Hacker News Reader

Comment by plater | Hacker News Reader

Not sure what a cold basement has to do with it? You suck in air from outside, extract the energy, and blow out colder air. The warm water is stored in an isolated tank (sure if the basement is cold it might cool off slightly quicker, but you warm up the cold basement which is a good thing). It is then used for heating the house via radiators as well as for warm tap/shower water.

We use it to warm a 240m2 house with four inhabitants. It works well down to -10 to -15C.

During the night we pay only have the price per kWh to the utility company (as they have excess capacity during that period).

It works great and it pays itself back quite fast.

It's not controlled by any cloud. You can regulate on the control panel how and when you want it to run and it adapts with the weather too if you want.

View on HN

The problem is in "extract the energy".

If your refrigerant can reach -20 in your system, and it's 30C outside, then your refrigerant is exchanging 50C of energy per cycle. But if it's -15C outside, the best you can achieve is 5C of energy exchanged per cycle. And at -20C outside, you can't exchange heat at all.

That's a bit hand-wavey and not exact units, but I think it accurately describes the problem.

What the above poster is saying is that as temperatures drop, the system gets less and less efficient. For some environments, it may become cheaper to use resistive electric instead.

The 'Technology Connections' channel did a series of videos about heat pumps that are fascinating and I highly recommend: https://www.youtube.com/channel/UCy0tKL1T7wFoYcxCe0xjN6Q.

That is why they are hybrid heat pump water heaters. When the air is insufficiently warm to heat your water, the heat pump can switch back to traditional electric heater rods.

They do run about 2x the price of a non-hybrid (My local hardware store has an 80 gallon hybrid for $2250 vs $1080 for the non-hybrid) but my power company is offering a $500 instant rebate as well as the county offering a $300 rebate, so in my case, it's a few hundred extra bucks for a few hundred dollar annual reduction in my power bill.

Wow 80 gallons is huge. I can see it making sense at that volume.

I find the benefits to be marginal when using an average 40 gallon or less and only using it for 1-2 people. The use of water conserving fixtures further reduces the benefits for smaller tanks with fewer people (not sure they even make them smaller than 40 with the heat pump). With the exception being for people in hot climates with the tank in the garage or similar space.

"Not sure what a cold basement has to do with it? You suck in air from outside, extract the energy, and blow out colder air."

The water heaters I saw were not running ducts to the outside, they use the room air. The point is that heat pump efficiency is tied to the ambient temperature. The hotter the air, the easier it is to collect the heat from it. So when it gets cold enough the "emergency" or "auxiliary" heating coil starts to be used (resistance based). This is very common in cold climates. My house heat pump auxiliary tends to kick in when the tempature gets below 40. This would also apply to water heaters if using outside air than the warmer basement air. Of course you are heating that basement air with another source, so that efficiency should be evaluated too (likely a heat pump because if you have a gas furnace, one would be likely to have a gas water heater too).

"It's not controlled by any cloud. You can regulate on the control panel how and when you want it to run and it adapts with the weather too if you want."

The proposal in the article is that smart water heaters could be controlled by the utility companies.

"It is then used for heating the house via radiators as well as for warm tap/shower water."

That is one type of system. The article is talking the US, in which the type of system you describe is relatively rare. The 10 million annual water heaters they reference are overwhelmingly used just for tap/shower water.

Reply on news.ycombinator.com