# Solar hot water - hard to find kits



## Habeed

Update : more google revealed that SRCC is the main rating agency. And they rate a _lot_ more panels. Evacuated tube collectors give the best and most consistent performance because they prevent conduction of the hot water back to the environment. So, like this one. 

The easiest setup to install would be to stick one array facing East and the other West. A third component of my roof faces South, which is optimal, but it means much longer plumbing runs. 

I would also need an accumulator tank. Thinking about it, there has to be a few temperature sensors and a pump. And a relay and a controller, though the logic the controller has to be able to do is pathetically simple.


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## Habeed

And, ok, you need a storage tank. Apparently, the good ones are the Rheems. They claim they will never fail, which doesn't seem impossible - no metal in the tank to rust, and there's no heating happening in the tank. Only way it could fail is scale buildup and I don't think that happens if there is no heat source. 

So, cold water goes into the tank. It gets pumped out of the tank to the evacuated tube array, circulating in a pressurized loop between the array and the tank.

Naturally, it's actually cheaper to purchase an electric hot water heater version of the same tank instead - home depot has it for $900. 

So $900 for the tank, $1200 for the array, probably at least $500 for balance of system components. 2600 bucks. Or I can buy 1400 watts worth of solar panels for that.


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## concrete_joe

a well insulated PEX tank seems to be a lifer.

why east and west facing. all south facing should give you best overall performance ??

a on-demand gas water heater in a system that is coupled to solar heated tank seem like a good setup. just feed the tankless from the solar tank so that tankless doesnt go on unless the feed water is at/below the on temp of the tankless. however, i am not sure if those tankless on-demand unit can be controlled by inlet water temp, i think they are all flow based on/off control.


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## Habeed

concrete_joe said:


> a well insulated PEX tank seems to be a lifer.
> 
> why east and west facing. all south facing should give you best overall performance ??
> 
> a on-demand gas water heater in a system that is coupled to solar heated tank seem like a good setup. just feed the tankless from the solar tank so that tankless doesnt go on unless the feed water is at/below the on temp of the tankless. however, i am not sure if those tankless on-demand unit can be controlled by inlet water temp, i think they are all flow based on/off control.


Because the South facing portion of the roof adds another 40 feet of pipe. I may end up using that, however. 

On demand gas..with a mixing valve...hmm...


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## concrete_joe

i am not sure you want to mix the two outputs together as this kinda defeats the purpose. i believe most tankless on-demand heaters simply go "on" when water flow is detected as the unit is expecting cold water supply on the inlet. it would be nice if the tankless also had a inlet water temp sensor so it doesnt go on if the water is already hot (hence being fed from the solar tank).

if the tankless is an 240v electric type then it should be simple to add a temp sensor and a solid state relay to control the electric heating element of the tankless. electric units will be the most efficient.


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## Habeed

concrete_joe said:


> i am not sure you want to mix the two outputs together as this kinda defeats the purpose. i believe most tankless on-demand heaters simply go "on" when water flow is detected as the unit is expecting cold water supply on the inlet. it would be nice if the tankless also had a inlet water temp sensor so it doesnt go on if the water is already hot (hence being fed from the solar tank).
> 
> if the tankless is an 240v electric type then it should be simple to add a temp sensor and a solid state relay to control the electric heating element of the tankless. electric units will be the most efficient.


Tankless heaters do have inlet water temperature sensors.

The problem is that tankless gas heaters can only safely throttle down so far before the flame goes out. 

Electric tankless heaters can in principle throttle to 1% power no problem. I know one of the engineers who worked on the Seico brand tankless heaters, and they can do this.

So I'm proposing sending pre-heated water through an electric tankless heater. Only if the water isn't hot enough - because there wasn't enough solar power - would the electric tankless heater come on.


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## concrete_joe

Habeed said:


> So I'm proposing sending pre-heated water through an electric tankless heater. Only if the water isn't hot enough - because there wasn't enough solar power - would the electric tankless heater come on.


yes, seems like a good design.

if you tank the solar heated water with a circulating feature, you should be able to extend solar water heated use well into the night, thus the bigger your solar tank the better.


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## Habeed

concrete_joe said:


> yes, seems like a good design.
> 
> if you tank the solar heated water with a circulating feature, you should be able to extend solar water heated use well into the night, thus the bigger your solar tank the better.


Yes. The hotter the water, the more thermal energy is stored in a given tank size, but the faster heat is lost through the tank walls, the circulating pipe plumbing, and out the heat collector itself. 

But, the bigger the tank, the more money is spent on constructing the tank wall and paying for the floorspace to hold the tank.

So there must be a mathematical way to find out at what temperature you want to store the water at. If you store 50 gallons at 145 fahrenheit, and the inlet water temperature is 65 fahrenheit (this is true where I live), you are storing the equivalent of 100 gallons of usable hot water at a showering temperature of 105 farenheit. Each 50 gallon tank costs $900.

The rate of heat loss is proportional to the temperature difference between the inside of the tank and the outside. It's a complex problem to solve as there are other factors such as the average holding time and so forth.

Also, very large tanks are disproportionately cheap because the volume increases faster than the wall costs. But, very large tanks may have problems..


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## concrete_joe

a PEX tank wrapped in good styrofoam should make for a good storage tank. not sure how your $900 tanks are constructed.

considering solar energy is free i would opt to install the most tank i could w/o worrying so much about heat loss. when solar water is hotter than tank water, circulate it to put heat into the tank water, thats really the best you can do. the tankless is only there to heat when solar tank water is below your threshold for hot water temp, etc.

your example is not 100% accurate because you dont always heat up 65F water. the solar tank may have no flow, but you still heat the water by circulation to extract heat from the solar source, etc. this means the solar tank could reach max temp even when solar source has plenty of more heat to offer, but you stop the heat transfer at max temp threshold to avoid water scolding, etc. if you incorporated a water mixer based on water temps then you could essentially get the solar tank very very hot but mix in cold water to its outlet to bring the temp down, etc.

the issue with solar water heating is that you are basically limited to extracting heat from the solar to heating the water to a temp that is safe. this basically restricts how much solar energy you can extract for any given tank size, etc. if you could get your tank water up to 300F you would have enough energy in the water that your tankless would likely never go on (depending on solar tank size and water demand, etc), but i think you get the point. since water temp is basically restricted the only way to compensate is to install bigger solar tank, etc.

and just for clarity, i would not heat cold water source and then dump that into the tank. i would use a circulating method to move solar heat into the tank water, etc. heating the cold water source before the tank requires water flow, which usually wont be there, etc.


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## carmusic

i would simply use 2 tanks, the first for heating through the solar panels with recirculating pump and the second one would be a standard electric or gas hot water tank. the electric or gas one would only come on if water in first tank is not hot enough.


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## concrete_joe

carmusic said:


> i would simply use 2 tanks, the first for heating through the solar panels with recirculating pump and the second one would be a standard electric or gas hot water tank. the electric or gas one would only come on if water in first tank is not hot enough.


this would mean you have two low thresholds. one low for the solar circulator, and another for the gas/electric circulator. the gas/electric would have to be set lower than the solar threshold. 

but why two tanks? with a tankless as the bkup to the solar you really only need one tank. having two tanks means 2x the thermal losses, etc.

i really like the idea of a solar tank backed up by a tankless, this seems to be the most efficient setup.


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## Habeed

I read several articles arguing that at this point, solar thermal is a dead technology. I agree. 

Instead of doing solar thermal, I can stick panels up on the south facing portion of my roof (higher up, but since electrical wires are a lot easier to run than plumbing, this isn't a problem), paying slightly over a buck/watt capacity. 

I then just purchase a heat pump driven hot water heater like this one. (rheem performance platinum) I set the controls to always use the heat pump.

Then I have a sensor or electronic monitor on the solar array. When the solar array is producing enough power, it turns on the heat pump. Ideally I'd also have an amp meter clamp ring on my main power input, so it only runs the heat pump when 100% of the electricity comes from solar, making it "free".

This heat pump heated water then goes through a cold water mixing valve set to about 115 farenheit followed by a tankless electric heater, a little bit undersized, set to 110 farenheit (enough to run a shower and a sink starting with cold ground water, or the whole house if the water is preheated like this).


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## Piedmont

Habeed said:


> I read several articles arguing that at this point, solar thermal is a dead technology. I agree.


I would read more articles :vs_worry:

PV panels are 11-14% efficient at turning sunlight into electricity, solar thermal (hydronic solar panels) are 65-85% efficient at turning sunlight into heat. PV panels, the hotter they get the worse their efficiency so at noon when you should be reaping the benefits, PV's are at their worst efficiency of the day. Then if a PV panel gets a cell blocked by shade, that cell turns into a resistor. Unless the panel has a bypass, that panels efficiency will reduce 50% because a leaf. If it has bypasses, you lose a section of the panel. Lastly there's cost. Here's a site that will sell a complete solar hot water and heating system for $9286. A comparitive PV system a guy from work just installed at his house was $45,000. I look at him like he's an idiot... installing a $45,000 system that saves him $1800/year in electricity when he could've installed that hydronic solar heating system and saved $1800/year as well but in 5 years would've paid for itself and been free money in the bank after that. 

I don't recommend evacuated tubes if doing hydronic solar heating for many reasons, FP's are the better bet. But, I think you should look at efficiency, cost, and reliability as used for heating you need so many PV's to compensate for their poor efficiency their cost is astronomical and is NOT the right tool for the job. Sunlight converts to heat very well, make use of that by getting solar hydronic collectors if you want heat.


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## mske390

"I read several articles arguing that at this point, solar thermal is a dead technology. I agree. "


many moons ago we installed these panels on the roof to a commercial building in MA:

http://www.viessmann-us.com/en/residential/solar/tube-collectors.html

We made so much hot water that we kept blowing the valves on the storage tanks so installed a second, still were making more than we needed so found ourselves dumping hot water. 

How about install PV panels to run an electric on demand hot water system? I didn't read everything on this thread so if already mentioned ignore


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## concrete_joe

mske390 said:


> How about install PV panels to run an electric on demand hot water system? I didn't read everything on this thread so if already mentioned ignore


and at night this works how?
maybe the on demand can run from AM to mid afternoon, and then circ to storage tanks before sunset, this way you have some hot water when there is no sun, and then no need for batteries.


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## Oso954

Your whole house on demand electric water heater will need somewhere between 80-120 amps at 240v to run it, depending on the flow you require and the incoming water temperature.

Many homes need a major electrical upgrade just to handle the electric load.


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## mske390

All solar PV have to be tied into electrical system & switches between the two depending on sun. At night will the system will need house electric but should offset by the electric that us sold back to power during the day when hit being used.


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## Oso954

If you want to put $20,000 or more into a hot water system, you've picked a good way to do it.


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## langit

an interesting thread. I too have experienced similar things. and have not found the answer


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## crackedactor

Habeed said:


> Update : more google revealed that SRCC is the main rating agency. And they rate a _lot_ more panels. Evacuated tube collectors give the best and most consistent double beds performance because they prevent conduction of the hot water back to the environment. So, like this one.
> 
> The easiest setup to install would be to stick one array facing East and the other West. A third component of my roof faces South, which is optimal, but it means much longer plumbing runs.
> 
> I would also need an accumulator tank. Thinking about it, there has to be a few temperature sensors and a pump. And a relay and a controller, though the logic the controller has to be able to do is pathetically simple.


I've never seen this used to generate hot water.


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## Habeed

crackedactor said:


> I've never seen this used to generate hot water.


What are you implying?


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## Gary in WA

They injected an ad for beds, already notified Adm.

Gary


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## Piedmont

Habeed said:


> Update : more google revealed that SRCC is the main rating agency. And they rate a _lot_ more panels. Evacuated tube collectors give the best and most consistent performance because they prevent conduction of the hot water back to the environment. So, like this one.


This is old, but others may come across this. Yes and no. Yes the SRCC rates panels and can be trusted. However, evacuated tubes DO NOT give the best and most consistent performance. Here's the issues with evacuated tubes.

1.) They advertise they collect more morning & evening sunlight because of their round shape. That's true, however morning/evening sun is only 1/3rd -1/4th as strong as noon sun so the name of the game is noon-time sun, it's the full cake whereas morning & evening are the crumbs. Do you want more full pieces of cake, or more crumbs? Because of the space between each evacuated tube at noon they can let 50% of the noon sunlight fall wasted between each tube when sunlight is at its strongest. 

2.) Evacuated tubes are often rated in aperture efficiency/size instead of the physical size of the unit. That's to say, it will say it's 85% efficient per square foot of sunlight it can collect vs. a flat panels 75%. The problem with this is, to collect 32 sq ft of sunlight an evacuated tube system can take up 64 sq ft of your roof (again spaces between each tube) whereas a flat panel to collect 32 sq ft of sunlight will only take up 32 sq ft of roof. So they're comparing a 64 sq ft evac system to a 32 sq ft Flatpanel system and finding it's 10% more efficient. Compare a 64 sq ft Evac system that collects 32 sq ft of sunlight to a 64 sq ft FP system that collects 64 sq ft of sunlight and the FP system kills the evac.

3.) Longevity. FP's typically have a 25-30 year warranty on the fins which is the component most likely to fail (My AE-32 panels are this way). The glass face won't fail, the copper pipes in them won't fail before 25 years, the aluminum frame won't fail, the fins are about it. Evacs depend on a gasket that must maintain an air tight seal with temps swinging from 20F - 300F+ each day. Think it will last 25+ years? I don't. The evacs are designed to be easily replaceable, which tells me they fail. They also come with a 5 year warranty not the 25 like many flat panels. Lastly, if you have an evac fail in 7 years think you'll be able to find the exact model/replacement? How do you tell if they fail, do you go on the roof and inspect them regularly? There's nothing on FP's to really fail.

4.) Some will say a failed evac whose seal has broken will behave just like a flat panel. Their heating process is similar to dumping a bucket full of boiling water into a cold tub to warm it up. With a broken seal, it becomes dumping a bucket full of warm water into a cold tub... Good luck having it be much use. An FP system warms the whole tub.

5.) Evacs can't melt snow because they're evacuated. This is killer if you have snow they can be buried for weeks or months under snow.

6.) They say because they're evacuated they produce more energy in the winter/rain/storm. If it snows, they don't melt snow you can go weeks/months without collecting. Because their aperture (sunlight area they collect) is smaller than FP's, your temps need to be 10F or less at noon to be more efficient. How many times are temps 10F or less at noon in winter where you live? They don't collect any energy when it's stormy, and the little bit more they collect when cloudy is enough to get you a few more degrees, but not enough to warrant their purchase since clouds block 50%+ of the sunlight reaching them. Stick to the motto "the cake is noon sun", not the crumbs of morning/evening/cloudy/stormy sunlight. 

7.) Their temps are incredibly variable. What they do is all the sunlight's energy they collect in the tubes rises to the tip (called the header) where it dumps all the energy into the small tip. That tip can reach over 300F with evacs and there's glycol running through it. Those temps will fry most glycols (even car engine), you need special high temp glycol. Even so, if there's a power failure in summer evacs will fry high temp glycol... just ask my friend who has them. The most I've seen in my panels is 225F when I shut the pump off, not enough to damage the glycol. My friend has had all the glycol dump out of his evac system twice from turning the glycol to steam. Now he has special high temp glycol, but if he has a power failure in summer we're waiting to see if it dumps. Since there is no limit to the temps evacs can reach I'm pretty sure he'll be able to exceed even the high-temp glycol. Just remember, he's depending on the gaskets to keep the evac seal and he's reached over 300F on his system that's got to be hard on a gasket.


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## YerDugliness

Habeed said:


> So I'm proposing sending pre-heated water through an electric tankless heater. Only if the water isn't hot enough - because there wasn't enough solar power - would the electric tankless heater come on.


You are correct…to a point. I want to do much the same with a solar batch heater to hold preheated water, which would be used to supply my electric HWOD unit. Theoretically, that should make the demand on the electrical tanks much less, because the HWOD unit has a preset "maximum" temperature and the amount of electricity being used would only be enough to heat the incoming water to the maximum output the HWOD is designed to create.

Things get a little more mixed up in the winter, because regardless of the input temperature each HWOD is capable of only a certain "gain" in degrees. If the water is cold, of course, as it is coming into the basement during the winter, the unit will use all of its heating capacity and perhaps if you are in a VERY cold climate not heat the water to a temperature you would prefer. If the water going into the HWOD, however, is of a temperature that it does not require full power to heat it to the pre-programmed maximum temperature (such as that having been pre-heated in a tank with a solar panel), then you'll get the full temperature the HWOD can provide with less energy use.

I've been toying with the idea of using radiant floor heat in my almost 100 year old home…that would be a closed-loop system with the heat provided by an HWOD and a pump to circulate the water. IF, however…IF I were to build or buy a solar system that will allow a large tank of water to be heated and just put a long piece of the PEX tubing that ultimately transmits the heath to the underneath of the flooring (perhaps a full roll of PEX, stretched and uncoiled like a "Slinky" in the heated water) into that tank, the process of pre-heating the water that will be recirculated to heat my house would mean that the HWOD would require much less energy to heat to temperature because of the pre-heated nature of the water.

My house's main structure is 30' X 30', and is a flat-roof structure, so I have plenty of opportunity to put solar panels on the roof (if it weren't for that ONE huge elm tree that provides valuable shade during hot summer weather). My only issue is do I want a drain-back system or not…it would be better if I did not need a drain-back system, but there will be a lot of time during the winter when overnight temps fall low enough to freeze the water in the tubing, so I expect I do need a drain-back system. it just complicates the issue, that's all.

We all hear that there ought to be a great price break coming on solar…but when? I've been waiting for 15 years….hasn't happened yet. I hate to rely on rebates or tax breaks, but every penny helps, that's for sure.

Carry on….ONWARD, thru the fog!!

Cheers!

Dugly


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## Habeed

YerDugliness said:


> We all hear that there ought to be a great price break coming on solar…but when? I've been waiting for 15 years….hasn't happened yet. I hate to rely on rebates or tax breaks, but every penny helps, that's for sure.


I concluded a couple weeks after I made the OP that solar hot water is a dead and obsolete technology. It's simple arithmetic and the alternative gets better with every week that passes. 

The alternative is simple : PV panels (they are under 50 cents a watt now) and SMA inverters (the 6.0 model supports 3 strings, 2000 watt batteryless backup power, and is 27 cents a watt). 

You use the PV power to drive a heat pump hot water heater. The GE model is $1000 at the hardware store and this year has a $300 rebate on it. 10 year warranty. Also it's a lot easier and safer to just replace 1 hot water heater and run a simple conduit to your roof carrying the solar wires than it is to try to plumb a heavy heat collector on your roof. In hot climates you get an extra bonus that the heat pump hot water heater should be inside your house so the cold air it produces as exhaust helps cool the house. (or it can make it less sweltering in a garage, I suppose)

No risk of freezing, no risk of plumbing leaks, and it's cheaper. A no brainer, and this also is why it was so hard for me to even find available kits for solar hot water and why they were so expensive.

One custom thing I did think of doing was setting up a data link between the hot water heater (which has USB and bluetooth) and the solar inverter (which has wifi). Basically, if the sun's out, fire up the heat pump. In series right after the hot water heater would be an electric tankless heater. So it would "top up" the possibly cold water coming out of the heat pump heater, since the heat pump would only run when the power is "free". (and the tankless would only run when hot water is needed but the free power didn't make the water hot enough)


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