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Old 12-27-20, 07:59 AM   #1
solarhotairpanels
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Default What's better 40 gal of hot water or 3 gal? What's your 2 cents cuz I'm not SOLD yet?

What I'm referring to is those who are building solar hot water collection systems for domestic hot water assist.

Some people build big solar heated tanks then submerge a coil of pex or copper inside to run their domestic hot water thru on route to their faucet/shower. Doing it this way keeps the domestic hot water separate from the solar heated water in the tank and I get that... however how much volume of water do you actually have inside that big coil of pex or copper pipe? 3 to 7 gallons total maybe if you're lucky? Heck you better be taking some super short showers and forget the soap right?

Others use a 'purchased' (new or used) insulated solar preheat tank that contains a coil in the bottom heated by solar collectors which intern heats the 40 gallons of domestic hot water surrounding the coil which is then fed to your hot water heater or tankless hot water heater. Now to me this makes sense and is what I use at my house.

My collectors heat up what I call a 40 gallon domestic hot water preheat tank. When the hot water faucet is turned on that pre heated water (from my collectors) is fed to my tankless domestic hot water heater so that my tankless water heater is not taking in 48 to 50 degree ground water.

I don't understand how doing it the first way running your domestic hot water thru a coil is going to help you much at all unless I'm missing something here?

The EPA estimates that the standard shower heads use 2.5 gallons of water per minute. That's 20 gallons of water for the average shower!

Do you have 20 gallons of water inside those coils that are submerged in your solar collected hot water tank?

Please note that this post pertains only to DIY solar projects pertaining to domestic hot water systems. NOT house heating assistance

PS: I also got my ambition to build my own drainback hot water collection system by researching Gary's Build it Solar website. A wealth of information can be found there for sure, including analizing the amount of water that is actually inside the coil of tubing pex/copper that is submerged in the solar heated storage tank. I'm going back to that website again now to see if I can find those posts again just to refresh my memory.

Your views / comments are encouraged and welcomed to respond to this post.

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Last edited by solarhotairpanels; 12-27-20 at 08:30 AM..
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Old 12-31-20, 10:47 PM   #2
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Just about all the systems that I've seen that actually work well use the second method you describe. Small e-tube collector loop volume, exchanging heat into a larger tank, usually insulated well.
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Old 01-04-21, 12:46 AM   #3
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The coil inside of the water heater would be exchanging the heat from the coil into the hot water in the tank, so it's not going to "run out after 3-7 gallons" as you said it, the capacity of the hot water tank remains the same because the water being fed to the house is in the tank, not the coil. The coil is connected to the solar heater. You've got the configuration backwards.

..but with that being said, it's not so great to have a small differential in temperature for the sake of efficiency. Ideally you would want to store the solar heated water in a tank not exposed to electric or fuel heating, you ideally want it to be exposed to water incoming at ground water temperature and have the solar heating heat that up as much as possible. From there, I think it would be ideal on a cost basis to use a tankless natural gas water heater capable of handling a warmer than expected(not ground water temp) water source. ..or I suppose you could use a second standard tank type of cold water heater. This whole thing gets expensive when hot water tanks generally only last a decade.

I would almost want my solar heat collection tank to be one of the 275 gallon(1 kiloliter) cube tanks if they can handle the heat without getting soft. ..but that would be a problem in the summer because you'd be heating your house, but it wouldn't be too difficult to build an insulated enclosure out of XPS or polyiso. Even a heavy layer of rockwool insulation would work if the whole thing is air sealed and thermal bridges are minimized.
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Old 01-06-21, 02:49 AM   #4
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PEX wouldn't work so great, copper would be fine. plenty of old oil fired systems used those without issue (well, until they caked up with minerals). it's all about rate of heat transfer.
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Old 01-06-21, 07:39 AM   #5
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Quote:
Originally Posted by MN Renovator View Post
The coil inside of the water heater would be exchanging the heat from the coil into the hot water in the tank, so it's not going to "run out after 3-7 gallons" as you said it, the capacity of the hot water tank remains the same because the water being fed to the house is in the tank, not the coil. The coil is connected to the solar heater. You've got the configuration backwards.
What you described above is exactly how mine is setup.
My collectors drop hot water down thru a coil inside my insulated preheat tank then back up to the collectors.

The water surrounding the coil is my domestic hot water which is run to my tankless hot water heater. It is preheated to about 64 to 80 degrees in the winter and much hotter in the summer.
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Old 01-22-21, 01:14 AM   #6
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I guess it depends partly on your feelings about large amounts of stored water that you then use.

With the more typical large storage tank that you then consume water from you are using 'older' water (think legionella), with the fresh water only passing through the internal coil you won't run this risk.

Interestingly there is a company in Germany (OEG) that sell 'Hygienic tanks' that have a stainless steel corrugated coil (I guess to increase surface area and promote disturbance) OEG Hygienic tank.

As your cold water passes through the coil it will pick up heat from the tank so you won't be restricted to a short shower!
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Old 01-22-21, 11:27 AM   #7
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I guess it depends partly on your feelings about large storage

As your cold water passes through the coil it will pick up heat from the tank so you won't be restricted to a short shower!
Thanks for your input Acuario

My storage tank is only 33 to 40 gallons so one shower and most of it is replaced with fresh city water. Doesn't sit around in that tank any longer then a regular electric hot water heater would.

My wife just used the hot water at the kitchen sink for awhile on some dishes then went up and took a shower. That's it.. it's history / gonzo. :-}

Sun is shining great so my solar pump is kicking in now re heating the storage tank.

So far so good.. everything is working as planned now.

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