# Need help with solar hw options



## btuser (Mar 6, 2010)

Right now our DHW is heated by oil.   As near as I can figure I'm using at least 1 gallon/day for hot water ( 80 degree rise x 8.4 x 200g/day) .  It comes to about 135k btu/day.   At 350g of heating oil per year I've got an incentive to try an trim it down.

If I do the numbers on what I can get from a panel I'm only looking at 20k/day on an average day.  Not enough to flub up my roof and waste 4 grand.

Am I missing something?


----------



## DBoon (Mar 6, 2010)

Hey btuser, you're not missing anything, in my opinion. 

If it's just two people in a household, these solar hot water systems will never pay for themselves.  There is a huge fixed cost to just getting the infrastructure in place to support one panel, and if that's all you need, then the cost is really high per gallon of hot water heated.  

If you have a big family, teenagers, use lots of hot water for a commercial business (laundry, hospital, etc.), then they make a lot of sense economically.  Personally, I have a historic house and don't want to mar the appearance with one of these systems anyways.  

Before I invested that $4k in a solar dhw system, I would invest it in reducing water consumption (front loading washer, reasonable low flow showerheads, efficient dishwasher, etc.).  You can buy all of that for $2k easily, and cut your usage a lot.  Then, I would take the remaining $2k and spend some of it on an electric hot water heater (you may even have a night rate for electricity that is pretty cheap) and heat water that way.  Oil boilers to heat hot water are really pretty inefficient - probably less than 50% efficient with all the piping, standby and tank losses factored in along with burn inefficiency.  I spend only $20/month on electricity to heat my hot water.   If you needed to install a new 240V breaker, run wiring, and pay a plumber to buy and install an electric hot water heater, it would probably still only cost $2000. 

If you had a few extra bucks to spend and want to spend it on the best electric water heater you can get, check out the Marathon polybutylene electric hot water heaters. 

Let us know what you decide to do.


----------



## DBoon (Mar 6, 2010)

> 1 gallon/day for hot water ( 80 degree rise x 8.4 x 200g/day) .  It comes to about 135k btu/day.



I should have mentioned this before, if you back-calculate, that 1 gal/day of oil = 140,000 btu/day.  If you are at 50% efficiency for heating hot water (realistic, in my opinion, and you definitely can't do it with oil any better than 85% with even the most efficient system), then that means 70,000 btu/day is heating the hot water and the rest is up the chimney or heating your basement.  

That means you are using a little over 100 gallons of hot water/day.  Maybe less if your efficiency is worse than 50% with the oil hot water heater (and it could be less). 

Try to figure out where this is all going.  Is someone taking 30 minute showers?  Does someone leave the hot water running for dishes?  There may be a few simple lifestyle changes that can save you a lot without spending any $$$


----------



## btuser (Mar 7, 2010)

I'm a friggin' hawk when it comes to the oil bill.  Front load washer, shower heads and dishwasher are all checked off.  I've go six people, 4 of which are women.  I only mention that because if they were boys I'd just hit them with the hose, or buy a case of old spice.

6 showers x 2.5 gpm x 10 min (shaving, hand washing, ect) is about 150g/day.  I figure another 50 gallons for everything else.  It might be less but I've got a meter on my oil burner and I'm using about 1-1.25 gallon/day in the Summer.  I figure Summer usage is right around 70% eff.   Electric rates are pretty high for us at around $.17/kwh, but I'm looking into maybe getting a large tank and doing some off-hour heating at a reduced rate.  So far the numbers don't add up for this.


----------



## DBoon (Mar 7, 2010)

Well, you are using a fair amount of hot water, and you've already made all the efficiency gains that you can.  A solar dhw system might be the next right move if you can take the aesthetics.  The tax credits are pretty reasonable nowadays also.


----------



## btuser (Mar 7, 2010)

where's a good place to get reliable info on btu output?  I'm in Souther NH, and my exosure is pretty Southern in the morning, but I lose it over to the other side of the roof at night.


----------



## dougstove (Mar 7, 2010)

I am in New Brunswick Canada.
I did not do all the thermodynamic and cost estimates, but a nearby friend did.
In our situation, solar DHW makes sense for a family vs. electric or oil heated hotwater.
I got a solar 60 gallon DHW pre-heat system from ThermoDynamic in Dartmouth, NS, that feeds into a backup 60 gallon electric tank.
The payback is about 7 years, and it cut my monthly electric bill by about 40-50%.
Hotwater was by far our biggest electric draw (teenagers, lots of laundry, outdoor sports getting things muddy...).
I have a perfect southern exposure but my incidence angle is lower than optimal because of wind concerns.
I get significant solar gain in all months, except December;  in the summer it generates more hotwater than we can use.

All that said, certainly, conservation, insulation, good quality electric water heater (or gas) would be higher priorities for most people.
I got it because I am semi-stuck in a house with very limited options for increased efficiencies, but I had the ideal location for the solar DHW.


----------



## DBoon (Mar 7, 2010)

Check out this link for a primer: http://ases.org/index.php?option=com_content&view=article&id=301&Itemid=23.  The system I was researching (but declined to install) would have been a two tank system with a big 80 gallon tank being heated by the solar dhw system and feeding my regular hot water tank.  I was going to use a Marathon 80 gal tank for the solar hot water tank since it can take quite a bit more overtemperature than a regular hot water tank. 

This link has a list of states with a PDF of solar incidence data in kWh/square meter.   You can convert that to BTUs.  There are system inefficiencies - if you figure about 75% efficiency, with the linked data, you can pretty much get an idea of rough system cost and size.  Rough rule of thumb is one collector per two people in a house.  You are probably looking at at least a two collector and maybe a three collector system from what you previously indicated re: usage.  http://rredc.nrel.gov/solar/pubs/redbook/#maps


----------



## sesmith (Mar 8, 2010)

If you're handy, you can do solar hot water for a lot less than 4k.  This link should get you started.  Many good ideas for a quick payback.

http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm


----------



## btuser (Mar 8, 2010)

Yeah, I have to be carefull with the redneck options.  I plan on buying the components and installing myself, but there's no way the wife would let me string polytube on the roof like Christmas lights.


----------



## sesmith (Mar 8, 2010)

btuser said:
			
		

> Yeah, I have to be carefull with the redneck options.  I plan on buying the components and installing myself, but there's no way the wife would let me string polytube on the roof like Christmas lights.



That site is far from "redneck options", if you bothered to read though any of the designs there.  Most of it is well thought out by some very smart people.  Just because it isn't off the shelf expensive doesn't necessarily make it less effective.  BTW, most of the commercial solar installers around here get close to 16K for domestic hot water installs.  They deserve to make a profit, and have to stand behind their work, so I'm not knocking the price.  The truth is, a knowledgeable do-it-your-selfer can get the job done with good looking professional results for less than 2K.


----------



## btuser (Mar 8, 2010)

I've been to that site off and on for two years.  I agree there's some great stuff but it comes down to two things:  Glazing and storage.  If you're going to heat HW in Northern climates you need glazing.  If you're going to heat more than 2 showers a day you need expensive storage.   I need about 100,000btu/day.  I'm not going to get that out of a coiled garden hose.

I agree how the installers are being pigs about it.  I had one quote over the phone for 9k-the very next sentence was "REBATE!" down to $5500.  Problem I have is a lot of the good stuff/equipment is locked up with dealer/reps who won't sell if they don't install.  Explain to me why evacuated tubes have a 1000% markup between here and China.


----------



## Badfish740 (Mar 8, 2010)

btuser said:
			
		

> Yeah, I have to be carefull with the redneck options.  I plan on buying the components and installing myself, but there's no way the wife would let me string polytube on the roof like Christmas lights.



This one hardly qualifies as redneck though I know the one you're referring to.  The only reason I'm not putting this in my house tomorrow is because though it looks (and is proven) bulletproof to me, we are going to sell one day and I'd imagine that it might scare off more than a few buyers.  However, I'm already formulating plans for something on a larger scale for the home that we will build one day.  I figure that with a larger tank, more insulation, and twice the square footage of collectors a family of four or five with heavy hot water usage would be easily accommodated.  As built it cost $1000-if I incorporate such a plan into my original build and consider the fact that whatever house we build will be our last, it makes a lot of sense.


----------



## btuser (Mar 8, 2010)

Definately nicer than black coils on your roof.


----------



## WES999 (Mar 9, 2010)

The problem with the commercial solar systems as some have mentioned, is the cost is too high.  For a 5 or 10K system the payback is not there.

I am working on my version of the homemade system as seen on the Build it solar website. Here is a pic of my progress so far, it is about 75% done.

 One 4' X 8' collector costs about $200. According to the site author the homemade collectors nearly as efficient as the commercial ones. What you lose in efficiency you can make up in size or number of collectors.


----------



## DaveM195352 (Mar 9, 2010)

one thing to always keep in mind - as you draw down (use) the hot water out of the hot water storage tank, new COLD water is replacing it.  So, having 50-80 gallons of pre-heated water at 150 degrees IS NOT going to give you a 60 gallon hot tub full of 150 degree water.  I believe in most sucessful solar DHW systems, the solar is thought of as a pre-heat to another form of hot water system.


----------



## Badfish740 (Mar 9, 2010)

DaveM said:
			
		

> one thing to always keep in mind - as you draw down (use) the hot water out of the hot water storage tank, new COLD water is replacing it.  So, having 50-80 gallons of pre-heated water at 150 degrees IS NOT going to give you a 60 gallon hot tub full of 150 degree water.  I believe in most sucessful solar DHW systems, the solar is thought of as a pre-heat to another form of hot water system.



The system I referenced above is designed to take preheat cold water from the street/well to 140 degrees before it goes into the actual electric water heater.  The heat exchanger coil itself holds 9 gallons of water so for some hot water uses (quick shower, washing dishes, washing hands, etc...) the system is 100% efficient because the hot water heater never kicks on.  I'm wondering about scaling the system up-specifically, what would be the point of diminishing returns?  How does collector square footage compare to tank size?  The collector used in the installation I referenced is 48 square feet and the tank is about 180 gallons.  Would doubling the capacity of the system simply mean making the collector size to 96 square feet (I think I would go with two 4' x 12' banks on either end of the house if this were the case) and using a 360 gallon tank?  Or would it be a better idea to leave the collector the same size and just increase the heat storage tank? 

In terms of the tank itself I was thinking of building it into the foundation of the house utilizing poured concrete.  It would not be difficult to form a tank along one of the outer walls of the home leaving plenty of room for thick foam insulation on the sides and bottom and an EPDM liner.  The advantages would be a greatly decreased chance of leaks, and a more robust tank.  I'm thinking a tank (inside dimensions) roughly 4' wide by 4' high by 12' long which would give me nearly 1500 gallons of storage.  It would also give me room for a much longer coil that would store more water.  Is there anything I might be missing here?  Finally, I'm still not sure how the heat exchanger from the wood boiler would come into play.  Would it make sense to have cold water from the well first pass through the solar heating tank for preheating and then to an insulated holding tank containing a flat plate heat exchanger fed by the wood boiler?  This would avoid the problem of the solar heating tank becoming too hot, but I would certainly have to put an anti-scald valve on the domestic hot water tank.


----------



## benjamin (Mar 9, 2010)

DaveM said:
			
		

> one thing to always keep in mind - as you draw down (use) the hot water out of the hot water storage tank, new COLD water is replacing it.  So, having 50-80 gallons of pre-heated water at 150 degrees IS NOT going to give you a 60 gallon hot tub full of 150 degree water.




If yur fillin yur hot tub with that hot a water yur hogs is gonna come out too crispy, I fill my hot tub with water bout 140 so as ya can put yur finger in for a tad an the hogs come out real scrapeable.  The chickins pluck durn fine two!


----------



## DaveM195352 (Mar 9, 2010)

it's the wife's tub....  I don't ask to many questions


----------



## benjamin (Mar 9, 2010)

My redneck system, referenced above, is almost 300 sf of first generation commercial flat plate collectors, some are wood, some are al. It works great for heat.  My connection to dhw is a bottleneck at the moment, virtually no contribution.  

I agree with nearly everything said so far.  A professional install is well worth the money, and you can also get the same result for a fraction of the cost if you're willing to do a lot of work.  

Whichever system you decide to go with the primary consideration in my humble opinion, is will you be around long enough to make it pay off.  I don't believe I have EVER seen an active solar system of any sort, over the last 30 years that has paid for itself.  The ones that came closest were the redneck type where there was very little money involved from the start.  I'm thinking specificaly of a hot air system that a neighbor kid did as an FFA or shop project to help heat an old farmhouse. ALL the rest have been just recently installed or are/were no longer functioning.   I recently glanced at the MREA's solar tour of homes online, none of the few systems I was familiar with had any chance of being  payed off by themselves without subsidies.

I'm not cutting MREA in any way, and I love the work done by Gimme shelter, I just think that the economic feasibility is far from proven for expensive, copper intensive, complicated and failure prone systems like a typical active solar system.  The redneck versions come out ahead to the extent that they are not copper dependent, and are cheap enough to size up to work really well with simpler components.  

Doitsolar is an excellent resource, their designs make more sense than anything else I have seen anywhere, and anything I do in the future will be along those lines.


----------



## benjamin (Mar 9, 2010)

DaveM said:
			
		

> it's the wife's tub....  I don't ask to many questions



That's terrible, I can't believe anybody would say such a misogynistic and cruel thing in the 21st century.  You should be ashamed of yourself joking about domestic violence.


----------



## DaveM195352 (Mar 9, 2010)

When I say to friends  "I have a hot wife", 
 I really mean it.


----------



## Badfish740 (Mar 9, 2010)

benjamin said:
			
		

> DaveM said:
> 
> 
> 
> ...



Huh?  Can we get back to talking about solar hot water?


----------



## WES999 (Mar 9, 2010)

See post #6 for a pic of my tank setup. It is 2 55 gal drums with PEX coils for heat exchangers. Right now I am using the wood stove only to heat the water, but when the solar panels are finished they will be connected to the tank. The PEX does a good job as a heat exchanger, the water exiting the coil is only about 5 °F cooler than the tank.
The tank is a two stage so you get more usible heat.
https://www.hearth.com/econtent/index.php/forums/viewthread/49253/

As for payback, if you are using oil to heat DHW you may be paying almost $1000/ yr ( that's about what I was paying). If you build your own system ($1000) the payback could be about 1 year. If you have electric DHW the payback could be about 3 years. Seems reasonable to me.


----------



## benjamin (Mar 9, 2010)

Badfish

I think your system makes perfect sense me. I assume you're talking about your future long term home, given your comments on the resale value of questionable technology. 1,500 gallons seems like a good amount of storage. You could use less if it was pressurised and had a wider usable temperature range or you could use more and get away with an even lower collector temperature. 

My own system is a combination of passive solar home and active solar combined with the boiler in the same system. They don't work at the same time although they could. The storage consists of the concrete, the soil underneath it and the mass of the building, sort of along the lines of annualized geo solar or AGS. It works fairly well, except for the dhw, but I'll get that done some day. 

When I start from scratch, I plan to do a combination of Radiantec, doitsolar and AGS. There are lots of great systems out there on the web. Everything from evacuated tubes combined with phase change storage to pop can collectors and corrugated drain tube. To me, the main thing is how much it costs you to get the job done, it doesn't matter if you're storing lots of low temp heat or small amounts of high temp heat. Unless you're talking about butchering hogs (and I have no idea why DaveM mentioned using a hot tub for scalding, wives or otherwise) you don't need 150 degree water if you have enough 110 degree water.

I also agree that this forum is no place for inappropriate comments. The next thing you know we'll have Dago, Mic, Midget, Lesbian and Amish jokes. I for one am thankful that our fine country was enlightened enough to break through years of discrimination and elect that groan and her houseboy to the presidency, or wait, is it that houseboy and his groan?


----------



## Badfish740 (Mar 9, 2010)

benjamin said:
			
		

> I think your system makes perfect sense me. I assume you're talking about your future long term home, given your comments on the resale value of questionable technology. 1,500 gallons seems like a good amount of storage. You could use less if it was pressurised and had a wider usable temperature range or you could use more and get away with an even lower collector temperature.
> 
> My own system is a combination of passive solar home and active solar combined with the boiler in the same system. They don't work at the same time although they could. The storage consists of the concrete, the soil underneath it and the mass of the building, sort of along the lines of annualized geo solar or AGS. It works fairly well, except for the dhw, but I'll get that done some day.
> 
> When I start from scratch, I plan to do a combination of Radiantec, doitsolar and AGS. There are lots of great systems out there on the web. Everything from evacuated tubes combined with phase change storage to pop can collectors and corrugated drain tube. To me, the main thing is how much it costs you to get the job done, it doesn't matter if you're storing lots of low temp heat or small amounts of high temp heat. Unless you're talking about butchering hogs (and I have no idea why DaveM mentioned using a hot tub for scalding, wives or otherwise) you don't need 150 degree water if you have enough 110 degree water.



Thanks for the info.  I did come up with something interesting but I need some more information in order to make it useful.  I found the following formula online for finding heat loss from a tank:

(Square footage of tank surface area) * (Water temp - Ambient temp) / R-value of tank walls = BTU/Hour heat loss

My calculation looks like this:

(224 SF) * (140 degrees F - 60 degrees F) / 30 = 597 BTU/Hour lost which comes to 14,328 BTU lost in a 24 hour period.  What I can't seem to find is a calculation that will tell me how to use this number to determine what the temperature of my tank would be over a period of little to no sun.  Any ideas?


----------



## DaveM195352 (Mar 9, 2010)

you guys are rough.

The point I was trying to offer (in my feeble way)  was to point out.  If your solar panels heats the water in a tank to (say) 150 degrees.  You should not plan to have all 50 gallons available for use at 150 degrees.  As the new water (from street/well) comes into the tank to replace the outgoing hot water, it will cool off the water in the tank.

we have a 50 gallon indirect tank  that my solar panels easily heats up to 150-160 degrees.  However, I can't not come close to filling our 60 gallon jetted tub and make it a comfortable 110 degrees.  Thus, the furnace does come on when the tank gets down to about 120 and finishes off the requirement.


----------



## Badfish740 (Mar 9, 2010)

I may have figured something out on this-but feel free to jump in anytime folks 

Ok, so I've determined that assuming tank wall r-values of 30, a basement temperature of 60 degrees F, and a water temperature of 140 degrees F, heat loss _should_ be 14,328 BTU per day.  I say _should_ because at first glance this seemed like a large number, but when compared to the actual amount of heat energy that 1500 gallons of water heated to 140 degrees F actually holds, something smells fishy I think:

1500 gallons of water = 12,513 pounds of water, which at 140 degrees F contains a whopping 1,751,820 BTUs!  So based on the amount of heat energy I'm starting with, a constant basement temperature of 60 degrees F, and the r-value of 30, after 5 days the water will have lost 71,640 BTUs, which only lowers the amount of heat energy in the tank to 1,680,180 BTUs.  1,680,180 BTUs / 12,513 pounds of water = 134.3 degrees F.  I'd love to believe that this tank could stay that warm for five days but my common sense tells me it's not possible.  Where am I going wrong/what am I not taking into account?  I should also add that this is a completely hypothetical calculation anyway in order to establish a baseline for the tank.  Obviously usage would completely throw this off.  I'm trying to determine what would happen if you heated the tank to 140 degrees F, then turned the system off (closed the valve to the collector) and let it sit-no hot water usage at all.  A 5.7 degree temperature drop in 5 days just doesn't seem possible.


----------



## btuser (Mar 9, 2010)

150 degree hot water in a bathtub?  I'm not making soup!


----------



## benjamin (Mar 9, 2010)

Badfish

Your math is right on.  Temperature is usually just figured relatively, as in 20 degrees change of 1,000 pound of water = about 20,000 btus, because the water still has lots of absolute heat in it even when frozen solid at 0 degrees.  But who am I to judge, we're ok with that if you're ok with that.  

Remember that the heat that is lost from the tank will warm up your basement, keeping it dry and also reducing the heat loss as the temp of your basement rises.   Also it will take a lot of collectors and a lot of time to make up the heat lost and used over those five days. 

Funny that DaveM's description of a scalding hot tub inspired me to give a boy scout speech about tempering valves or at least suggest other uses for scalding water,  but the tempering valve is probably what is keeping him from being able to utilize all of the hot water in his tank, as the tempering valve dilutes the hot water with cold so he cannot use all of the available hot water in his tank.  This is a purely hypothetical example, I don't want anybody to disconnect their tempering valve and risk scalding or even damaging their hot tub.


----------



## WES999 (Mar 10, 2010)

Badfish, you may find this useful
http://www.leaningpinesoftware.com/hot_water_pipes_Newtons_cooling.shtml
I put the formula in to a spread sheet, it may be a bit hard to follow but here it is.

If you want the real spreadsheet I can Email it to you.

I put in your numbers and came up with about 9 °F temp loss/24/hrs.

Electric water heater                                            
CDS8220RT973                            est hot water per day GAL    30    
D98631765                            est effiency %        95    
    0    H    IN    0    0.0    FT^2        BTU to heat 1 gal water    553    
    0    DIA    IN                    KWH to heat 1 gal water    0.162    
                                KWH to heat 30 gal water    4.86    
                                cost to heat 30 gal .17/KWA    $0.80    per day
T(t)        Temperature at time t                cost for electric water heater    $24.10    per  month
Ta    60    Ambient temperature (temp of surroundings)                        
Th    140    Temperature of hot object at time 0            7/22/09 to 8/22/09    32.5 hr        
A    224.0    Area FT^2                                1.05 hr/day    
m    180    gal * 8.34 lb/ga    1501.2    lb                    4.725    kwh/day    
c    1    1 btu/lb/F for water                             0.17    cost of elect
R    30    R value                                0.80    cost/day    
t    24    Time                                17.1        

            T(t) = TA + (TH - TA)e-(A/(mcR))t            So the coefficient A/(mcR) = 37.5/(667*1*16) = 0.00351

                                Substituting into the equation:        

                                     T(t) = 60 + (120 - 60)e-0.00351 t    
            130.9983        0.004974                            
                    0.119371            Setting t=24 hours:            
            9.0    delta    2.718282                            
                    0.887478                 T(24) = 115 F


----------



## btuser (Mar 10, 2010)

Most indirects have about a degree per hour loss.  R19 is about the economic break even point of a water heater/storage.  After that you better start looking at heat traps and pipe insulation, or so I'm told.   

The more I look into solar, the more I think its a great counter-balance to an oil boiler with indirect.  I'll make the most of my hot water in the Summer, when its the least efficient to fire the boiler.


----------



## mainemac (Mar 10, 2010)

BTUSER

I am not sure how far south you are in NH but this outfit in Portland ME comes highly recommended from a friend.

I have been saving money to get a system with them.

Good Luck

PLEASE post what you go with, savings payback etc.

Tom

http://www.revisionenergy.com/solar-hot-water-solar-power.php


----------

