e., mud, sand and binder, such as chopped straw). Check internet for “recipes”. Quite a thick layer, adding to wall mass, useful for sucking up heat in the summer, and releasing it in the winter.

A green roof can be pretty heavy as will soak up a lot of moisture at times.
Both Earthship Brighton & Groundhouse Brittany included their water storage in containers in the hillside in the back – so that rain water from the roof could gravitate to them.
To keep the roof structure light, and ensure less silty water (which is potentially used for drinking), one used steel, the other a membrane over plywood.

The opening sky-lights at the back are sensible (& multifunctional). Obviously allow light in, but also allow air flow (when open). So, on sunnier days, hot air generated by the large sough-facing windows naturally ventilates though the building out through the sky-lights, creating a coiling breeze.

Hope that helps.
9 years ago
Hi Paul, I've listened to a few podcasts, then got rather overwhelmed. So I've foldered the email links you've kindly sent out to listen to "on a rainy day". Thanks for a huge amount of work though, Don