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Bruce L. Bergman[_2_] Bruce L. Bergman[_2_] is offline
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Default Heating Elements for the soil

On Sat, 27 Dec 2008 23:34:08 -0800 (PST), trg-s338
wrote:

I'm building a 4 x 8 foot greenhouse planter and would like to embed
an electrical heating system, some kind of grid/mesh in the soil.
Just need to keep the soil around 70 degrees. Accomplishing this with
the least use of electricity is a plus if possible. Is this something
I can build or salvage off of an existing system? I would appreciate
any suggestions or direction where I can research further. Thanks.


Depends on what energy sources you have available, and their costs.
The electric resistance heat cable route is easiest to set up, but can
be the most expensive to operate by a huge margin. The only time it
would be cheaper is if you get hydroelectric power straight from the
dam.

If you have natural gas or propane or fuel oil available, the
simplest way is to make a heating loop out of copper or poly tubing at
the bottom of the planting beds, a small wet-rotor circulation pump,
and a water heater or a small boiler. Rig a thermostat to start and
stop the circ pump, and let the water heater stat run the burner. And
be sure to install ball valves so you can balance temperatures between
the different beds, plumb them in parallel.

Doesn't even need to be a new water heater, you could get a used
one. The water system is easy to cobble together, you just need an
expansion tank that is mounted high to supply make-up water, and rig
up a float valve to supply makeup water (with an air gap so it can't
siphon back) and a float switch to kill the water heater burner if the
water level gets low.

This style system would be the easiest to fill with antifreeze in
case the system gets shut down in freezing weather. But use
non-poisonous propylene glycol antifreeze for safety, especially if
the plants are for consumption.

The proper water feed method is a "Boiler Trim Kit" - pressure feed
with a double check valve and a special boiler feed regulator, and a
bladder style expansion tank - but they all cost money. If the
expansion tank is open to the atmosphere, technically you don't even
need a T&P Relief valve, but leave the old one on anyway. And it
can't be used in freezing climates.


-- Bruce --