I've been lurking for a while as well......but this attic heating post got my attention. I've been working on a similar heater for several months now.
In short, I've arranged four radiators (new, of course) in a cube. It has a top and bottom. I have a 24" fan mounted to the bottom of this unit. Inside, I've built input and output manifolds to put cold water from the pool into the bottom inlets of the four radiators in parallel....the water rises through the four radiators, is heated by the hot attic air drawn in from the fan, and flows in parallel to the output manifold. The key here is the parallel flow......my unit can flow a ridiculous amount of water. Numbers? I've got about 1500 sq in of radiator surface; I draw hot air from all four sides; the fan does about 2700 cfm on it's lowest speed. This unit is suspended at the highest point in the attic. I intend to use a 24" flexible duct to exhaust the cold air the unit will produce into the garage - essentially a/c for the garage.......cold air out, warm air in through the soffit vents is heated as it rises.
The highest risk in this is a catastrophic failure resulting in a two-inch pipe pushing water into the attic at full pump output. A lot of gadgetry and controls will be utilized here, but even shutting off the pump will not prevent heavy water intrusion; it would definitely catch and intercept a small leak, but not the full failure of an inlet or outlet fitting. And no, a small catch pan won't be sufficient. My solution is to basically "tub" that part of the attic under this unit. There's really no other way to make this safe. I'll panel the floor of the attic and seal it up with about an 18" surround. The purpose isn't to hold that water for any significant time, it's to direct that water through a huge, perhaps 4-6" outlet (maybe a pair of them) to the outside. Overkill? My unit is essentially over my garage.......each installation is different. As an additional safety measure, I intend to shut off the main drain and draw water only from the skimmer line - this will inherently limit the amount of water at risk and will preclude possible catastrophe #2: a drained pool popping out of the ground.
Please share your ideas.....I'm still doing some fabrication on this and I welcome your input.