# Hot Water Coil for supplementing solar?



## Mr4btTahoe (Oct 4, 2016)

Hey guys...

Well I'm working towards building a solar hydronic heating system for my house.  It will be an open system that heats a large (250+ gallon) insulated water tank.  From what I've researched, the solar heat panels will be able to keep the tank in the 120-150F range. 

I'm looking to supplement this for long periods of clouds, etc...  by adding a heating coil to my stove.  Again, an open system...  water pumped from the bottom of the tank through the coil and returned to the top of the tank.  The tank will not be sealed so no risk of pressure build up, etc...   if a pump fails, the coil would just drain back to the tank.

I'm not overly worried about extracting every bit of heat that I can from the stove...    Just want enough to help keep the tank warm.

The fluid in the tank will then be pumped through radiant baseboard heaters in several rooms and returned to the tank (again, open system here too) or heat exchangers placed in the duct work of the original forced air furnace.  May also add a large coil to the inside of the tank plumbed inline with the electric water heater to pre-heat the incoming water and reduce the amount of time the element is used. (Not a priority at the moment)

So.. with all that said...   what would be the best way of going about this?  I've seen copper coils placed outside of the fire box (between the fire box and outer sheet metal skin)..   I've seen coils placed inside the box..   I've seen them in the flue.  I personally want to avoid anything placed in the flue...   so either inside the box on top of the baffle plate or on the outside of the box.

What would be best?  Would a copper coil hold up to internal firebox temps?  Has anyone had any luck getting real heat from outside the fire box?  Really leaning towards a large copper coil on top of the firebox (not internal).

I'm leaning towards building the insulated tank in the crawlspace as to make the heating coil the highest point in the stove/water system so any pump failure would result in draining of the coil back to the tank.  I would think that with an open system, even if water was still in the coil, it would just gradually boil off and re-condense/return to the tank (worst case scenario).

Any input on this?  The stove burns hot/clean...  has a vertical 8" double wall flue from stove to cap (double wall from stove to ceiling, double wall class A from ceiling to cap) and I want to keep it burning clean.


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## Woody Stover (Oct 4, 2016)

You might try posting in the "Green Room" forum, as well...


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## electrathon (Oct 4, 2016)

I have a long write up on here a year or so ago explaining my system.  Search my name and you will find it.

Fast simple answers:  You need a stainless coil, inside the stove, above the baffle plate.  A copper coil outside the stove will extract very little heat.  No copper inside the stove.  You will get a lot less heat than you are hoping for, but you will get some.  With your idea of a tank low, allowing the water to "drain back" during power outages you will find you will have an air locked system, rise the tank and let the water self circulate.


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## Mr4btTahoe (Oct 4, 2016)

Hmm...  not really worth the hassle of trying to lift/raise a 200+ gallon tank to make it a thermosiphon system.  Many people use drain back systems for solar heated hydronic panels (to prevent freezing)...   so why would it not work the same way?  Don't get me wrong...  I don't doubt that you are correct...  just curious as to what would cause the issue in this type of system versus the large grid in the solar panels.

If it won't be enough to maintain the tank's temperature, I'm not sure that it will be worth the extra work.  Most people just use panels and their tanks (very well insulated) seem to maintain useable heat for a few days.

I read through your thread earlier today actually...  quite a bit of useful information.  I didn't however see any thermal storage tank mentioned in your threads.  Did you use any type of large water tank (150+ gallons) to store the heat being produced?  Most of the research I've done is based on systems using large, very heavily insulated tanks as a "heat bank" so to speak.  The solar panels (and stove in this case) would just heat the water in the large tank.  I'd say the solar panels will do most of the heating..  just wanted the stove in the loop in case of cloudy days..  I could keep the radiant heat going a bit longer.

I'm not doing any type of floor heating.  Planning on having a few baseboard heaters or heat exchangers.

Its either this or I'm going to go with a large air/air solar heater.  Either way, the solar collectors will be 100+ sq. ft.

Basically, I'm trying to cut down on my wood consumption and decrease the required run time of my stove.  Last year was our first year heating 100% with wood and I loved it...  but If I can cut down the wood use a good bit by using solar heat, I gladly will.


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## electrathon (Oct 4, 2016)

I am not very familiar with drain back systems, but have spent many hours purging air out of hydronic systems. Once air gets into the top of a loop it is hard to move it out without vents or high velocity. I suspect you will have trouble with it. 

I have a 50 gallon water tank for storage. It has no additional insulation.  One layer of pipe insulation on the pipes from the stove to the tank. I feel the heat lost ends up in my basement which is where I want it anyways. The water never gets much over about 110 degrees, and that is rare even with the stove running all the time. If you want more heat than that you may need to locate the coil in the firebox and not above the baffle plate.  I have another system similar to the one I wrote about out in my shop. It is in an older stove and the coil is in the firebox, it produces a lot more heat a lot faster.  I had a coil outside myself tank when I first made the system. I got almost no noticeable heat gain from that coil.

I really like my system. It heats most of my water in the winter time.  If you set it up properly you will get hot water.


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## electrathon (Oct 5, 2016)

On my system I pull water from the bottom and return it through the cold water inlet on the water heater tank.  Meaning it is returned pretty low in the tank.  The circulation is controlled by a solar controller, so when the water is hotter in the stove than the bottom of the tank the pump turns on.  The warmer water rises in the tank and the cooler water settles, I do not want to add semi-warm water to the top of the tank and loose the warm water I have there.  I am not sure how you would get pressure for the "lift" of the water on your open system.  You would likely need a diaphragm pump instead of a circulation pump, but I am out of my area here. You will need about 10psi to lift the water from the basement to the first floor (about 1 psi for each 2 foot of lift).

I don't know how to do it, but would suggest you have someone run the math on the BTUs you will need to heat what you want to.  I am betting it is far exceeding the capacity of what a woodstove coil can produce.  A furnace to heat a house would be in the 80,000 BTU range, more or less depending on your house.  The woodstove provides a lot of that, but only in a part of the house.  The hydronic would be a leveling of heat to the far side of the house, but I am not sure if you can get enough to be happy.

I took a shower yesterday and today without the insta-hot water heater on.  Toasty warm and no energy used (or money to the gas company).


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## maple1 (Oct 7, 2016)

Mr4btTahoe said:


> Hmm...  not really worth the hassle of trying to lift/raise a 200+ gallon tank to make it a thermosiphon system.  Many people use drain back systems for solar heated hydronic panels (to prevent freezing)...   so why would it not work the same way?  Don't get me wrong...  I don't doubt that you are correct...  just curious as to what would cause the issue in this type of system versus the large grid in the solar panels.
> 
> If it won't be enough to maintain the tank's temperature, I'm not sure that it will be worth the extra work.  Most people just use panels and their tanks (very well insulated) seem to maintain useable heat for a few days.
> 
> ...



I doubt you will get results you are hoping for with the solar.

For starters, baseboards & HXs need hot water. (Floors can be effective with warm water). Quite a bit of it, especially when trying to warm the house up.

You should do some number crunching on BTUs you could realistically expect from a thermal solar setup, before you do anything. I have friends who have solar thermal setups for DHW. I think it might cover DHW use in the summer, but not much more than that - at a cost of $4k or so for their solar setup. And from my experience at our place, space heating demands are something like 20-30x DHW heating demands.


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## woodgeek (Oct 7, 2016)

I would second @maple1 concerns....and add one thing.  While solar thermal systems DO work, at a certain cost per BTU, many of those BTUs come in the wrong season for space heating.  You should look up your solar thermal resource in IN, and break it down by month.  In New England, it is very poor in Dec-Feb, just when you want space heating, even though there are enough solar BTUs for the other 9 mos to drive a decent solar DHW system.

Even worse....the efficiency of flat plate collectors drops significantly with outdoor temps.  Evac tubes do better....but are $$.

I used to draw up and design all kinds of solar heating schemes, before I decided that the seasonal timing mismatch would be a killer at my location...not so in the SouthWest and Mountain areas, BTW.

If you want to get solar heat out East, you are better off putting in a big PV array with grid tie, and burning up your excess electrons in the winter using a mini-split heat pump.  Sell the utility a lot of electrons all spring and summer, and buy them back in the dark, cloudy winter.


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## georgepds (Oct 11, 2016)

I built a combo solar/wood hot water heater.. that I eventually took down. It never worked really well, and , though I worried about  freezing, the real problem was overheating.  I eventually put in a hot water dump before I gave up. when the sun didn't shine , the stove took over. Problem was it worked OK in Spring and Fall, but was way too hot in Summer and Winter.

These days I have 18 panels of solar PV on the roof (250 w nominal each) and net metering. The panels pay for all my electricity, including hot water, a split duct heat pump,  and an T-ZEV ( 2016 chevy volt, about 1/2 my miles are electric))

You might want to consider these articles

http://www.greenbuildingadvisor.com/blogs/dept/musings/solar-thermal-dead

http://www.greenbuildingadvisor.com/blogs/dept/musings/solar-thermal-really-really-dead


re"If you want to get solar heat out East, you are better off putting in a big PV array with grid tie, and burning up your excess electrons in the winter using a mini-split heat pump. Sell the utility a lot of electrons all spring and summer, and buy them back in the dark, cloudy winter."

Amen.. (I live in New England)


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## electrathon (Oct 13, 2016)

georgepds said:


> I built a combo solar/wood hot water heater.. that I eventually took down. It never worked really well, and , though I worried about  freezing, the real problem was overheating.  I eventually put in a hot water dump before I gave up. when the sun didn't shine , the stove took over. Problem was it worked OK in Spring and Fall, but was way too hot in Summer and Winter.
> 
> These days I have 18 panels of solar PV on the roof (250 w nominal each) and net metering. The panels pay for all my electricity, including hot water, a split duct heat pump,  and an T-ZEV ( 2016 chevy volt, about 1/2 my miles are electric))
> 
> ...


I read through the quoted articles and must say they are about as lopsided as you can get.  I have no panels on my roof, but have looked into it enough to scoff at his quoted prices.  He uses contractor top end prices to get the hydronic quotes, but uses made up engineer speculation for photovoltaic prices.  I doubt few people are able to get panels for what he is saying.  Also, he is saying how great the pricing is on PV panels but omits the fact that they are only so cheap because the government is subsidizing them.  True dollar for dollar pricing is likely 3 to 10 times what he is quoting.  I was quoted I would need to spend $30,000 to supply my house with 1/3 of my electricity, keeping in mind I have a gas water heater and gas furnace (offset by wood). The government would pat half of that through subsidies.


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## maple1 (Oct 13, 2016)

$30,000 to generate 1/3 your use?

Either that quote has something severely whacked to it, or you use a LOT of electricty.


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## woodgeek (Oct 13, 2016)

I looked into solar thermal....I was quoted $8000 for a system that would make half my hot water, the rest on electric backup.  Doubling system size (to $15000) would get me to a 75% solar fraction on HW.  

Instead I got a HPWH system for $2k MSRP + $2k installation (relocation of heater) that makes all my hot water for <50% of the elec of a conventional electric system, and less elec than the first solar system would have used.

If I wanted solar HW, I could get add about 1.5 kW of panels, for a few $k, and run the HPWH and get 100% solar HW, and still be less than the $8k solar thermal system would have been (and only made 50% of my HW).


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## georgepds (Oct 13, 2016)

electrathon said:


> I read through the quoted articles and must say they are about as lopsided as you can get.  I have no panels on my roof, but have looked into it enough to scoff at his quoted prices.  He uses contractor top end prices to get the hydronic quotes, but uses made up engineer speculation for photovoltaic prices.  I doubt few people are able to get panels for what he is saying.  Also, he is saying how great the pricing is on PV panels but omits the fact that they are only so cheap because the government is subsidizing them.  True dollar for dollar pricing is likely 3 to 10 times what he is quoting.  I was quoted I would need to spend $30,000 to supply my house with 1/3 of my electricity, keeping in mind I have a gas water heater and gas furnace (offset by wood). The government would pat half of that through subsidies.





He used a price of $3.74/watt... ten times that would be $37.40/ watt... Is it possible you are exaggerating for effect?

G


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## electrathon (Oct 14, 2016)

I have a bill of about $150 a month usually.  I was told (similar price quotes from two places, about a year ago) that $30,000 for the install, half paid by taxpayers, half by me.  So about $15,000 out of pocket. I was told to expect up to $50 per month savings.  If I didn't use electricity during the day and used it during the night that ratio would go way better since the power company would buy it for more than double during the day than I would pay for it at night. If I used an electric water heater, I would need more, a heat pump, even more.  So, from what I was told, real cost of a system that would fully power my house would be over $100,000. I am not sure if that is high or low compared to other parts of the country or how much most people pay.  In this area power is about $.10 per kWh.


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## georgepds (Oct 14, 2016)

$150 a month is 1500 kwh per month.. about 3 times the average user

To better understand the bid you received one would need to know thhe size of the system in watts that was proposed. Divide the cost in dollars by the size in watts to get the installed price in dollars per watt

Three years ago,when  I upgraded my system,  the installed cost was~$4 per watt  ..before any rebates.. rates for the installed cost is now less


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## georgepds (Oct 14, 2016)

Just fyi

"
*How much can you save with solar?*
So how much are your neighbors actually saving over 20 years as a result of installing a solar energy system? As you might expect, this depends on where you live. For example, homeowners will save about $38,000 on average in Boston when they go solar. In Los Angeles, homeowners will save nearly $22,000 on average, and in Portland, homeowners can save a whopping $43,000 over 20 years."


http://news.energysage.com/how-much-does-the-average-solar-panel-installation-cost-in-the-u-s/



They use a price per watt higher than nrell ($3.09/watt for 2015)


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## maple1 (Oct 14, 2016)

Our consumption is right around 18kwh/day, on average. Up to 22kwh/day in the summer when heating DHW electrically.


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## NP ALASKA (Feb 18, 2018)

So; am I gathering that solar power is the way to go instead of thermal HW solar options??

Put some 220v coils in 250 gallon thermal tank and heat the shop radiant floor for instance?


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## NP ALASKA (Feb 18, 2018)

Nothing like entering a conversation almost two eyars later-


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## georgepds (Feb 18, 2018)

NP ALASKA said:


> So; am I gathering that solar power is the way to go instead of thermal HW solar options??
> 
> Put some 220v coils in 250 gallon thermal tank and heat the shop radiant floor for instance?




Better yet... buy an air source heat pump and get 3 times the output


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## electrathon (Feb 18, 2018)

NP ALASKA said:


> So; am I gathering that solar power is the way to go instead of thermal HW solar options??
> 
> Put some 220v coils in 250 gallon thermal tank and heat the shop radiant floor for instance?


The reason photo voltaic was so much of a benefit over hydronic was the fact that other peoples taxes were used to pay for your solar system.  It cut the cost in half, saving the homeowner many thousands of dollars.  Now that other taxpayers no longer have to pay for your system I think the benefit will no longer be there.

Also, no where near the ability for a roof solar system to be able to make enough energy to run a floor heat system (unless you have a huge roof and a tiny floor).


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## jebatty (Feb 19, 2018)

Some hard facts. In my area of northern MN current cost for a solar PV system is about $3/watt before any incentives, rebates, credits, etc. My 12.3kW system produces an average of 15,500kWh per year (4 yr average). System is net meter. At the current kWh rate of $0.12/kWh, annual value of 15,500kWh is $1,860. Current value divided by current gross cost is 5% simple rate of return. With only the FTC, current cost would be $25,830, and simple rate of return would be 7.2%. Total maintenance cost over 4+ full years of operation is zero. Electric rates are certain to rise. Effective simple rate of return is certain to increase. Whether 5% or 7.2% simple rate of return, solar PV is a no-brainer investment, not a cost. Plus, 100% clean air, water and soil impact and 100% fossil carbon free.

Our annual effective kWh household usage is 9,000 kWh on a 3,000 sq ft walkout total electric house, with heat from a wood stove in the living room being the primary heating source, electric is supplemental as needed. That leaves an excess of 6,500 kWh/yr from the PV. That excess now is being used to the extent of 3,750 kWh/yr at 3.2 miles per kWh to "fuel" our new Bolt car with estimated annual usage of 12,000 miles, still leaving 2,550 kWh/yr to "fuel" a future mini-split to provide shoulder season spring/fall heat and reduce need for wood for the stove.

From a personal moral and environmental ethic point of view, my wife and I are on a mission to achieve 0 fossil carbon energy use, and we now are very close to achieving that goal. I see nothing not to like about solar PV.


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## jebatty (Feb 19, 2018)

ditto - noting like reviving an old thread with current info.


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## NP ALASKA (Feb 19, 2018)

Thanks much


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