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jezter6

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Reply with quote  #1 
I got a free hot tub. The electronics are 100% shot, and I've cut the cords on the motors to help aid in moving it, and had to completely rebuild the wood frame around the fiberglass.

Fixing up the minimal leakage, and it's holding water pretty nice, enough for testing and having some good soaks without any jets.

The "plan" is to include 3 heat sources (redundancy may just be my middle name).

1) Solar collector. Planning on building the standard one from Build It Solar. there's a 4x4 unit near me for $75, it's expensive but might get me up and running with a test solar sooner.

2) Wood heat/copper coil. That's currently working. Using a submersible HF sump, running out some garden hose to the fire pit with 5/8 x 50' copper coil and back into the tub via garden hose.

3) Camping "on demand" propane heater...for those cold winter days where solar isnt cutting it, and I'm too lazy to build a fire.

I'd like to re-use the 110 circulation pump (presuming it works) that handles the spa filter and existing heater that I'm taking out of the equation. Since I don't have 220 out there and don't want to pay to run it, for now we're playing with standard outdoor extension cords.

So, I'm hoping the solution is to run these somewhat in series, starting with the collector, then through the fire loop, then if all else fails, the propane heater. That is, unless there's a much easier way to bypass some of the options that isn't opening and closing 6 valves or something. Or I have to have 3 different pumps and I guess figure out how to not pump water through one of the heat sources not in use? Not sure how that even works.

To keep chemicals out of the collector, I'm thinking of insulating a 55g drum to hold the collector water (with its own pump hooked to snap switches) and only using some tubing to heat exchange into the hot tub. Pending how nicely it holds heat and any excess, a second HEX may go into the drum for some pool heat.

Opinions and ideas (and healthy criticism) welcome.

sundug

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Reply with quote  #2 
Do you know how many gallons the tub holds? If you do series, you will lose solar heat thru the fire coil when there is no fire. Parallel with valving seems the more efficient. I just use solar, directly from the thermal collector, or indirectly from my PV array. Here's mine- https://simplysolar.supporttopics.com/post/new-evacuated-tube-spa-swh-7477891?highlight=spa+hot+tub&pid=1304990455 I added a bunch of insulation, but the 4 x 8' collector can't keep up in winter-I'm in TN- and produces too much in summer. Going to a steeper collector angle is not practical for my situation. Doug
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stmbtwle

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Reply with quote  #3 
Jezter6,
That's a really complicated system and it will be a lot of work to build and maintain. I'D Try to simplify it some.

Hot tubs are really tricky due to the narrow operating range, and once the tub does hit the desired temperature (rather low in solar terms) you have to shut off the pump, which stagnates and overheats the collector, with possible damage.

I think you might try an unglazed collector of black poly pipe or even garden hose. In summer it will air-cool if it gets too hot, in winter you can cover it with clear plastic. It's simple, cheap, and a good place to start. You can make improvements over time if needed.

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jezter6

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Reply with quote  #4 
Thanks all. I live in MD, so a bit further north (actually not far from the forum's owner). So I'm going to have a similar winter situation, if not worse. Also, I'm on a north facing slope with some tree infiltration in the winter.

The series thing I found out wouldn't work due to most of the propane heaters kicking on directly with water flow, regardless if it's at/over temp anyway. So that will have to be a manual on/off thing. And losing a ton of heat via the coil...yeah, I didn't even think of that one.


I think for this summer I'm going to do a black poly "X" coil that I've seen around the web, and will consider putting it in a box or replacing it with a PEX-AL inside the box closer to winter. I'm ok with a little overheating in the summer because I also have a 3000 gallon pool that I'd like to keep at a mid-80s level. So I'm planning on using a collector with the 55 gal insulated drum and plumbing little pumps for the pool and hot tub through heat exchangers there.

Hopefully the overheat and a moderate amount of storage will allow me to warm the tub at night if it drops during the summer and not need to light the fire.

Collector size will start at 8x8 uninsulated, and when I get the pergola installed on the tub, it'll go to something like 12x16 insulated and glazed.

Can you use 2 snap switches on 1 pump? IE: 1 in the tub, if it's cooler than operating temp, run the pump through the HEX to gain heat, and a 2nd one in the hot water drum to not turn on the pump if the storage tank is "cold" so I'm not accidentally cooling the tub?



jezter6

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Posts: 24
Reply with quote  #5 
Quote:
Originally Posted by sundug
Do you know how many gallons the tub holds?


400 gallons. Apparently it's a Thermospas Chesapeake model. I only guess because it looks like the pic on their website, I don't see any info on the tub anywhere.


Love your setup. I'm only hoping I can maintain maybe 85 in the winter (any higher would be awesome and use one of the secondary sources to make it hotter as needed. I live in a highly wooded area, so falling branches gives me a fair amount of "free" firewood along with what we buy for our wood stove (mainly a visual thing for family gatherings).




sundug

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Reply with quote  #6 
Quote:
Originally Posted by jezter6
Thanks all. I live in MD, so a bit further north (actually not far from the forum's owner). So I'm going to have a similar winter situation, if not worse. Also, I'm on a north facing slope with some tree infiltration in the winter.

The series thing I found out wouldn't work due to most of the propane heaters kicking on directly with water flow, regardless if it's at/over temp anyway. So that will have to be a manual on/off thing. And losing a ton of heat via the coil...yeah, I didn't even think of that one.


I think for this summer I'm going to do a black poly "X" coil that I've seen around the web, and will consider putting it in a box or replacing it with a PEX-AL inside the box closer to winter. I'm ok with a little overheating in the summer because I also have a 3000 gallon pool that I'd like to keep at a mid-80s level. So I'm planning on using a collector with the 55 gal insulated drum and plumbing little pumps for the pool and hot tub through heat exchangers there.

Hopefully the overheat and a moderate amount of storage will allow me to warm the tub at night if it drops during the summer and not need to light the fire.

Collector size will start at 8x8 uninsulated, and when I get the pergola installed on the tub, it'll go to something like 12x16 insulated and glazed.

Can you use 2 snap switches on 1 pump? IE: 1 in the tub, if it's cooler than operating temp, run the pump through the HEX to gain heat, and a 2nd one in the hot water drum to not turn on the pump if the storage tank is "cold" so I'm not accidentally cooling the tub?
===========================================================
Yes, you can use two snap switches like that. I had black poly melt with stagnation in my homemade collector with twinwall cover. The PEX has withstood the same conditions just fine, altho I have eliminated the hi temp switch which caused the stagnation. I use a roll up shade to keep summer temps down. What is the tub side HX coil made of? You also have to consider how to keep things from freezing. Doug


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stmbtwle

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Reply with quote  #7 
Yes you can use two switches, a "normally off" in the collector, and a "normally on" in the tub.

For the collector I'd use one that comes on at 120F and goes off at about 110F. If the "off" is too low it will actually COOL the collector in the evening. For the tub I'd get a mechanical hot tub thermostat designed for the purpose. They're adjustable and more accurate than a snap disk. Connect the two switches in series.

https://www.spadepot.com/Thermostat-14-Bulb-36-Capillary-P821C702.aspx?zmam=55673001&zmas=1&zmac=1&zmap=BX9918&gclid=EAIaIQobChMIgZ_DioWo4wIVhIjICh18cwIMEAQYASABEgIRDPD_BwE

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
jezter6

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Posts: 24
Reply with quote  #8 
Haven't decided  yet. Ultimately, i want to buy a couple big rolls of PEX-AL so I can eventually continue to expand my collectors for DHW and some space heating if I can.

Trying to figure out what best to run spa chemical water through that wont destroy it too quickly.



sundug

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Reply with quote  #9 
Haven't decided  yet. Ultimately, i want to buy a couple big rolls of PEX-AL so I can eventually continue to expand my collectors for DHW and some space heating if I can.

Trying to figure out what best to run spa chemical water through that wont destroy it too quickly.


===============================
 Plastics are the only practical plumbing to use with spa chemicals. 
I used a differential thermostat to keep the tub pump from running if the collector was not more than tub temp. Used to be a member here selling DIY differential thermostats. Doug 

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jezter6

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Posts: 24
Reply with quote  #10 
Quote:
Originally Posted by sundug
Used to be a member here selling DIY differential thermostats. Doug 


I see a couple links for some basic < $100 controllers on the builditsolar site.

How did you plumb the temp probe into the tub?

I saw one of the links posted earlier about a thermostat and in the related links was an elbow with a "built in" spot for the probe they sell.

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