DIYbanter - WATER/AIR HYDRONIC SYSTEM WITH HEAT-PUMP

I've been designing and working on this for over a year and have a home
that facilitates experimentation fairly easy.

The system design so far: 4 Ton RUUD Air Handler factory equipped with
R-410A coils, matching 4-Ton heat pump. Rinnai water heater in place of a
boiler to a water/air heat exchanger(s) at the discharge of the air
handler.

Rinnai because of their controls and efficiency. The output of their
water heater is restricted so I may need to purchase a unit specifically
made for a boiler, which is the same except the control.

Questions: I have examples of a couple of applications where this was
done in Colorado successfully, and I live in Colorado. They used the std.
water heater in a modified open loop circuit; meaning it is closed loop
except they tee off with a check valve and also use the circuit for the
overall hot water demand. I assume the hot water demand for the house
could easily affect the HVAC demand when they parallel. I think this
would be tolerable and the system would work a bit longer to maintain
temp. after everyone takes their shower. I believe I can put together a
circuit for this allowing room to modify.

However, I would rather have two water heaters and put one on a dedicated
closed-loop system for HVAC. In doing that I am not sure how to maintain
adequate pressure. I think the Rinnai will want at least 45 psi or it
will boil at the heater. I am scratching my head a bit on the initial
fill of this system using an expansion tank and if a std. expansion tank
is the right device to use for fill & makeup? I see the air bleeding
devices and fill valves associated with the expansion tanks and I know
enough to incorporate a check valve so there is no reverse flow but I am
still thinking on this. I was also considering using Ethylene or
Propylene glycol to prevent corrosion only.

Also, I would like to control this so I have normal AC & Heat-Pump control
down to 30 deg F at which point the control switch over to water heat.

I've looked at normal hydronic units using boilers and those systems make
sense but they're low pressure. They're doing this in Japan but their
systems are packaged so you cannot see how they're doing it exactly and
the hardware is not available in the US except special order. I am also
enjoying putting this one together with COMMON STUFF that is obtainable
off the shelf in the US.

The guy in Loveland is converting a standard gas forced air furnace by
abandoning the burners and putting a water/air heat exchanger in the
discharge air flow path. His circuit is super simple: Two check valves,
one on the main inlet so he can complete a hot circuit to the heat
exchanger and another on the discharge to the house demand for hot water.
He has a circulation pump in the circuit, which I assume he's hooked up to
the thermostat. When the pump is turned on the Rinnai detects flow and
turns on... no need to talk with the Rinnai what so ever except setting
the temperature. This works but trying to convert this over to a closed
loop system causes me to think a bit and I am not sure how to maintain
fill especially using chemicals in the circuit.

I talked with a couple of the manufacturers of hydronic air handlers and
they had no problem but all they could say was, "The air handler does not
care what provides hot water to it... just so it gets enough hot water at
a minimum of 140 deg F".

*** Feel free to lend any knowledge even if it is only a portion of the
system, such as a controller/thermostat. I need just a tad more
intelligent control for that outside temperature input threshold at 30 deg
F to switch to water heat. Any circuit diagrams you've run into would
also be helpful. If you're an experienced HVAC designer/tech you might
have some ideas to enhance the system or flags may have gone up reading
about this... let me know what you think.

Setup for this system will be totally safe. I live alone and the house is
open on both floors... walk out basement. I am redoing the entire house
so I can set this up and run temporary ducting for testing and rely on the
existing heating system.

I have the Ruud hardware, air handler and heat pump purchased. I was
waiting on the rest of the hardware as I tie down exactly what will be
needed. I will also allow room in all of the circuits for resizing and
change to include duplex water heaters if needed.