Hot Water for grid-tie household: Gas vs. Hybrid Electric vs. Direct Solar

soylentgreen

I have an 1980s-era house with a natural gas hot water heater that's over 12 years old.   A few years ago we went solar and have a 5KW grid-tie system.    The house was also built pre-plumbed for solar hot water heat (e.g. there are two pipes from the utility room that go up through the roof).  Oddly, though I live in a housing complex with 100+ identical dwellings, I think only one or two have their solar hot water heat connected.  

The existing gas hot water heater works fine, but is getting old and I'm thinking about preventative replacement.    This is in coastal southern California.  


Seems like I could...

* get another gas water heater
* get a GE GeoSpring
* use the rooftop solar water heat plumbing and do something sensible

The GE GeoSpring is tempting:

Pros:
* relocating it to the garage would move it out of the enclosed part of the house (if it leaks, floods the garage rather than killing the hardwood floors)
* the garage has a south-facing door (it's often warm in there) - free heat!
* it's on sale  (black friday sales = only $700 at the big box store)

Cons:
* non-trivial plumbing & wiring changes to move to the garage
* online reviews are very mixed - sounds like GE had some manufacturing issues?

Any advice?    Why does it seem like nobody in southern California uses rooftop solar water heat?

Photowhit

soylentgreen said:

Any advice?    Why does it seem like nobody in southern California uses rooftop solar water heat?

I think this is the only part I can address, Mostly it's regulation to save our souls...

A simple 'bread box' heater could likely provide all the hot water you would need but wouldn't meet code, Solar water heaters aren't simple any more in general, they require heat exchangers typically and regulation of heat and pressures, generally pumps to move the heated median around that will heat the water.

Raj174

Hi Soylentgreen,
I bought a Geospring and I'm really glad I did. It only draws 2 amps and produces 3 to 4 thousand BTUs cooling effect, but it is only noticeable in the laundry room where mine is installed. The compressor noise is hardly noticeable in the rest of the house, although this would not be an issue in a garage. It is an easy install if just a replacement. It's been installed for a little over two years now with no issues. I used to have a gas water heater in my previous home and It was very economical. I would think that if I had your options though, I would go with the gas heater. It probably cost about the same in month by month cost and the initial cost is a lot less. I would also think the possibility of mechanical failure is considerably less with gas, and if you ever go without power for more than a day you will still have hot water. 
Just my thoughts.
Rick

inetdog

As long as you have piped natural gas available, there is usually no point in trying to go to electric water heating. No way for resistance heating and heat pump type units are comparable to rather than better than modern efficient natural gas units. The one exception would be if you have excess solar capacity and are not getting any money for your yearly surplus.
A solar thermal preheat tank would make sense for either IF you can do the necessary plumbing and get the permits and approvals. As Mike said, the business of installing solar thermal is not a simple as it used to be if your are in an area that requires permits and inspections.
Pool heating is still pretty accessible for DIY, but things like variable speed pumps and pool covers can have a better return on investment.

LucMan

If you have a roof with a southern exposure I would suggest finding a way to install a couple of flat plate solar thermal panels and a stainless storage tank with an electric backup element. The install cost would be much higher than replacing your existing water heater, but in about 7 years you will have recouped your original investment on the install if your state offers any solar incentives.
If you size the tank properly the electric backup will rarely operate unless you get a few cloudy days.

Dave Angelini

LucMan said:

If you have a roof with a southern exposure I would suggest finding a way to install a couple of flat plate solar thermal panels and a stainless storage tank with an electric backup element. The install cost would be much higher than replacing your existing water heater, but in about 7 years you will have recouped your original investment on the install if your state offers any solar incentives.
If you size the tank properly the electric backup will rarely operate unless you get a few cloudy days.

This is also why Europe is so far ahead of us in lowering energy use. It is such a no-brainer to not use solar water as the heating source. In San Diego? Flat Plate collectors are the way to go if you do it right.

solar_dave

Dave Angelini said:

LucMan said:

If you have a roof with a southern exposure I would suggest finding a way to install a couple of flat plate solar thermal panels and a stainless storage tank with an electric backup element. The install cost would be much higher than replacing your existing water heater, but in about 7 years you will have recouped your original investment on the install if your state offers any solar incentives.
If you size the tank properly the electric backup will rarely operate unless you get a few cloudy days.

This is also why Europe is so far ahead of us in lowering energy use. It is such a no-brainer to not use solar water as the heating source. In San Diego? Flat Plate collectors are the way to go if you do it right.

Same in AZ, a single 4 X 10 flat plate collector provides all our hot water for 10 months out of the year and the other 2 months provides pre-heat for the gas unit making its duty cycle minimal. 

Dave Angelini

Even if you go open loop in snow country and just shut it down in winter it saves alot of gas. Gotta love Arizona though.  I did freeze my back end there for awhile on some of those desert nights. Alot of friends out in the hills south of Tucson. A great state!

solar_dave

Dave Angelini said:

Even if you go open loop in snow country and just shut it down in winter it saves alot of gas. Gotta love Arizona though.  I did freeze my back end there for awhile on some of those desert nights. Alot of friends out in the hills south of Tucson. A great state!

I hear that, my controller will pump from the storage tank to the panel if the temps get too low. I have never seen it stay cold long enough in the valley to actually think about draining it however. Gotta love AZ for the hot water we get. 

soylentgreen

Hey, thanks for the great comments (I wasn't getting email notifications so didn't know).   More thoughts to come.

soylentgreen

I once ran some back-of-the-napkin calcs. about the following question:

Is it more efficient for me to buy natural gas from SDGE, burn it and use the energy to heat water, vs. having SDGE burn natural gas down the road from me, convert it electricity, send the electricity to my house, where I convert it back to heat with a heat pump?

My answer was surprising : it's about a wash, thermodynamically.  

Here are my rough numbers

Utility-scale natural gas plants are on average about 33%-40% efficient converting heat to electricity.   Deduct a few percent for transmission losses.  Hybrid water heaters are about 250%-300% efficient, so (depending on details) you end up with about a 1.0 net efficiency.

By comparison, my 12+ year old natural gas water heater is probably only 50%-70% efficient.  But a brand new one is probably in the 90% or higher range.     

My conclusion: it's roughly the same either way, and details such as "how much does installation cost" vs. "how noisy is it" could easily sway one's decision.

Dave Angelini

It won't be a wash if the old water heater fails at Christmas. It will be merry for the installer who will charge you near 3K to deal with it.
Maybe you changed the zincs and it will last? Most do not....

m151

 Probably the only way to compete with natural gas is DIY thermal.  First, I would use a regular natural gas heater for the family, because if there is no hot water to take a shower in the morning,  they will kill you!  Then, I would look on craigslist for used components and install a tempering tank perhaps outside so you don't have to worry about leaks.  If you keep the cost down, the payoff will be quite fast. If you don't already have a hobby making  this yours can be quite satisfying. I know it was for me. 

Dave Angelini

m151 said:

 Probably the only way to compete with natural gas is DIY thermal.  First, I would use a regular natural gas heater for the family, because if there is no hot water to take a shower in the morning,  they will kill you!  Then, I would look on craigslist for used components and install a tempering tank perhaps outside so you don't have to worry about leaks.  If you keep the cost down, the payoff will be quite fast. If you don't already have a hobby making  this yours can be quite satisfying. I know it was for me. 

Kill you and then go out for Pizza.
 For Offgrid there is the beauty of not needing much of any gas. (propane) Just farmers harvesting what you need in your environment sustainably.