Aquifer Residential desuperheater for HVAC heat recovery
So I found this heat recovery unit to be tied between HVAC and water heater. I’m a layman but the idea seems simple enough, waste heat from AC heats water for water heater. I run my AC a lot here in central Florida. I mentioned it to my HVAC guy and he seemed excited about it, said it would lower water heating costs and prolong the life of my AC outside unit.
Mentioned it to a plumber and he quickly said don’t do it, they fail, they were popular in the 80s but that’s why you don’t see them anymore.
I really don’t see much about it on the internet for residential. I can see where maybe the economics didn’t make sense so they’re not popular anymore but I don’t know. My house is not efficient or tight, so the AC pumps away. Both the outside unit and my electric water heater are 16 years old.
Anyone have experience with these things? Was there a time when they were popular then went out of style?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
The principle is sound. The reliability problems are real. It's very hard to tell what your luck will be with the current crop of products. One point made in this article:
https://www.greenbuildermedia.com/blog/good-news-geothermal-heating-where-have-all-the-hot-water-generators-gone
is that a better high-efficiency A/C will have less waste heat available to heat water. Given that your A/C is 16 years old, you'd like get better overall efficiency plus a boost, instead of a degradation, in reliability if you install a new A/C instead.
For heating water, a heat pump water heater will give you a similar benefit: it cools and dehumidifies your house while heating the water. These are sometimes called "hybrid" water heaters.
The article says, "As federal regulations over the past 20 years have required seasonal energy-efficiency ratio (SEER) ratings of air-conditioning units to be increased, the availability of “waste heat” for operations such as domestic hot-water recovery diminished." But doesn't back it up.
On its surface the claim makes no sense. The "waste heat" is largely heat that is removed from the dwelling, that's going to exist (and be substantial) even if the unit itself took no energy to run.
What I will concede is that heat pump efficiency is largely determined by the "jump" -- the difference in temperature between the two ends of the system. The way to increase SEER is to lower the jump, making the indoor side warmer and the outdoor side cooler. So the outdoor side could be cooler in a high-SEER unit. But it's still going to be plenty hot for making domestic hot water.
Yes, most of the heat removed from the building is still expelled at the condenser. But there are two things that make it less attractive nonetheless. One you mention, which is that the condensor doesn't run as hot. The other is that the amount of superheat, which is what is theoretically removed by the desuperheater, is reduced. The net result is that you can do great pre-heating, but not necessarily bring it up to the temperature that's needed. And then you need two tanks. So all in all, with such great heat pump water heaters available now, that's the way to go.
I see, thank you. I almost want my water heater to break down so I can get a hybrid, almost.
Better to replace it before it leaks and makes a mess.