convert a soil warming cable into a heating mat

ronald208

Hi I have a soil warming cable and thermostat that I used to have in a sand
box on a bench in my greenhouse. However it was very heavy and my bench
collapsed. So I thought I would update my bench and propagator and use the
soil warming cable to make a pad similar to this.

http://www.keengardener.co.uk/product/view/5864/bio-green-warming-pad-40-x-65cm-(42w).rails

So what type of material should I use to make the pad from? And how would I
fix the cable to the material? I suppose I could put it in the middle of
two layers and sew it in place with nylon fishing line but would that melt
with the heat?

I did try using a sheet of expanded polystyrene but the cable is melting
into it plus I used cable clips and they keep coming out of the polystyrene
and I am worried it might catch fire.

So any suggestions other than use perlite/vermiculite?
I do want to convert it to a pad so that I can roll it up when I no longer need it

Jenny

Photo_Stylus

you need something to help radiate the heat. not insulate it.

A thick metal plate or some form of radiator.

Davesnave

The two things are rather different. Heat mats use narrow cable only about 1/5 the thickness of soil warming cable, so at any point the amount of heat to be dissipated is less. That's why the aluminium foil is sufficient. The soil cable is designed to be used in a larger heat sink. i.e sand. With the one I use, the manufacturers specify damp sand too and say it will burn out in dry.

You could buy some aluminium sheet and then attach your cable by nailing clips through the sheet into some thick ply or blockboard, used as backing. This might spread the heat enough to avoid cable burn-out, but I'm just guessing. You could place rock wool type loft insulation under this to avoid too much heat loss without the dangers associated with polystyrene.

Sand boxes are indeed very heavy, but the sand itself is a great rooting medium & I wouldn't be without one. I'd put my energies into finding a suitable support for the box instead. I use an iron stand that once supported a pottery kiln. Maybe Freecycle?

Lastly, with energy prices rising, the sand bed makes best sense, especially used with a pay-for-itself thermostat.