Solar heat for hotub?? Homemade solar thermal box??

Question:

HI, I live in an all electric Condo and my  8×8 Hotub is costing approx 50 a month to heat.  Recently I visited a friend that had a home made box for solar heat to his pool.  just a wooden box with pvc pipes crisscrossed and every thing inside painted black and a glass cover. Did not get into what else he had with the box. I am interested in making something similar. A home made thermal unit, to heat both tub and provide hot water to house.  I have just been given 8 old hot water heaters that have excellent tanks, were replaced because of faulty elements or electrics. I want to utilize a few of these in the attic to store maybe 300 gallons of waters heated by a solar box. I need help, have little or no solar experience. (do understand the principles) 1- constructing the solar heating box.        - best size of box, piping 2- what type of paint???  An special type of pvc. 3- what type of controls?? Any help would be appreciated. thks BG

Response:

First thought that comes to mind… Why do you want to store solar water, and please do not do it in the attic of a condo…. something about roof loading!.  What is wrong with storing the water in your hot tub?.  The system that my brother and I put together for his redwood tub ( with a cover ) is … I found some take off glazed panels that were frozen ( only a little damage and $25 each!) and repaired them. The panels are 4*6 foot and like I said, GLAZED. ( gets the water hotter). We used two of them for a small tub, and the tub gets HOT.  The controls are off of a house unit, but as we have been watching the unit run, if we just use a reverse photo switch ( on in the bright, off in the dark ) that would work fine.  The system is set up so when the pump stops flowing, the water drains back into the tub, preventing the possible freeze in the night.  The biggest problem with his system is that in the afternoon he has to start monitoring the temp to keep it from getting to hot, and if so he adds cold water so he can get in. Also, go find a system to look at.. ( something about the picture and a thousand words) CAP

Response:

What is wrong with storing the water in your hot tub?

The comfort range for a hot tub is fairly narrow–104 F is too cool and 106 is too hot. We can only store about 300gx8Btu/Fx2F = 4.8K Btu with that temp swing, but a 4′ R24 cube at 105 F in a 70 F room needs (105-70)6×4′x4′/R24 = 140 Btu/h to stay warm with the cover on, ie we can only store solar heat for about 34 hours that way. Nick

Response:

Would make a lot more sense to just concentrate on a solar collector – the hot tub "is" the storage tank. Make sure it is well insulated and get or build a solar panel that you can use to circulate the spa water through. The typical problem is the size of a circulating pump in a typical system (ie. panel on roof, pump at or below pump level, separate suction & return line, temperature control moderating temperature). Usually, the smallest pump to use in this application is a 1/12 hp pump. If you choose to do a "breadbox" type of heater for a spa, you have to figure that there is gonna be alot of energy expended to make the boxes useable and well insulated. Old heaters would have to have a pump to circulate through a solar panel, unless you remove all the insulation on the tank and place it in a well insualted box with a glass cover. Believe me, it’s just not worth it… The best of all possible installations would be a typical installation:     1/2 hp Grundfos circulator     150% solar sizing (ie 60 sq.ft. of spa surface area — 90 sq.ft. glazed solar panels)     Heliotrope HM5000C fully automatic solar controller Mount the solar panel as low as possible on the roof directly above the spa. Drill new suction and return lines in the spa (suction low – return to upper 1/3 of spa). Use a suction screen to prevent a foot or toe from getting "sucked" into the suction. Directly out of the spa, install a ball valve on the suction and return lines. This will allow you to isolate the system if you have problems. Install the pump next and a hose bib and run the line (almost certainly should be copper, though your compromise becomes the weakness in the system) up into the solar panel. Return back to your return port on the spa and if possible, locate that port in a place that might be somewhat protected from the hot water that might come down that line. Insulate the lines with Rubatex or the best insulation you can find. Install a temperature sensor on the supple line (preferably an immersion sensor) and another at the collector outlet. Hook the solar control to the sensors and the pump and you have a solar system! There are other ways to do it, but this is the most sensible system to continue to heat your spa for 10-15 yeas of use. Keep the chemistry accurate in the spa and you’ll avoid problems with the copper. Now, I know this is probably not what you wanted to hear… It seems that your firend planted the seed that get started and it might seem easy, but without even seeing his system, I can tell you it probably doesn’t really add that much to his pool. Pool solar systems are very large in size (square footage of solar panels equaling the surface of the pool) and again the pool is the storage tank. You can’t take a little tank of water, even real hot water, and add it to a 150,000 gallon pool and expect it to raise the pool very far. A typical pool solar system uses the fact that the filter pump runs typically 8 hours a day, and if you can route that water that has been filtered through solar panels that are bathed in sunlight, you can pick up 1-3 degrees on each pass through the solar collectors – turn the pool over (meaning filter the pool once completely) 2-3 times a day and you have an accumulated 3-10 degrees. 10 degrees would be ideal, raising a 65 degree pool to 75 degrees and on… Interpreting what you mentioned in your message, it seems like you would just like to heat these tanks of water up and drop the water into the spa. While possible, and if you are really into homebrew, you "could" pump the water up into "breadbox" type solar heaters (the tank, painted black and built into a well insulated and glazed box, hence breadbox…) from the spa (emptying the spa) and drop it back into the spa at the end of the day. The major problems with this setup would be that you aren’t necessarily filtering the spa while the water is in the tank, though you could use the filter pump to fill the tanks; you have no method of temperature control (a very major problem…) and if you had an inground spa, you "might" float the spa (if the ground around the spa is saturated…) and ruin the underground plumbing. It has happened to people before, fiberglass spas mostly….  A typical spa holds about 150-300 gallons of water. and that would require (3-6) 50 gallon tanks to heat the spa and that is a lot of work and a lot of effort and would be major ugly in a typical backyard. I suggest against this method. I am in the solar business (22 years) and know that there a lot of solar panels available on the used market. You may have trouble finding them where you are, but I doubt it (unless you are in the northern climate zone). In San Diego there are loads of used panels enough to fill all the needs of anyone wanting to do this sort of thing. By using a pre-manufactured panel, you can avoid all the homebuilt aspects or the tanks. If your particular installation allows it, you could even mount panels below spa level and use thermosyphon technology (thermally fueled syphon pumping action). You’d have to have all your chips lined up for this to work, too. South facing location for solar panels, panels completely lower than the base of the spa, copper piping or CPVC, and again NO TEMPERATURE REGULATION…. I can’t stress that enough. People have been killed from burns received in overheated spas (skin being the largest organ of the body and burning it off leaving you open to infection…), or so they say… Anyway, I have said enough. I don’t check this forum very often, so you might want to continue in private email. Remove the NOSPAM from my email address and we can chat that way. Joe To answer your questions directly: HI, I live in an all electric Condo and my  8×8 Hotub is costing approx 50 a month to heat.  Recently I visited a friend that had a home made box for solar heat to his pool.  just a wooden box with pvc pipes crisscrossed and every thing inside painted black and a glass cover. Did not get into what else he had with the box.

PVC and glass glazings are going to lead to a meltdown eventually or very brittle pipes that make this setup sort of temporary. A Black rubber hose might do better than the PVC if it is an EPDM rubber hose (~$30) , but it might lead to a rubbery smell in the water. I am interested in making something similar. A home made thermal unit, to heat both tub and provide hot water to house.

Not going to be very easy, but can be done. Actually, one of the best ways to heat a spa is by first making a very powerful domestic hot water solar heating system and extracting the heat off that system into the spa through a heat exchanger. Very expensive and not for the timid. You still need solar panels, a heat exchanger, pumps, controllers, etc etc…  I have just been given 8 old hot water heaters that have excellent tanks, were replaced because of faulty elements or electrics.

Better off selling these repaired and using the $$$ to fund your solar project….  Really, the heaters are better of remaining heaters. I want to utilize a few of these in the attic to store maybe 300 gallons of waters heated by a solar box.

Tanks in the attic are a real problem. Weight (300 x 8 lbs per gallon – 2400 lbs in the attic). Don’t think I’d wanna be sleeping below them if you know what I mean. I have R&R solar storage tanks in the attic with block and tackle and believe me, it’s a drag…. Never could figure out why they weren’t just installed at ground level. (They were older homes in Coronado and I suppose it was thought that they were in a great location, but, unless you do major drain pans, it’s a waste. I need help, have little or no solar experience. (do understand the principles)

That’s why I am trying to help. 1- constructing the solar heating box.        - best size of box, piping

No bigger than it has to be – a refrigerator makes a good breadbox model, but wouldn’t hold a larger tank. Weight, aesthetics weatherizing, etc all major considerations (this is asssuming you were talking about mounting the bread box on the roof…) 2- what type of paint???  An special type of pvc.

Typical flat black Rustoleum would do as well as any other. Copper pipe if possible, CPVC if copper doesn’t work. CPVC is rated to 200 degrees. Anything less will meltdown under certain circumstances… like when your spa is heated and there is still water in the solar system. That water has to go somewhere, right??? 3- what type of controls??

Heliotrope General in Spring Valley, CA has some of the best controls on the national market.  Has to be a differntial temperature thermostat, knowing when to circulate a pump and when the spa is heated up to the thermostat setting. Good luck with whatever you do…. – Hide quoted text — Show quoted text – Any help would be appreciated. thks BG

Response:

Sorry my long winded message got posted three times…  My news server wasn’t set up properly and kept failing – when I corrected the news server information, all 3 attempts went through….

Response:

…the hot tub "is" the storage tank.

That won’t work very well. Hot tubs have a very narrow temperature range for human comfort, so they can’t store much solar heat–104 is too cool and 106 is too hot–150gx8Btu/F-gx2F is only 2400 Btu. Nick

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