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Comment by csours | Hacker News Reader

I wonder if octovalve/superbottle is coming to home hvac. I spend energy to make my water hot for showers and washing. I spend energy to make my air cold so I can work without sweating. I spend energy to chill my groceries.

I can't help but feel that coordinating those better would lead to reduced energy usage (thus reduced cost). I don't know if I can feasibly do that right now with residential/consumer tech.

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There would be decreased costs for the entire house as you then no longer need a heat pump in you fridge, water heater, etc if you tried to emulate current hot/cold use. If I designed the system, it would just be a hot and a cold loop that goes throughout the building, with a standard outlet that had both the hot and cold lines present.

For residential, you could install faucets that had a internal heat exchanger that would let you have actually cold or hot water for cheaper than a traditional on-demand water heater. HVAC would be better as individual control of the heat for each room wouldn't add much extra cost, so your south facing rooms can get more cooling without having to freeze everyone in the other rooms.

For restaurants, the system would be particularly attractive as they can have heated/chilled food holding (and work surfaces) and they would be able to be swapped between the two by flicking a switch, so the winter menu (or the lunch menu even) could feature more hot items, and the summer (or dinner) menu more cold things without wasting capital (and space) on equipment that isn't used all the time.

That's a compelling idea. I've seen systems that use essentially solar panels on the roof that heats up a liquid in a loop then goes to a heat exchanger which heats up your water before it goes to the water heater, thus making it so the water heater doesn't have to work as hard.

Could we use the exhaust from your fridge, A/C, furnace, etc. to also heat coolant in a similar system that circulates to your pre-water heater, thus boosting efficiency. Especially if you can dump the exhaust of your fridge outside rather than paying to heat up that air, then cool in down again with your A/C.

The question is if the cost savings would be worth the cost of the system.

> The question is if the cost savings would be worth the cost of the system.

I think it might be, if it was built into the home from the beginning, and the engineering cost was spread out over a large number of units. In other words, I don't expect it to make sense for one homeowner to retrofit their house.

It would make more sense in a condo/apartment/multi-unit/commercial situation.

The closer you get to the equator, the more you're likely to see these. When I was visiting Turkey, I didn't notice them in Istanbul because most rooftops are not visible from the street, but then I visited Safranbolu, a small tourist town, and there almost every century home had this recently-made metallic hot water tank on top which completely destroyed the bucolic architectural character of the town.

One of my uncles here in Canada built a passive system by just putting a giant 1" PVC tube coil on the backside of his roottop to heat up water for the swimming pool, since the swimmable season is pretty short anyway.

Not sure if it’s exactly what you’re talking about, but something similar is pretty common in China as well. I’ve heard people call the system “solar power“, but what they’re referring to is effectively a tank of water on the roof that is heated by the sun which is used as the hot water source for showers, etc.

It is very much solar power, it's just not fancy high-tech photovoltaics.

Solar water heating has been on the decline recently because photovoltaics & heat pumps surpassed them thanks to ridiculous scale & modularity.

A refrigerator consumes 200 watts. That is equivalent to 2-3 old lightbulbs. Sorry, but there is nothing to heat up with this power, even if you could use it 100% efficiently. For reference a water kettle is 1200watts.

I understand the coefficient of performance for a heat pump in a refrigerator can be 5, so consuming 200W of electricity, you can get 1200W of heat. (But that heat won't be hot enough for boiling water.)

It could be hot enough for a pre-stage on a water heater. Many houses don't have the fridge close to the water heater, but some do, and if you were designing a house around a system like this, you could choose your layout.

Sure it only consumes 200W but it moves a lot more than that, which is what can be taken advantage of instead of radiating/convecting it off the back of the fridge

Sounds very interesting! I'd love to learn more if others could chime in

A version of it is a heat pump water heater:

https://www.energy.gov/energysaver/water-heating/heat-pump-w...

In my home, we leave the garage door open when it’s warm so the cold air that the heat pump water heater expels can come into the house.

It’s basically an air conditioner that dumps heat into the water, instead of the outside air.

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