# PV water heating



## EatenByLimestone (Dec 14, 2016)

Does anybody run PV panels straight to a water heater?  Since the little one started taking showers my bill to heat water has gone up 50%.  Now that cold weather is here I expect the wife to extend her shower time too.  I currently have a 30g indirect tank heated by the boiler.  

I'm thinking about setting up a 40g tank before the indirect tank heated by PV.  At the very least, it would pre heat the water.  At best, it would stop the boiler from firing.


Has anybody done this?  I'm not interested in powering the grid.  It seems like an extra, and unneeded step for this project.


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## semipro (Dec 14, 2016)

Are you talking about a resistance-electric or heat pump water heater? 
If the former, I've heard of some that remove one 120 or 240 VAC element and install an element rated for the same voltage as the direct output (DC) of their PV system.   Since larger WHs have 2 elements one is powered by the PV and the other by the grid or whatever.   
Overheating is a potential problem so you'd want to make sure you have an expansion tank, working T&P valve, and perhaps a tempering valve on the outlet to add cold water to prevent scalding to users.


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## EatenByLimestone (Dec 15, 2016)

I was thinking about a simple resistance heater.  I wouldn't want the cold air off the hp being pushed into the basement.  

I can see where temp and pressure could be an issue.  I hadn't thought of an expansion tank.  Doesn't the t and p valve cover that?

I figured the 30 gallon indirect tank would prevent sudden scalds.  Any water from the preheat tank would have to mix with the indirect water.  I can see where the indirect temp could go up, but do you think it would go up so quickly that you wouldn't have a chance to adjust the shower knobs?


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## semipro (Dec 15, 2016)

EatenByLimestone said:


> I was thinking about a simple resistance heater.  I wouldn't want the cold air off the hp being pushed into the basement.
> 
> I can see where temp and pressure could be an issue.  I hadn't thought of an expansion tank.  Doesn't the t and p valve cover that?
> 
> I figured the 30 gallon indirect tank would prevent sudden scalds.  Any water from the preheat tank would have to mix with the indirect water.  I can see where the indirect temp could go up, but do you think it would go up so quickly that you wouldn't have a chance to adjust the shower knobs?


The T&P will handle the over temp/pressure but does so by venting.   That would be wasteful.  
An expansion tank on the outlet of a WH is a good idea in any case.  In a house with any kind of check valve before the WH its required by code. 
I'm not sure the indirect tank would provide enough tempering of overheated water.   
Water definitely stratifies within a tank so slugs of hot or cold water passing out do occur.  A tempering/mixing valve is relatively cheap insurance.


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## Rhone (Dec 21, 2016)

You don't want to use PV for heating if you can avoid it.  PV is only 14% efficient at converting sunlight into energy, and the hotter it gets the worse its efficiency.  I know back in the day, PV also had the issue that if one single cell got blocked (maybe a leaf was on it) that cell became a resistor all the other cells had to overcome and a single leaf would reduce the efficiency of the entire panel by 50%.  I know they've alleviated it by putting in bypasses now that shut off sections of the panel if there's any shade but it still seems to be relevant.  My friend has PV panels, and one of his panels has a row of cells blocked by a roof overhang much of the day and looking at the chart online of power output of that panel compared to the others it produced much, much less.

If you want to heat something, you should do hydronic solar (pipe glycol through copper piping and have the sunlight heat the fluid).  This is 60% efficient in winter and 85% in summer.  It's also a lot cheaper than PV and lasts longer.

Three 4x8 flat panels running glycol will collect enough sunlight in New England to equal about a gallon of fuel oil in energy per sunny day.  I recommend flat panels, they make evacuated tubes which do similar but aren't as efficient or reliable, I would argue evacuated tube manufactures do false advertising or misleading claims.  Be aware the secret to hydronic solar is collecting noon-time sunlight.  Not morning, evening, cloudy, snowy, nor rainy the energy reaching the panels is miniscule at those times/instances compared to noon sunlight that's why FP's are the better bet.  Also any comparisons of the two that compare apertures instead of footprints is also misleading, typically meaning they're comparing a 50-64 sq ft evac system to only a 32 sq ft FP one.  Compare a 50-64 sq ft FP footprint to a 50-64 sq ft Evac footprint and the FP kills it because it doesn't let noon sunlight fall between the tubes and noon sunlight is what matters.

If interested here's a site that sells kits you can install yourself, I got one.  My hot water was $674/year and the solar reduced it to $97/year last year (I have a data logger on my solar to measure how much it costs to heat the water and it only kicked on in the months of Dec and Jan and a handful of times the rest of the year).  Here's the site http://www.radiantsolar.com/solar_packages_and_pricing.php and be aware there's federal and state incentives.  Some states give incentives based on the total cost of the system, while others give you money based on the output.  I've seen some states based on the output that in the end pay you for each flat panel.  That's to say, each panel costs $879 but you get $964 in credits and in that case... load up on the panels each one will pay you $85.


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## lml999 (Feb 12, 2017)

Rhone said:


> If interested here's a site that sells kits you can install yourself, I got one.  My hot water was $674/year and the solar reduced it to $97/year last year (I have a data logger on my solar to measure how much it costs to heat the water and it only kicked on in the months of Dec and Jan and a handful of times the rest of the year).  Here's the site http://www.radiantsolar.com/solar_packages_and_pricing.php and be aware there's federal and state incentives.  Some states give incentives based on the total cost of the system, while others give you money based on the output.  I've seen some states based on the output that in the end pay you for each flat panel.  That's to say, each panel costs $879 but you get $964 in credits and in that case... load up on the panels each one will pay you $85.



Thanks, this is helpful. I particularly appreciate the link provided. 

We moved last June and in September put PV on our upper roof, 37 panels for a 12.75KW system. So far it's been wonderful, even though we're in the lower production portion of the season. Haven't had a good summer day yet with the system!

We had to replace a poorly installed NG hot water heater, and I picked a dual supply 80 gallon indirect, with the intention of installing two or three SHW panels on our lower roof, which gets full mid and late day sun. The indirect was not inexpensive, but the dual supply option added only a couple of hundred dollars and will allow a 30% federal write off on the cost once I install panels. That write off should pay for much of the rest of the system... 

The vendor at the link above offers packaged systems - 2 panels with 80 gallon tanks, 3 panels with 120 gallon tanks. Hope I didn't undersize my tank!  I guess I could always add a second tank inline...


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## woodgeek (Feb 12, 2017)

A HPWH would have better payback than any solar kludge.  If you want to go cheap, put one of Tom's retrofit units on your indirect.


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## lml999 (Feb 12, 2017)

woodgeek said:


> A HPWH would have better payback than any solar kludge.  If you want to go cheap, put one of Tom's retrofit units on your indirect.



NG costs run me $25-$30 per month in the summer -- hot water, cooking, grille. So I'm not really looking for a big payback...

That having been said, I've already bought the indirect tank, and I figure that adding the solar collectors and plumbing will run me $4K ish. I'll get $3K back in taxes next year, so the net additional cost will be $1K. The indirect is a sunk cost...

So for $1K I'll have reduced cost or free hot water for a long time, plus another conversation to have with the grandkids when they arrive.

Why would this be a "solar kludge"? And I'm not familiar with Tom's retrofit units...

Thanks,


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## georgepds (Feb 12, 2017)

Rhone said:


> You don't want to use PV for heating if you can avoid it.  PV is only 14% efficient at converting sunlight into energy, and the hotter it gets the worse its efficiency.  I



A different point of view

The new lg 315 panels are 19% efficient. Couple them to a heat pump with a COP of 2 to 3 and you have ~50% conversion efficiency... 

Now net meter and use the grid as your battery and you transfer all that energy from the excess of summer to the needs of winter...Can't do that with solar thermal

See


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

"it's time to admit that a PV array is cheaper and less troublesome than fluid-filled solar collectors on your roof.

Read more: http://www.greenbuildingadvisor.com/blogs/dept/musings/solar-thermal-dead#ixzz4YVYojXPo
Follow us: @gbadvisor on Twitter | GreenBuildingAdvisor on Facebook


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## peakbagger (Feb 12, 2017)

I have solar hot water it works but is not ideal and quite expensive. There are several moving parts plus some ongoing maintenance. If you go on Ebay and search heat pump hot water heaters, look for a seller called Heating Stuff. It is far lower cost better option than SHW for most folks. $365 compared to several thousand dollars sound good to me.

The fundamental problem with SHW is you get most of the hot water when you don't need it in the summer and not enough in the winter. I ended up having to move mine to a much higher angle then my roof to optimize spring and fall heating and back off on summer heating. It did work but looks a little strange. I heat my hot water with my boiler in the winter so I use the SHW for preheat on my incoming cold water.

About the only reason not to use a Heat pump hot water heater is if you have a cold basement as it needs 50 degrees minimum


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## georgepds (Feb 12, 2017)

FWIIW.. I run a surplus  using PV and get a credit from my electric company (*). I use an electric element HW heater, but it's only 10 gallons.. suits the two of us just fine. Myself, I'm for simple no moving parts if possible

When we have guests in the summer... they shower outside ( house is on the beach).




(*) The excess increase has disappeared since I started charging the electric car.


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## peakbagger (Feb 12, 2017)

I also run a surplus and could just hook up the element on my SHW tank. I keep getting close to hooking it up when I have a couple of days of clouds in the fall.


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## woodgeek (Feb 12, 2017)

lml999 said:


> NG costs run me $25-$30 per month in the summer -- hot water, cooking, grille. So I'm not really looking for a big payback...
> 
> That having been said, I've already bought the indirect tank, and I figure that adding the solar collectors and plumbing will run me $4K ish. I'll get $3K back in taxes next year, so the net additional cost will be $1K. The indirect is a sunk cost...
> 
> ...



Show me the numbers, and that a BTU of HW will cost less than a NG BTU.


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## semipro (Feb 22, 2017)

lml999 said:


> And I'm not familiar with Tom's retrofit units...


See this post:  https://www.hearth.com/talk/threads/space-heat-pumps.160601/


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