The Hot Water Chimney Effect
This thread is not to be a passive solar 101 but rather a passive solar 201... I kind of expect you to come to the table with some understanding; and
So you have a tank in your house and a solar panel outside. The tank is situated above the panel and you have lines running to the appropriate
but no thermoflow.
Think of your solar collector like a wood stove...and the water flowing inside like the smoke flowing out of the stove.
One thing I have realized that has made the world of thermosiphon clear is that the "pump" of the system comes from a well designed solar collector
exhaust; that is, the hot return to tank.
Simply plumbing your panel exhaust to your tank hot connection using the "easiest" route possible will NOT produce thermal flow.
My systems all heat 20-40 gallon tanks and I usually use 1/2" copper lines to connect everything together... except for the exhaust riser, for which,
I use 3/4" copper.
The exhaust riser, aka solar panel out, is your "hot water chimney" and you want it to be a strong flowing pump. The idea is to get the natural
thermal heat rise to propel your water upward.
Immediately upon leaving the solar collector the exhaust riser must pitch upwards and continue pitching upwards until it is above the top of the hot
water tank. Doing so allows the system to capture the flow of heat and create a flow of water. At its peak it can then extend towards the tank
horizontally... a distance no greater than twice the total rise for straight shot horizontal runs. All vertical piping on the exhaust end I keep in
3/4... when it reaches the top of the "hot water chimney" I convert to 1/2 to increase flow rate during the horizontal run; thereby decreasing heat
Again, out of your panel: VERTICAL FIRST... horizontal second.
I find it best to just run straight up with my exhaust riser
... although I have found that a pitch of 1 inch rise for every 10 inches run will
work effectively. This is much steeper than one would pitch conventional sewer drain lines, where 1 inch of rise over ten FEET would be sufficient.
Bottom line, the steeper the better. You are fighting the tendency for heat to rise right through the wall of your pipe; which is why an immediate
vertical run makes the best pump. All of your piping should of course be insulated.
You'll find that at the top of your water heater you have more connections and valves than a "conventional setup" all this additional metal will
act as a heat wick and draw energy out of your tank. I usually insulate this cluster of valves and pipes with a laundry basket worth of old rags,
towels, and sheets... much more effective than lots of little pieces of "conventional" foam pipe insulation; EVERY HOT WATER TANK...conventionally
heated or not... IMHO should have a blanket over top of it to conserve energy. Does yours have a blanket? Is your meter spinning? Go give it a
blanket; its chilly for a 125 degree warm blooded body.
The cold return from the bottom of the tank to the bottom of the solar collector can be 1/2... this return is like the air intake on a woodstove and
needn't be as wide a diameter as the exhaust.
In future designs I will be experimenting with a 1" riser and 3/8 soft copper horizontal and return lines. I expect the 1" riser would capture
more of the thermal flow. The 3/8 return lines should speed the water along faster on the horizontal runs so less heat is lost in transit. Using
soft copper return lines will also eliminate many of the 90 degree elbows which should increase flow. On my most recent system I have opted for
double 45's in place of elbows for this same reason.
Those are my thoughts for today... I'll be back with more.
[edit on 25-3-2008 by Sri Oracle]