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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorEd Davies
    • CommentTimeMay 10th 2016 edited
    I would be interested to hear of an inverter suitable for Steamy's suggested setup. I know of none.
    • CommentTimeMay 10th 2016
    I just assumed that a small 600W inverter like my mate has on his off grid system would do. Think his is a Victron or something.
    I shall have to see if I can get hold of him, but think he is off sailing the Pacific again.
    Would advise not to paint the wall black all it will do is overheat during the day and kill your plants off. Far better to have an activated alumina wall and a fogging system which can be temperature activated when the glasshouse gets close to freezing.
    • CommentAuthorEd Davies
    • CommentTimeMay 10th 2016
    All the ones I know of are intended for use with batteries. Let's not derail another thread with this discussion but I'd like the OP to be aware that Steamy's suggestion is basically untried.
    • CommentAuthormike7
    • CommentTimeMay 11th 2016 edited
    Quoting OP:- "The floor slab will be well insulated whatever we do"

    Not sure if that is a good thing in your situation. It would be the opposite of using the thermal inertia of both the floor and the ground beneath to help stabilise the air temperature. That is what the barrels of water would also be doing - if not also warmed by solar panels. If your floor slab is very thick it may not make much difference over short cold spells. Some vertical perimeter insulation might be worthwhile, but not to any great depth.

    Why would you need an inverter for PV if the output is only being used for resistive heating? Probably there's a good reason but I don't know what it is.
    • CommentTimeMay 11th 2016 edited
    Posted By: mike7Why would you need an inverter for PV if the output is only being used for resistive heating? Probably there's a good reason but I don't know what it is.
    Impedance matching and switching.
    Though with low loads and a non critical application, it may not make any real difference.
    • CommentAuthorjondavies
    • CommentTimeMay 11th 2016 edited
    mike7 Quoting OP:- "The floor slab will be well insulated whatever we do"

    Not sure if that is a good thing in your situation. It would be the opposite of using the thermal inertia of both the floor and the ground beneath to help stabilise the air temperature.

    If I was going for the "insulate well and then heat with a fan heater" option I would probably use the slab as a thermal mass to absorb the sun during the day and then look to insulate under the slab to prevent heat loss.
    • CommentTimeMay 11th 2016
    Try a simple experiment with using water as a heat store. All get up early on Saturday, get washed and then fill the bath up with water that is as close to the air temperature as you can get (work out the amount of water in the bath too).
    Then every hour (or half hour), check the air and water temperature. Do this for as long as you can, then plot the results. Will give you an idea of how stable it keeps the air temperature. You can repeat with more and less water in the bath, then sort and plot all the results by temperature differences, time differences and water volume differences.
    We use the difference of height within a glasshouse to our advantage. Any plants wanting a cooler environment are grown on the floor higher temperature plants grown on benches. Delicate frost susceptible plants ie cucumbers on benches but protected by plastic cloches. Typical temperature difference between floor and bench 5C.
    • CommentTimeMay 20th 2016
    Sorry for the late addition, but it's worth having a look at http://www.builditsolar.com/Projects/Sunspace/sunspaces.htm for ideas.
    • CommentAuthorskyewright
    • CommentTimeMay 20th 2016
    Posted By: djhSorry for the late addition, but it's worth having a look athttp://www.builditsolar.com/Projects/Sunspace/sunspaces.htm" >http://www.builditsolar.com/Projects/Sunspace/sunspaces.htmfor ideas.

    Works for us. See pic in


    Other parts of that Topic (Solar corridor - insulated divider - book case?) might be of interest too?
    I'm also starting to look at this as I'd like to install a woodshed and greenhouse combination in our garden to help produce vegetables over a longer growing season. There would be space on the woodshed roof for two solar thermal flat panels (angled to optimise winter sun), which would be connected in a drainback system to solar pipework that ran from the panels, into a drainback tank and down into an insulated concrete slab. The pipes would loop up and down the slab and be covered with a layer of shredded tin cans and other scrap metal to help conductivity, then covered with a concrete slab.

    The shed and greenhouse house face south with the north, east and west facing walls of the shed and greenhouse (and the south and roof of the shed) being insulated with 100mm kingspan that would extend from the foundations. The greenhouse roof would be twin-walled plastic and the front of the greenhouse would be glass.

    I've attached a diagram showing the proposed foundation design.

    The question is how long heat would take to transfer up through the concrete and into the shed/greenhouse and whether a better option would be to lightly insulate the top of the concrete slab to slow down the heat 'loss' into the greenhouse and shed?

    Would this work?
    I've attached a diagram of the greenhouse sat on top of the slab. As mentioned, the rear northfacing wall and the two ends (east & West facing) would be insulated studwork. The roof would be twinwalled polycarbonate and the south facing wall would be glass.

    At one end would be a shed for storing tools, and other bits and bobs with solar thermal panels on its insulated roof, with a small PV panel to drive the solar thermal pump.

    Obviously there would be a lot of heatloss through the glass, a little less through the polycarbonate roof and hardly any through the rear and side walls. The question is whether the solar thermal panels and insulation could keep the interior of the greenhouse above freezing.

    To be fair, if it evened out the often large summer day and night temperature changes and extended the growing season into the shoulder months without using energy and then just needed a bit of input from a heater in winter, then that wouldn't be too bad.
    • CommentAuthorsam_cat
    • CommentTimeJan 9th 2018
    Stick large water butt IN the garage, Radiator in greenhouse, plumb through garage to water butt with submerged pump to circulate (Eheim 1250 or similar matbe). Solar thermal on the garage roof to heat the water in the butt. But gut instinct says that it wont work as you expect.

    1-2kw of solar PV, electric bar storage heater and run it directly from the solar PV? Or do solar PV and grid tie, it offsets a chuck of your bill. Install a normal electric heater in garage and set temperature to something fairly low (2-5oC) so that it comes on below that and goes off when it gets to temp.. This will prevent front, improve the warmth situatuion (even in a week or 2 of no light and very cold... snow?) and the solar PV offsets any costs by binning some of your electric bill.

    My gran used to run a couple of large greenhouses at her house (each was about 10+m long 3m wide) and they always had 2 black water butts at the far end to the door, these would always be full of very warm water, even in winter it warmed up well. Large thermal mass.

    I like the idea of painting the wall black and heavily insulating the other side (inside the garage). Do the same with your slab, insulate down the edges of greenhouse into the ground and under the slab, hopefully slab then is 'inside' the green house and only place stored heat can go is into the greenhouse. Might need to build greenhouse slab with a course of bricks and a cavity of somesorts..
    I keep my 40 mtr long polytunnel frost free by using a double drum sawdust burner but the anti biomass brigade will be along shortly.
    • CommentAuthorsam_cat
    • CommentTimeJan 9th 2018
    Reading through in more detail I see most of my thoughts have already been covered off..

    Another thought, build it to house standards, insulated walls, double glazed, roof with velux or similar. More stable temp year round, not as hot in the summer, not as cold in the winter.. (almost) year round growing.
    • CommentAuthorgyrogear
    • CommentTimeJan 9th 2018
    Posted By: Pile-o-StoneThe roof would be twinwalled polycarbonate

    I used corrugated polyester on mine, it offers optimal light-transfer.

    Also, stick a couple of water tubs INSIDE the greenhouse, Personally I would not bother warming the earth, it will warm itself as and when...

    (Especially if you include a composting pit at one end...)

      solar wood-driers.jpg
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