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Comment by henrikberggren | Hacker News Reader

Heat pumps are really amazing and can be used both for cooling and heating with the same hardware. If it wasn't for natural gas they would make sense for many American homes

https://medium.com/hackernoon/the-magic-of-heat-pumps-fd2721...

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They even beat natural gas on operating cost for many installs these days:

Gas Furnace: 1 Therm = 29.3kWh, Cost $1.5 (at least where I live), Furnace efficiency = 95%. Cost per kWh heat delivered = $0.053.

Heat Pump (COP=5, EER=17): 1 kWh = $0.14, COP =5, Cost per kWh heat delivered = $0.028.

Heat pumps are quite a bit more expensive to install though.

That depends on the cost of electricity, which varies widely across the continent. I ran the numbers a few years ago for central Ontario (napkin math), and natural gas was 2x or 4x the cost (I can't remember). Anyway, definitely not worth it, especially since it falls back to pure electric heat when it can't keep up. And the house is heated with NG anyway, no point in making that energy transfer even less efficient.

(NG fireplace heats the air, air heats the hybrid water heater coils, which heats the water via refrigerant and a noisy compressor. Instead of NG just heating the water directly.)

It might solve some of the solar demand problem. Heat water up during the day when the sun is out, and then store that in an insulated tank.

Have both bath water and central heating run off that.

Makes sense for hot water, but with central heating less so. The energy is mostly needed during the months when there is not a lot of sun, over-sizing the solar collector is financially untenable, as is large enough insulated storage to cover months of heating.

That's already a thing, they just skip the intermediate steps and run the water through black pipes in a sunny spot.

Black pipes are great, it would make some interesting calculations to compare:

Pipes vs. PV taking heat pump characteristics into account, whose efficiency depends on climate. Installation costs, maintenance costs. The Pipes compete for real estate space against the PV, so if pipes are more efficient, does it off set the lost electricity (that would be more expensive, but cheaper than from the grid, but only when it's sunny and you can use the appliances at that time, unless you have a battery which is more capex, so does the interest/repayments on that cost get offset by using electric 24/7). Etc. Etc.

It might need some ML to figure out the best solution. And by ML I mean linear regression.

*if it wasn't for cheap natural gas

Gas is super expensive here in Australia and I plan to try and put 10kw solar on the roof next year. Our hot water is currently gas but I would like to move to electricity. The solar will take 10 years to pay off, I expect adding this to the mix would take another 10 years.

10kW on the roof and a Sanden heat pump hot water set to only run 9-5. I get credit from the Energy Australia for all the summer months which I spend on on the AC heat pump in winter. 25 year warranty on the panels and 20 on the micro inverters. The economics stack up.

Sanden do seem to be the recommended option for this. The problem is if you have a functional gas, the economics rarely stack up to change. If I knew for sure the Sanden would last 10 years yes, but unlike very simple gas water heaters, heat pumps have a disturbingly high failure rate.*

*At least that was the way it seemed from my research, similar situation 6.66kw solar on roof, functional gas hotwater. We do have both Gas and Heat pump house heating/cooling. Heat pump is much cheaper, gas is much faster, even running the largest size possible ducted heat pump split system. I have the gas warm the house from 6am to 8am, then heat pump take over for the rest of the day.

My Daikin is near instant, even when near freezing outside. The compressor is yuge though, so maybe that’s why.

Daikin RXYMQ6AV4A 2 x Daikin FXMQ80PAVE

Also check out https://brighte.com.au/ for an interest free loan for this stuff. backed by Michael Cannon-Brookes.

I have a AO Smith hybrid water heater for the last year and half. Seems to work. Made sense in my case because I wanted to move water heater from the house to the garage. Using a hybrid unit meant I didn't need to plumb gas and figure out how to install a vent.

My suggestion is when you get your solar system installed have the electricians run a 30 amp circuit and disconnect to the water heater location. That way you'll be ready to go. Otherwise you can get stuck if you current water heater dies. You'll be basically forced to replace it with a new gas heater.

Also I saw a youtube video by a off grid nut talking about installing a heat pump water heater. It worked, the only problem it had was the AO Smith unit he was using would test it's heating elements on startup. Which would overload his inverter. I think he fixed that by getting higher resistance heating elements. Otherwise the current draw in heat pump only mode was 300 Watts.

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