I am building a custom home and would like to use solar domestic hot water.
However I cant find any GOOD information about whether to use a tank or tankless heater as the backup.

Since you need a storage tank anyway it seems like using a tank heater would be less redundant.  (in both space and maintence)

However folks routinely spout the advantages of a tankless system not heating water needlessly, etc.  

A tank system would not be heating needlessly either if it were receiving already heated solar water. When you have a solar storage tank then all the space advantages of tankless are gone.

A tankless heats from 50(?) degrees supply to 110(?) degrees output, (50 degree increase) but if you are feeding it solar heated (but not quite hot) 70-80 degree supply water, how efficient is a tankless at heating just 20-30 degrees?

Anyway I get lots of OPINIONS but nobody is speaking from EXPERIENCE.
Does anybody have any practical performance experience with Solar/tankless systems?

Thanks

OK if not experience, how about summarized real test data? :-)

If it's a gas fired HW heater the standby losses of a tank are atrocious. If electric, not so much, but still far more than most solar storage tanks or indirect-fired boiler HW tanks, which tend to have better insulation. Most solar storage tanks have ports for electric elements, even if they don't come pre-installed, but that's probably not what you're after...

The drafting flue of the gas-fired tank convecting heat out of the tank 24/7 whether you're using it or not, and the fact that you can't insulate the tank under the burner gives it a signficant heat leak there too. The EF test is just plain WRONG and way too optimistic for intermittent or occasional-usage. (They're about right if you use ~60-70 gallons/day, every day, and drop dramatically from there. They gain efficiency slightly if you're using tons & tons of hot water though.) To infer just how bad it can be as a solar store backup, check out the low-use efficiency profiles of the standard tank HW heater in this test summary:

http://www.aceee.org/conf/08whforum/presentations/1a_davis.pdf

While it's actively heating up the solar storage it'll do OK, but the standby losses will kill ya if your using it AS your solar store!

Furthermore, the raw combustion efficiency of tanks are limited to about 75-80%, due inherently low turbulence on the water side of the heat exchangers. Tankless units run typically in the mid-80s for combustion efficiencies, even with mere ~30 degree delta-Ts as measured in this combi-HW & heating system:

http://dsp-psd.pwgsc.gc.ca/collection_2007/cmhc-schl/nh18-22/NH18-22-106-108E.pdf

They didn't specify the delta-T, but air handler coils typically run ~30 degree drops, and this is but one of many combi designs where a tankless is used to run coils in air handlers (most less exotic than the one with the integrated HRV they used in that test). I've not seen data for tankless running radiant heat systems, but they tend to have similar deltas. Note that the combi system running off the gas fired tank heater had significantly lower performance than the tankless in the shoulder (and non-heating) seasons- an artifact of it's standby losses mounting during low duty-cycle periods.

Where tankless heaters lose efficiency is in very short low volume draws, which is never going to happen in a solar-store backup situation. See:

http://www.energystarpartners.net/ia/Water_heaters/Documents/WHPAGette_Final.pdf

Look at figure 5.1, p 42 (p 48 of the PDF). Any time it has pumped more than 5 gallons through the heater it's hitting near it's raw combustion efficiency. There is a ton more information in that document that is relevant (and far more info than you ever cared to know about heating hot water, eh? :-) )

If you're going to cheap out on the solar tank, a tankless propane or gas HW heater driven by a pump switched on by an electric-tank's thermostat can work (it's been done- haven't seen any test data on such systems though.) But I'm not sure using an electric tank as the solar store will qualify for any subsidies (in countries & areas where solar HW are subsidized). I'd think that only appropriate solar tanks are fully qualified in most places (could be wrong, often am.) In subsidized situations, the difference in cost between a solar tank vs. and electric HW heater is only a few hundred dollars after subsidy, but the performance difference will likely be in double-digit percentages.

Last not least, this guy's lo-cost homebuilt uses a Takagi tankless as "finish heat" to his pre-heated solar HW:

http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm

but the tankless suffered some heat-exchanger damage from a freeze-up when it got to -20F overnight:

http://www.builditsolar.com/Experimental/PEXColDHW/Performance.htm#MonthlyPerformance

(scroll down to the bottom for the tale of Takagi woe.) If you live in a climate where temps that low are common, be sure to add some length and better backdraft control to the tanklesses flue and/or keep it inside of conditioned space.

Now that is the type of information that I need, thank you

The heat loss through the draft flue of the gas tank was not something I had seen discussed. I saw the reccomendations to use an electric style tank but did not understand why.

As an FYI (I shoudda stated it) I am in the Los Angeles area so freezing is not a big problem.

In considering all the aspects of both energy efficiency AND space, perhaps the best 'complete' system is to use a true solar storage tank and a tankless back-up heater.

I am building a new home and I want maximinum energy efficiency designed in, not added on. Unlke most people's thining I am looking at economizing in the decorator items and not going to try penny pinching with the operational items. So I am looking at installing a high quality professional system not a 'home made' type set up.

Thanks again for your infomration and links.

How are you heating the place?  In LA the heating season is typically short, and none too deep but you still need it when you need it.  Many/most just opt for a gas-fired forced air or similar, but if you have a solar-tank with a tankless for backup, it's relatively easy to turn that into a combi-system, either as low-temperature pumped water baseboards, etc or a coil in an air-handler comparable to the system in the eKoComfort experiment.  If you have central AC a heating coil in the air handler can work. (OTOH, if you have central AC it might be more cost & space efficient to use it as an air-source heat pump to heat the place in your climate.)

If you want condensing-type efficiencies, Navien and several others now have condensing tankless HW heaters.  (Navien has even engineered a space-efficient unit designed to simplify combined heating/hot water system design.)

I looked at water radiant type heating systems but gave it up and will be going with forced air. The problem was that the more I talked to people about it (architect, contractors, HVAC folks at home shows) I found nobody who was familiar with it. Sure, I read up on it and started making comparisions and when I got no reasonable feedback I began to relaize that if nobody knew how to design or install a such a system, there would never be anybody to service it over the years.

Good or bad forced air is the accepted Los Angeles, California technology. There is no shortage of designers, installers and parts sources for FA out here. So I am looking toward a very high efficient furnace (and A/C) with well insulated ducts in conditioned spaces. Even this has been difficult as folks have reccomended systems that are tacked on with no design thought.

Some quotes:
"Oh I always reccomend a 50 ton sysem in all my homes, I never calculate a system"
"Ducts in conditioned spaces? Whatta ya mean, ya wanna have the ducts stick out in the room?"
One MEP 'engineer' never did understand the difference between solar water and solar electricty and had no understanding what so ever of net-metering!

As for radiant I am looking at perhaps an electric style system installed under the hard tiles in the bathrooms and maybe the kitchen. Some electric systems say they can be placed under "wood floors" too, so maybe it might find its way into a family room(?) I do not see it used as a full house heat system.

It will be an insulated slab on grade and althrough LA doesnt get THAT cold (how do you folks deal with below zero temps?) I hate cold floors and having something to break the chill would be nice.
As for the electrical power there is already a 7kW solar PV system already up and operating (waiting to power the new place) so electricy should not be too big an issue.

OK,
Don't give up on the radiant floor, slab on grade is about as easy as it gets, LA area is an easy design target. We design these packages regularly.
Next tank-less VS Tank, with a solar system our best recommendations are a drain down system. Depending on the size of your home baths, and all will guide the size of tank for solar storage. Next don't rule out the cheep home depo electric 40-50 gallon H20 heater as additional storage. The solar tank feeds the electric tank, this provides back up, additional inexpensive storage, no stack loss. So the electric tank is pre-fed with the solar heated H20, if it enters anywhere above the set point of the electric tank, the electric tank will not come on. Entering water temp on the electric tank can be as high as 140 deg with out causing harm to the tank. A 3 way mixer valve will temper the delivered H2o down to 120. A tank-less operation will go on and of to simply wash hands, cost 3x's that of a 40 gallon electric, and offers no storage.
Now depending on the size house it may be possible to apply that solar heat to the slab. Again more steps forward in the design process but not complex.
If warranted look at a tax rebate on a larger portion of the system, current law is %30 of the domestic solar system labor and materials, no limit. If you have 5 collectors that are directed at the solar domestic H20 side first that meets the definition of the tax credit.
Dan

From a carbon footprint POV radiant-electric is about as lousy as it gets- identical to electric baseboards or electric-resistance forced air, twice the carbon delivered to the atmosphere (at least until the grid gets a whole lot greener than it currently is!)  It's cheap to install, but more expensive to operate-  about 3x more expensive per delivered BTU as compared to even a bottom of the line natural-gas furnace in my utility market. YMMD.

CA Title 24 notwithstanding, it sounds as if there's still a wealth of ignorance around, eh?  :-)

Dan the Blueridge man is right when he says radiant slab-on-grade is something of a no-brainer for light heating loads like LA, and an ideal situation for keeping the water temp low, the flip side of which is higher collector efficiency on the solar and higher combustion efficiency on boilers.  At higher efficiency, lower temps takes measurably less collector area.  Almost every solar heating contractor would find this an easy-to-design-for installation, and these days most go with a tankless (condensing or otherwise) as the backup heat source for the solar, particularly if natural gas is available.  Designing & installing from scratch most solar designers would make it a combined heat & HW system too. 

Almost any hydronic (forced hot water) heating designer these days can deal with a radiant slab, and hydronic heat is usually at least 10% more efficient than forced air (even at equivalent AFUE numbers for the furnace/boiler) since hydronics don't induce pressure differences between rooms that get short-circuited via the outdoors in all but the most extremely tight houses.  If you don't do a radiant slab, consider low-temperature baseboards (designed for 140F water, maximum), radiant panels or other hydronic heat.  I'm sure there are dozens of contractors in LA that can design & maintain reasonable hydronic heating systems.  But a "heating and solar" outfit will probably be the best fit for integrating systems for maximal efficiency with minimal space requirements.  A quick google turned up these guys:

http://localbusinessreview.com/LocalBusinesses/p/Jo-Mi-Plumbing-and-Solar-Inc._102476791/

and these

http://www.acmegreen.com/index.html

http://www.acmegreen.com/page29/page29.html

(note the radiant slab-ly-ness goin' on in their home page picture...)

and many others...

Seems there are people in LA who are up to the task- the trick (as is true anywhere) is to find them.  A tiny condensing boiler running the radiant floor that also acts as the backup to your solar HW, or a full on solar-thermal heating/HW combi system that can use the solar for space heating are all possible here.  It's silly to have multiple burners these days- one for the HW, the other for space heating- make those appliances work for you!

I am considerating geothermal and want to know of ideas for boosting the heat for floor radiant in bathroom, MBR and possibly the kitchen. Should I consider seperate  Tank inexpensive heater so I can turn off in late spring/summer. 

Lisa 6801

6801, Lisa as it were, you might consider asking this question in the Geo forum also!