Im looking for some ideas and thought you guys would be able to assist. I currently utilize a 4 ton Tranquility 27 water to air geothermal unit for both heating and air conditioning my home that is about 2800 sqft. I would like to add radiant floor heat to approx 600 to 700 sqft of the lower level. Most of my home has tile floor and I do not want to tear it out so my only installation option is to install some type of water to water system to the underneath side of the floor joists. I was wondering what my option were. I would like to be able to utilize my current geothermal unit to heat the water if thats possible. I am willing to entertain almost all ideas! Thank you in advance!

depends on your heat loads, but it's almost guaranteed you will want to use heavy gauge aluminum transfer plates. not the lightweight plates, but a nice thick rigid plate. definitely a room by room load calc should be done before anything happens though. that would be a fair amount of money to spend on something to not be sure it's going to work to your satisfaction.

Figure 26 gauge aluminum conducts heat better than 4" of concrete. There is more to it, but it gives you an idea.

Every proper heating system, but more especially radiant floor heating systems start with a room-by-room heat load analysis, performed on dedicated hardware by and experienced professional. All sub-floor radiant heating systems are enhanced by extruded aluminum panels as Rob suggests. Also implied is the legitimate concern that the modest - yet still efficient-water temperatures produced by a water-to-water ground source heat pump will be insufficient in many applications (most especially retrofit). Radiant ceilings are for nearly all intents and purposes a better match for the low temperature geo systems, but only the heat load can qualify the best or even the plausible match for your home heating system. Were I heating AND cooling the ceiling would be thing.

A water->air heat pump isn't a good match for tubing. If you are up for something non-standard, you can box in the area under the floor and circulate warm air through it (I'm sitting in such an area right now - works well).

Okay guys I have not given up on this yet. The radiant heat that I want to install is not going to be a replacement for my geothermal heat pump. A substantial amount of square footage on the first floor of my home is tile. Which means it feels cold on your feet all winter long. And since the Geothermal heat pump does a decent job of heating the home, the goal here is to take the chill off of the floor in these tiled areas, not necesarily to heat the room(s).

I am thinking of utilizing some sort of low voltage hot water heater with a couple of low voltage, low flow pumps with the appropriate harware (tubing underfloor aluiminum plates, etc) in creation of two or maybe three zones. The system would be solar/electric/battery powered (12 volt?) with a 120vac backup inverter. I am thinking that once the hot water and system were up to temperature then it would not take much energy, ie electricity, to mainain the loop(s) temperature. Were not talking about acheiving a water temperature of 120+ degrees F, but only 80 or 90 degrees F. I could use an indoor thermostat to turn the loops off at a certain room(s)temperature and also set the hot water heater thermostat to a reasonable temperature as well. Say mabye 95 degrees F.

My concerns are:
1) Is this a reasonable idea?
2) Could a small 12 volt DC Hot water heater enough to maintain a floor temperature of 80 degrees F. (approx 700 sqft)
3) Has anyone done anything like this before?

Your thoughts and ideas are appreciated.

700' of 80F floor would dumping something like 20 BTU /foot into a 70F room, which makes that about 14,000BTU/hr.

14,000BTU/hr is (14,000/3.412=) 4100 watts, which would take about (4100/12=) 340 amperes of 12V.

That's a heluva lot of DC-amps.

If you're going to use low voltage DC, there are mesh-mat solutions that would probably be cheaper than the all-hydronic solution using heavy extrusions.

But a standard grid-attached electric hot water heater could deliver that much heat, provided the water temp requirements fall within the output temp range of a HW heater. Even if you're mounting the tubing in concrete/gypcrete above the wooden subfloor you won't be getting floor temps anywhere near that high with 80-90F water (but that's probably a good thing.)

Also note, 14,000BTU/hr is almost half the design condition heat load of my whole house, and WAY more heat than is necessary just to take the chill off the floor (an 80F floor is approaching the uncomfortable range too), and probably more heat than would be required to fully heat the room.

So, go back to first-principles: Calculate the heat load of the room at design condition using Manual-J or I=B=R type methods, and don't overbuild the input radiant floor aspect. If the goal is only to take the chill off the floor, a 72F floor is plenty comfy in bare feet, and won't roast you the way an 80F+ floor would. If electric, the low-voltage mats are pretty cheap to install compared to hydronic (and at least as efficient as a hot water heater) but 3-5x as expensive per BTU as your ground source heat pump, so run it off a floor thermostat. If the power to the mats maxes out at something like 1/8-1/4 the design condition heat load it's more than enough to take the chill off the floor, and won't "take over" from the heat pump for heating the house, displacing high-efficiency heating with low-efficiency heating running the power bill through the roof. Most of the mat solutions are designed for installation above the subfloor, but would still be able to deliver more than enough heat through an inch of wood + thinset + tile to be felt.

"the goal here is to take the chill off of the floor in these tiled areas, not necesarily to heat the room(s)."

You have one without the other. The best way to fight cold floors is to hook up a hydronic radiant floor to outdoor reset. The radiant panel is "on" more of the time at lower operating temperatures and fuel bills.

But if the "fuel" is resistance-electricity the difference in operating cost between ODR & hydronic and electric mats is pretty small, and the difference in upfront cost is huge.

With a GSHP system as the primary heat there might be some economic rationale for hydronic heated floors if the GSHP were providing the floor heat rather than an electric boiler or hot water heater. Even tepid 80-90F water in sub-floor aluminum extrusion heat exchangers would be enough to take the chill off the floor, even if heating the space entirely with the floor would take much higher water temps, killing the efficiency of the GSHP. I'm not sure how easy/difficult/expensive that would be to hack into an existing tepid-air GSHP system though.

It is possible to move (vs add) heat from the air to the floor - with a heat pump water heater and radiant tubing. ~$700 more in upfront cost, but it will draw much less power than resistance heat (increasing efficiency by leaving more work for the GSHP) and can create comfortable floors with far less chance of overheating the house.

The raw BTU/hr rating of a heat pump water heater in heat-pump-only mode isn't exactly awe-inspiring though. The heat pump a GE Geospring draws 550W, and with a COP/EF of about 2.4 that's only ~1320W or 4500BTU/hr of output. With 700' of floor that works out to 6.4 BTU per square foot at the floor, which is enough to feel, I s'pose. It may not actually achieve as high a COP though, since the return water is never actually cold, reducing the heat transfer efficiency somewhat below what it would be under water-heating EF test conditions. It might be close though.

There may be longevity/warranty issues with running a nearly 100% duty cycle on the compressor for months on end too. It would see fewer and shallower thermal cycling (which is being nice to it), but a few orders of magnitude more raw hours of run time.

Another option is to splice a refrigerant->water heat exchanger into the existing water->air GSHP. Would probably pay for itself although I'm sure it would void any warranty.