Hi Bill

I added as 650 gallon water tank to my greenhouse which helps quite a bit. We were able to extend the growing season to Thanksgiving. (see link below). I also added a new feature to the tank that isn't pictured yet - I took a bunch of old baseboard radiators and attached them near the peak of the greenhouse. When it gets warm from the solar gain, I pump water from the tank through the radiators which extracts the heat and stores it in the tank. On a good day, I can reclaim about 7-8 degrees back into the tank. If the aren't running, it's maybe 1-2 degrees. The only problem is they sweat a lot!

Here is a link that might be useful: my greenhouse

I don't find thermal mass alone worth the effort given how much space it takes. In a passive system, it tends to release heat before you really need it (ie, not at 2am but instead as soon as the temp differential favors loss of heat in the afternoon). Web4deb appears to have an active system, and that may be more efficient.

I built the GH to house plants, not water. YMMV.

SB

I second stressbaby's observations. A lot of minds smarter than mine have tried to make passive work, and it just won't on a small scale in a hobby gh. Try as folks might by squinting and tapping at their thermometers, they just can't measure much effect...if any. The mass required just isn't efficiently charged enought during the day, to release enough BTUs at night to counterract the cooling through the walls.

An active system is something else entirely, but one that works, and won't sacrifice space or light inside the gh, needs some planning and effort. If you've already erected your gh, then doing an under-floor system (the best choice) will be problematic.

-Bruce

Bill has a lot of wall vs. poly like I do. He said he insulated it well, so I believe he can do as I have, and line the north wall (assuming his poly faces south) with water barrels and effectively add a month at least on both ends to his growing season. It depends on his purpose. If like mine, it is to grow hardy vegetebles, it is a slam dunk without adding heat. If he wants to grow orchids or tomatoes, he needs to add lots of external heat.

The argument for water barrels etc taking up room doesn't always apply to a structure like Bill's or mine. His growing area is not 100% of that structure. ~tom

"The argument for water barrels etc taking up room doesn't always apply to a structure like Bill's or mine. His growing area is not 100% of that structure."

True indeed.

SB

I am new here. I built a passive solar greenhouse from a plan I found in The Passive Solar Energy Book. All walls are solid except the south face. I poured an 18" high perimeter foundation, put in chicken wire, then large gravel, and bricked the center area. Under the shelves I placed water filled black barrels. I don't know how well this works as I have nothing to compare it to and I assume it depends on your individual climate. However, here, in the foothills of central California we get sun in the winter (T-shirt and jeans OK outside) though it gets cold as soon as the sun goes down. The GH is not finished and has openings at both ends where I plan to put in an exhaust fan and some intake louvers. Even with these unfinished openings it gets very hot. . .this month it was up to 115 degrees. I don't have the electricity ran to it yet, so leave the door open and can keep it between 90-95. I hooked up a weather sensor in the GH and have that linked to my computer and see that it rarely gets below 45 degrees at night even when it is about 30 degrees outside. Again, I am new to this and still learning what I need to know and cannot say how effective the heat sinks are. I plan to put in an exhaust fan, two intake vents (trying to decide whether to have motorized or gravity louvers for the intakes). I will put in a HAF fan system. . looking at one of those rectangle thermostatically controlled window fans with two small fans both reversible and with speed controls and set one one direction and one the other direction to get circular horizontal air flow - these used be about $40 at Lowes. I also had a spare 6" fan in a section of heating duct pipe so I installed that to duct runs to opposite corners of one side of the GH - about 6" up from the floor and then ran it up to a "T" in the center of one shelf and then and blow air into the center of the GH . . even when it is hot in the GH it is considerably cooler under the shelves - really amazingly so. Still experimenting. Everything I have planted has really grown rapidly even with a lot of heat an no fans as of now. At times in the winter it can get down to 20-25 degrees. . . this may happen a few times a season. I will have an electrical thermo controled heater to turn on when it gets that cold. Hopefully I won't ever need to heat it very often. I think my problems will be to keep it cool enough and air circulation.