Solar water heater in Greenhouse

We are building a high tunnel in WA (zone 7b).  The main purpose of the high tunnel is to get a whole bunch of starts rocking early spring... however if it is there we would like to grow whatever we can in it all winter.   So we are looking at heating options.

We are going to build cold frames inside the greenhouse which we will remove and place over some beds in the spring.  This got me to thinking if every layer of greenhouse material bumps the zone up... how about a solar water heater inside of a greenhouse.. heating up our barrels.

Has anyone done this?  I know theoretically it should work, but I'm wondering if the juice is worth the squeeze.   I haven't found any info about this type of setup on the web.

Hi Jared, welcome to Permies!

So are you proposing a solar water heater collector inside the greenhouse to heat water tanks that are also inside the greenhouse?  That's interesting and I'm not sure how much it will do.  The greenhouse will only accept a given amount of sunlight (energy).  Whether it hits the collector and goes into the tanks OR if it just hits the tanks directly (or the floor or plants), the same amount of energy was transferred into the thermal mass of the greenhouse.  But, you may prefer it to be transferred to you tanks instead of the ground, since water is a better thermal mass.  So if you goal is just increasing thermal mass energy accumulation, it should help.  But day in and day out, the average temp of the greenhouse would remain the same (I think).

If the collector is outside the greenhouse and absorbing energy that wasn't going to hit the greenhouse, then I think you get a better boost.

Or if you want your thermal mass to be hidden on the North side of some banana trees where the sun can't hit it, this would be a way to transfer energy there without putting the tanks in the sun.

If your high tunnel runs E/W, you may want to consider making part of the North side of the tunnel out of an insulated material.  Holding heat in would really help.

Is this going to be a NRCS high tunnel?  If so, I imagine that what your are proposing would be "out of contract".  If not, Look into solawrap covering.  It is like bubble wrap for greenhouses.  A bit spendy, but when we build our next house (already have the property) my GH will be covered with it instead of traditional GH poly.  Supposedly will last for 20 plus years.  If not that look at using a double layer of conventional GH poly and use a blower to inflate between the two layers.  That coupled with more thermal mass in the HT and maybe even a wood stove could let you grow several extra months and possibly all winter depending on you climate and winter severity.  I would suggest a RMH, but have no personal experience with them, especially in a GH.  Could be a viable option as well.

Mike Jay wrote:Hi Jared, welcome to Permies!

So are you proposing a solar water heater collector inside the greenhouse to heat water tanks that are also inside the greenhouse?  That's interesting and I'm not sure how much it will do.  The greenhouse will only accept a given amount of sunlight (energy).  Whether it hits the collector and goes into the tanks OR if it just hits the tanks directly (or the floor or plants), the same amount of energy was transferred into the thermal mass of the greenhouse.  But, you may prefer it to be transferred to you tanks instead of the ground, since water is a better thermal mass.  So if you goal is just increasing thermal mass energy accumulation, it should help.  But day in and day out, the average temp of the greenhouse would remain the same (I think).

If the collector is outside the greenhouse and absorbing energy that wasn't going to hit the greenhouse, then I think you get a better boost.

Or if you want your thermal mass to be hidden on the North side of some banana trees where the sun can't hit it, this would be a way to transfer energy there without putting the tanks in the sun.

If your high tunnel runs E/W, you may want to consider making part of the North side of the tunnel out of an insulated material.  Holding heat in would really help.

I am suggesting that...  I read that 55 gallon drums often do not fully heat in greenhouses due to their size/thermodynamics.   I figured the solar collector would circulate the water, fully heating it (and hopefully heating it more).
I'm not attempting to raise the daytime temperature of the greenhouse.  I would like to simply add thermal mass for nighttime temps.
Makes sense to put it outside of the greenhouse now that you say this - never though of the fact that this adds area for the sun to hit.

Any easy (cheap) ways to insulate high tunnels?  

Walt Chase wrote:Is this going to be a NRCS high tunnel?  If so, I imagine that what your are proposing would be "out of contract".  If not, Look into solawrap covering.  It is like bubble wrap for greenhouses.  A bit spendy, but when we build our next house (already have the property) my GH will be covered with it instead of traditional GH poly.  Supposedly will last for 20 plus years.  If not that look at using a double layer of conventional GH poly and use a blower to inflate between the two layers.  That coupled with more thermal mass in the HT and maybe even a wood stove could let you grow several extra months and possibly all winter depending on you climate and winter severity.  I would suggest a RMH, but have no personal experience with them, especially in a GH.  Could be a viable option as well.

It is not.  I will look into the bubble wrap as well as the blower.  We are keeping costs as low as possible for this house as we have the windows to build a permanent green house... we are running the high tunnel as an experiment to see where/how/what to heat etc our 2nd greenhouse.   The high tunnel will then become a starting greenhouse and a nightshade machine (we hope!).

We plan to put an old woodstove we have in/in a connected shed outside the greenhouse with the stovepipe running under the entire thing.   We may build a RMH if we have time.  I would also like to incorporate as many "simple" systems such as compost/solar hot water to cut down on our wood and the amount we have to tend the fire.  

Jared Van Denend wrote:I read that 55 gallon drums often do not fully heat in greenhouses due to their size/thermodynamics.   I figured the solar collector would circulate the water, fully heating it (and hopefully heating it more).
I'm not attempting to raise the daytime temperature of the greenhouse.  I would like to simply add thermal mass for nighttime temps.

I'm struggling with this in my mind as well.  I've read that if you use one gallon water jugs for thermal mass they heat up fast and cool off fast so you lose the heat back to the greenhouse before the night is over.  10-20 gallon jugs were suggested to hold and release heat in about a day which would be perfect to offset the lack of sun at night.  55 gallon drums sound like they are more of a 3 day gain/loss so they can help out over cloudy stretches.

In my mind, it makes sense that circulating water into the drum from an efficient solar collector would boost the temp of the water and give you more heat to release at night.  But "conservation of energy" is telling me that the amount of energy into the building is the same either way.  So I'm guessing that with the solar collector inside the GH it would reduce your daytime temps slightly (by hiding that energy in the drums) and increase your nighttime temps by allowing the extra energy in the drums to come out then.  Overall, average GH temp would be the same but it would help by leveling the daytime highs and nighttime lows.

Another thermal mass collection idea I had (and probably many others) is to put some black pvc pipe at the highest point of the greenhouse and circulate water from it to your tanks.  All day the sun would hit that pipe (solar collector).  Plus the rising heat of the greenhouse would thermally warm the pipe as well.  That would let you store that heat before it escapes through the thin greenhouse plastic.  At night you'd have to shut off that pipe so it doesn't work in reverse.

Insulation ideas:
If you do the double poly glazing with the blower, maybe you can insert some packaging peanuts between the layers on the North side.  Or fasten some 1" styrofoam to the ribs between the layers.
You could fasten styrofoam to the inside of the ribs on the North side and cover them with a white or reflective material to bounce light back down to the plants.
Concrete curing blankets are another possibility.  Just drape them over the GH on the North side and tie them down.

Hi Jared,

how much space are you allocating for the solar water heater ( I guess the water storage/ transfer portion)? I'm moving towards the GH phase of our build and have been thinking of this as well. I've thinking of using the black barrel wall idea as a winter heat storage mass and summer drought water supply that will serve as a work bench of planter shelf (hoping the warmth will add additional seed germination awesomeness). For the water heat however I've been playing with two ideas, the solar gain ( basically the high pressure black hose on the roof, I already blew up the black garden hose) and the compost pile water coil. The compost pile heater coil has been on my mind for the past two years for emergency hot water and it's thermal properties while providing ample compost for the spring planting. I've seen compost piles in corners of greenhouses or in one section that keep the GH mild even in snow.  

Here's a possible different approach.
I know running hot-water tubes through the planting beds may seem like a budget buster, but would be more efficient to focus on soil temperature than ambient temperature? One could collect solar heat in tubes in a panel or heat from a large compost pile and run the tubing through the bottom of planting beds so that you spend your energy only heating the places where the plants are and not all that cubic feet of air. In extreme low temperatures, use row covers to hold in heat and deter frost. A small solar powered pump can circulate your warm water through the system.

There are some other posts on permies.com that touch on this topic extensively.

Here is a video of a small high tunnel with double layer and small blower. The person in the video is showing about a 60 degree gain with the double layer.  

Here are some interesting ideas presented here by Verge:  

Mike Jay wrote:

Jared Van Denend wrote:I read that 55 gallon drums often do not fully heat in greenhouses due to their size/thermodynamics.   I figured the solar collector would circulate the water, fully heating it (and hopefully heating it more).
I'm not attempting to raise the daytime temperature of the greenhouse.  I would like to simply add thermal mass for nighttime temps.

I'm struggling with this in my mind as well.  I've read that if you use one gallon water jugs for thermal mass they heat up fast and cool off fast so you lose the heat back to the greenhouse before the night is over.  10-20 gallon jugs were suggested to hold and release heat in about a day which would be perfect to offset the lack of sun at night.  55 gallon drums sound like they are more of a 3 day gain/loss so they can help out over cloudy stretches.

In my mind, it makes sense that circulating water into the drum from an efficient solar collector would boost the temp of the water and give you more heat to release at night.  But "conservation of energy" is telling me that the amount of energy into the building is the same either way.  So I'm guessing that with the solar collector inside the GH it would reduce your daytime temps slightly (by hiding that energy in the drums) and increase your nighttime temps by allowing the extra energy in the drums to come out then.  Overall, average GH temp would be the same but it would help by leveling the daytime highs and nighttime lows.

Another thermal mass collection idea I had (and probably many others) is to put some black pvc pipe at the highest point of the greenhouse and circulate water from it to your tanks.  All day the sun would hit that pipe (solar collector).  Plus the rising heat of the greenhouse would thermally warm the pipe as well.  That would let you store that heat before it escapes through the thin greenhouse plastic.  At night you'd have to shut off that pipe so it doesn't work in reverse.

Insulation ideas:
If you do the double poly glazing with the blower, maybe you can insert some packaging peanuts between the layers on the North side.  Or fasten some 1" styrofoam to the ribs between the layers.
You could fasten styrofoam to the inside of the ribs on the North side and cover them with a white or reflective material to bounce light back down to the plants.
Concrete curing blankets are another possibility.  Just drape them over the GH on the North side and tie them down.

I think perhaps you're stuck on the potential energy of the greenhouse...  It is working by trapping some of that energy inside its structure.   If a cold frame inside a greenhouse preforms better it is logical that a solar hot water heater would preform better as well.   BUT what is the trade off as far as area?  

I like your idea of the black PVC pipe.   To go a step further perhaps bury those barrels in the ground to heat the actual soil.  

Vern Life wrote:Hi Jared,

how much space are you allocating for the solar water heater ( I guess the water storage/ transfer portion)? I'm moving towards the GH phase of our build and have been thinking of this as well. I've thinking of using the black barrel wall idea as a winter heat storage mass and summer drought water supply that will serve as a work bench of planter shelf (hoping the warmth will add additional seed germination awesomeness). For the water heat however I've been playing with two ideas, the solar gain ( basically the high pressure black hose on the roof, I already blew up the black garden hose) and the compost pile water coil. The compost pile heater coil has been on my mind for the past two years for emergency hot water and it's thermal properties while providing ample compost for the spring planting. I've seen compost piles in corners of greenhouses or in one section that keep the GH mild even in snow.  

I have not really come up with a number.   The greenhouse will be 30x18'.   I think barrel tray is a good idea... we will be doing something similar, or burying them.

I've also looked into the compost water coil.  From my research it takes a very large compost pile to make it feasible.  I plan on trying a smaller system in a barrel.  Hopefully with the ability to swap out the coil/barrel when it needs it.   The problem with this is if you run your pump too fast and pull too much heat from the compost it will stop composting  (which is why i'm doing it in a barrel).

For us the big thing will be having many options all which will slightly boost the heat till (hopefully) we can grow some fruiting plants.    I think if we set up a good barrel system and have a few ways to heat them that will be the best for thermal mass etc.

What i'm worried about is if you put a bunch of thermal mass in your greenhouse and it is not getting heated daily it will simply be cooling the greenhouse rather than heating it.

if that is the case we just have to drain the barrels.    My biggest questions now is if its worth it to bury the barrels in the ground (if they are being heated by compost/solar hot water/coil in the stove) and if the solar collector should live inside our outside the greenhouse.

Dan Grubbs wrote:Here's a possible different approach.
I know running hot-water tubes through the planting beds may seem like a budget buster, but would be more efficient to focus on soil temperature than ambient temperature? One could collect solar heat in tubes in a panel or heat from a large compost pile and run the tubing through the bottom of planting beds so that you spend your energy only heating the places where the plants are and not all that cubic feet of air. In extreme low temperatures, use row covers to hold in heat and deter frost. A small solar powered pump can circulate your warm water through the system.

There are some other posts on permies.com that touch on this topic extensively.

Here is a video of a small high tunnel with double layer and small blower. The person in the video is showing about a 60 degree gain with the double layer. https://youtu.be/LlEczaE0-eg

Here are some interesting ideas presented here by Verge: https://www.youtube.com/watch?v=b1p-Dm1bUjs

I think you are 100% correct about focusing on soil temperature... this is why i was thinking about burying the barrels directly under the beds.

I have seen these air transfer systems and they look great.... we will likely do something similar when we build a permanent greenhouse.   For our high tunnel we are just looking for low cost semi-simple solutions to add some heat.   The tubing is a great idea and perhaps we will have that, but the biggest issue in my opinion is night time temps.   But perhaps I'm going the wrong direction and if we heat our beds enough in the daytime then they will hold that heat into the night?  

My observation is that at this latitude the winter sun angle is so low that it pays to have the high tunnel running east to west with a reflective north side.  If you use black PVC for purlins to hold a mylar lining on the inside of the north wall then you could pump water through them to absorb heat when the temperature is too high or radiate heat when it is too low.
The thermostat in my house for example is set to come on when the temperature is above 73F so when the solar gain from the sout windows or the wood stove heats to that setting the circulator comes and moves the air to the north end of the house.  In the summer the inside air intake is closed and it draws air from under the house and exhausts the hot air through the bathroom vents.
If you set up such a thermostat to pull air through lighter larger drain pipes and you have three long raised beds with pipes returning through the soil with less  expense and less digging

Hi All,

I have been using a water heating system that is working so well, that it needs to be shared. It is efficient with space as well as time and energy, and I think it has great potential for greenhouse use--far better than compost piles or even building fires.

I am using it as my primary water heating system in my house and it works great.

I have an electric water heater (mine happens to be a two element 220//240 type). I replaced the lower factory element with a 24V, 600W element, and direct wired two 200W solar panels set up in parallel so it produces a 24V current. These two panels were mounted on my roof above where the water heater is located. The direct current heats the water beautifully.

In a greenhouse, the hot water could be piped through the earth in shallow pipes to release the heat gently, or passively thermo circulated into other water drums etc, or, directly below planting beds, etc. There could be options, of course, depending on the heat needed for the time of year (valves to send water here or there). The panels would be outside, out of the way, perhaps on the ground below window height, but as close as possible to the water heater tank location, since DC power doesn't travel well.

When I first installed it (in early spring this year), the panels received less direct sunlight (due to less sun time and the angle of the roof) so there was not really adequate heat produced, so I actually backed up the system with my regular (battery style) solar system--not an efficient use of solar--but I did this on a timer, so it only went on at peak sun times. (I could do this back up system because I have a two element (220/240V) water heater, where I hooked up the main system to the upper, factory original element, set to go on only when the water fell below 100F, and having my direct current system attached to the bottom 24V element). But it turned out that I could turn off the inefficient back up by about March, and I had plenty of hot water all summer and now, almost into October.

*AND* I have since found out that I can produce even more heat from the direct current system by using a 24 V element with less resistance... a 24V, 200W element, instead of the 600W element. So this fall, as the sun lessens, and I need more heat, I plan to drain my tank and install the 200W element instead, and see if that provides enough hot water in winter with out the use of my back up battery based system. I got this idea from some local solar guys who like to tinker with ideas. They started me off with the 600W element, and have since found that the 200W works even better.

I don't know if the thermostat on the direct current system works (it is wired to work, but not sure if it does), but never actually tested it, since I have it on maximum heat, and I use all hot water if I notice there is extra. So there may be danger of it overheating if not looked after (mine does have a pressure release valve with exit port). If I leave the house for a few days, I pull a fuse that disconnects the solar panels from my tank. But in a greenhouse, the extra heat can be automatically directed to a safe heat sink, so no need to monitor it.

My tank is only a 30 gallon tank, and I am only one person, so any a larger system would need a larger tank and or additional panels. Yes, there is some upfront cost, but panels are getting cheaper all the time, and a tank is a good investment. For the bargain hunters, 'Broken' water heater tanks may even be usable as long as they don't leak. I can find out where to get the 24 V elements if needed, but probably a google search will do.

I hope some of you have fun with this idea. I think it has great potential.

Well I have a 34' dia dome greenhouse, with a 1/2 loft . While building my home adjacent, I moved in to the north 1/2, insulated that, so I have 1/2 glazing. For DHW, I put inside  3 ea  4x8 flat plate collectors, I had salvage, a 150 gal Stock tank for reservoir and a small taco pump to circulate. As long as there is nice sun, we have plenty of hot water. And as the greenhouse never freezes, ( also have close 3000 gal water storage in there in various ways,)  so that tempers the temps, I do not need heat exchangers or any such, just pure, ( hard as hell) well water.

Michael, that's great!  What climate zone or approximately where do you live?

I have played around with the solar water heater concept before . I ran 200 feet of black 1/2" poly tubing inside a 4x5 bow with a glass face. While the water heated up in the tube as soon as the water started flowing the temperature difference was negligible. Although some folks say that this works for them it definitely didn't pass the test for me. I have an aquaponics GH and have decided if and when I update I will be building a RMH and set the fish tank(s) on top of the mass bed to heat the water too which will act as an additional storage for heat. In theory running the RMH a couple of hours a day will provide all the heat necessary for the aquaponics and GH. I have a Sketchup drawing if you're interested.

You could heat the water like this:

https://www.ourfigs.com/forum/figs-home/227858-new-greenhouse-heater

You could make a collector like this:

https://www.youtube.com/watch?v=qXvrYNOZY04

But then you would need 1) a pump 2) a drainback system if you have freezing weather.

I'm thinking about both these systems for my DIY build, also digging down 2-3 ft for some "poor man's geothermal" and insulating the north wall. I'm in Georgia 7b.

Keep us posted on what you do.

I know this question was posed over a year ago, but this is what I did on this idea. I put the heat collectors on the inside. I know this has resulted in less heat gain, but I don't have to worry about freezing water in the tubes either. I entertained the idea of an outdoor freeze protected glycol system and heat exchanger, but didn't want to spend that much money.

No matter how you heat, perimeter insulation going to a depth of 2-4 feet, east/west long axis, secondary covers (coldframe or low tunnels) close to plant tops and as near the ground as practical, and a (edit, minimum) ratio of 15 to 1 interior floor area to perimeter length are the main technology to apply before fuel, solar or any other heat source.

http://www.newfarm.org/features/0404/moore/greenhouse.shtml

If you can find the article from home power magazine, its a treasure trove! The family built 15 or so greenhouses over many seasons in their home business of supplying organic produce before arriving at these simple specifications. It is a wealth of practical knowledge that can also be applied to other structures.

Many greenhouses suffer having a footprint so far under the golden ratio that heating is cost prohibitive or futile.

Steve said back in 2002 or so, ' the small greenhouse farmer must spend half or more of proceeds to keep a greenhouse in production all year, if they must pay heat it'. That was in north east pensylvania.... the design requires no external heat at that location.

I have used Steves wisdom to clobber an international graduating class of industrial engineers who could only prescribe two forced air funaces per 40x90' greenhouse...... for an off grid and low cost alternative..... to wood!

No wonder we cannot advance.

frank li wrote: and a (edit, minimum) ratio of 15 to 1 interior floor area to perimeter length

Hi Frank, can you explain this ratio?  I've heard that a good ratio for passive solar greenhouses is a E/W length double the N/S width.  I followed that on my greenhouse which is 40x20.  That gives me 800 sqft.  If the perimeter should be at most 1/15th of that, I calculate a maximum perimeter of 53 feet.

Mike Jay wrote:

frank li wrote: and a (edit, minimum) ratio of 15 to 1 interior floor area to perimeter length

Hi Frank, can you explain this ratio?  I've heard that a good ratio for passive solar greenhouses is a E/W length double the N/S width.  I followed that on my greenhouse which is 40x20.  That gives me 800 sqft.  If the perimeter should be at most 1/15th of that, I calculate a maximum perimeter of 53 feet.

Yes, you have it correct. A 40'x90' yields 3600 sq' and a perimeter of 260' length. Almost 14 to one and close enough for a manufactured greenhouse.

Square or round shapes work best for the ratio, but not for light admittance where a rectangle with a long axis east/west and a gothic arch are generally better. I would ask a mathmatician or actual engineer or energy evaluator on those details, i just read stuff! There are good online calculators for the purpose. I can do the simple calculations for electrical and pv system design, but heat loss of a structure over the year makes my brain hurt!

The wider the ratio, the less heat loss through the walls and footing, mainly the footing because of the conductivity of the mass of frozen earth on the other side. Air has less mass for heat to move to and so the rate is less unless it is circulated to the same effect and that is a reason for secondary covers in the greenhouse. The heated soil will have less loss to the outside via conduction/convection and the resulting heated air can only rise to the top of the secondary cover before having to conduct to the other convection loop in the envelope. The temp differentials are in steps and not as wide, further slowing heat loss and keeping a higher average temperature down where the plants are instead of in the top 3 feet of a 14 foot tall envelope.

The arrangement is proven to widen the season and in some regions do away with the requirement for added heat when growing cold tolerant plants. Either way heat requirements are reduced to a minimum without heavy insulation of a north wall or having to operate or buy an insulating cover.

An interesting exercise that i like is to do the calculation on this ratio for various available sizes of greenhouse..... the best ratios are more expensive.... span costs!

A 60'x75' is 4500 sq' and 16.6 square feet of absorber/storage/radiator per lossy foot of perimeter! Again to the heating degree/day mathmaticians for the answer on performance vs cost difference of the resulting 2.8ish to 1 between a 40'×90'. So as an appeal to the "Gandhi of Greenhouses" Steve Moore ' 40'x90' is the most cost effective size and not untill that size, is the ratio starting to be advantageous' heavy para-phrase from 10 years and more ago, but not this gem!

After the greenhouse is set a greenhorn can make $30k per year and after cultivating proper clients, technique and varieties a pro fan produce $60k per 40x90 house.

" Find 20 customers and quit your job" Steve Moore.

The best inspiration ever!

Gotcha, so the only way to hit that ratio is to get commercially big.  I just ran the numbers for my greenhouse and failed but I didn't scale it up.  Thanks!

Mike Jay wrote:Gotcha, so the only way to hit that ratio is to get commercially big.  I just ran the numbers for my greenhouse and failed but I didn't scale it up.  Thanks!

Being outside the ratio is not to be considered a fail! More thermal mass, more insulation at the ground and on the cover or north wall are workable, but may cost as much or more than an upsize in floor to perimeter for the same effect. The thread and others on the topic lay these out many times.

I have come into farms that looked like a shipwreck of propane containers and heaters and couldnt imagine the cost or time involved.
At one place the people had propane heaters (salamander type) firing under tables with low tunnels over them with plant starts in february, 3 houses full and they were losing the battle. I suggested electric water heaters and hydronics on insulated tables full of sand and pex with double bubble covers to be way more efficient and safer..... they said it was a great idea but couldnt justify the cost......

All three green houses and all the plants, an outbuilding, and all the equipment and heaters, their small 20 and 100 pound fuel bottles and the large bulk propane "pig" were lost to a raging fire in march. Im told the propane explosions were spectacular.

Space in the greenhouse is at a premium and is rareley enough, so i lean toward bigger. We have about 6 months of winter here!

I wish I'd had these numbers before I built my 8' x 30' greenhouse. LOL It's always been too narrow.

Water is indeed an excellent heat sink. So adding a water mass to the greenhouse should help moderate the temperature of the greenhouse.
This is the first year that I will have a high tunnel/greenhouse to use. It was set up in January of this year so I wasn't able to use it last winter.
The plan for this year is to put a 275 gallon IBC tote in the greenhouse and use a pump like the one running my aquaponics, '12 vdc low wattage'
to circulate water through a 100 foot black hose rolled up outside the greenhouse. The pump will run straight off the charge controller of my
100 watt PV panel. Since no battery is used it will only circulate water on sunny days. This is the only time that heat can be added anyway.

Good luck. I tried the same setup in my aquaponics greenhouse. Didn't work but then again we don't get a lot of sun in Michigan in the winter. I replaced the solar with a chiller coil running from my wood heater. That didn't do much either.  Electric bucket heater was the only thing that would heat the 330 tank and that didn't do much to warm the GH.  Only practical way to do it is with cold water fish. Tilapia will die below 65 degrees. I kept the water at 70.

On average, we get between 205 to 216 sunny days per year here in Louisiana and we get some kind of precipitation, on average, 102 days per year.  I'm thinking that even if there isn't much added heat to the water it will stay above freezing. This should or may at least give me a place to pot up some starter plants on top of the tank nearest the mass of non frozen water.
The average number of days with temperatures of 32° F or colder decreases from 36 at Shreveport to less than 10 at New Orleans I am between these two so my average should be somewhere 8 to 16 days. We have a very mild climate here although it can get bitter cold and freeze with snow on occasion.
Part of my greenhouse plan for this year is to keep chickens and rabbits in half of it. They may not add much heat but some is better than none and maybe a compost pile as well.

Jared Van Denend wrote:We are building a high tunnel in WA (zone 7b).  The main purpose of the high tunnel is to get a whole bunch of starts rocking early spring... however if it is there we would like to grow whatever we can in it all winter.   So we are looking at heating options.

We are going to build cold frames inside the greenhouse which we will remove and place over some beds in the spring.  This got me to thinking if every layer of greenhouse material bumps the zone up... how about a solar water heater inside of a greenhouse.. heating up our barrels.

Has anyone done this?  I know theoretically it should work, but I'm wondering if the juice is worth the squeeze.   I haven't found any info about this type of setup on the web.

Beside heat storage, there is another side, that is the insulation property of your greenhouse. I have tried everything I could think of. To cut the story short, Richard Nelson's SolaRoof approach is the best. This system is basically a double layer greenhouse, creating a cavity that we cool it in summer time by spraying water in there and keep it warm in winter by filling the cavity with soap bubbles. That's it. We cannot find any other system that is better than this. Period.