heating the soil is certainly more efficient than heating the air if the plants need warm roots more than warm air. Using water is probably easier than ducted air, and safer than heater wires. You may have problems with the water getting cold before it gets to the end of the pipe - I would go for a larger diameter pipe.

You might be able to solar heat a tank of water using a separate collector from your greenhouse (perhaps corrugated iron with glass over the top and trickle the water down the gullies) to stockpile some heat and save on your fuel bills.

A significant advantage to storing heat is that you can heat the water with a short, hot wood fire and use that heat over the night. This would be much safer than trying to maintain a fire over night and wood can be quite cheap for heating. Short, hot fires are more efficient and produce less particulate emissions (smoke).

What is your climate, your space requirements and your budget?

Hi, Thanks for your reply. I was wondering in the back of my mind about the possibility of supplementing with solar heat, but hadn't gotten as far as the "how to do it" nuts and bolts. So as far as wood heating a tank of water, would I have to construct some sort of wood burning chamber under the tank, or is this something you can buy off the shelf? Also, where would be the best place to put a tank holding solar collected hot water...in the greenhouse? Nearby, and somehow insulated? What about coils of black poly hose set out in the sun and put on a timer so the hot water would get pumped into holding tank every so often? ...would the corrugated iron/glass setup be a better solar collector than black poly? Winter climate here (on a south facing hillside at 2,000 ft elevation in Northern California) usually dips down to low to mid 30's overnight, but can get down to mid 20's at times, with very occasional snow that doesn't stay on the ground too long. More likely rain than snow. Winter days range from cloudy/rainy to sunny mid-50's-low 60's. Thanks for any advice.

There are certainly plenty of water jackets for wood stoves here, ranging from pipes in the firebox to double walled flue pipes. However, if you are feeling suitably DIY you could probably make something yourself. First confirm that wood is a reasonable option there!

black pipe with a thin film cover is probably more effective than straight pipe, and the problem with pipes is always that they could freeze and burst, potentially emptying all your nice warm water the next day. iron + glass will be much more effective than black poly and probably cost less if you look around (we have the remains of our garage roof, for example). iron + polyfilm probably works too, although I haven't tried it. Remember that for collecting the sun you are after maximal area per dollar. Pipes don't have much area.

On the other hand, there are tricks you can do to increase the solar exposure of the black tubes. I'm reminded of the setup for Apricus's evacuated tube solar water heaters. They have tubes spaced apart by about their width, forming a "blinds" pattern. At most solar angles, you'll get almost as much sunlight hitting them as you would if the entire sheet was covered with black tubing. You can increase the effect by mounting them over a reflective sheet. They call it passively tracking the sun.

The important thing is to have a "greenhouse" setup for your tubes. The worst thing you could do would be to have exposed tubes -- you'll be losing heat through radiation, through conduction into the surrounding air, and new, cool air will be continually cycled past, thanks to convection.

You get the same effect with the corrugated metal as the ridges collect heat and conduct it through to the water.

I agree that it is most important to minimise heat loss of the collector!

So, I'm trying to envision the angles of the corrugated metal/glass or the tube in blinds pattern setup. Would this be lying nearly flat, or at more of an angle...maybe corresponding to winter angle of sun? Should the corrugated be painted black?

When I tried this last year I had a 5m length of corrugated metal with a fall of about 1m, so 1:5 fall. I think flatter would be better as the longer dwell time would make for better absorbtion. The angle probably should be based on your latitude too. I don't have a good idea about the optimal angle, and I suspect that if you can't get the best angle you can make up for it with a larger collector area.

Not sure about painting the metal. Zinc is a good selective surface. You might try it though. Spray on bitumen might work well.

Why don't you try it out with what you can find. One note - don't use polycarbonate as your only glazing because it is corroded by hot humid air. PC with a plastic film would probably work well on the other hand. I'm hoping to get some time to play with this this weekend.

Heating a greenouse needs to be geared to your growing climate. Take into consideration the outside conditions the greenhouse may endure on a regular basis during your specific winter conditions.

Black poly water pipe,

Umm.... There is NSF and non-NSF

One is twice the price of the other.

One has a blue stripe, one has a yellow (or is it green) stripe.

On sale here 400 ft of 3/4in non-nsf was $29

400 ft of 0.75 inch = about 2.3m^2. Are you going to try and keep the pipe in one single piece, or use lots of connectors? A single 5m length of corrugated iron has an area of about 4m^2 and cost me nothing :)

Hi Chris in Iowa,
What does NSF and non-NSF stand for? What I wanted to do was use the black poly tubing in rows a couple inches underground, and connect it to a propane hot water heater in a closed system to run hot water and heat up the soil at the plant root level, much like the electric cable bottom heat, but hopefully cheaper than electric and able to cover a much larger area. This would hopefully decrease the air temp needed to get same level of plant growth and save on energy costs to heat the darn greenhouse in the winter. This would probably involve multiple connectors, and I'm not sure how far the water would travel down the tubing before it cooled off. Maybe I should run it the width of the bed rather than the length? And/or use larger diameter tubing? A lot of things like that to work out in practical terms, and I was looking for advice from someone who may have a similar system. If possible, I might like to supplement the hot water heater with solar heat, by heating water during the day and storing in a tank, then circulating at night. This might involve more black poly exposed to sun, or something like Nathan is describing, trickling water down glass covered corrugated metal exposed to the sun.

National Safety Foundation

The non-NSF stuff is not for drinking water.

A product commonly used in commercial nurseries for root zone heating is called biotherm. It is one fourth inch EPDM (rubber-like) tubing. It is efficient at 140 deg. F water temperatures. The hot water source can be a water heater, solar water heater, etc.

Sounds like just the ticket cuestaroble. How does it compare pricewise to low pressure PE irrigation tubing? Google can't find me anyone willing to sell it.

I dont think a water heater wouldnt be efficent enough, or up to the task. It would be a lot of water. I think you would need a heat pump or boiler. Maybe one of the instant on gas units would work. Talk to a radiant heating place about it. Most radiant heat systems are laid out 8 - 12 inches apart, all accross the surface. Thats a lot of pipe. Anyone done the math as to how much water that would be in gallons ?

numbersix,
Using a typical small greenhouse as the example, with a 10 foot long bench, 3 feet wide. The 5/16 inch biotherm tubing is layed into groves in styrofoam or between a lattice. This allows pots to be placed directly over the hot water (145 deg. F) tubes for bench heating. The tubes are placed 2 inches apart, the length of the bench. In this example that would require 180 feet of tubing. The volume of water in 180 feet of 5/16th inch tubing is just under 3 gallons. The flow through the tubes would depend on the amount of heat needed. The biotherm releases 0.22 btu/ft/degree F with 145 deg F water.

Here is a link that might be useful: biotherm bench heating system

cuestaroble, the original post seems to indicate they want to heat the entire greenhouse with hot water, not just to make heat mats.

I live in North Carolina and completed 2 years ago a greenhouse with a radiant panel in the floor for space heating. I used Zurn/pex pipe and controls (zurn.com), with a 50 gal LP gas heater. My first layer was 2" styrofoam with the 1/2 pex pipe clipped to the styrofoam. I then put about 2" of granite screenings covering the pipe. I then put down some 12" surplus ceramice tile for a floor between my benches so there would be something stable to stand on. I then brushed some more screenings in the spaces between the time for a finished looked.

The best way to heat your plant beds with warm water is to use 3/8" PEX pipe connected to a Noritz gas/lp instantaneous tankless water heater,circulate the water with a Taco pump using a Goldline thermostat control.Install the thermostat sensor in the plant bed soil.Make sure that the bottom of the beds are insulated with styrofoam and the top of the beds have sawdust or small wood chips.You can build yourself a passive water heater out of a couple of 50 gallon water heater tanks.Remove the tank shells and insulation from electric water heaters and install inside a concrete coffin crypt all painted with chassis black.The crypts can be mounted in the ground facing true south with the crypts tilted on angle to your geographical degree of latitude for the best solar exposure through the year.These passive solar heaters can preheat the Noritz heater.It is also very energy savings to build the greenhouse below the ground level for added thermal mass and insulation.

Here is a link that might be useful: INDEKSOLAR POOL HEATING

how much insulation on the bottom of the beds?

Use 2-3 inches of sprayed in polyurethane foam on bottom ,cover foam/interior box with 20 mil pvc tile liner.The liner can be purchased at a good tile supply.It can be stapled to wooden box and glued with Christy's Red Hot Blue glue found at most Home Depots.Be sure to staple liner to the outside of boxes.Interior staples can be glue patched over with pvc liner.