Our house is 20 yrs old, 2 stories, 2500 sq. ft., central CA, faces due south with slab foundation, flat roof with 7" of rigid foam insulation, 2x6 walls with Fiberglas bats, rubber gaskets btw. framing and slab and at the interior drywall corners, and 1" styrofoam below the slab in the corners and adjacent to the foundation walls.   It has Pex tubing throughout with a Wirsbo zone manifold, and we have a 55kbtu gas fired water heater used strictly for space heating (we are on the third water heater).  There is no controller for the radiant heat, I adjust the recirculation mixing valve every 2-3 months or so and we get a constant 69-70 deg.  We have a Takagi tankless water heater for dhw.  We have never had a period when the space heating could not keep up with the heat demand in 20 yrs.  Our gas bills for water and heating run $60/mo from Nov. thru April and $30/mo. the rest of the year.

When I built the house I ran insulated 1" copper pipes from the utility room to the second story roof for future solar hot water, but never got around to installing solar.  However, the water heater is about due for replacement, and I am thinking this is probably a good time to replace it with a more efficient heat source, and over the next 6 mos. add solar to the mix.  So I have been looking for a design that integrates a tankless water heater, radiant heating and solar.  I'm thinking that I can use the existing Takagi tankless water heater for both dhw and as the heat source for radiant heat,
and so I can replace the existing water heater with an 80 gal storage tank with dual heat exchange loops.  The upper loop would be for dhw, the lower for solar, and the storage tank would be part of the radiant heating closed loop system. 

This is where I need some design help.  I want to be able to use the upper loop for dual purposes, with automatic valves to control the flow:
A) as the radiant heat source (when solar is not producing heat),
B) as a pre-heat for dhw (when solar is producing heat).

In A) the input to the loop would be heated water from the Takagi and the outlet would return to the Takagi inlet.

In B) the input to the loop would be the cold dhw inlet and the outlet would return to the Takagi inlet.

I also need to know what kind of control & sensors to use.

I looked at BuildItSolar.com, and found lots of designs for a single heat source heating system, but none with a dual, hybrid capability like I described.

-Dave

I can point you to some illustrative diagrams at Houseneeds.com at least. http://houseneeds.com/shop/heatingproducts/heatu/heatinguniversity.asp You will find a couple of approaches to solar/tankless heating and dhw, although none that implements your strategy directly. A second approach is to use a large vented tank, like the diagram here: http://www.stsscoinc.com/Products_SolarAndDHW.ASPX With heat storage and extra collectors, solar could handle more of the heating load without creating undue heat dump issues in the summer. Lots of variables here, and pitfalls too.

toddm,

thanks for the link to Houseneeds. This diagram:
http://houseneeds.com/shop/heatingproducts/heatu/solar_space_heating.asp
is close, although the storage tank contains domestic water rather than radiant heat water. i.e. the top coil is radiant heat, and the system I want would have the top coil be domestic water.

I also found a diagram in this Caleffi book that comes very close to what I want (Fig 29 a, b & c):
http://houseneeds.com/shop/manuals/caleffi_idronics3.pdf

Again, the storage tank contains domestic water. After reading the doc. on Legionaire's Disease on the same website, storing domestic water at 130 deg. seems to be a risk.

The Caleffi design also has this note:
"The two circuits intersect in the hydraulic separation chamber in the HydroLink." (the two circuits are the boiler circulation loop and the tank circulation loop). So the Caleffi design requires a $pecialized manifold. I could not find any additional information in Caleffi technical publications about this manifold.

-Dave

Have you looked at the more energy efficient hot water heaters? AO Smith Vertex 100 - supposed to be 96% efficient. Does both dhw and radiant through side taps through a heat exchanger

http://www.hotwater.com/products/residential/rg-vertex100.html

-Rosalinda

Rosalinda, that would be a solution if all I wanted to do was replace the existing water heater, but I have several objectives, and a new water heater only solves some of them:  go solar, get rid of the pilot light based water heater, go to a single heat source for dhw and radiant, increase thermal efficiency of the system, replace the mostly manual control system we now have with something that is fully automated and reliable.

-Dave

Been there. Worried about that. Opted for a vented 300 gallon tank with room for lots of heat exchangers.

You may be able to mash together the caleffi approach with this Houseneeds solution http://houseneeds.com/shop/heatingproducts/heatu/tankheatandwatermain.asp

The difference is an external heat exchanger for the hydronic. I wonder if your shared HX also opens you to legionaire issues without a hard separation of dhw and hydronic. I have a very capable plumber. You may need some help yourself, even if it is calling Houseneeds.

thanks for pot this information..its very interesting to visit this websites..

We decided to go with a Navien combination unit (ch-240).  This allows us to do this in two stages: first replace the existing water heater with the Navien, which gives us a combined dhw and radiant heat source.  Then next year we will add solar hot water using a storage tank with a single coil.  That part of the system still needs to be designed.  The radiant heat system controls the Navien primary loop circulation pump, so I would need a smarter control, one which will turn on the pump through the solar tank whenever there is heat demand and solar heat available, but which would turn on the Navien pump when there is heat demand and no solar available.

-Dave