Steve, I think you figured it out already...SB

Do I have too much water in there or is this the way that it should happen....its kind of wierd because I figure that things will grow best from 70-90 degrees...and if this passive heat does work like that...that means that a heater is needed during the day lol....sounds very confusing...kind of using solar heat to increase other heating needs....I am going to clean the GH thourghly tommorow...its needs to be cleaned but its not to the point that would cut back on that much day heating...

Steve

Any unused space in your greenhouse should be filled with a thermal mass. You cannot have too much!

A while ago I posted some real data from my empty (of plants) greenhouse at night. I think it was about 34 outside, 32 inside. It was actually colder in the greenhouse than outside!
If you need to rais your daytime air temps in there just hang a chunk of black cloth, a couple of black trash bags etc against your north wall.

this morning it is colder inside......54 outside 45 inside...this has never happened to me before...temps during the day were always hot in the 70's 80's this time of year....I actually opened the vent with hopes of warming it up....Im just glad that we are in a warm trend right now....chris is this what you meant by it biting me in the butt...Im thinking of removing one 5 gal to get temps back where they use to be....and leaving it outside in the sun for today...also what is up about having to log on everytime...its like that all over gardenweb....

Steve

you want as many thermal masses in your greenhouse as possible if you want to save heat. these include rocks,bricks,black plastic/clothe,water,anything that has mass really, personally i like water the best.

fill all your unused space with rubbermaid containers filled with water and put a black plastic tarp on your ground floor.

i might disagree a bit with the thread here.

just as steve has discribed, not only is water a thermal sink but its also a thermal sink. what i mean is that it tends to balance out the hot and the cold parts of the day. not too hot, not too cold.

so one could have TOO much water in the house, only to make the temp stay low in the day.

I think I'm missing something in this discussion. Is it correct that you have a 4x6 green house with 2 5 gallon buckets of water for storing heat? If so, this is way too little storage to affect the temperature in your greenhouse.

Water stores 1 BTU/lb-degree F. If you have one lb of water and heat it from 50 degrees to 80 degrees you have stored 30 BTUs.

10 gallons of water is 83.4 lbs
(10 gallons /(7.48 gallons/cu ft) * 62.4 lb/cu ft)

Even with a temperature rise of 50 degrees, you can only store about 4000 BTUs.

What is the size of the exposed surface area of the greenhouse and what is it covered with? You heat loss is going to be on the order of 0.5 to 1 BTU/sq.ft-degree.f-hour. In a couple of hours, your stored heat is going to be gone.

How much sun does your greenhouse get? Usually, small greenhouses have problems with overheating during the day (even in the winter). It don't think you have so much water in the greenhouse that on a sunny day you only get a 2 degree temperature rise.

if you are worried about the water bins taking in all the heat and not letting any out to heat the greenhouse than make a water to air heat exchanger.

the more water bins the better!

I probally have on the order of 20 gallons total...This is the first time I have noticed that the GH hasnt been heating up during the day..it will usually heat in about half an hour...and like stated earlier temps can get to extremes...I get about 6 hrs right now of full sun...temps did get up to almost 70 degrees today and it was sunny until around 11...then got cloudy...I have noticed that the temps have been slowly risiing since the last few nights have been in the 50's...the differential now is about 2 degrees...and it hasnt been colder in there last night compared to the outside temp...

As far as how I have it layed out...the front 6ft wide is facing east...the side wall south recieves about 6 hrs of direct sun right now...it is next to my house but not attached...so after 2 there is no sun..this weekend when I first noticed it the water buckets were cold...the 2 liter soda bottles were warm though....although the heat sink theory seems viable, it is not what I have expected ... a little upsetting this time of year...like stated above it seems to be moderating the temperature...and instead of having a 50 deg difference from day to night...has been roghly 10-20 degrees...thought that this may helpout in the summer as well...well atleast if we get a hot day out of the blue before hook my mister back up againg...my plants wont be dead on me when I return from school or work...ut sharpie can you explain the water to air heat exchanger...

thanks
Steve

you could simply get a u shaped pipe that is perforated. make sure the holes are submerged in the water. one end you connect to a blower, air is forced down the "U" where it comes into contact with the "hot water", it is than forced out the other end of the "U" into the air.

basically picture a "U". at the bottom of the U drill a bunch of holes all around it, this is the portion that is submerged in the water so when air is forced down into it the air will contact water.

you may need to experiment on pip size and hole many holes.

I am at a loss on this one.

The theory of thermal mass seems to be working as in warmer at night and cooler in the day but I cannot understand why your air temp in there in sunlight is so low.

From my own limited experience I found that I had to turn off the fans under the bench that blew air amongst the trashcans because my legs got cold even though I was wearing a T-shirt.

(80-90 above the waist, 50 on the legs when planting seeds...)

Don't it make you yearn for having enough money to get heaters and fans and thermostats and coolers and just program the day and night temperatures in and to hell with the cost?

If you are serious about the forced air heat-exchanger you might be better off getting a 10W pool pump and a car radiator with fan, both activated when the temperatures are suitable (for heating the water or cooling the air I would kick the heat-exchanger in at about 28C, for heating the air at about 15C). A low difference in temperature is very inefficient for any heat-exchanger and the three things to optimise are surface area, surface conductivity and counterflow length (unintuitively for low differences the counterflow length term dominates practical designs so good designs are very long). Air bubbles have good surface conductivity but are hard to make a large surface area and counterflow length is very short. Better is a shower heat and fan combination (called bong-coolers in computer circles), and presumably a car radiator is very good too.

It's probably a bit hard for most people as you really want a two set-point differential thermostat, something you'd have to build yourself (though it can probably be done with about 10 components).

Alternatively you could use the air-exchanger for heat storing/air cooling, and under bench piping for plant heating. This would simplify the logic at the cost of two separate loops. I'd head in this direction if you are serious about thermal mass. You also want lots of water - gbrendemuehl's calculations are right on the money - we're talking up in the thousands of gallons. But you were all going to collect rainwater anyway, weren't you!

thanks for the ideas...it is appreciated...I may try something like that...just kind of hard to rig something up with only 24 ft of floor space..and have plants as well lol..I may try to collect those 5 gal containers and do away with the soda bottles...and stack them along my north wall next year...the black plastic also sounds like a good idea...along with bubble wrap..which I was planning on doing this year but didnt have time before it got cold...(finals and turnpapers)....thanks all for the help an ideas

Steve

You can use the water store as benches too if you have suitably strong containers. 44gal steel drums are still fairly easy to get (find ones used for food stuffs like maple syrup rather than random chemicals) and are very strong and about the right height for a bench.

thanks nathan

Saw a British TV prog where the heat sink was a hole in the ground, lined with polystyrene. A plastic tube manifold was fitted into the hole which was then filled with crushed glass(from bottle recycling). Finally the hot air from the top of the green house was pumped through the manifold using a small solar powered fan.
Dimensions, flow rates and the other calculations (size of heat sink cf total greenhouse) are unknown to me, but, the designer swore by the results - presumably, the fan circulated the hot air when the sun shone, and in the dark, the fan stops and the stored heat rises from it's grave.
Any comments and what would be the design criteria for an efficient heat sink of this type?

The underground crushed glass/solar tube idea is fascinating. Any guess on the dimensions of the tubes or other details of the system?

My HFGH is in the Finger Lakes Region of NYS. It gets pretty cold in this area, but if the sun is out, my greenhouse can get very warm and even hot especially towards the ceiling.

The floor I have now is about 10"+/- of crushed stone. I have room to add more. My thought is to take PVC soil pipe and bury it in the crushed stone in a looped fashion. One end will be in the uppermost area of the ceiling with a fan mounted above the hole in the pipe. This would pull heat out of the ceiling area and push into the loops buried in the crushed stone. The other end of the loops would project out of the stone and back into the greenhouse. Screen would cover the ends to keep the critters out.

I would think that the warm air being pumped through the loops would transfer through the pipe walls and warm the crushed stones that would in turn act as my massive heat storage area.

Anyone done this? Any comments or suggestions would be appreciated.

Thanks in advance.

Joe

Here is a link that might be useful: My HFGH is ready for spring

I have a friend who built a 20' x 20' greenhouse. He insulated the ground by digging out 4' down, and lining it with 2" styrofoam on the sides and the bottom. Need to do this so the ground in the greenhouse does not freeze when the ground outside freezes. He did many tests with things like pools, black swimming pool heaters tubing on the north side of the greenhouse- and storing heat in coy ponds. He found the best way is to use the ground under the greenhouse as a thermal heat sink. Get 4" perforated tubing and bury it under the ground. Coil it back and forth. Pump air through the tube during the day. Taking air from the top of the greenhouse, through the coiled 4" tubing that is underground, and the air exit is on the other side of the greenhouse at ground level. Heat is held in the soil and given off in the evening. He found that the tubing needs to be perforated, because as the air cools, it creates condensation, which collects in the tubes. After a while they filled up with water. LOL Can have the fan be solar powered and/or triggered with a thermostat. Also nice to have a window from the greenhouse that can allow the warm air into the house. A bit of work though if you are planning a greenhouse from scratch, I hope this helps.