Hacker News Reader

Comment by moralestapia | Hacker News Reader

If heat pumps are pretty much "fridges with reversed orientation" why is it that they're so much more expensive compared to them?

Genuinely curious about this. Do they have to use different materials? Is it because they have to be outdoors? What is it?

View on HN

Fridges cool a smaller contained space and don’t need as big of a compressor. ACs need to dissipate a lot more heat and you need to move that heat outside. You can get a decent mini split AC for $500 these days, and pretty much all mini splits have the valve to reverse the flow to make it heat pump.

But I guess the $500 won't be enough to warm your house in the winter ... ?

A $500 minisplit is often a ~6,000BTU unit. Most houses require more like 48,000BTU depending on their size, insulation and climate. More expensive mini splits use higher pressure and fancy inverter technology so that they can deliver 100% of their listed capacity at very cold temps where the $500 one is likely downrated below something like 15°F

I think two things. One is just size. The other is complexity.

A fridge uses a few hundred watts. A HVAC heat pump uses a couple thousand watts.

A fridge has a simple hermetically sealed compressor. With a condenser and evaporator that are just long tubes. All the connections are brazed at the factory. Dirt simple.

A heat pump has a external motor driving a compressor. So you need seals on the shaft. And then the condenser and evaporator are more complex. Some designs need blower to blow air over the evaporator. And because it's reversible you need valves as well. More complex.

Your house is a lot larger than a fridge. Also, fridges rarely have things like windows in them, and do not usually contain breathing humans who need their air ventilated.

Size, your fridge needs a compressor and cooling system that is an order of magnitude or two less powerful than something needed to heat/cool your house. Even a car aur conditioning system is way bigger and more powerful than a fridge/freezer.

heat pumps are bidirectional - they can move heat into or out of the same space. fridges are unidirectional.

The ability to be bidirectional only requires a $20 valve (first result from a search is https://www.amazon.com/Fdit-Conditioning-Reversing-Refrigera...). That's not the reason for the cost.

I don't believe that this is correct. Yes, you need the valve.

But at each end you need a device capable of either radiating heat to the atmosphere or absorbing heat from the atmosphere. Although the principles behind both processes are similar, heat pumps & minisplits (the indoor part of the system) are fairly distinct from each other.

Others have already mentioned the much greater volume of air that a heat pump typically provides service too, additionally.

Reply on news.ycombinator.com