There is a greenhouse and structures forum here on GW that has a lot of info, but realistically, there isn't enough heat exchange between barrels of water and the air in the greenhouse to warm the air enough without a supplemental heat source this far north and at this low an altitude. It is more successful if you are at a higher altitude and a lower latitude. If you want to grow cold-tolerant veggies, look at Elliot Coleman's book on four season growing. There is quite a bit of info on the web on solar greenhouses.

:D I've read his book, I have it. And love it. I didn't know about the Greenhouse forum here, sorry. I am new here.

For sure this will not work all year where I live, I know. But with a cold frame I can have stuff out of the garden until december. I wonder if the same setup on my porch would work. If I can get 2-3 hours of heat extension everyday, it is still pretty good!

I suppose I will try it, it will only cost me the plastic. Although I am not sure it will hold up properly. I wish someone else attempted this haha. Sometimes I get snow on my porch during winter (the roof goes above a foot further then the porch roof, so usually I get no rain or snow on the porch). So I wonder if a regular, cheap, thin plastic would hold or with the wind and snow hitting it, it will tear. I'm thinking it will tear. I'll get some tarp instead but they cost more and let in less sun... humm.. well, thanks for your input, I'll go find the greenhouse forum :)

One problem with "thermal mass" is that it will also retain "cold" and tend to slow down the warming up period.

Using a row cover over the plants, inside the hoop house and removing it during the day to let the soil warm up would be more effective. Dirt stores a lot of heat if you let it.

Thanks!
I have now bought and receive some wireless thermometers. It's really fun, they feed live on my Iphone.

I've set one in the low tunnel and I monitor it.
This weekend I will divide the tunnel in three parts.
1 - Just the tunnel
2 - A row cover inside the tunnel
3 - Row cover with water containers, inside the tunnel

This way it should give me the real numbers :) It's true that the thermal mass will also retain cold, that makes me wonder why it is talked about SO much in winter gardening! I have a hard time understanding this.

This weekend I setup a temporary inner row cover.
Outside it went to -3C and inside it was +2C. This was encouraging already, and it's just an old, white shower curtain (I have not received my floating row cover yet).

"It's true that the thermal mass will also retain cold, that makes me
wonder why it is talked about SO much in winter gardening! I have a hard
time understanding this."

Because they are gardeners, not engineers who study thermodynamics for a living.

:)

Thermal mass acts to delay changes of temp, and can narrow the range of temperature fluctuations, but all that heat it gave off on Monday night has to be replaced Tuesday for use on Tuesday night.

Preventing heat loss - insulating the North side of the green house, insulating around the base, and using multiple layers of protection to keep warm air from transferring heat to the outer skin - is more effective as a starting measure.