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Old 04-01-2009, 09:17 AM   #8 (permalink)
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It's true that it takes a lot more energy to move a unit of heat in air than in water. For its mass and density, air is pretty viscous so it takes a lot of power to shove it around, and for that matter the conduits we have for moving it, unlike plumbing pipe, are generally pretty rough on the inside. If you could have it, about the best material for moving large volumes of air would be large-diameter PVC pipe - which is really expensive.

My smallest HVAC system here at work uses a 1/3hp motor for running the blower. Obviously none of this has been optimised for the best possible performance but it isn't the worst case scenario either. So that blower moves around 800cfm, which after a little math with a REALLY GENEROUS fudge factor equates to about 300 pounds of air per hour.

My largest water heater has a permanent circulator pump, in this case about 1/2hp so it isn't a direct comparison, but that's moving, assuming the lowest numbers on the scale, over 400 pounds of water per MINUTE. Now, you can't heat water to as high a temperature as air, but water's heat capacity knocks air into a cocked hat.

I've thought about projects like yours from time to time. The one I kept coming back to was, what if I got a small window-mount heat pump unit, detached its exterior coil and built a water box around that? Then just circulate your heat exchanger water from the ground loops into the water box. It's quick and dirty, but I don't see any big reason why it shouldn't work. You get to eliminate half of the fan noise from the AC unit by pulling off that outdoor fan, and redirect that fan's controller to operate the ground loop pump. Since the water is entering and leaving via insulated pipes, you can place the heat pump pretty much wherever you want it.

As you say, your mileage may vary.

Lead or follow. Either is fine.
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