Monday, July 18, 2016

Holistic home temperature management

This is a draft of an article to be published in the SA permaculture magazine. Your thoughts and comments would be very helpful in making it better.

I have deliberately chosen the term management instead of control. This relates to the idea that, in a retrofit situation, there is no way to control the internal climate of your home without using many resources (expensive, unsustainable). This article will give a quick overview of how to maintain a comfortable home economically and sustainably.
Note that this is in the context of a retrofit -- for new builds and renovations, there are other opportunities that are not covered in detail here.

Quick thermodynamics primer

Heat flows from warmer places to cooler places. A hot cup of tea left on the bench cools down to room temperature, while a block of ice left on the same bench will melt. In both cases there is a temperature gradient, heat moves from a warmer place to a cooler place and eliminates the gradient. The greater the temperature difference, the steeper the gradient, the faster the heat moves. You can slow this movement of heat by adding insulation -- a tea towel wrapped around a mug of tea will keep it warm for longer.
It is exactly the same with your house. If you want to maintain a house that is a different temperature from the environment (eg. cooler in summer or warmer in winter) then the natural movement of heat will be slowly removing that temperature difference. You must work with this process.

A way to think about heat and your house, in an ideal situation

These examples aim to work only with the resources at hand -- they try to avoid using lots of energy from the grid or other stores.


It’s morning, outside it’s very cold. Inside, your house is at its coolest. When the outside temperature is greater than the inside temperature, you want to bring that heat inside as much as possible. When the sun rises, you allow it to shine through the windows into the house. When your solar PV is producing significantly, you switch on your reverse-cycle or heat-pump hydronic system to start pumping heat into your house. When the sun hits your solar heat collector or greenhouse, you pump the warmed air into the house (this can be thermostat-controlled). At the end of the day, as the sun sinks and the outside temperature drops below the inside temperature, you close up the house to slow the escape of inside heat into the cooler environment. You avoid space-heating inside the house now (because your solarPV is no longer producing), and use heating direct to occupants as they need it (see below).


It’s morning, and the coolest it will be all day. Your house should already be open to allow warm air to escape. If you have a solar heat collector and pump, you can pump warm air out of the house. If you have East-facing solar PV, you might consider switching on your reverse cycle to cool the house further (taking advantage of the cool outside temperature to improve the efficiency of the heat-pump), otherwise you might want to wait until a little later. Now might also be a good time (depending on the humidity) to use an evaporative air conditioner if you have one.
All outside blinds should be down before the sun hits any windows, and as the outside temperature increases above the inside temperature you should close up the house to exclude as much heat as possible. Every curtain, door and blind should be closed.
At the end of the day, as the sun sets, the outside temperature is at its highest. You should not open up the house until the outside temperature drops below the inside temperature, but at that point, you should open all doors and windows to let the warm air out of the house.


I will now talk about some ways of being more comfortable, in order from cheapest and most sustainable to most expensive and least sustainable.

1. Rug up

It’s a lot harder to change the temperature of an entire house than it is to change the temperature of one room. It’s also a lot harder to change the temperature of a room than of a single person. Try temperature changes that target the human body directly:


  • put on a jumper, hat, thick socks 
  • put a throw (electrified throws are also available which are extremely effective)
  • have a hot drink
  • do a small amount of exercise
  • sit in a snug chair (one that surrounds you and stops heat loss)
  • use a heated chair


  • use a fan to move air over your skin
  • wear fewer clothes
  • have a cool drink
  • Avoid unnecessary movement
  • Put your feet in cool water
Doing these simple things allows you to be comfortable in a much greater range of inside temperatures.

2. Target heating to the person

Heat that targets a body is much more direct than heating a whole room. More of the heat is warming the person so it is more efficient. [0]
I’ve already mentioned some direct active heating methods that can be used. An electric throw (eg [1]) is an electric blanket that is designed to be worn when using a computer, reading or watching TV. They use little power but are very effective. Similarly, a heated foot pad (eg [2]) will help keep your feet warm when it is cold.
Conversely, a cool foot bath is very cooling on a hot day.
A cold bedroom doesn’t matter if there is warm bedding and an electric blanket. You can even buy a refrigerated bed (eg [3]) cover to keep the bed cooler in summer.

3. Stop heat movement through the building exterior

When I say “stop” I really mean “regulate” (it is not possible to stop the movement of heat, you can only slow it down). You want to control when your house lets heat in and out. If there’s a warm sunny winter’s day, you want to get lots of heat into your house (ie. open the doors and windows, allow winter sun to shine into Equator-facing windows).

This means you must insulate your house. I approached it in this order (based on bang-for-buck and easiest-first):
  • stopping drafts (caulking around doors, skirting boards, windows)-- reduces convective heat transfer
  • roof cavity (loft) insulation (batts) -- reduces conductive heat transfer
  • window coverings (curtains, bubble glazing, external blinds, secondary glazing, double glazing) -- reduces conductive and radiant heat transfer
You definitely want to keep the sun off all windows in summer. That makes external blinds a very good investment. Judicious use of deciduous trees/vines can be helpful, though their leaf-free time may not perfectly align with your desire for winter sun.

4. Use passive solar gain to warm your house in winter

Adelaide has very hot summers, but still has a “heating” climate for most of the year (April - October). Hence, thinking about winter warmth is very important. You want to get winter sun shining into your home, preferably onto something dark and heavy, such as a stone wall or floor inside the house. Dark things absorb energy (heat) from sunlight more effectively, and heavy things can hold more heat (thermal mass). This is like charging up a heat battery in your house.
If you don’t have good Equator-facing glazing that will let lots of winter light into your house, it can be expensive to retrofit. Some other options are to build a greenhouse on the Equator-side of your house, then let the warm air into your house. Another option is to build a solar heat collector and pump in hot air [4]. These are less effective than true passive solar, but are a cheaper retrofit.

Note that thermal mass in the floor can be problematic if it is inadequately insulated from the ground. It can be very difficult/expensive to maintain a desirable floor temperature in that situation. It may be better to "insulate the floor out of the space" instead of heating it.

5. Active heating

There will be times when you want to employ active heating in your house.
If so, remember that it is much cheaper and more sustainable to only heat (a small) part of your house.


I don’t recommend gas heating. Gas is expensive to connect initially, the ongoing supply cost is expensive and the gas itself is expensive and becoming more so. Also, gas is a fossil fuel and thus contributes to climate change.

If you live in the city, think carefully before installing a wood combustion heater. I have been using one sparingly this winter, but they produce so much smoke and soot (if you want efficient, low-smoke combustion, you will quickly burn your wood and lose lots of heat up the chimney. If you throttle the fire to reduce chimney heat loss and conserve wood, your fire will produce a lot of smoke and soot). [5]
Rocket-mass-heaters or masonry-heaters [6, 7] are a possible exception to this, but I suspect most people wouldn’t install one of these as they are too expensive and/or too labour-intensive.
Separate to the question of smoke/soot, where will you get your firewood from if you live in the city? Is the wood sustainably grown/harvested/cured/transported? (I’m betting not)

We don't rely our wood heater a lot, and I don't really want to change that approach.

What does this leave? There are three main options, as I see it:

1. Hydronic heating

Has the advantage of allowing various sources of heat (eg. evacuated tubes, heat-pump) and can store heat from times of abundance (eg. sunny day, solar PV) to release later. It is fairly expensive to install.

2. Reverse cycle space heating

Cheaper to install, but less effective. There is no heat storage, except in the thermal mass of the house itself.

3. Electric radiant heating.

This is nowhere near as good as an electric throw, but is still more efficient than space heating of any kind, because it can be targeted more to the building occupants. [8, 9, 10]

A few thoughts

This article is not exhaustive, by any means. I don't think that sustainable home heat management is a "solved problem". There are some other, more explorative, options that I've considered but not researched in more detail.
  • Home fuel-alcohol distillation and its use for heating/cooking. This would provide only very meagre heating, but could provide cooking on the few days that solar PV is inadequate. Finding a sustainable source of sugar for fermentation could be tricky in the city.
  • Home ethanol synthesis using surplus summer PV electricity. This could be a good way of storing surplus solar PV energy in summer for use in winter. A catalyst is in the early stages of development, and no commercial product is available
Really, we just need to learn to live with cooler houses in winter, and warmer houses in summer.



  1. Chris,

    I'm particularly interested to hear your thoughts on my recommendation against using wood fires in (sub)urban areas, given your experience with wood heating in rural areas.

    Cheers, Angus

  2. Everything I ever knew or said about green sustainable design was probably wrong

    no heat pumps required One Step Off The Grid
    Hi Angus, some links for your consideration. Im not a fan of thermal mass floors myself, cold feet equals unhappy me! insulation under a thin low conductive surface ie (polyurethane sealed wood) is barefoot anytime of the year.
    Wood heaters are tricky, they need to reflect enough heat back into the fire and secondary combustion area to keep it above a minimum temperature. Going out on a limb I'm guessing 800-850 C for the primary and at least 500 C for the secondary chamber. Much below that and the high heat generating clean burn collapses and takes the chimney draw with it. Some of the super low emission heaters lose their draw occasionally, so I am thinking of backing off the ragged edge and trialing a local heater next winter *800LE. Perhaps modifying it to run pressurised with a tiny fan for overnight burns and playing around with the insulation to keep the fire internal temp up a little higher. I'm confident it will run well without the mods but Hey! where's the fun in that. I have looked at a lot of wood heaters in the last 5 years and I am not very impressed, I think we are paying for form rather than function.

  3. Hi Steve,

    Thanks for your great comment and interesting links.

    The TreeHugger article is interesting. I think it's highlighting the tension between new-build and retrofit scenarios, as well as the greater context of urban planning. The comments are interesting too. I think that a lot of consideration needs to be given to the specifics of the site. For example, if shading means that winter sun is affected, then it doesn't make sense to have a lot of Equator-facing glazing since even the best glazing loses a lot more heat than a well. This is precisely the dilemma I face at my house -- a large gum tree means our North-facing wall only gets winter sunshine form about midday onwards. I don't want to put too much glazing there!
    The One Step Off The Grid article is intersting too. I am planning to buy one of these radiant heaters, and wire it with a movement sensor so that it automatically switches off if no one is there. See how that goes... I don't think I agree about using "excess" solar PV to resistively heat water though -- I still support either solar HWS or heat-pump. This is for the simple reason that solar PV energy that is exported to the grid is not "wasted".

    Your thoughts about flooring are interesting. I guess historically, floors would be heavy and cold, and in winter they'd be covered with a rug (that was put away in summer). That has the best of both worlds, perhaps?

    Our current combustion heater is about 60 years old, and exhausts into a chimney. It's very inefficient. Perhaps it would be better to replace it with a fan pumped one so that I can have a cleaner burn and also capture the heat in the house without it going up the chimney. Certainly having an exposed flue like on the 800LE (which looks good, btw) helps a lot.
    I am presuming that a strong secondary burn will consume most/all of the smoke and soot?

    I will try to incorporate some of these ideas/links into the article prior to publication. Thanks very much for your suggestions!

    Cheers, Angus

  4. Hi Angus,

    The term management is spot on. A great idea. Control does lend the wrong mindset to the whole issue.

    I also have to second Steve's comment about thermal mass under floors. I have thick R3.5 batts under 19mm timber tongue and groove flooring and even on the coldest day I can walk around barefoot if I so chose. Do that on a concrete floor and you immediately know that it takes a huge amount of energy to stop the thermal mass draining the heat out into the soil and no amount of insulation can stop that loss to the soil. And the cold radiates at you if the house has not been mechanically warmed for many long days - even if the air is warm.

    Why do we make houses on concrete slabs then? Because it is quicker and takes less skill to do than strip footings or stumps. And labour costs a huge amount. Nuff said really. Stumps are not a cheap way to build a house.

    I agree with your overall strategy. Local has the benefit of being cheaper and easier to manage too as you get immediate feedback.

    I don't reckon people think much nowadays about actively managing for the inside temperature in a house. They used to do that when I was a kid.

    I can't really speak for Adelaide, but there is no heat in the winter sun here, so you can have as much sunlight streaming in through windows as you can manage to achieve, but it won't actually heat anything. Otherwise you'd be able to grow tropical plants in a greenhouse all year around down here - but that is not possible this far south. You know, I've often wondered whether this viewpoint favours the societal preference for large windows in houses? Dunno, but if you look at older houses you generally notice that the windows are very small in proportion to the walls. Sure, glass was expensive back then, but it isn't cheap nowadays either.

    Again, I'm generally suspicious of talk about eaves too instead of a proper full sized verandah shading external walls and windows. We do things in buildings because they are cheap and not because they are good and thus that is what is promoted as being the correct solution. And many of the processes involved in building are skewed towards project builds.

    Wood heating is great as I can manage that energy resource from start to finish, although it has taken me at least six years to come to such an understanding. It is a complex resource. In a city, to use coal fired electricity means that you are laying waste to somewhere else. You should travel to the Latrobe valley someday and see the smoke stakes from the brown coal power plants. Oh my!



  5. Thanks, the excess PV thing shouldn't be an issue long term if the utility/retailers work with their customers and trade electricity, society will find daytime use for it. The resistive water heating saga is created by some short sighted policy (SIGH) I'm hoping it will pass before the utilities get their customers too far offside and destroy their own business.

    Lloyd makes a good case (treehugger) sash windows are great for cross ventilation but they also leak heat like a sieve. If we dont have the tree for afternoon shade as well, its going to be bleak, its all about combinations that work together. The alternative, insulation, double glazing, a heat exchanger for fresh air and air conditioner will certainly do much better on some sites particularly if there is a chance of mould from condensation.

    A thermal mass floor that is not insulated underneath is almost impossible to warm with heated room air, the walls change temperature a lot faster than a floor because they leak less heat out to the environment. My observation about floor coverings is the plusher the pile the colder it feels, the captured air layer in the carpet is the same temperature as the mass underneath. I think short tight pile or weave would be better for a rug. The benchmark I go by is standing barefoot on a flattened cardboard box, try it, if a floor surface can match that, its a winner.

    Given a choice, I would insulate a thermal mass floor out of the living space rather than leaving it in.

    A pressurised firebox is not something that is is available on wood heaters in Oz.. yet, there are some tech hurdles to clear. I see some merit in choosing a freestanding heater over the inserts, mainly because with an insert the circulation fan has to run to extract heat. Very little radiant or convective heat gets into the room without it running. It's hard to maintain a temperature with an insert heater (particularly a big one) they are a blast furnace when you stoke them and dribble to nothing at an idle.

    Yes, the heat in the secondary burn area when that extra bit of hot oxygen is introduced is the key to complete combustion, however, if the primary firebox is not hot enough some of the combustible molecules are never formed in the first place. Fires that run hotter need furnace grade firebrick or cheap easily replaceable parts, not popular in a price sensitive industry. It is easier to sell brass trim rather than proper efficiency.

    I was tossing a few ideas around with somebody last week, we reckon a heat pump with a small inverter driven scroll compressor pushing a hydrocarbon refrigerant through a solar collector could be extremely efficient, we could heat water, thermal mass and interior spaces all from the one unit with no frost risk on the collector side. In theory it could work down to -40C.

    I guess it is all relative, burning 4 tonne of redgum is worse that 1.5 tonne for the same result, is worse than burning low grade chip off-cuts or plantation trimmings, is not as good as not having to stoke a fire in the first place.
    PS Just read Chris's post before I hit the publish button he has covered it well. Architects talk about thermal bridging, every time there is a join or fitting that cuts across the thermal envelope of a building heat is lost, window frames and joists are a large part of that.


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