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How much water for thermal mass?

 
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So I am building a new greenhouse this week. It will be a barn style with the north wall traditionally framed and insulated. The south, east, and west walls will be covered with 6 mil double wall polycarbonate. I struggle to get alot of sun where I live so I can only count on 5 or 6 hours of direct sunlight a day in deep winter. Without adding lights I know I wont be able to grow year round. My question is how much water should I add to keep me warm enough to extend my season and start a little early. I currently have 2 250 gallon ibc totes and 6 55 gallon drums available. Is that enough for my 10 x 12 greenhouse? I hate losing any more floorspace, but will have them all set up so I can use them as shelves to plant on top of. I also was wondering if I should link them all together and circulate the water or if that will just cause heat loss. I am fortunate enough to have free natural gas where I live and will probably add a small gas heater, but want to get as much passive heat as possible. Also, is there anything that will help water hold its heat better (adding salt, or something like that)?
 
gardener
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Location: latitude 47 N.W. montana zone 6A
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Hi Stephen;
As far as I know , leave the barrels separate, paint them black, place in the sun and forget about them.
Wow ! Cool free natural gas !  
I was going to suggest your building a rocket mass heater to keep things warm, but with free gas you got that covered!
Keep us posted on your build with lots of photo's!
 
steward
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I think for your goals that is plenty of water.  I think in colder/cloudier places (north or north east US) you want 5 gallons per square foot of South facing glazing.  

If they're under benches and not getting any sun, they should still contribute to the thermal mass effect but painting them black won't really help.  I believe there are other tricks like keeping them away from walls a bit and allowing for air circulation to heat them from all sides.  Insulation underneath could also be a good idea.
 
pollinator
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Location: Thunder Bay, Ontario, Canada
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Thermal mass acts similar to physical mass, except that it acts on heat.

Think of a physical mass for a moment. A large mass like a truck going downhill picks up speed more slowly than a smaller mass like a passenger vehicle. When it gets to the bottom of the hill, the truck will coast along the flat further than the vehicle will. When the bottom of the next hill is encountered, the truck will initiate the climb with more energy, but there is a point at which the vehicle will overtake the truck as it looses it's initial momentum.

Thermal mass acts the same way. You have the daily thermal cycle of heat absorption when the sun is shining, and heat release at night. You also have a seasonal cycle that acts on a much longer timescale.

The larger the thermal mass, the more "momentum" it has just like that truck. But it is a two way thing. You might get more of a buffer from the nighttime cold with a larger mass, but it will also take longer in the day to replace that lost energy. And in the fall a larger mass may extend your growing season a little, but in the spring, that mass will delay your growing season from initiating because it will suck the heat out of your greenhouse.

If you live in an area that gets overcast weather, the chances are that there won't be enough energy to charge a large thermal mass fully and so when the sunny days appear, the greenhouse remains cold, then the next weather cycle appears and your greenhouse ends up colder than the outside ambient temperature.

Unfortunately this idea that a large water tank painted black will transform a greenhouse simply doesn't work in the majority of real world situations without some sort of additional energy input. Geothermal is an option. Compost is another although it tends to be problematic when it comes to decommissioning.
 
Stephen Cummings
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Thanks for the feedback. That is why I was considering linking them all together and having a copper pipe coil next to a heater if for very cold or cloudy days. If I can help circulate hot water through I thought it might help. I will be building next week and will just experiment for a few months before I try any plants in there. It will be mostly greens and carrots to start so hopefully I dont kill much.
 
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Location: Somerset England
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I don't know if it will or won't apply, solar water heating attached to the tanks? Or solar panels and heating elements?


I'm looking into solar hot air, for drying my wood for the wood stove while I build the RMH
 
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