May I have your advice on two alternatives for solar DHW, please? For my new off-grid house in central Ontario, I'll be using a closed-loop solar water heater. I had been planning on storing the hot water from the solar heat exchanger in a cheap electric water tank without the electricity connected (as just an insulating shell), and then feeding this into a gas tankless water heater to bump it up to 122 degrees when needed.

However I just realized that I could save money and simplify the hardware by dropping the tankless heater and storing the hot water from the solar heat exchanger in a power-vented gas water heating tank. The tank's internal gas heater would only turn on when the tank's water temp was below 122, so sunny days wouldn't require any gas.

Wait wait wait, I see the logic flaw. Unlike the on-demand tankless water heater, the gas water heater/tank is always on. So as soon as cold water fills the tank, the internal gas heater goes on at the same time as the same water picks up heat from the solar heat exchanger -- and we know that the gas heater will be doing most of the work in getting up to 122. Is there any way to make this system work?

One way is to keep the gas heater turned off in general, relying on the solar water system alone until I had a hot water need that wasn't being met. I could then turn the gas heater on for that situation (say, a shower on a cloudy day, or the fourth shower of the morning) and then turn it off once Mr. Sun comes out. I guess the problem here is having to test the shower water temperature whenever it's cloudy or we've used lots of hot water recently, and then go to the utility room to turn on the gas heater and wait a bit. Is this the reason the simpler combined storage+heater isn't practical?

Thanks for your assistance,
...Terry

2 tanks.

Pre heat using solar in one.

Feed pre heated water to the second which has the heating unit.

como has the right approach. 
I have one tank heated by solar feeding another indirect tank heated by a  mod con boiler.   If you have a boiler for heating the house anyway, this is the way to go.  My boiler has only come on 2-3 times since May to heat the water in the indirect tank.  The solar tank is usually around 130+ deg F measured near the center of the tank.  The tank heated by the boiler is set for 120 F.  As long as I use 20 gallons or so of hot water each day, the indirect stays above 115 F witch is the cut on temp for the boiler.  IF I don't use any hot water for 24-36 hours the boiler fires briefly to bring the indirect up to temp.  If you have a boiler, it is better to use an indirect tank as you can get very well insulated indirect tanks. I use an 80 gallon solar and a 50 gallon indirect.  

Make sure you add a tempering valve  on the hot water out to prevent scalding.

Cheers,
Eric

Posted By Como on 27 Jul 2010 10:45 PM
2 tanks.

Pre heat using solar in one.

Feed pre heated water to the second which has the heating unit.

My system has done the above for the last 23 years, had to change the controller for the first time this summer, and the pump once about 15 years ago!

John

In Winter you may want to bypass the preheat so that you are not actually lowing the temp. Many people use a differential thermometer and circulating pump to turn the circulation on and off to the solar or power the circulating pump with it's own PV panel so that it only runs when the sun is bright and shiny.

You also, in case nobody mentioned, need to have an expansion tank on the closed loop solar heater side, and use a non-poisonous anti-freeze in it.

I forgot to mention, even just a tank, placed inside, not heated, will warm supply water a significant amount, easing the burden on your water heater. I'd imagine, well or city water is pretty chilly in Ontario!

Hi Terry,

My family has a single large tank with 2 coils -- one for solar exchange, the other for indirect heating from a combi-boiler which also heats the house hydronics.  The top coil comes from the boiler and the bottom coil comes from the drainback (solar exchange).  The aquastat for the boiler is up top, so the boiler only kicks on when the hottest water in this stratified tank cools down.

This has worked out very well for us, even in cloudy Vancouver.  On days when we use less hot water than the sun provides and no space heating is required, the boiler doesn't fire at all.  The DHW loop of the boiler is also on a timer, so that it will not fire overnight (am currently fiddling with the timing to match my family's habits).  I have taken late-night showers using the last dregs of heat in the tank before, so this has worked out fine.  In your situation where it sounds like there is just DHW, it might work out ok to put the power-vented tank on a timer.

At the risk of inciting a thread war, I've not seen very good things about efficiency of pairing tankless with solar.  eg, http://www.swinter.com/news/documents/CostDesignPerfSolarHW.pdf  Other postings I've read here suggest that even pairing tankless with solar reduces the warranty on many brands.  I'm curious about the experience of the professional installers on the forum with such an arrangement.

Bill

Posted By billdoors on 02 Nov 2010 06:42 AM

At the risk of inciting a thread war, I've not seen very good things about efficiency of pairing tankless with solar.  eg, http://www.swinter.com/news/documents/CostDesignPerfSolarHW.pdf  Other postings I've read here suggest that even pairing tankless with solar reduces the warranty on many brands.  I'm curious about the experience of the professional installers on the forum with such an arrangement.

Bill

The inefficiencies in that system had more to do with the recirculation loop from the storage tank, not the presence of the tankless. The other operational issues with the tankless were related to the min-modulation of the gas-fired tankless selected and the fact that it was plumbed in series.  While there are good/better/best models to use when plumbing a tankless in series, that is a sub-optimal configuration for a GAS tankless, but works pretty well with an ELECTRIC tankless, which has no minimum-modulation constraint to deal with. These are all fundamental design issues when putting together a system, but in general, plumbing a gas tankless in series with a solar tank will always have a tank temperature point of operation where it's sub-optimal.

Thanks for your comments, Dana.  That was helpful.

Interestingly enough, I pulled the reference for this article out of a greenbuildingadvisor.com blog thread where one of the senior contributors was trying to use it as an indication of why solar thermal does not pay back.

Solar to heat water has been used for centuries.

For example, solar energy was used to heat roman baths, i.e. to increase the temperature in caldarium (room with hot water) without warming the frigidarium (room with cold water).

You can I am sure design a bad system to do it.

If you look at any Instantaneous Tankless system you can see how much greater the flow is as the incoming temperature increases. Using solar to do this is an obvious choice.

Posted By billdoors on 03 Nov 2010 09:10 PM
Thanks for your comments, Dana.  That was helpful.

Interestingly enough, I pulled the reference for this article out of a greenbuildingadvisor.com blog thread where one of the senior contributors was trying to use it as an indication of why solar thermal does not pay back.

Solar may not "pay back" from a net-present-value analysis of the reduced fuel even with a DECENT design, in areas where utility rates and solar resources are both low.  In MA/CT where that study was doen natural gas is cheap, and the sun less than reliable.  In Hawaii utility rates are sky high, but the sun is also high & reliable.  In Florida or Texas, you still have to do the math though- lots of sun, but mostly cheap or average utility costs.

Even in New England there's usually payback if comparing it to the fuel costs of heating your hot water with an oil-fired boiler (indirect or embedded coil.)

Hi Dana,

Totally agree with you.  The bogglement I initially had to the article from someone I otherwise respect had to do with the fact that I live in Vancouver (Pacific Northwest), where the solar potential is similar to MA/CT, if not lower, and the energy rates (NG or electricity) are among the lowest on the continent, and I still managed simple payback 2 to 4 times faster than the article implied, taking solar thermal payback firmly out of the "when my grandkids graduate college" to at least the "when my mortgage isn't even paid off" category.  That's before factoring in government incentives, cash/borrow analyses, etc. which tend to make the picture even rosier.  Anyone living anywhere where there's even a modicum more sun ought to be able to do better than I do at this game, or at least that's what I thought.

B

Simple payback isn't the best measure for any investment longer than 5- 10 years, but with reasonable assumptions on fuel inflation and reasonable discount rates you can still get to NPV+ within the lifespan of the system. But not if you're blowing half your solar uptake with a ridiculous recirculation scheme and long uninsulated hot water distribution plumbing within the house.

Still, it's worth considering whether the same size investment might buy you more energy savings elsewhere in the house. Most homes (even in mild coastal PNW climes) leak far more heat out of the house with air infiltration ever use for heating hot water, and the cost of air sealing to a better standard would be less than half than even the subsidized cost of solar hot water. If yours is a showering family, (rather than tub-bathers), and have a full basement, the payoff for a drainwater heat recovery heat exchanger is also faster than solar (and will increase your solar-fraction by about 10%.) If you're in Vancouver B.C. (as opposed to WA), there are subsidies available for that too.

Rare is the home where active-solar thermal is the next-most-cost-effective energy investment, even when it's clearly cost-effective. The exceptions still tend to be high energy cost energy-import island communities in warm island areas with low heating/cooling loads such as Hawaii or Okinawa. In colder high energy-cost areas there's usually bigger payback in things like air sealing, higher insulation levels, high-efficiency heating systems, etc., and in hotter areas higher-performance heat rejecting windows, insulation, and higher efficiency air conditioning seem to work out better.

Most people who invest in solar thermal for their homes just plain LOVE SOLAR more than doing math on a gazillion other projects (and there's nothing wrong with that.) Domestic hot water systems are well bounded, and for the most part pre-designed, taking much less overall design effort than many other types of projects. And unlike solar space heating, there is a substantial demand when the solar resource is most-available, making the odds of showing at least SOME return on solar likely. For most other solar thermal applications expectations tend to outpace performance by an order of magnitude until you really REALLY do the math and decide to spend at least 1/2-3/4 the money on lowering the load. (The cost of superinsulating a house would be far less than the solar it would take to heat even half a typical code-minimum insulated house.)