Jim Griff

Update: Since it was to hot in the greenhouse this summer I decided to change my plans so I ran AC current to the greenhouse for lighting, ventilation and to run the heater cord on a transformer.
I was also working on getting another 6kw more solar for the house and figured it would be best to run my greenhouse on more consistent AC current. Unfortunately while replacing the roof that the solar was to go on I ran across a lot of wood rot and structural deficiencies which required much more work than expected. 🤷‍♂️
Before this roof problem occurred I started putting a 2nd layer of polycarbonate on the greenhouse roof on southern walls and a radiant insulation on the northern walls which should make a difference with retaining heat when the colder months come.
Hopefully soon I can get back to working on the greenhouse and wire up an Inkbird and relay to control the heating of the sand and rock bed below the greenhouse so I’m not wasting electricity .🤞

I love my 100 ah LiFePO4 battery for led lighting but the only drawback is it can’t be charged if the battery gets to freezing temperature.
Most of my needs are for outdoor lighting.

So I finally finished filling up the bed, insulated and floored!
As soon as I was done I plugged in the transformer for the heat cable to see how long it could hold heat. It started at 63° then got up to 90° after 16 hours. I unplugged the heat cord and checked it 20 hours later and it dropped down to 77°. 9 hours later I went to check it again expecting to see the temperature to be close to the temperature before heating it up but to my surprise it actually raised up to 79°. 🤷‍♂️
I’m speculating that during the last 9 hours the sun finally came out and heated the greenhouse really hot and maybe the manifold inlet and outlet pipes sticking up from the floor may have radiated heat back down into the media.
After seeing this I decided to try taking a black painted pipe and place it near the top of the greenhouse and connect it to the inlet side of the manifold and T off the outlet side with valves so one side goes out of the greenhouse and the other stays in. I’ll also put a fan at the beginning of the black inlet pipe.
I’m hoping this will do 2 things. 🤞 Hopefully it will help heat up the bed and help cool the greenhouse when the sun is out then switch the valves at night so it will blow heat up into the greenhouse when it gets colder. If I find that it does work in this manner I’ll replace the manual valves with 12v automatic dampers on thermostats.
For now I have to wait till parts to be delivered.

4/22 Update:
I finally had a decent day to get some stuff done yesterday.
I was able to complete my aluminum air pipe manifold but unfortunately I found that 1 of the boulders underneath was sticking up to high which is making the manifold higher than I wanted. In turn this will also require me to purchase more pool sand since what I have put in so far hasn’t reached the bottom of the manifold yet. 🤷‍♂️
Hopefully next weekend weather will cooperate so I can finish the fill. 🤞

I also forgot to mention, one day before I built the manifold I had a party sunny day so I plugged in my digital thermo coupler. I was seeing big swings in the temperature which I believe was from the sun going in and out from behind clouds. When the sun was out the temperature would jump up to nearly 90° but when a cloud came it would drop to high 70° to low 80°.  I believe that is a good sign that the heat is getting transferred through the 1 foot of sand gravel mix. 🤔 I think I may increase my gravel to sand on my next 1 foot of fill around the manifold.

C. Letellier wrote:Definitely insulate the north wall.  If the roof ridgeline runs east west also probably insulate the north half of the roof as your sun in winter should reach around it.  On the insulated north wall add a bunch of mass.  Since your building is small guessing you don't want water barrels which would be the common answer so the other 2 common one are shelves with partially water filled 2 liter bottles painted black or the tire tubes cut in long hanging arches filled with water tied off at both ends.

Beyond that most of the stuff suggests you get roughly 1 zone for each layer of plastic.  As best I can tell it is actually closer to 1 1/2 layers per zone and of course a law of diminishing returns applies.  Now you might be able to add cold frames inside all of this for another layer.  And I would set up for the chinese greenhouse inside sort of so you can add an insulating blanket over this too.  So if you put your greenhouse under a greenhouse then added and inner greenhouse and a cold frame that would give you 4 layers so probably 2 zones worth.

Swedish skirt insulation around the building on the ground outside would probably help too.

If you look up the Mike Oehler greenhouse book another one of his tricks was a raised bed to the cold had room to fall off around the plant giving another tiny buffer.
greenhouse book

Beyond that one in my dreaming is to add an active foam insulation system.  My mid size greenhouse dream would have one side as glass and inside that I want to build a giant plastic bag of greenhouse plastic set in an insulated trough with a heat tape under it so it can't freeze.  Run a fish tank stone into that liquid.  The liquid will be weed sprayer marker foam(sodium laurel sulfate) and water  Turn the air pump on when the sun goes down and hopefully fill the bag full of what amounts to dish foam.  It is supposed to be good for a R per inch roughly so a foot of foam would give me an R-12 window at night.  When the sun comes up turn it off.  The foam will die down and let the light in during the day.  Now there is a bunch of thinking that goes into keeping ice from damaging the bag that if you want just ask and I will share.

some links.

soap bubbles

foam 2

soap bubbles canada

There are a number of others.  Best one is out of the UK from about a decade ago but I can't find it at the minute.

I will definitely insulate the north and west walls next fall and will probably do a full layer of greenhouse plastic sheet over everything outside since snow slides off more easily. When it gets closer to fall and I get a better look on how much sun comes through the north side roof I’ll make a determination to insulate that roof as well.
I wanted to stay away from water as heat storage because of the potential of freezing and taking up valuable space inside my small greenhouse.
The ground heat battery I’m making is looking promising at only half way filled which it has stored heat for a full 24 hours after heating it for only a couple hours but I hope it will improve when it gets completely filled up and insulate the top of it.🤞When that gets done I can start doing some long 4-6 hours of heating to see what temperatures it can get up to and how long it will last.
Yeah this is a different style of thermal mass that I believe will work and will save a great amount of heat electricity by incorporating solar pv panels and a 600 watt transformer as backup when several cloudy days come around. But of course the true test won’t be until fall and winter comes again.

Mike Philips wrote:Conductivity is a useful parameter if the temperature is not changing.

However, in dealing with thermal mass, the whole point is that the temperature *is* changing over time.  Here’s the reason why this matters. Before heat can be conducted it over some longer distance, it first has to raise the temperature of the material in-between. That material in-between has a heat capacity. Heat is required to raise its temperature. Heat cannot be conducted further along down the line until the temperature of the in-between material is first raised.  That’s why (in the case of dynamic temperature changes) the time it takes to transfer heat depends on the distance squared.  

On a good note I came home from work today and decided to check the temperature on the bottom of the media and it was 7°F higher than the outside air temperature. So just from the sun hitting the sand and with the greenhouse fully open with 1 side with no wall panels the bottom heated up. Currently it’s only 1 foot deep but after I make the aluminum manifold and top it off it will be 2 feet deep.
I believe that’s a good sign! 👍🤞

Mike Philips wrote:Are you trying to make a 3 or 4 season greenhouse?  

What are you growing?  Some plants like spinach and arugula can freeze solid and thaw without dying.

In my opinion the most economical passive solar greenhouse is a “Chinese” greenhouse.  Insulated, opaque north wall with thermal mass, and insulating nighttime blankets over all the glazing.  

As far as thermal mass, often the bottle-neck in heat transfer is convection.  Typical passive convection (solid to air heat transfer) is only about 20 watts per square meter per degree C temperature difference.  For this reason, open gaps in loose gravel might allow air to naturally convect easier over more surface area, even if the total mass is a bit less.

If you want to get a feeling for how far heat will conduct through a solid in a given amount of time, look up heat “diffusivity”.  By the way, with diffusion, time is proportional to distance squared. So, in a dynamic sense (diurnal temperature change) for heat to transfer twice as far takes 4 times as long. In other words, heat transfers much faster over short distances than over long distances.  

It would be nice to get 3-4 seasons but I’m not holding my breath that it will happen. 😆
I have seen many deep winter greenhouses and my cheap Harbor freight greenhouse is no match to them. I could possibly insulate my north and east walls and maybe put another layer of greenhouse plastic sheet on the outside so snow slides off better which “possibly” could get me 10 months of growing cold weather crops in my 6b growing zone. Worth a try though. 🤷‍♂️🤞
Your information is very interesting but I believe it’s beyond my math skills and I’m beyond the point of being able to change things if I have not used the best media. I’ve done lots of research and bench testing different sand, gravel and sand mixed with gravel so I opted to go with the mix that held heat the longest. I’ll just hope for the best that it will work good enough to get 10 months a year use with it and that the solar panels will provide the majority of electricity for the heat.🤞

I finally had some time with some good weather to get some stuff done.☀️ I picked up 1/2 yard of mason sand, 1/2 yard of wash 3/4” gravel, 1/8 yard of 2”-4” crushed granite, 18 bags of pool filter sand (silica sand) and 4 - 4’x5’ 16 gauge steel sheets.
 I pulled out the granite boulders, laid 2” of mason sand on top of the bottom foam board, laid the steel sheets double layered, put the granite boulders back in, snaked the elastic heat cord between the boulders, mixed the pool sand with gravel and started filling, laid the 2”-4” stone between the larger stone and filled more pool sand/ gravel mix to the top of the boulders.
 Next step is to build my aluminum pipe manifold on top of the boulders and fill more pool sand gravel mix between the pipe and top with mason sand, foam board and plywood. 😅
I purchased a 24v 600w 25amp transformer that I purchased on Amazon (late delivery ☹️) so I can plug up the heat cord and start heating up the bed before I top it off and mount my solar panels. I also hope to cook out any moisture because a few bags were damp before adding more. I figured the transformer would give me a backup option in case of several days of no sun. I also purchased a digital thermocoupler which I have already buried the probe at the bottom on the steel sheet.
I just have to hope for more good weather but unfortunately more rain is forecasted. 🤞

Mart Hale wrote:

Interesting take on sand battery,   PDF attached....

Very useful to compare the heat ability, and how fast the meterial can release that heat...


For those of you who build rocket stoves you already know how well cob holds heat in the PDF it spells that out.

Pure silica sand (pool filter sand) I have tested and does some amazing stuff. Definitely more pricey but well worth it. Good for the smaller projects.

I forgot to mention that my heating cord is a 50ft long 30v (32v max) which will be connected to 2-30v 395 watt solar panels.
Yes the solar wattage is on the lower side reason being is because the heat cord is rated up to 32 amps and both panels could put out 27.4 amps. From what the heat cord manufacturer told me is that the cord will heat up to 200°-250° at 27-32 amps. Of course solar panels won’t produce these amounts all the time so plan B is to take hot air from the top of the greenhouse during the day and push it down to the manifold in the media to help the heating up process and cool the greenhouse. 🤞