One of the many reasons I haven't invested more time in the design is my mining equipment is still producing a profit in excess of 50% of the input costs.
Not sure I'm following you. Are you suggesting that it doesn't make sense for you to mod your gear [to be a part of your hot water heater] now, because your profits are high without doing it?
Are you planning to modify the gear eventually, once the profits drop? If so, why not mod it now? Unless the mod will cost you less at a later date, you're simply losing money. Especially since you claim that you've underclocked your gear to 60% due to heat issues (summer). It either makes sense to do it right away, or it *never* makes sense.
If you're not planning to mod your gear, then my point is made.
2) the concern you point out is valid but not problematic, if the water in the tank is hot, the miner runs but is not used to heat hot water. (so your wast heat is wasted) but for the rest of the time, the cost of hot water is free, or the bitcoin are being mined for free. you can still mine with 100% efficiency 100% of the time and not mine if your efficiency drops below say 99% or whatever your threshold is.
But that adds further costs--the need for dual cooling systems & switchover valving/control circuitry.
In short, impractical.
1) It won't be hard to integrate into hot water heaters when the chips are no longer obsoleted every 6 months or less. That will have to happen eventually.
2) The miner will shut off when the heat is no longer needed. If it continued running it would be at a loss, because the re-use of heat is the only thing keeping it profitable.
You're making this out to be way more difficult than it really is.