Can you confirm that its necessary/desirable to keep the sand moist to fill any voids within the sand,ensuring that you get an even temperature spread and avoid hotspots /chilly areas?
I wet my sand for a number of reasons for example:
1) I think it preserves the cable and keeps it flexible. My cable is around twenty five years old.
2)I found that by putting a plastic sheet over the sand this keeps it from drying out due to evaporation, plus it keeps the sand free of compost that may get washed out of pots/trays onto it.
I am still using the same sand that I put on the bed when I built it.
Back to the moisture retention; What happens is when the heat comes on the moisture condenses on to the underside of the plastic,then when the heat goes off the condensates seep back into the sand, meaning I only wet it at the start of the season.
In fact that was what I did today!
When I removed the cover today I found that a thin film of green algae had formed so I scraped it off then wetted the sand with water containing some fungicide.
How long does your sandbed take to reach set temperature, and how quickly does it respond to adjustment?
Difficult one this as I have never timed the process, but I would say it is up to the desired temperature within twenty four hours of switching on. In terms of adjustment I never need to do this as the thermostat takes care of this.
Have re-done mine this year but it wonder if its working correctly.
Currently seems to be air temperature plus 8 degs C (Around 13deg C)
I will not know if my system is working OK until I check it tomorrow hence me using a soil thermometer to find out. Once I have checked the temperature I will set my thermostat +/- to my desired temperature.
Once I have done that this will be the last adjustment I will make all season.
In terms of general temperature in the greenhouse this is done by a 3kw fan heater.
All winter this has been set to keep my greenhouse frost free usually around 37*-38* F about 3-4*C.
I keep check of this with a wireless sensor that relays the temperature in the greenhouse to an other sensor in the house so that I have no need to run up the garden to my greenhouse to check it.
Once I start my sowing season I adjust the greenhouse thermostat to give me an ambient temperature in the greenhouse of around 45-50*F (8-10*C)
So with the relative high bottom heat of the hotbed and the general heat in the greenhouse all I do is move plants around to ensure they get the best light available, until it is time to put them into a cold frame or cool greenhouse.
Some seasons I may protect the plants in the cold frame by covering them with fleece if the weather forecast is scheduling low temperatures.
Another thing I find with this system is that the air temperature is usually a little warmer above the hotbed than the greenhouse in general and that it is slightly humid.
This humidity is particularly usefull if taking cuttings, hence its development during the time when I exhibited Chrysants & Dahlias.
I hope that answers your questions.