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.
A system I saw years ago at a friends workshop was:-
he ran a continuous length of galvanized fencing wire in a snake
pattern , suspended about half the thickness of the slab above ground
- he used bits of garden hose for this. He then poured the slab. (He
had brought the 2 ends of the wire out of the slab before he poured
it). He then connected the wire ends to a 30v transformer, from fading
memory it was supplying approx 3 amps. 90 watts doesn't sound much,
but it kept the place warm (and dry) throughout winter. Cost to
install was minimal.
No idea where you live, Melbourne gets to perhaps 3 dgrees C on a cold
night, 13 degrees C is a cold day.
Just an idea - you would need to do the maths for your location....
And if anyone on the side remembers Roy Hartkopf VK3AOH now SK and
has more data of his shack heating, please post it here...
"accomplishing with the least use of electricity" is the tricky part.
Resistance soil heating cables are a common greenhouse item, plenty
cheap enough to buy new. It's running them that costs money.
More complexity, less electricity if you used the same electricity to
power a heat pump - but prohibitively expensive for heating one planting
Insulate the whole planter well - that will make a big difference. The
whole thing - including underneath. Use good quality double or triple
glazing for the greenhouse part. Use at least 2" extruded styrofoam.
Depending how much sun you get and how handy you are (not cost effective
if you are not handy and have to buy the collectors) a solar thermal
collector (perhaps with PV powering pump or fan) coupled to the soil
(via pipes or ducts) could do well, and relegate your electric to
night-time & cloudy-day back-up.
If you have hot water heat in the house, often the most cost-effective
(to run) route is to couple a heat exchanger to a antifreeze filled loop
running out to the planter, if the planter is near the house. Insulate
the pipes well, of course.
Do a google search on "greenhouse soil heater" brings up several
suppliers. "driveway heat cables" will get you some more. ditto on
"gutter heat cables"
Around here we use a lot of heating cables for ice control in the
winter. While not exactly UL approved for your application, they are
certainly some tough cable. These are 2 wire plus a ground sheath.
Normally run 3 watts per foot and about $.50 per foot in 40' to 120'
Just how much you would need to run these would depend on what soil temp
you need, how much heat loss through the bottom, what spacing you decide
on, etc etc. I looked at some of the suppliers for some watts per square
foot info, didn't find too much. But a 60' cable would give you about 3"
max distance to a heat source, run 180 watts, cost around $30 to $40. I
would put it on either a timer or a thermostat.
> 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
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.
Which is why i said to improvise. If he's in a fairly mild climate
(non Snow) running the electric heaters won't break the bank.
But if it gets Sub Zero in his area, you start needing lots of
electricity to keep the starting bed toasty warm, and that starts
getting seriously expensive - more money than you would save buying
fruits and vegetables at the store. That's when you improvise.
You don't go buy a new oil-fired boiler to rig this up, you call
your local friend with the Heating and Air company, and your local
Plumber, and see what useful things they have in their scrap piles.
If they know you are looking for a used unit, the next time they
take one out they can do it a bit more carefully so it still works.
This is R.C.M - Adapt, Improvise, Overcome. The first step is to see
what you have to work with.
If he's living /way/ out in the boonies and Oil and Propane are
expensive to deliver, then you start thinking about building a wood
fired boiler. Temperature regulation can be a challenge, but there
are ways to do it.
Or you tap a line off the boiler for the house heat - build the
greenhouse as a "Lean-To Shed" off the south side of the house to
minimize the pipe runs.
If this is an indoor system you can buy standard under-bed planter
heaters for this. Or you can go the lower tech route and use a water bed
Another option would be to go with a solar/water heater system.
I use a small water heater and used nylon tubing routed through sand,
then over the sand I have a layer of root stop fabric and then the
planting soil. The water heater is set under one end of the bench and
there is also a solar collector on the side of the greenhouse. I have a
thermostat control system that circulates the water through one or both
depending on temperature. For the months that it runs it costs about 75
bucks to keep the greenhouse and the planting beds warm.