Hello! I am looking for a tank less water heater (single point or whole house)that has a temp sensor so that it can be used as a back up for a solar hot water system. Any luck with particular brands or preferences on single point vs whole house? The heater(s) will be supplementing the sinks showers laundry and radiant floor heat system. House will be 500-900 sqft. Thanks so much! -Abby Jayne

Almost all tankless units sold in N. America sense and regulate the output temperature. Putting it in series with the solar hot water distribution, when the incoming water from the solar is at or above the setpoint on the tankless, the tankless doesn't fire.

What is the water temp & total BTU requirements of your radiant floor? (Are we talking slab, staple up, suspended tube or...????)

Abby Jayne-

I looked into whole-house tankless water heaters earlier this year as a backup for my solar hot water system that provides domestic hot water only (no space heating) for a new house. I wanted a natural-gas fired unit rather than electric, since natural gas was available, and the generation of electricity by fuels to use for water heating typically uses 3+ times as much fuel as burning the fuel directly to heat the water. Therefore, my comments pertain only to fuel-fired tankless heaters, while I think that you did not specify your heating source.

The builder for my house normally uses units made by Noritz, and I really liked their combustion design. However, I noticed that their warranty was reduced from 12 years to 3 years if it was used with preheated water. An engineer that I talked to there said that they did not recommend it to be used with preheated water. A Bosch unit that I looked at did not have a sealed combustion system, which the builder recommended for the relatively tight house, so I dropped that one from consideration. I ended up with a Tagaki unit as recommended by the solar installer, and I think that all their heaters are suitable for use with preheated water. It also had a wide dynamic range, i.e., the heat input could be varied over a wide range, making it more suitable for a wide range of inlet water temperatures. The selection of this manufacturer was partly based on what brands the plumbing contractor was comfortable working with, and there might be many other brands that would work as well as the brand that I selected.

In conclusion, some features that you might consider for solar back-up applications are:
- correct heating range, and it seems it might be quite high for space heating
- suitability for, and warranty impact, when used with preheated water
- sealed combustion system if your house is "tight," and therefore might be subject to poor combustion if there are pressure imbalances between the inside and outside
- wide dynamic range of heat inputs (min. to max.)
- a brand choice that your plumber or solar installer is comfortable with if you are contracting the job out

Lee

Takagi is the only manufacturer that doesn't pretty much void the warranty when using water heaters as boilers, but Navien also promotes them for combined-use. Noritz has a separate line of wall-hung boilers (slightly more expensive than and controlled differently from their on-demand hot water heaters) that would also have potential, but what make sense depends on the peak load & temp requirements- could be a fossil fired system would be completely off the table.

Without knowing more the actual loads or temp requirements, or whether you're talking electric vs. fossil-fired it's hard to say for sure what approach is the most-appropriate. In many apps point of use or larger electric tankless units plumbed in series with the potable solar would make the most sense, especially if the space heating radiation was low temp, and the design day heat load within the range of backup elements in the solar storage tank (set for a maintenance temp suitable for the lower space heating temp.) If the radiation temps and design day heat load were higher a different approach might be taken, maybe fossil-fired, maybe not. A well insulated sub-1000 square foot home with slab radiation might be perfect for a condensing boiler, if they made them small enough, but if the peak heat load is under 10KBTU/hr it would be a silly expensive short-cycling waste of a fossil-fired system, and an electric boiler or simple electric element in the solar store for backup might make more sense.

If it's higher temp radiation with a higher heat load, a grid-attached natural gas or propane-fired cogenerator like the Marathon rather than a tankless water heater or might make more sense.

But we know nothing other than it's 500-900 square feet of radiant floor space heating & solar combi system. The design heat load could be 2.5KBTU/hr or 25KBTU/hr, (but probably not 250KBTU/hr :-) ) and the space heating water temp requirements may be anywhere from 80F to 150F, depending on the particulars.

Got a zip code, an R value, and a fraction of glazed wall area, and the type of radiant floor?

As Dana said above, you need more info on max heat loss needed ie btus per hour as well as hot water requirements.
You should also figure out what your costs for each type of energy available ie cost of natural gas, electricity, propane, etc. Convert them all into btu’s per dollar, and figure out how much you need to use per year.
You may find that electric resistance heat + passive solar and solar hot water is quite economical.

Personally I would think about using the solar for DHW only, which simplifies the system design and requires less panels(maybe only one) vs space heating.
If I was going to use gas as the primary heat source, I would look at a munchkin or other small gas boiler and an indirect tank. The solar loop would warm an 80 gallon storage tank which would then run into a second indirect tank. The second tank would be heated by a zone of the boiler whenever it needed extra heat. The boiler would supply heat to the radiant floor as a separate zone. If it is a well insulated slab on grade, the temps would be fairly low and the system would have enough mass that it should not shortcycle. The little boilers can fire down to ~15K BTU which may still be too high, but needed for hot water.
 You can also buy an indirect tank that has 2 heating loops in it. The lower one is fed by the solar panel; the upper one is fed by the boiler. This eliminates one tank, but I think it would decrease efficiency of the system.

The other technology to consider, if you are in a sunny area is using a solar air heater, either premade or site constructed. These are very simple devices and provide a lot of usable heat on sunny days, without a lot of heat loss at night (or heat gain in the summer). www.yoursolarhome.com is one example.


Tell us more!

Good Luck
Eric

I am designing this home for a senior project and am very new to the game. So thankyou everyone for your interest and awesome responses!

The radiant floor system we chose is hydronic in slab. The project is in Ogden UT (84401). I am unsure of what temps will be needed for the floor or btu's required. We haven't been able to calculate our R value yet. We will be doing 18 to 24" on center 2x6 framing with blown in insulation and are planning on making the home as air tight as is healthy/possible. I should be able to get a fraction of glazed wall area to you by the end of the week.

The reason we hope to use the solar water system for both the floors and the domestic hot water is we are working on making this home as inexpensive as possible to run. I need to do more research but considering the aprx 226 sunny days per year I'm hoping it can pull a large portion of the weight most days.

I havent heard of the solar air heater but will definiately put some research into it! Thanks for the tip

A little more info on our project. It is a small single family home with a base floor plan of 528 sqft (one bed one bath) it will have 3 options: A 2 bedroom, 3 bedroom, and an ADA compliant. So we quite possibly will end up needing different systems for heating considering our 3 bedroom could reach as much as 900 sqft (almost double our base plan). We want to spend less than $90,000 to build and have as many environmentally friendly features as possible. We have just began our design process, so very little is set in stone.

Thanks again for all the great input if you have any other questions feel free to ask!

-Abby Jayne

Oh yes and we are hoping to go with electric (if we choose all tank less) but if it turns out we need a tank system we will most likely switch to gas from the local grid gas company Questar.

With that type of construction your radiant slab will never need more than 90F water, and the solar should be set up as a 2-zone hydronic heating system with the slab as the "priority" zone, switching over to the DHW storage only when the slab isn't calling for heat. The tighter and better insulated the building envelope, and the less glazed area it has, the lower the heating water temp requirements, which dramatically increases the collection efficiency of the solar.

If you run a ~100' coil of 3/4" or 1" PEX in the slab to pre-heat the incoming cold water from the street to the solar hot-water the DHW heating will be more efficient too. It would also allow you to use a smaller electric tankless (plumbed in series with the output of the solar hot water's tank). With an embedded PEX pre-heater in the slab, during the coldest part of winter when the slab is at 75F you'll get at least 60F water out of the pre-heat unless it's a very long draw. And in summer it'll provide a very modest amount of space-cooling as well.

The slab's backup heater can be a small tank HW heater (it may require a custom or separate thermostat to allow low temp operation) if it has sufficient wattage to the element, but you'll need to do a Manual-J type heat load calc on the final design to know what "sufficient" is. If code won't allow a water heater in a space-heating application, the very smallest of electric mini boilers will almost certainly be enough (and are pretty cheap. See: http://www.houseneeds.com/shop/heatingproducts/boilers/electricboilers/electroboilerbuy.asp )

Dana1
"If you run a ~100' coil of 3/4" or 1" PEX in the slab to pre-heat the incoming cold water from the street to the solar hot-water the DHW heating will be more efficient too. It would also allow you to use a smaller electric tankless (plumbed in series with the output of the solar hot water's tank). With an embedded PEX pre-heater in the slab, during the coldest part of winter when the slab is at 75F you'll get at least 60F water out of the pre-heat unless it's a very long draw. And in summer it'll provide a very modest amount of space-cooling as well."

Never thought of that, but I will incorporate it, good idea!

John

Dana1-

You lost me somewhere. You mention preheating the in-coming cold water going to domestic hot water (DHW) system by running it thorugh tubes in the slab. But of course, this just takes enthalpy from the slab, cooling it in the case of the in-coming water being cooler than the house temperature. Then the house needs to supply that same enthalpy back to the slab. The second law of thermodyanamics would say that there is nothing to be gained by this effort, but there will be losses since the heating processes have losses. There will also be unnecessary costs for the tubing and labor.

Lee

Posted By Lee Dodge on 14 Oct 2010 11:40 PM
Dana1-

You lost me somewhere. You mention preheating the in-coming cold water going to domestic hot water (DHW) system by running it thorugh tubes in the slab. But of course, this just takes enthalpy from the slab, cooling it in the case of the in-coming water being cooler than the house temperature. Then the house needs to supply that same enthalpy back to the slab. The second law of thermodyanamics would say that there is nothing to be gained by this effort, but there will be losses since the heating processes have losses. There will also be unnecessary costs for the tubing and labor.

Lee

What's gained by this effort in winter is the efficiency of the solar collection, since it's running at least 30F cooler to heat the slab than when it's boosting the temp to DHW temps.  There's no attempt to break laws of thermodynamics here. 

Think of the slab as a heat buffer running at 70-75F, and the DHW store as heat buffer running at 120F+, and what that does to the losses at the solar collector. It's far more efficient at the collector to do the first half of the DHW temp rise with a lower temp collector, which is what you get when the space heating is the priority zone for the solar.
 

As speaking to the tankless water heater working with solar I can say that these little devices work superbly. They work great to bring up the temp needed especially for when 10 or 20 degrees are needed. They actually pulse the amperage until a greater flow is required then pulse more (observed with an ampprobe). They are also deadly accurate on temp.
That said there are a few things to consider. You may spend more on the electric service than you do for the unit. If you hire an electrician and it is inspected you may be required to derate by 20% requiring larger wire. My unit took a 60 amp breaker down after no sun for a week so I had to up the wire and the breaker. (10/3 twice cause I had it).
Another thing to consider is that they work on flow and as long as that doesn't change the temp stays the same, however, when someone uses water at another source within the building for a short moment or two the water temps lags and if you are in the shower you are going to feel the hiccup. Hope that helps!

It's somewhat outside the scope of the question, but with a %90K budget it's probably more cost effective to superinsulate the place PassiveHouse style and do the bulk of the heating/cooling by controlling ventilation rates with an HRV (or using building-integrated active thermal air panels) than going with solar hydronic & radiant heating. In UT if you take a 2x6 studwall full of wet spray cellulose (air sealed & blower-door verified prior to the cellulose), then add 3-4" of foil faced iso (layed up in 2 layers, with staggered seams, and the seams of each layer FSK taped) to the exterior over the structural sheathing you end up with a ~10-11" wall that's over R40. Putting R20+ of EPS under the slab and using R25+ insulated concrete forms for the stem walls to thermally break all concrete & wood from both ground & exterior, and using R8-R10 insulated exterior doors, and at least R50 of cellulose in the attic, with well managed/placed glazing type & size the total heat load would be low enough that you could heat the place with a small amount of resistance electricity, using the same order of magnitude of power that your pumping power on the solar/radiant slab would have been.

It takes a sharp pencil and a lot of thermal analysis to figure out the breakover points, but the size of an active hyronic solar array that would provide the fraction of the space heating in Ogden with standard 2x6 framing insulation levels would be larger than the house. Designed for passive tempering and cutting the heat loss by half or more, and it's a very different picture. In terms of lowered space heating bills, you'll see a much bigger ROI by boosting the insulation levels than you will with low-temp hydronic floors and hydronic solar. Using the slab for pre-heating the DHW is still viable, as is active solar hot water. But a low-temp thermal air panel and R40 walls will likely cost far less than a radiant slab and a solar array big enough to count. With R40 walls and R20+ under-slab insulation, using the slab as radiation makes little sense, and NO difference in actual comfort level- it'll all be pretty much room temp, not noticably warm to bare feet.

Thanks Dana1, your last comments were very insightful and will make me rethink my design!

John

JohnnyH: If you haven't already, DO spring for the PassiveHouse tools and use them to help break down where the costs/benefits lie. In Ogden with a heating outdoor design temp of 5F doing active solar space heating only starts making sense at ~ R40-ish clear wall values and up. In colder places you may need more, but in warmer areas R30 would be fine.

But at the proposed ~R20 2x6 18- 24" o.c. framing with no exterior foam it might work in Santa Barbara-San Diego, or maybe even in Las Vegas, but not in northern UT. To reach full PassiveHouse levels on it would likely require something like R55-R60 in Ogden, which isn't practical with a simple studwall + exterior foam approach. A 3" R19-R21 exterior iso wall sheathing package for a 500-900square foot 1-story house is less than $3K in material, less than $10K installed if the exterior shape is simple, like a rectangle or square. It won't cut the heat loss fully in half (the windows & doors will dominate at that point), but it'll likely be over 1/3 in a low glazing area house with an insulated foundation. Under-slab EPS at R20-ish would be a couple or three grand. With those improvements a solar array big enough to support most the remaining heating load might actually fit on the house, but solar air panels would be both cheaper & simpler than a radiant floor & flat panel/evacuated tube hydronic system. At mere R20 walls you'd be burning up about third or more of the $90K construction budget on solar panels & radiant floor. Solar is cool and all, but it's higher maintenance & lower reliability than insulation.

Thankyou so much everyone for your overwhelming support on this subject!! I'm so lucky to have stumbled upon this web site. So much better than a million Google searches!

Another article arguing for taking the superinsulation approach over solar enhancements:

http://www.greenbuildingadvisor.com/blogs/dept/musings/solar-versus-superinsulation-30-year-old-debate

Dana1-

I think Martin Holladay's commnets apply to people that talk about complicated solar schemes to switch solar hot water systems between domestic hot water and radiant slab heating depending on various temperature measurements. Many of us today are more interested in tight, well-insulated homes the include passive solar heating using high solar gain, low-e windows (if we live iin heating-only climates), and relatively simple, dedicated active solar hot water systems. And if we have extra money to play with, we throw in a solar PV system and see how close we can get to net zero-energy homes.

I think Robert Riversong's response to Mr. Holladay's comments comes closest to my philosphy when he says:
"What Martin fails to note, is that the "debate" was settled 30 years ago when passive solar was married to super-insulation and the offspring were efficiency, comfort, and a more ecologically-and biologically appropriate approach to shelter."

Lee

For Dana1

If you were going to put R20 under the basement slab, wouldn't it be prudent to put pex tubing in? I don't know the cost but it could be utilized at a later time you don't have to use it but you could!

John

Posted By Lee Dodge on 22 Oct 2010 12:14 PM
Dana1-

I think Martin Holladay's commnets apply to people that talk about complicated solar schemes to switch solar hot water systems between domestic hot water and radiant slab heating depending on various temperature measurements. Many of us today are more interested in tight, well-insulated homes the include passive solar heating using high solar gain, low-e windows (if we live iin heating-only climates), and relatively simple, dedicated active solar hot water systems. And if we have extra money to play with, we throw in a solar PV system and see how close we can get to net zero-energy homes.

I think Robert Riversong's response to Mr. Holladay's comments comes closest to my philosphy when he says:
"What Martin fails to note, is that the "debate" was settled 30 years ago when passive solar was married to super-insulation and the offspring were efficiency, comfort, and a more ecologically-and biologically appropriate approach to shelter."

Lee

That's exactly the topic here- active solar heating + DHW, gets complicated messy & expensive fast, when compared to lowering the load and designing passive gains appropriately.

Riversong and the PassiveHouse folks are all in harmony:  Design it with sufficient passive solar and insulation that active solar for space heating becomes irrelevant.