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Comment by porphyra | Hacker News Reader

Interesting but federal tax credits for heat pump water heaters is ending next month [1]. It looks nice though.

For people wondering what's up with the "150 gal virtual" capacity, it's actually a 47 gal water heater but it heats water to a really high temperature (if needed) and mixes it with a thermostatic valve to output the desired temperature. For example if cold water was 15 degrees C and you want the hot water to be 40 degrees C, it can triple its "capacity" by heating it up to 90 degrees C and then mixing one part hot water with 2 parts cold water to give you three times as much hot water. Of course, this comes with a grave penalty to efficiency so you should only do it for "party mode".

[1] https://www.energystar.gov/about/federal-tax-credits

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They must be doing something different to get that much capacity. There’s a bunch of water heaters with mixing valves but they don’t talk about that much capacity. What worries me is, doesn’t higher temperature make your tank rust faster?

> They must be doing something different to get that much capacity.

Yeah, they put the word 'virtual' in front.

I had one of these types of water heaters in an old apartment. Thing worked kinda okay-ish? The main issue was that it was electric and if you used the hot water for really any time at all (bath, dishwasher) then it would pop the fuse from current draws and overheating. Likely this was some issue with the water heater and the electrical system of the apartment.

Still, really annoying when half way through your bath, the fuse would pop, colder water now coming out, and you'd have to completely dry yourself to reset it and wait for whatever got too hot to cool down enough to get the other half of the bath going again.

Sounds like you're using a totally different type of water heater (tankless) and not a heat pump water heater (which has a tank, so even if the fuse pops, the water won't get cold instantly).

Nah, same type. The tank was about half the size of a normal one. The water didn't get cold, just not as hot as I'd liked it to be.

Yeah this sounds disingenuous. I can also see my heater to 70C if I want, that does not increase its size...

It is a scalding hazard though. At 70°C skin burns occur in about a second or so.

The thermostatic valve makes it so that the water that comes out of the water heater is at a more reasonable temperature.

It's supposed to be HOT water, not lukewarm... I have my water heater set to 150F because it makes the hot water last longer, especially in the winter when the incoming supply is barely above freezing, but that doesn't make the tank bigger.

I think what this "smart tank" does is mix the super-hot with cold.

If the automatic mixing feature malfunctions to super-hot, then it could be risky...

I was surprised at how cold "hot" water actually was. I thought it was 60-70, but apparently what feels "hot" is around 45-50. Especially for me, that finds anything beyond my shower's "middle" heat as uncomfortably hot, I must be showering with around 40 C water, which is basically "hot day" hot.

Going less than 55 °C tap temperature / 60 °C in the tank is bad though, at least in larger installations - otherwise, you risk legionella and other microbial infestations [1].

[1] https://www.verbraucherzentrale.sh/pressemeldungen/lebensmit...

Oh definitely, I'm purely talking about "feel".

Then again, I grew up with thermostatic valves in the shower mixer, and a "child safety" latch to restrict it to even less hot water in normal use. I don't see why putting the mixer in the water heater is that different.

Why do you care what size it is though? It's only the capacity that matters. For example a tankless only has a few gallons inside, but that doesn't limit how long of a shower you can take.

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