Heating cable

I have made a new propagating bench which is sitting on top of a layer of insulation on top of a slatted bench, the box is made with 1 inch thick timber, internally it is lined with two layers of polythene and filled with a 2 in. layer of sand and a heating cable placed over it this was then covered by a further 2-3 in. layer of sand the sand was then sprayed with a watering can of boiled water and the cable switched on through a thermostate the temperature went to 17.5c then started to lower so I then plugged the cable directly into the electricity and the temperature held a 16.5c overnight. I have set up propagators several times over the years but have never had this problem, the cable has never been used before but was purchased over 18 months ago so can't go back, but is there anything that I could be missing

What temperature did you expect it to be?

It should go up to 25c but the temperature I want is 21c

Wouldn't the thermostat control the temperature?

Could be a faulty thermostat

I first connected it via the thermostat but the temperature didn't go above 17.5 I then plugged it directly into the electric socket but it sat at 16.5 I just don't understand why the temperature won't increase with a direct power supply, its as if it wasn't getting full power but I don't how that could happen

You seem to have a lot of sand on top of the cable Spec? Maybe if you had less it would 'up' the temp?

Would vermiculite not be better at transmitting heat if it was used instead of sand, above the cable.

If you include the sand above and below the cable - its 4"-5" deep. That sounds like a lot of sand to warm before you could expect to be able to control the temperature?

This is the way I have set up my hot beds in the past, the 2in. layer at the base ensures that there is no overheating of the wooden floor and the same with the cover above the cable, and though the most important factor with a hot bed is keeping it moist, I have now removed the heating cable and replaced it with a shorter cable, but before fitting it I run both around the greenhouse (if the cable is warm it is easier to install as it is more flexible) and there was a definite difference in temperature between them, the new one we a lot cooler, so evidently it's a faulty cable, so if you ever buy a cable test it immediately to make sure its ok

Vinny I don't think vermiculite would do as it's more of an insulator and damp sand holds and spreads the heat once it's at temperature, I am now waiting to see how the cable I have now fitted goes, the cable I am having problems with only went to 16.5c overnight