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You can also do this at a household level. It is called heat recovery drain pipe, basically a heat exchanger that replaces a vertical section of drain pipe.

You can also do this at a household level. It is called heat recovery drain pipe, basically a heat exchanger that replaces a vertical section of drain pipe. The wastewater heat is used to pre-warm the fresh water going to the water heater. Water falling down a vertical pipe tends to spread out and cling to the inner surface, giving you more surface area for heat transfer. Last time I priced this system out the installed cost was going to be about $1000, at that price point the payback time is pretty long for a single family home but much better for an apartment building. https://www.homedepot.com/s/power-pipe?NCNI-5 https://www.homedepot.com/s/power-pipe?NCNI-5

switchbak 3y ago
It would seem like a water heater using heat pump tech (these exist) could use this as a source of heat to scavenge in addition to the surrounding air. I've also wondered why we eject the hot air from running an AC to the outdoors, while we then pay to heat the water in our hot water heaters. That seems like a no-brainer to use that to help heat the hot water.
- kagakuninja 3y ago
  Yep. Most homes have multiple sources of waste heat. Clothes dryers, ovens, AC, refrigerators... The heat from refrigerators is vented into the home, which is inefficient if you are running the AC. I've been wondering for a long time why we don't have whole-house heat recovery systems.
  - brtkdotse 3y ago
    > I've been wondering for a long time why we don't have whole-house heat recovery systems. Every house in the Nordics built after 1985 or so has this.
  - philwelch 3y ago
    In theory, you can have an AC or heat pump that shares a refrigerant loop with your fridges and freezers. But the efficiency gains are extremely marginal and it adds a lot of extra trouble. Right now you can just buy a fridge or a freezer, plug it in, and it just works. You don’t have to plumb refrigerant through it. Which is good because it reduces the risk of refrigerant leaks.
  - nullc 3y ago
    I spent a while looking to see if split-unit refrigerators exist-- not really (except giant commercial room size stuff). The reason is that mostly refrigerators are so well insulated that their waste heat is extremely small making it worthless to do much of anything with it.

switchbak 3y ago

It would seem like a water heater using heat pump tech (these exist) could use this as a source of heat to scavenge in addition to the surrounding air. I've also wondered why we eject the hot air from running an AC to the outdoors, while we then pay to heat the water in our hot water heaters. That seems like a no-brainer to use that to help heat the hot water.

kagakuninja 3y ago
Yep. Most homes have multiple sources of waste heat. Clothes dryers, ovens, AC, refrigerators... The heat from refrigerators is vented into the home, which is inefficient if you are running the AC. I've been wondering for a long time why we don't have whole-house heat recovery systems.
- brtkdotse 3y ago
  > I've been wondering for a long time why we don't have whole-house heat recovery systems. Every house in the Nordics built after 1985 or so has this.
- philwelch 3y ago
  In theory, you can have an AC or heat pump that shares a refrigerant loop with your fridges and freezers. But the efficiency gains are extremely marginal and it adds a lot of extra trouble. Right now you can just buy a fridge or a freezer, plug it in, and it just works. You don’t have to plumb refrigerant through it. Which is good because it reduces the risk of refrigerant leaks.
- nullc 3y ago
  I spent a while looking to see if split-unit refrigerators exist-- not really (except giant commercial room size stuff). The reason is that mostly refrigerators are so well insulated that their waste heat is extremely small making it worthless to do much of anything with it.