> I've always wanted to co-locate the heat output from the compressor of the A/C+refrigerator+chest freezer, dryer, and dishwasher into the same equipment space as my water heater

I worked at a McDonald's in the 1980s that did that. All the heat from the ice maker and drink cooler condensors was captured in a "pre-heat" tank that fed into the water heater. The idea was that you'd need less electricity for heating water for the dishwasher.

It worked, but they removed it several years later so I guess it wasn't a net benefit.

Thanks for sharing that historical tidbit! My understanding is compressor manufacturers don't like to work on co-generation on a small scale. However, early attempts like you describe tended to use some kind of water jacket because heat pump technology back then was finicky and much more expensive than today. Using air to conduct heat is hugely inefficient, but we're dumping the waste heat into the air anyways and then later spending more kWh to cool down the air, so the current batch of heat pump-based water heaters are a possible starting point.

In idle moments I've wondered if I could retrofit the multiple small compressors with a single large unit with multiplexed hoses and electronically-controlled relays that was designed to operate submerged in something like polyalkalene glycol (PAG fluid), insulate the heck out of that to minimize thermal conductivity to the surrounding air, then extract out the heat of the PAG fluid into the water heater. Would be a maintenance nightmare to be sure, but it would be a cool hack to play with exploring and measuring those boundaries of efficiency.

Its also possible that labor/maintenance costs outweighed the cost savings. Or they couldnt buy the equipment any more :)

Speaking of everything being a heat pump, another interesting supplementary solution is https://en.m.wikipedia.org/wiki/New_York_City_steam_system

Um, your link is hardly an endorsement for foamed glass:

"But it's also two or three times as expensive as one of its chief rivals, extruded polystyrene (XPS), and its R-value per inch (R-3.4) is about 30% lower."

Thank you for pointing that out, I should have set out my qualifiers, it is definitely not for everyone nor every use case. I favor the extremely stable properties and far lower environmental impact over XPS. For my specific use case, I want an application where the lifetime of the building is measured in >100 years, without having to go in and replace the insulation during that period, and without significant deterioration from the ocassional penetration of water or dampness. I wanted the natural flame retardant properties without doping in chemicals that have to be extracted out later with solvents (yielding its own set of recovery, recycling and disposal challenges) during recycling like we currently have to employ with recycling XPS (if the recycling facility extracts it out at all). I really like the concept of building this insulation once, and being able to re-use it indefinitely until it is physically compromised, capturing the embedded energy to make the insulation in a one-and-done cycle.

I'm not a huge fan of plastics or styrofoams outside of very well-contained use cases since its breakdown in the outside into extremely small pellets is already easily detectable in our seawater-based salt supplies, and unabated, I'm not sanguine about our keeping it from pervasively entering our food supply. The actual plastic itself I'm not too happy about mainly because it represents energy waste to me, and at least those types that have phytoestrogenic effects are still debatable. The doped compounds attached to those plastics however, I'm not terribly enthusiastic about that stuff leaching out into our environment and letting the sun and weather eventually breaking them down before they materially damage our bodies.

I'm not sure if with foamglass they mean an aerogel made of glass. That stuff has wild thermal properties.

https://www.youtube.com/watch?v=kz95RnIG0NY

If Chinese factories figure out how to mass produce aerogel, with similar durability and environmental footprints to Corning Foamglas (or similar foamed glass blocks), then I'd be all over that. So far though, I can't get very good field reports of people using the much cheaper pellet-form factor aerogel products commonly coming out of China.

I don't know the true manufacturing costs of sheet and block form aerogel, but they're priced so high that even multi-generational payback periods didn't pencil out even close, even granting them a 30% premium. Current manufacturers simply aren't aiming those form factors at building insulation as far as I can tell, though I'd love to be shown wrong.

For the time being, when I need near-aerogel-like qualities insulation but without the volume penalty foamed glass incurs, but can't justify the cost of aerogel due to the required quantity, I look to vacuum insulated panels to cover the big expanses of area and volume, and aerogel to fill in the gaps.

Be careful retrofitting insulation to a house. It can cause damp issues.