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Passive Heat, what REALLY works?

Posted by spogarden Wa5 (My Page) on
Fri, Mar 19, 10 at 0:37

I watched the weather report late last night, &$%*! They were predicting 28 degrees. I have a 6 X 8 HFGH, I like it very much but I had just put a tray of starts out there today that seemed doomed. I got up early today and ran out to rescue the survivors. As I made coffee and researched heat retention, I thought about the nice spot I had for the thermometer, right by the door at eye level, on the south wall. It often said 90 degrees or higher and so I would open the vents. I read that water was supposed to help with heat retention, but my water bucket on the floor was always ice cold, even when the thermometer said it was 90+. HMMmm, was I being misled? DUH, yes I guess heat rises.

If I put the therm. on the floor next to the water bucket, what would it read? It dropped like a rock, to the more predictable temp of 20 degrees above the outside temp. crap!!. SO I spent the day ripping up and reinstalling the brick floor, starting where I knew I should have in the first place, at the NEcorner, by the door. Originally I just paved the pathway.

I keep adding plastic containers of water hoping that they will hold the heat, but they are cold when I use them to water. What do you all use for passive heat? I dont have electricity but could maybe see using a solar fan to move the hot air. 30 percent of the roma tomato starts died, and the one stray green been in the corner of the tray, RIP.


Follow-Up Postings:

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RE: Passive Heat, what REALLY works?

You need dark containers of water to absorb sunlight. And some extra insulation in the GH on the north side.

Dan


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RE: Passive Heat, what REALLY works?

The dark containers need to be in direct sun or they won`t gain much. They will also reduce the amount of reflected light in the greenhouse.


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RE: Passive Heat, what REALLY works?

So those water filled pop liters won't help? I did also put some strips of floor vinyl up against some of the north wall windows. They look kinda white trashy but they were just the width of the windows so I thought I would give it a try. I should have most of the floor covered by bricks this weekend.
Does anyone know, if you dig the floor down deeper, does that keep the greenhouse cooler overall or prevent heat loss? I was thinking of doing that if there was a heat retention benefit.


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RE: Passive Heat, what REALLY works?

I removed the water bench out of my greenhouse and installed permanent raised beds. I'm think it has regulated the temps as much as the water ever did. They were white containers filled with black die. Never followed through on painting them black. The more you pack into your greenhouse the better,IMHO. It's like your refrigerator, the more that's in there the better it works. I believe this is true. Nothing to back up my thoughts.

Cucumbers on water bench
Photobucket

Greenhouse today, raised beds.

Photobucket

Looking at the photo, I see, I need to install more cross bracing.

Eric


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RE: Passive Heat, what REALLY works?

I'm a newbie, having built my "solar" greenhouse in the late fall and early winter.

I have a subterranean system with ADS covered by 14" of 1" rock. I have nine- 55 gallon plastic drums and a black masonry wall. I found my greenhouse stayed in the low 40's when outside temps dipped into the low single digits. Amazingly, when outside temps were in the low 20's, inside temps were in the mid 40's at best, a testament to the thermal storage properties of water, I suppose.

Initially, I painted 3 black and left the other 2 (4 aren't in direct sunlight) a light aqua blue. There was a noticeable, though not huge difference. After a bright, sunny day the black ones were probably 5-10 degrees warmer, a lot of BTU's when one considers the volume of water. I experimented with the masonry wall and 2- one gallon jugs as well. The black water jug as well as the black wall were considerably warmer after a sunny day than their lighter counterparts.

I will be adding at least 5 more water barrels when they become available later this spring.

Mark


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RE: Passive Heat, what REALLY works?

I went out to my greenhouse to see. I put over 600 litres of water in it last week. Last night the low was -9C outside and -3.9C in the greenhouse. We had several warm days and nights since I started wintersowing and I have a variety of sprouts. They look fine, other than the marigolds I should have watered yesterday. I do have water directly under the majority of flats (ice cream pails and a large tote). Although the ice cream pails are in dark garbage bags and the larger containers are covered with black garbage bags, they don't get a lot of direct sunlight. Yesterday was cloudy and the high in the greenhouse was around 10C. Sunny days run the interior temperature into the high 20's.

Last spring below freezing outside translated into 0C inside. The interior temperature followed the outside temperature down until it hit 0C.


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RE: Passive Heat, what REALLY works?

I should have added that the walls are insulated @ R-19, the ceiling @ R-30 and the stem wall@ R-10 (foamboard to a minimum depth of 32"). The door and windows are insulated and south facing roof is doubled polyethylene.

I'm also investigating the option of installing some solar panels for the heating of water to be stored for nighttime circulation.

Mark


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RE: Passive Heat, what REALLY works?

Markmahlum, you don't have a passive system, though, do you? You have a fan, right?

Spogarden, you have a 6x8 HFGH...sounds to me like you need better air movement and circulation inside. That is quite temperature differential.

In my GH most of my thermal mass is soil within the raised beds, like Eric. Little water bottles are a waste of time. You will see, after a couple of years, they leak, they break, and your time is much better spent insulating that north wall or maybe hanging another layer of poly film inside to create another air gap.

Disregard Tom in 6bzones results in the link below...his structure is really not a greenhouse in the sense that we think of a greenhouse (although his results are impressive!). Study the other posts and the temps reported and I believe you will find it not worth your time.

Here is a link that might be useful: water temps and thermal mass


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RE: Passive Heat, what REALLY works?

I do have an active subterranean system with a blower but I seldom remembered to turn it on. I will ultimately purchase a thermostat. Of course I did not scientifically measure and record any data, but through observation, I'd conclude most of the thermal storage occurred in the 495 gallons of water.

Before addition of the water (which blocked some black painted masonry wall and covered 1/4 of my rock floor), temperatures were at or below freezing when outside temps were in the low single digits. Adding the water improved that by 12-15 degrees, I believe.

I had a similar experience with a 8' x20' cold frame that was used to get a 2 month jump on the gardening season. For several year I relied solely on "earth materials" solar collection. After I added water in 5 gallon buckets along the back side, I noticed more moderate temperatures.

I used the National Sustainable Agriculture Information Services' "Solar Greenhouse Resources" site as well as several books in the design of my greenhouse. Water stores nearly 4 times the BTU's that "earth materials" do although finding space can be an issue. I don't know what the life of my heavy duty plastic barrels will be, but unlike metal, there won't be a corrosion issue. If you doubt the thermal storage properties of water, look at how much later in the winter a deep body of water will freeze than a shallow one.

Anyway, just some observations so far.

By the way, it is essential to adequately insulate all non-glazed areas of your greenhouse. As a designer and builder of homes, that I can emphatically state. Your local building department can tell you what R values and U values are required for your zone.

Mark


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RE: Passive Heat, what REALLY works?

I haven't yet added any passive solar storage yet but will consider a few things.
Heat doesn't rise!!-hot air rises and hot water rises but heat moves from the warmer to the cooler object regardless of the direction
so
I am considering driving aluminum rods into the floor of my greenhouse intending to drive heat into the soil when its hot and retrieving that ground warmth when its cools off. I intend to have a mesh radiator at each end. I will use aluminum because it transfers heat so well.
I am curious about any critiques of this before I attempt it.


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RE: Passive Heat, what REALLY works?

stressbaby,

The link you provided shows that water storage can moderate temperatures. The critical factors are: insulation, the volume of water/sqft, space for storage, the amount of sunlight, ambient temperatures, and purpose. Stored water will extend the season but won't keep tropicals in northern zones happy all winter.

My greenhouse is an uninsulated extension on a shed. Water had no effect until I hit 400 litres. As I stated the value for me appears to be keeping the temperatures in the greenhouse from going below freezing. For me the primary goal is extending the season and getting plants out of the house before the bugs and powdery mildew get them. Some days the tender annuals aren't happy but they live. I have my water under benches or acting as benches so I'm not giving up space to water.


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RE: Passive Heat, what REALLY works?

knotbored
It wont work, your limited by the surface area for capture and transfer. All transfer is conductive so the area in contact with the soil matters :)

Mark,
Water may store nearly 4 times the BTU's of "earth materials" but if you can`t transfer the heat into the water it wont make an ounce of difference.

At the end of the day, the effectiveness of a system (passive and active)is limited by the availablity of sunshine.


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RE: Passive Heat, what REALLY works?

How about having the floor made with real dark red or black paving brick?


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RE: Passive Heat, what REALLY works?

Sunshine where Mark lives is not a problem to obtain; Colorado, USA is an excellent place for solar. In many areas sunshine storage is the way to go, and dark brick absorbs heat but does not store for as long as water. A mesh radiator may work but you will be limited by the surface area for conduction. The current favored ways to do this are subterranean forced air and rock storage, and water storage. Every other passive method is cost-prohibitive (and I'd argue for my blood forced air is costly, unless you can obtain the material less than cost).

Dan


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RE: Passive Heat, what REALLY works?

It has been great hearing everyones ideas. I knew random bottles of water were too easy for heat storage. I don't think I have room for large quantitys of water. I have lots and lots of river rock where I am, will use that where I can.
I asked about lower terrain greenhouses because I think I will add a second 6x8 HFGH on to this one. I will take more time and the north wall may be made of different materials than the kit. I would like to go deeper inside, think it might give me more months of above freezing temps. Perminent beds may be a consideration too, seems like that is a plus were temp control is an issue. From the outside they would look like one long greenhouse.


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RE: Passive Heat, what REALLY works?

hex2006,

If you have very limited sunshine it will be difficult to obtain much heat transfer into water. The thermal conductivity of concrete is some 3 times greater than water. I'd imagine in an area without much sun that is a consideration. If the sunshine where spogarden lives is that limited, any benefit from passive solar will be difficult to achieve.

At the end of a sunny day the black masonry wall (in my greenhouse) is quite hot, much warmer than the water barrels. However, within 3 hours it is cold, much colder than the water. My masonry wall is well insulated, by the way. I read somewhere that only 4" of an 8" thick masonry wall picks up significant BTU's. I'm not sure how correct that is, though.

I have a thermometer for measuring air temps in the greenhouse but nothing to accurately measure the masonry wall or water temperature.

I researched phase change materials a bit and they still sound good in theory. I couldn't find either a good, affordable source or much information on this method, however.

Mark


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RE: Passive Heat, what REALLY works?

Your shcs uses phase change to transfer heat. Each lb of water condensed from the air represents 1000btu`s of latent heat.
A black wall surface may reach 140F in full sun which creates higher convective and radiant losses, which could draw heat out as fast as its going in :)


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RE: Passive Heat, what REALLY works?

A black wall surface may reach 140F in full sun which creates higher convective and radiant losses,

In my view you want as much white in there as possible in winter.

If spogarden is in SPOkane, that is a challenging place to store heat in winter, as there are several periods in winter that low clouds settle in for many days, and this year was esp bad.

One of these threads had pics of 5gal square containers which may be economically ideal for several reasons until a system is perfected. No sense going whole hog right away unless you want a small commercial op. Water and those food-grade containers is pretty cheeeep.

Dan


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RE: Passive Heat, what REALLY works?

I am aware that the schs is a phase change system. Thermal storage occurs in the water and the rock, I believe. I was referring to the Glauber salts material.

So, the belief that a dark earth material such as tile or a painted concrete floor make good thermal storage materials is incorrect? All of my walls, except the masonry wall are dead white.

At the risk of sounding stupid, I'm not sure I understand the principle behind painting the masonry wall white other than the fact that I reflect useable light.. I'd planned for some dark pavers down the center aisle. I guess they should be light?

You're saying I'd have better thermal storage results if I painted the masonry wall white?

I appreciate the input from both of you.

Thanks,

Mark


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RE: Passive Heat, what REALLY works?

No, I don't believe that is the thought on the white.

In winter, light conditions are much lower than other times of the year, so you want to get as much light bouncing around to the plants as possible. The darker color will allow for more absorption but the point is that they lose their heat so quickly that it really doesn't make much of a difference.

Personally, my foray into garden structures ends at season extension. I am only looking at a month (at most) on either side of the frost. Best of luck to you.


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RE: Passive Heat, what REALLY works?

Here are the containers. They are foam concentrate containers from the fire department.

Photobucket

Hope this helps

Eric


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RE: Passive Heat, what REALLY works?

Mark
Eaglesgarden has it right with lower light levels but even more importantly lower angles, its only 14 degrees elevation at noon here in the dead of winter.
A dark floor will need the sun to be directly overhead to heat it :)
The north wall will probably be the only thing getting any direct sun depending on obstructions to the south.
Any amount of sun from any direction will ultimately heat the greenhouse air for your shcs to make use of.


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RE: Passive Heat, what REALLY works?

I see. I am aware of the benefits of white for light reflection and it sure works in my greenhouse, at least in terms of blinding me. Over half of my masonry wall has been shielded from the sun by the water barrels, The rest will be when the remaining barrels become available. The upper part will be blocked by tall plants, e.g. tomatoes cucs, pole beans, etc. I may very well paint that upper part white, anyway.

I'm at 7400' elevation, 37 degrees lat., 5-10% humidity with absolutely clear cobalt blue skies much of the time (except this winter) so you can imagine the power of the sun here.

I stumbled upon the schs concept only after my foundation was poured, thus only 14" of rock and 1 layer of pipes. If I had it to do over again I'd do 3 layers of pipe in 4 feet.

Dan, you're correct about cost, schs isn't cheap- 14" cost me $550. Rock, backhoe, pipe, blower, pipe. As with everything, a cost/benefit analysis should be completed. Operational cost is minimal though, 76 cents per month at 8 hours per day.

Mark


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RE: Passive Heat, what REALLY works?

Hi Mark,
You don`t want to be digging it up twice, once is usually bad enough :)
You`re lucky to have such a great location to provide you with plenty of bright winter days.
Your greenhouse sounds like it may be quite large, my install cost around $300 all told. It includes a 700cfm blower, tubing, fanspeed controller and a 2400gph automatic sump pump i installed for flood prevention. I have 2 layers of tubing arranged in a radial pattern at 1.5ft and 3.5ft deep in fairly heavy clay.


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RE: Passive Heat, what REALLY works?

Actually my greenhouse is only 10'x20'. I don't need a sump, but I failed to add the cost of a yet to be purchased thermostat for the blower. I used 1" rock which set me back around $350 for the material alone.

Speaking of sunshine, the Mesowest Mesa Mountain, Colorado site is the closest to me, about 20 miles south and only slightly lower in elevation. The total solar radiation at that site for the past few days has been in the 570 ly. range.

Mark


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