Solar Hot Water on our Cottage Roof

After the success of our experimental hot water system using 300 Metres of plastic pipe on a steel roof, (See our Solar Hot Water System above), I decided to install a similar (but improved) version on the roof of our little cottage on the farm.

The roof of the cottage faces due North, and is 10m long by 2m wide on the north-facing side. This is a total surface area of 20 sq m, but it is in two pitches - one meter at about 35 degrees, and one meter at a shallower pitch, about 15 degrees.  (Which turned out to be rather too shallow for the winter - see below...)

So I decided to build two panels, 10m x 1m each, and cover the roof in two sections. You can see the framing and the start of the laying of the plastic pipe in the photo below (That's our 30 sq.m experimental solar panel in the background):

First I painted the north face of the roof with a DARK roof paint. (I wanted MATT BLACK, but all I could get was semi-gloss sort of Charcoal, so that's what I used...)

Then I built a frame of 75 x 25mm treated timber, and nailed it to the roof (trying really hard to drive the nails into the wood under the roofing iron, not into the air!)

I stretched 16 guage high tensile wire along the roof at 75mm spacings, to support the pipe, which I've attached to the supporting wire at 500mm intervals. Here's a photo showing detail of how I bent the pipe and fixed it at the ends:

After laying and fixing all this pipe (260m of it!), I covered the whole thing over with a clear plastic film (greenhouse plastic) to prevent the wind from blowing all my lovely heat away!

The hot water storage system is rather unique. I connected the system to a gravity-feed hot water tank, as shown in the diagram below:

As you can see, when the sun shines, the valve opens and lets water in to the solar panel. I've restricted this flow to around 150 Litres/hr.

The hot water from the panel goes to a flexible (rubber) inlet hose inside the hot water storage tank. This hose has a float attached, so that the hot water outlet is always near the TOP of the water in the tank.

When the tank is full, it overflows FROM THE BOTTOM, so that the coldest water flows out. (You can work out how this happens from the overflow pipe in the diagram - the vent prevents siphoning...) and it continues to overflow to waste while the sun shines.

When a hot tap is turned on in the house, the hot water is drawn from the TOP of the hot water cylinder, which is where the hottest water will be. (If the valve is open at that moment, some of the water will come directly from the panel to the tap, but if not, it will all come from the storage tank.)

This system performed just fine for one person, or possibly two. Our cottage is a very small one-bedroom affair, ideal for one or two people to live reasonably comfortably.

(I'll tell you how good it is when it's been operating for a while!)

3 years later....
Ok, it's been working pretty well - especially in summer, when the sun does all the work, and the tenants don't need to use any electricity at all to heat their hot water.  In winter, when the sun shines brightly all day, it works too.  But we don't always get bright sunny days in winter!  (And the slope of the roof isn't ideal for the winter sun.  (You probably guessed that from the info. above...)

So, we've replaced the system with a new one.  See our Central Heating Project for the Cottage  (on the Main Page)

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