I built a wood frame, put plywood on top, and mounted a couple of light sockets in it. I wired the sockets in series (not parallel). I use two 60 watt bulbs. The bulbs burn dimmer and use less electricity than if they were wired in parallel. The heat is gentle, and fairly even, so there's no fire hazzard. If I wanted to get fancy, I would install a thermal switch. These are the devices that automatically turn off a portable heater or hair-dryer, in the event of overheating. When the device cools down, it automatically switches back on. These devices are available from Digikey Corp. Do a search for bi-metal thermal switches. Get a "normally closed" one, at a relatively low temperature rating.

John

I don't think a rear window defroster will do much of anything, let alone you trying to power it. Putting a car battery by your grow area would be odd, and you'd have to charge the battery, which would be a pain. The price of a batter, if you don't have one, is also on the high side for that use.

The light bulb warmer seems like a good idea, although you don't have to go the route of changing the wiring. Series wiring is not needed, as it'll probably only confuse you, and make things a bit more dangerous, shock hazard wise. In a set up with 2 series wiring light sockets, if you remove the second light bulb, you now have a lot better odds of getting toasted by the socket, because if you bump the socket edge or something, you now get zapped and pick up the load of that other lamp, basically powering it through YOUR body. AMPS are what kills you, and any added load to a shock, and you get more amps, usually a LOT more amps. It hurts, and take my word for it instead of accidently feeling it. Just wire 2 sockets normal, in parallel, and use smaller watt bulbs. Or, go the custom route, put two 100w bulbs in, with a dimmer. Then you can change and adjust the light/heat, and find the perfect setting!

One way to heat a larger area evenly and relatively cheaply is to use a heating cable, which you run back and forth in a continuing "S" pattern beneath your flats, or in a layer of sand under a "hot bed." This is made easier if you insert wood dowels into planks on each side (or cut notches) and use them to guide the cable.

Here's another source.

I did exactly what nygardener suggested and even got it from CGHS, it has worked great for my needs and covers a large area.

I built two boxes from 2x4s and a plywood bottom. Filled with 2 inches or sand, put in a waterbed heater, covered with 2 more inches of sand and put the thermostate along the side. Worked great. Set them on the floor of my basement "greenhouse" area.

One of my goals when designing my bottom-heating fixture was to keep operating costs low. For that reason, and to even out the heat, I decided to wire two incandescent light sockets in series.

The waterbed heater sounds like a good idea, but I wonder how much electrical power it uses. I also wonder how much it costs to run the heating cable.

I disagree with joezkool about the shock hazard of series-connected bulbs. First of all, you should unplug the cord when changing light bulbs. Second, if you happened to get shocked as he stated, the socketed light bulb may act as a resistance, limiting the current flow through your body.

Regarding the dimmer: When you operate a dimmer at the low end of it's settings, it can be triggered off by a power surge or sag. You may wake up one cold morning and find that your plants froze from lack of heat at night. I prefer a more reliable system.

With the series-connected lights there is the danger that one of the lights would burn out, and therefore the other light would not burn. However, by operating the bulbs at half-power (because they're wired in series), they will last longer than their rated life. Still, you can replace the bulbs after a given number of hours if you wish.

Joezkool's suggestion of using two smaller-wattage bulbs wired normally (in parallel), is a good one. If one bulb burns out, at least the other will still provide some heat to protect your plants.

All things considered, the series connected bulbs may be the cheapest to build, and more importantly, the cheapest to operate.

John

The amount of heat you get per watt of electricity is the same for light bulbs and heating cables, and both come in an assortment of wattages. The cables are fairly cheap and most have thermostats built in. A cable arranged in even rows might provide a more uniform heat than a pair of light bulbs. Light bulbs also need replacing periodically. Those reasons might give cables a bit of an edge  but both should work fine.

I tried to use a heating cable once, but found out that it only heated up when the temperature was close to freezing. In other words, the internal thermostat kicked in at around 35 degrees. It would have served its intended purpose (keeping pipes from freezing), but was of no value for warming the bottoms of my plants in a cool growing room. The room temperature was never cold enough to allow the cable to heat up!

If you can find a heating cable that will provide gentle heat in a semi-warm room, then you should use that as nygardener suggested. Look for a heating cable without a built-in thermostat. Otherwise, the series-connected light bulbs with some sort of heat deflector is a good solution. To even out the heat, I mounted a metal plate inside the box.

John

Two bulbs, wired in series, should last an extremely long time. The voltage/current through the tungsten filament will be cut in half. Add a light dimmer, and you should be able to easily control the heat. Those bulbs also ought to last nearly forever.

How many people normally get shocked changing an incandescent bulb, anyhow? Just unplug it, if you ever need to change a bulb.

As a side note: For the last three winters, I have set up a 2-person heating blanket on my covered back porch, arranged with some boxes, carpet and blankets, to keep a varying-sized colony of feral cats warm. And I fed 'em, too. They liked that.

Half of them have loved it well enough to get spayed/neutered, and move inside. Now the inside cats keep me warm, and the outside cats keep the rodents out of my garden.

Zink

No one has mentioned the economy potential of using 2 1nch styrofoam beneath the heat source, perhaps protected by a sheet of something. How about it? fg

fg,

My heating box sits on top of an insulated floor. I'm not sure that the additional styrofoam insulation would help that much. I will not add styrofoam to the sides of the box either. My sunroom gets a little too cold in the winter, and I would like to somewhat warm the area surrounding the plants in addition to the heat that is rising above the box. I'm concerned that cold spots, near the windows for example, may harm my plants.

I have also completed the construction of a second warming fixture. This one has one bulb (incandescent socket), and a dimmer. I like that idea because I can easily change the temperature settings based on the needs of the plants. For example; tomato seeds need about 75 to 80 degrees for germination, somewhat less than 70 degrees for growing seedlings, and then warmer again for fruit production. This fixture will let me do it all, without changing bulbs or moving the plants. Also, I believe that day/night temperature variations are helpful. I'll probably turn it down at night, and up again in the morning. I'll use one or two thermometers to make sure I'm getting the desired results from my dimmer settings.

John

It makes sense that heating cables sold to keep pipes from freezing would have thermostats set to 35° or so. Those sold for plant propagation are set to about 70°.

A side note: you can consistently control any heat source using a separate thermostat with a probe that you place in the soil, but they're expensive. I used one like this to control the heat in the base of a homemade cold frame.

Here is another source for bottom heating supplies. They carry soil cables long enough to cover a large bench area or even an entire floor!

Here is a link that might be useful: Heating mats and cables

Quick question about heating cables. I am putting my tropical plants in my basement over the winter and plan to build a mini greenhouse. I have a skeleton of an 8x8 outdoor tent that I will cover with plastic. I want to make some kind of sandbox (like a big kids sandbox) as the bottom.This will sit on the basement floor. Should I give the box a bottom or can the basement floor (cement) be the bottom. Also I am looking at some heating cables. Do you just burry those in the sand and put more sand on top of them. Are they save enough if the sand get wet?? I am thinking electrical shock or should I not have to worry.
I have a small heating plant tray "greenhouse". All it is is a styrofoam box with a heatmat in it and you can fit a plant tray in it. It has a plastic cover and then you just plug it in. But very small. The styrofoam does insulate it really well, so I am wondering if I should add a styrofoam layer to my big kids sandbox.
Any input greatly appreciated.
thanks