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Comment by mrb | Hacker News Reader

It would be neat to combine a heat pump (water heater or general heater) so that the outdoor coil (cooling down the air) is put to good use instead of being wasted cold. For example the cold coil could be cooling down a fridge or freezer.

I'm not sure how feasible it would be in practice. There is an advantage in having appliances that are independently serviceable and replaceable. But then again one heat pump system cooling the fridge and heating up water is simpler and less likely to break than two separate systems (one for the fridge, one for the water heater.) Price-wise, however, the labor costs of installing one physically larger integrated system are probably high than the labor costs of installing 2 smaller systems. So many tradeoffs...

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LG makes a multi-zone mini split system that can use the waste heat from air conditioning to heat water.

https://www.lg.com/global/business/hot-water-integrated

I've always wanted to see what could be done to unify all sources of temperature change in a house to a central compressor.

fridge, dryer, water heater, forced air.

Bonus points, bring the sewage line into the fold, heat exchangers can extract heat off of hot water in the line from showers/baths in the winter, and dump heat into the line in the summer from the hot side of the compressor.

The big issue is timing: hot water, fridge use, and home heating would need to be well synchronized to take advantage of this. I.e. you don't necessarily want to make your fridge colder just because you happen to be heating water. The fridge and temperature is set so your lettuce doesn't freeze and your hamburger says solid. Your water heater turns on because you took a shower. You would need to store the thermal energy (a block of ice, perhaps), but this adds complexity and cost.

You don't need to be very optimal to save energy. Your hot water heater is a very good buffer of heat as it stores a massive amount of energy and you don't care if its a few degrees warmer or colder than its set point. In some modern homes the hotwater heater runs far above safe delivery temperature to prevent various issues and is mixed down to a safe temperature elsewhere in the plumbing (either at point of use or in the outlet of the water heater) so the water temperature is even less critical.

The biggest impediment to a system like this is political, as appliance manufactures need to be on board with it, when they have 0 incentive to do so, as their expensive refrigerator becomes a cheap insulated box with a light.

>In some modern homes the hotwater heater runs far above safe delivery temperature

Is this a modern feature? It has been this way for as long as I can remember.

Not sure if it's a modern feature per se. But if you have a tankless heater, you can set the temperature to be a little bit above the hottest water you require. If you have a heater with a tank, you need to keep it hot enough to prevent bacterial growth, specifically those that cause legionnaire's disease.

I've seen systems with a tank, heat pump and solar panels, where the system automagically drives up the temperature to kill bacteria every once in a while. Supposedly that's more efficient than keeping it hot all the time.

Its complicated, as at first you ran as how as you could to maximize the amount of hot water you had, then there was a period where it was seen as more energy efficient and prevented burns running at lower temperatures. "Modern" practice in this case started somewhere around 1970 where you have a termostatic valve (also called a Tempered Water System) to regulate the temperature of the resulting hot water to non-scalding levels.

If the link was optional, so they could either share heat movement or discard waste heat, then there’d be plenty of room for an algorithm to sort it out.

But if the link is optional (i.e. no storage), it's unlikely the timing works out often enough to justify the increased complexity or cost. (E.g. if the time overlap of water heating AND fridge cooling is 25% (which seems pretty high to me), you'd only get that percentage of the energy savings.

Maybe you can pipe all your heat and cold to central locations and have some kind of heat broker. There’s also a lot of heat in your attic.

perhaps maybe in a large tank of a thermally dense fluid.

I think it won't be so bad if only there were some standards. ie. a hot loop, a cold loop. And appliances can just use the standard sockets to hook into these energy sources/sinks.

I can imagine having modular heat sinks apart from the heat pumps which your various household heat pumps could tap into: a warmth sink your cooling devices dump to and warming devices draw from; a coolth sink that does the reverse. It would likely be expensive and you'd need a way to dump the surplus of either outside the building, but I don't imagine there would be much maintenance cost. You wouldn't need moving parts, just a liquid to move heat quickly within the sink via convection. Or maybe a chunk of aluminum would do.

The thermodynamic efficiency of the heat pump is higher the higher the temperature of the cold side, so at least in places where the outside temperature is warmer than the fridge or freezer this wouldn't be a win. This is why you can improve the performance of a heat pump by having the outside circuit run under the ground where the temperature is warmer.

There's also the point that the fridge/freezer is already a heat pump dumping its heat into your house.

Right could we focus that? I'm not sure how much heat it is but I dont like the idea of the fridge just dumping heat into my kitchen while the air pumps push cold air into the same room

The cooler your kitchen is to begin with, the less heat the fridge needs to dump.

I think that’s why heat pump water heaters are particularly popular in places that predominantly have A/C loads.

In cold country, the « inefficiency » of resistive/combustion hot water heating is still household heat. Just pricier/less efficient than your (typically) natural gas furnace.

In cold countries a typical new installation will have an air-to-water or ground-to-water heat-pump for heating with an integrated hot water tank (actually more like a warm water tank for efficiency reasons). Hot water is a considerable fraction of energy cost in a well insulated home and reducing it many-fold by the typical COP of a heat pump makes a lot of sense.

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