WtW Efficiency gain of lower temp. Load Tank
 

After studying more, I see the efficiency benefit of going to a 2 tank system.
As suggested by NiHaoMike, Thx.

Smaller (40-50 gallon) DHW tank, heated by NG boiler, 130-140*F to prevent bacteria.

Larger thermal accumulator (120 gallon) tank for radiant floors, 90-100*F
This tank can be heated from boiler or from 1000 gallon Solar storage tank via DIY WtW heat pump.

Radiant floors can also be heated directly (not using WtW heat pump & thermal accumulator) from the Solar storage tank, via a HX loop of 3/4" O2 barrier PEX in top of tank, if water temp. > 90*F

You can use a desuperheater to supply the smaller tank, allowing you to not use the boiler at all in warmer weather.

It's useful to look at degree days when considering advice.

Teller County, CO
HDD(68F) = 6198
CDD(75F) = 632

Austin, TX
HDD(68F) = 2172
CDD(75F) = 1540

Best,

-AC

The smaller tank is only for DHW. The demand for hot water won't change much with weather, especially with the preheat tank.

So with a desuperheater, the heat pump will supply the second stage hot water until the source water gets below the balance point, at which point it switches to the backup boiler.

It's also possible to use colloidal silver to kill bacteria but I don't know enough on how to design a system for that or if it's cost effective.

I will have no a/c cooling.

Open windows at night + 8,800' altitude is all that's needed with good house thermal envelope.

Boiler heats DHW tank via external flat plate GEA HX, approximately 180*F boiler water enters HX, approximately 160*F exits. On the DHW side of HX, approximately 120*F potable water enters HX & 140*F water exits.

If Summer/Winter switch is set to Winter mode, the approximately 160*F that exits the DHW HX goes on to heat the thermal accumulator directly (without any HX needed) then returns to boiler.

If radiant floors draw thermal accumulator down to 90*F, AND solar storage tank > 35*F, the new WtW heat pump will be turned on by 90-100*F aquastat and heat from the 1,000 gallon Solar storage tank will be used to heat the thermal accumulator up to 100*F.

I don't know much about desuperheater.

http://www.climatemaster.com/downloads/RP881.pdf
"
The hot refrigerant leaving the compressor has a temperature that is several degrees higher than its saturation temperature (e.g. The temperature at which the refrigerant gas is about to condense). The heat that causes this temperature elevation is called superheat, and it must be removed from the refrigerant before it can begin condensing. In heat pumps without a desuperheater, this heat is released to the fluid stream flowing through the condenser. However, when a desuperheater heat exchanger is present, this superheat is removed before the refrigerant gas reaches the condenser.
Water-to-water heat pumps equipped with superheaters are also equipped with a small, low power (1/150 hp) internal bronze circulator. When operating, this circulator creates flow of domestic water between a storage tank and the desuperheater. This water flow is what absorbs the superheat energy from the refrigerant, and ultimately moves it to the storage tank.

If the desuperheater circulator is off, the hot refrigerant gas simply passes through the desuperheater and thus carries more thermal energy on to the condenser.
It is important to understand that any thermal energy removed from the refrigerant gas by the desuperheater is unavailable for the heating load served by the condenser. When the condenser of the water-to-water heat pump is serving a space heating load, the heat removed by the desuperheater is used to heat domestic water and thus not available to heat the building. However, when the heat pump is operating in the cooling mode, any heat removed from the refrigerant by the desuperheater is heat that otherwise would be dissipated to the ground. Thus, it is correct to state that this heat is “free” since it would otherwise have no further value within the building.
"

Thing is: heat pump won't run in Summer when radiant floor is not needed.

If it's more efficient to use the heat pump instead of the boiler during the winter (at least until the source water falls below the balance point), it would be even more efficient during the summer when the solar panels have more sunlight. (In fact, you might be able to bypass the heat pump entirely during that time.)

I have had little luck trying to find plumbing diagrams.
One is main a loop with a loop pump going CW, the 3 heat sources are in order from low to high temp.
1st is Solar which can source temp. from 35->180*F, (1 pump)
2nd is Heat pump which can source temp. from 90->110*F, (2 pumps)
3rd is Boiler which can source temp. from 90->180*F, (1 pump)

1st load continuing CW is DHW tank, uses 180->150*F (2 pumps with external HX)
2nd load is thermal accumulator that can pull heat from loop OR add heat to loop \
(2 pumps)

So 9 pumps for what looks to be an elegant design.