Monday, May 18, 2015

Winter warmth

I have got the rooftop solar space heater working. It's quite simple. This is what it looks like from the roof:
My north facing roof. From left, the mini-kitchen-solar-HWS, the solar space heater collector, the solar PV, the main solar HWS
It's not easy to see in this photo, but on the top right of the solar space heater collector, there is a 6" galvanised steel duct connected, which goes through the tile. I picked up the ducting cheaply from a car yard for $10 (I saw it from the train and made an offer). In the roof, I connected insulated, flexible ducting to the galv ducting and ran it to a pump (the pump draws about 55 W -- not much). From there, it is ducted to a 3-way splitter, and then to three outlets that I installed in the ceilings. They look like this:

Installing them was quite simple, although sawing through my ceiling was a bit intimidating! I used a cheap thermostat from ebay so that the pump only turns on when the air in the solar collector is warm enough. It is available here (It works fine, but I can't recommend it yet as I don't know how it will last). The thermostat is designed to keep a space at a certain temperature, which is the opposite of how I'm using it. I've put the thermometer in the rooftop solar collector, and anytime that collector rises above a certain temperature (23 C at the moment), it activates the pump [1].

The controller seems to work fine. Note that I also installed a 1-way flow valve (like this), so that warm air can't flow out the house through the solar heat collector at night time)

Performance and improvements

It's early days to judge yet, but I think it has raised the inside temperature of the house from about 15 C to about 17.5 C, which I'm pretty happy about. The weather hasn't changed much from before it was running, and we've had a mixture of gloomy and sunny days.
  1. Insulate the back of the collector. The collector loses a lot of radiant heat from the back. I need to insulate this, which will make the pumped air warmer.
  2. Experiment with collector air flow. I may try to make the air flow a two pass thing, either by double-glazing and running the air intake between the glazings or running the air behind the corrogated iron, to preheat it and make the insulation less important. 
  3. Pump speed. I think the pump has a higher speed setting, and I'll try that
  4. Insulate. I've still got more work to do insulating and draft-proofing the house. That will help
When winter sun is shining in Adelaide, it is about 1000 W per square meter. My collector is about 3 square meters, and I think I'm achieving at least 33% efficiency,  which makes my heater at least 1 kW when the sun is shining (it costs about 50 W to run). This gives a coefficient of performance of at least 20, making it at least 4 times more efficient than the best-performing reverse-cycle heater (and also a lot cheaper to install).


I think that, when finished, this will show it is possible in Adelaide to do a cheap retrofit (I will itemise this sometime) on a 1950s house with relatively poor thermal performance and keep internal winter temperatures above about 18 C.


I have also just put a "lid" on the shower using some of the left over plastic sheet from the solar heat collector. I built a simple wooden frame, and attached the plastic sheeting with some screws, and have just sat it on top of the shower screen. It makes the shower much warmer by trapping the warm air. The shower is more like a sauna, and so it feels warmer (because heat loss by evaporation from the skin occurs less in a warm humid shower). It also has the benefit that not so much steam goes into the bathroom -- we don't even need to run the exhaust fan now. This is really easy to do (about 30 mins work, with children helping), and is really worthwhile.

[1] To work in this way, the thermostat is run in its cooling setting (where it switches on above a certain temperature). In summer, I will switch it to its heating setting, so that it only switches on below a certain temperature. Then I can tell it to pump air into the house, when it is below (for example) 24 degrees. This sounds confusing and backwards, but it works.

This article was written by Angus Wallace, and first appeared at


  1. Hi Angus,
    The duct looks very professional. Nice job. The one way flow valve was a good idea and the controller is very clever too. Nice to hear that you are getting some winter warmth from the system. Hope you received some of the recent rain and that your tanks are filling. They're full here.
    Cheers. Chris

  2. Thanks Chris,

    It was actually all a lot easier than I thought, tho I fudged the installation of the plastic sheeting a bit (I really should have waited until I had a second pair of hands to help -- impatience!)
    We've had loads of rain now -- tanks basically full -- we'll be shedding water soon, I'd like another 20 kL storage, but it takes up valuable garden room which I don't like. When things are a bit better defined, I might try and squeeze in 10 kL as a compromise... I'm thinking about the approaching el Nino...

    Cheers, Angus

  3. ps. the lid on the shower is *really* effective. Highly recommend you give it a try. It makes it a lot easier to shower in cooler water (ie. if your solar hot water is a bit marginal)

    Cheers, Gus


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