A Heated Raised Bed

Something for nothing and cheating the seasons are two things that always excite me, so plans for an outdoor growing space heated by the sun have been in the back of my mind for a long time.

Back in the autumn I came across one of those small solar powered pond fountains, which is basically a small aquarium type pump coupled with a solar panel, all waterproof. The idea is simple, the brighter the sun, the faster the pump, the higher the fountain.

The plan was to use this pump, not for a fountain, but to push water round a solar water heating panel, then through the raised bed to warm it.

I constructed the bed from decking boards and fence posts, it is roughly 6ft x 3ft and the posts stand high enough to support protective netting. I part filled the bed with surrounding topsoil, then a mixture of worm compost and seaweed. Above this I laid a coil of hose pipe, threading each end through holes drilled through one of the boards.

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Then the bed was filled with more soil /compost mix, and finally topped off with a layer of coir compost to try and deter slugs a little bit (this is actually the main reason for creating a raised bed, the more barriers the better).

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Then I created a small reservoir in an old plastic tub to hold the water pump.

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The final piece to the jigsaw was a coil of black hose, the type used in irrigation circuits, this was fastened to a black painted piece of thick plywood using cable ties and appropriately placed holes drilled through the wood, (much care needed, drills and hosepipes do not mix well). Once everything is tickety-boo this irrigation panel will be framed and covered with a piece of perspex, or even just clingfilm, to help trap heat inside.

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The finished bed

So far everything is functioning as planned. The pump is pumping when the sun is out and you can tell by the temperature difference between the in-pipe and the out-pipe of the solar heater that it is doing it’s job. It isn’t fantastic at the moment, but I know from previous experience that enclosing the heating pipe will make a big difference.

A word on air locks; the pump is quite small and though it should pump 160 litres per hour, I think it probably circulates much less due to the resistance of the pipes, but this isn’t a bad thing as a slow rate is best to warm the water on its journey round the solar loop. I had to prime the circuit by connecting it up to a hose pipe on mains pressure to blow out the air to start with, but now it looks like the pump is capable of pushing small air bubbles out into the reservoir bucket. I am monitoring this as I based my plan on an internet person’s experience of heating his swimming pool with a small solar pump like mine. He had problems with air locks, but his was a much bigger and more complicated circuit.

Why not use a solar syphon? A solar syphon is a very exact arrangement of pipes, like a sideways ladder that warms water, and as the heated water rises in the vertical tubes it pushes warm water out of the top and so draws cool water into the bottom. I have made these before, and whilst these are good for warming a tank of water, you would still need a pump to push the heated water around the bed, as there is not enough pressure generated from a solar syphon to force water through a few metres of pipe. This project is a simpler setup, with less components.

The idea of all of this is to cheat the seasons, like all chilli growers, and vegetable growers in general, even professional ones I, find that the spring months particularly are a time when there is always a need for more warm growing space than you can get your hands on, especially at night time. So my hope is that at that critical time, March and April, this bed will act as a heat sink and sit a few degrees higher than surrounding soil, and the warm sunny spring days, which are inevitably followed by a cold night will help to warm the bed to a point where spring vegetables get a head start. Lets hope for some good results. I will plot some temperatures over the next few weeks and see what a difference it makes. Time will tell.

3 thoughts on “A Heated Raised Bed

    • I have another identical bed next door without the heating, so I will be keeping an eye on the difference, it is a lot of mass to heat, but hopefully it will build up. Now the coil is enclosed it is producing much more heat. Here’s hoping! A similar setup will heat a water butt to 40c in a day, so it should make a difference.

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      • I really hope you prove me wrong. 🙂 But to me it seems that the increase can’t be more than the equivalent amount of sunlight that is falling onto the water container and it’s solar panel.

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