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Comment by lsaferite | Hacker News Reader

I think they alluded to it in the article, but when a heat pump is cooling the inside it's generating lots of heat on the outside. They could/should/likely are using that 'waste' heat to dry out the desiccant. Seems like a great plan.

I often wonder why hot water systems aren't linked into the HVAC as well. I know you can get heat pump water heaters, but it seems like integrating it into the HVAC system would potentially allow for system scale optimizations. I would assume the cost/complexity is too much for residential systems. For that matter, refrigeration in homes falls into the same category as they are a heat pump that's dumping waste heat into your living space.

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Combining refrigerator, water heater and heat pump implies the need for transferring the fluid used as refrigerant between these components. This is usually a gas that is not trivial to contain, so creating this shared pipeline increases the points of failure. Further, you now have to insulate the entire length of this system, using material to do so, and you will end up losing a certain amount of efficiency anyways in this piping system. Also, what if your need in hot water is much greater than your need to cool the house or vice versa, what is your backup system for disposing of heat or cold when not required? It’s not certain that after solving all these problems and others, that you still have a net efficiency gain.

I think you could get away with two insulated reservoirs (frozen and hot water) and then use something like glycol, alcohol or water to shuttle hot and cold to your shower, under floor heating/cooling. Frozen water can be mechanically stirred (like a slushy machine) to keep it from freezing solid, allowing you to pump it through pipes. Water can also be mixed with salts or alcohol to alter it's freezing temperatures, with allow you to move cold from your freezer to the refrigerated areas (fridge, computers or living spaces). You can also use air as a working fluid. Any excess can be dumped to the outside environment with a radiator as a last resort.

Modern cars are already doing this with heat pumps to manage batteries and the cabin temperatures.

It's pretty easy to see where building technologies will go by looking at car technologies...

I like to imagine a world where it's acceptable for houses to be designed around a utility room where all of these interchanges, insulation, isolation, exhaust, issues can be contained, and accessed, in a small, separate, space that is not part of the living area.

Unfortunately it's also a world where having a problem with your aircon means your fridge stops working and replacing a fridge means calling in a technician for the plumbing.

Where I live, few installers have specialized in heat pump water heaters, and it's still seen as a niche technology. This means the cost increase compared to a standard electric or gas water heater is high and installation issues are common. You also run into a lot of situations where the water heater is placed in conditioned space which makes any energy savings a lot more sketchy.

Adding different systems to the mix, to where we now need the HVAC guy, plumber, etc. to all set up a working system together seems like a recipe for disaster.

> You also run into a lot of situations where the water heater is placed in conditioned space which makes any energy savings a lot more sketchy.

Assuming these were the prior positions of a non heat pump system, you've gone from creating heat to transferring heat; while both radiate heat, the heat pump is much closer to net 0.

If this were a continuous commercial operation then heat from one process would definitely be used in another. That does save money. The problem is that residential use will be variable and intermittent, requiring a host of sensors and valves to turn loops on and off depending on random needs. And when say one process needs to dump heat, but that heat isn't needed elsewhere (ie the hot water tank is already hot) then equivalent backup radiators must on standby to take the load. That is all just too much control circuitry for residential devices normally controlled by just one bang-bang (on/off) control.

I have seen water heat pump systems. They are meant to replace boilers for radiators or under floor heating. Then the hot water can be stored. Or go to a heat exchanger for heating and cooling with ducts. Water is much easier fluid to work with than refrigerant.

One reason don't see combined systems is that refrigerator and water heater don't move that much heat. Refrigerator is fine dumping the heat into the kitchen, and water heater pulling heat from the basement. The air inside the house turns into the working fluid.

Heat pump systems in combination with underfloor heating are sort of common in new homes over here (not the norm, but common enough). The heatpump heats the floor in winter, cools it in summer. The heat is stored in a large water tank, and it can be transferred to potable water for showers and whatnot.

Best part is, you can supply heat to the tank in multiple ways, even concurrently. Think heat pumps, solar water heaters, or even a fireplace if you're so inclined.

Apparently integrating appliances that use refrigerant piping isn’t so easy, compared to self-contained units that just need electricity. It looks like mini-split systems require professional installation.

I wonder if there is another way to connect things?

Refrigerant lines tend to be high pressure (300+ PSI) and the system as a whole is intolerant of leaks - even tiny pinhole leaks - over time.

More than any of that is that a _proper_ install means you need a specific level of vacuum on the lines before refrigerant is introduced. That effects the performance of the system a lot. My friend is an HVAC nerd/wiz and has explain, in detail, the whole process for how HVAC systems function. It's really quite interesting.

One interesting thing he pointed out was that HVAC system load calculations are predicated on a max exterior temperature of something like 92 or 94 degrees (I forget the exact number). If the exterior temperature exceeds that, the system's ability to cool faster than the space heats is compromised. And if you think, oh, just oversize the system then, you'd be making a mistake as well. Over-sized systems don't run enough for proper dehumidification and most houses I've been in do not have a dedicated dehumidification loop. My friend has one in his house, but it uses dedicated vent lines and everything, so it's a sizeable investment. That's actually where things like this system start to become interesting.

Do you know if the vacuum is to purge contaminants or something else?

there are many mini-split heat pump systems that can be installed by a homeowner without a professional installation now. An example: https://www.youtube.com/watch?v=ZmwZVD2KEDY

These do exist but are indeed complicated. Still pretty new. Usually used with hydronic systems.

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