Tom tried a couple different hot tub collector designs during the summer of 2009 with mixed results as described below...
Tom has successfully heated a large pool with unglazed PEX collectors as described here...
This is certainly one of the best, if not the best, DIY pool heating collector designs, and works quite well.
In trying to adapt this PEX pool heating collector to heating a hot tub, glazing was added to reduce the heat loss from the absorber to the air. The absorber for a hot tube collector is going to run much hotter than a pool heating collector, so the glazing is needed to reduce these losses.
Here are the results from Tom for the glazed PEX collector:
We installed a Glazed PEX collector on my neighbors garage above his hot tub (connected to his hot tub). One day of sun on that collector without water circulation (pump malfunction/ airlock or power outage, we never did figure out why) melted the Lasco type "T" fittings. I also installed one of these systems on my hot tub and had the same plastic fitting failure. They obviously have a lower heat tolerance than the actual PEX. Those fittings worked great in "unglazed collectors. Please post this problem on the site with my pool collector info to make sure someone else doesn't stumble down the same path as we did.
In overcoming this fitting problem...
I'm back to the drawing board for a hot tub collector. If I use PEX, I will need to do it without fittings, like your serpentine design. Problem is; all my absorber material comes off my large aluminum coil which is 8" wide BEFORE pressing it out. My yield is just over 6" wide, which works great for 6" spaced collectors. I don't think the PEX will comply with 6" spacing without collapsing. Any ideas??
I think that with half inch PEX-AL-PEX you can use a 6 inch wide fin. This collector uses that arrangement... The 6 inch bend radius is just below the minimum that the PEX manual states, and seems to be OK as long as you are a little careful when bending it around the form.
It seems like even without the Lasco fittings, you would have to be careful not to get the PEX to hot. My solar hot water collector, which is PEX, is tilted at about 70 degrees to reduce stagnation temperatures, but it still gets up to around 220F when stagnated on a hot sunny day. I think it would go quite a bit higher with (say) a 45 degree tilt.
Maybe a little venting of the collector to the outside would take the edge off the stagnation temperature while still keeping the heat loss low enough to work?
I guess another option would be to use the copper collector (see below) and introduce a heat exchanger between the hot tub fluid and the collector fluid. It would have to be something like stainless steel to not react with the hot tub chemicals? A pretty ugly solution.
Tom also tried a glazed copper/aluminum collector, and reports this result:
My copper tubing hot tub collector lasted just two weeks before the hot tub chemicals reacted with the copper and discolored the water pretty bad. I guess chemical management is not enough to avoid this problem.
So, the combination of hot tub chemicals and copper tubing should be avoided. This probably holds true for pool heating collectors as well.
If anyone has any thoughts or ideas on this, please send them in.
Sometimes knowing the things that don't work can be as much or more help than knowing what does work -- thanks very much to Tom for passing these lessons learned along so we don't have to repeat them!
Tom's other projects on Build It Solar:
5.5 KW wind turbine on 140 ft Tom made tower
Simple and effective solar pool heating collector
Press for forming heat collection fins for solar water heating collectors
Gary October 5, 2009