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stmbtwle

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Reply with quote  #11 
I think it will work. 

I'd be tempted to put a coil of plastic landscape tubing in the tank, and divert water from the collector through this "heat exchanger" rather than the tub. The end results should be the same but with no chance of mixing.  

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walter matthews

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Reply with quote  #12 
I like the idea but wonder if I can reasonably expect a closed loop of water to stay in tact. An air gap, particularly a large one, would cause a problem with the pump servicing that system. Perhaps a small tank, say a 50 gallon water barrel, that I have from hydroponic stuff I do would work. 

Thank you for the suggestions and encouragement. My wife's son, a mechanical engineer assured me that hot air in August in Sacramento would be a pretty poor heat transfer vehicle and after suggesting that we could bury a coil of landscape tubing in the ground, similar to your suggestion, and talking further while we all came up with the external above ground polyethylene tank idea, he commented, wow, whether it works or not, WHAT FUN.

...Would people be interested in the wood working photos as i build this thing from Red Wood?
stmbtwle

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Reply with quote  #13 
Your loop isn't "closed", it's part of the hot tub system, and the hot tub is the reservoir. You shouldn't need the small tank. The water in the coolant tank never has to leave the tank.


I also installed a "bubble trap" (basically a vertical tee) just before the pump, air bubbles went up one leg and out a vent pipe, while the water went down and on to the pump. It worked well.

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walter matthews

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Reply with quote  #14 
OK, Final plan till someone suggests an alternative that's more attractive. I'm starting to purchase equipment for the solar part as the wood for the tub has been delayed due to rain in the area where the trees are located.  
So, I purchased a 4x10 commercial ELM panel. I'll put it into a box perimeter of 2x6 redwood with two  4x5 sheets of outdoor plywood, painted and pocket screwed about 1/2  off of the bottom of the 2x6 to keep the plywood off of the roof of the shed.Next will be 2 inch thick aluminized poly-iso insulation recommended by Scott Davis. Next will go the ELM solar pool heater made of stabilized polypropylene and there will be three water systems. The water for the hot tub, the water in the 600 gal tank that will serve as heat exchange medium and a 50 gallon tank of water that will circulate its water through the solar collector and it will flow back into the 50 gallon tank after it has passed through 100 feet of black polyethylene irrigation tubing. 
The plan for automation is as follows. A switch that goes off from a timer at say 9:00 AM. That will start the pump that pushes water through the solar collector and back into the hot tub. Next when a sensor-no idea what to use here or where to get it-passes say 106 a switch will shut down the pump for the tub, and turn on the pump that passes the cooling water through the collector. That will run until well into the evening on low pressure and at almost no electricity usage.


walter matthews

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Reply with quote  #15 
So, 1. how do I build a system that turns off the hot tub pump when the temperature in the tub is where I want it, then 2.  turns on the other pump when the sensor in the Hot tub reaches the desired level and 3. that resets itself to start the hot tub pump when power is applied again the next morning. 
stmbtwle

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Reply with quote  #16 
The simplest and easiest is snap-disk thermostats. 
[image]
You'd need one that comes on at about 120F for your heater pump, and another that comes on at about 140 for your cooling pump.  You'll also need a mechanical hot tub thermostat to control the temperature of the tub. 
[bx9919-1500-A-100x100]
The tub thermostat is in SERIES with the heating snap disk. The disk turns the pump ON when the collector is warm enough, the thermostat turns it OFF when tub temp is reached (1). When the heating pump turns off, the collector temp will rise till the "cooling" set point is reached, and the cooling pump comes on (2).

For more precise (and adjustable) control you could use digital thermostats, they'd be hooked up the same way.

Digital LED Microcomputer Thermostat Controller Switch Temperature Sensor
There are lots of different ones on Ebay.  

If you want to get REALLY fancy you could use a microprocessor such as an Arduino, but it would probably be more trouble to set up than it's worth.

(3) all the thermostats will reset when the temperature is out of range. A timer isn't needed.

(4) the polypropylene heaters you mention won't need a cooling system as long as you DO NOT cover them with glazing.  If you do glaze them, they will.

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Solar is like the wind. It may be free, but putting it to work isn't!
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Bruce

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Reply with quote  #17 
I would suggest you include a mixing temperature valve like you would a conventional water heater to control the heat of the water going into the tub.  Then you can store more hotter water in the tank and mix it with cold via an adjustable mixing valve that you can adjust to the proper temp, set if and forget it.  
walter matthews

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Reply with quote  #18 
Quote:
Originally Posted by stmbtwle
The simplest and easiest is snap-disk thermostats. 
[image]
After considering your suggestion for several days I finally see the how great your suggestion is.  The problem with my solution of just using a timer is that the timer is will  come on at say 9 am, and mother nature has provided us with a nice cool night and she left some morning mist in the sky. Sometimes, around here in N Cal the sky does not cleat till 11 or so. If the pump was on a timer on such a cool morning I would be pumping cool water into the tub which would take more time and effort to get back to the 104 range once things heat up from the sun.

I do need to make the system so that I can run the spa pump without any relays other than on and off at the source to heat the spa on days in the winter when the temperature in the collector only gets to say 110 as a max for that day. If the water in the tub is down to 60 110 degree water will make a big difference. In our area we typically get many days, sometimes even months of cloudy foggy days and of course I don't expect any heating at that time but as you mentioned in the beginning hot tubs take a while, ie a lot of pumping time when they are cold and I would want to be able to take advantage of, for example a week of direct sun. I think a simple A-B switch would be all I would need. In position A it would run automatically as you have suggested and in position B the pump would bypass all the sensors and switches and just run the pump. 
Quote:

  You'll also need a mechanical hot tub thermostat to control the temperature of the tub. 
[bx9919-1500-A-100x100]
The tub thermostat is in SERIES with the heating snap disk. The disk turns the pump ON when the collector is warm enough, the thermostat turns it OFF when tub temp is reached (1). When the heating pump turns off, the collector temp will rise till the "cooling" set point is reached, and the cooling pump comes on (2).


So far I follow but will I be purchasing a thermostat with a fixed temperature cut off point or is it adjustable? I haven't researched what is available here yet so I assume I'll find out about it when I dig in there. If the cut off point is close to 104 I'd be very happy.


Quote:

(3) all the thermostats will reset when the temperature is out of range. A timer isn't needed.


But this assumes that I don't need additional cooling of the heat sync tank. I suspect the non insulated 600 gallon tank that sits in the shade will stay cool enough to buffer the high temperature of the collector but that point does worry me. In any event it will be simple enough to install another A-B switch to just run the cooling tank pump if I find the tank getting hot. 
Quote:

(4) the polypropylene heaters you mention won't need a cooling system as long as you DO NOT cover them with glazing.  If you do glaze them, they will.  


OK here is a very important point and I really don't know what to expect. Can I get a hot tub up to 104 with a polyethylene solar collector that is not in a box, insulated and covered with glazing --ie. poly-carbonate. The collector supplier thinks not and I do have room for an additional 4x10 collector that would be pretty easy to install..  ???????????
Big unknown here on my part



stmbtwle

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Reply with quote  #19 
OK... the mechanical thermostat IS adjustable, up to around 104. It's made for hot tubs. Only issue I've seen is that like all mechanical thermostats it's a bit "sloppy", you may get 105 today, 102 tomorrow. Only way around that I know of is to go electronic. They're about the same price, but a bit more complicated to wire up.

The temperature attained in your collector will depend on the flow. A high flow will give you a lower temp, and vice-versa. You can adjust this with a valve or speed control, but the collector will do it automatically by cycling on and off.

An unglazed collector is more affected by external temperature and wind. In summer you shouldn't have any problems, but in winter you may need to cover it with poly sheet, which is a compromise. In summer you may (or may not) need to remove it. After a season or two you can decide whether to use polycarbonate.

There's a certain amount of trial-and-error involved.


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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
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