I do not understand why anyone uses water tanks at all. The tankless or demand-type water heaters tend to be more energy efficient, take significantly smaller space, less parts, and have been in use successfully since the 19th century.
Am I missing something?
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I don't know if this is the reason, but one potential advantage of tank water heaters is the ability to use off-peak power. This can be a big cost advantage, and even if it's less energy efficient it can theoretically be better for the environment by allowing water heaters to act as distributed renewable energy storage. At the moment, though, the efficiency savings probably make tankless water heaters a bit more environmentally friendly.
From a grid-optimization point of view a hot water storage is almost as good as a battery. You can "charge" it whenever unreliable local energy sources (wind, the solar panel on your roof) are producing in excess. This helps to consume electricity more locally, and reduces the losses and infrastructure required to transport electricity.
Energy storage. A tank of water can be used to store and release heat from a multitude of sources. Compost, wood, solar, wind, electric, heat pump, gasoline.. The best part is hybrid sources. Couple a wood burner to a solar heater and a heat pump and run the pipes through a fireplace as well. Combined with an intelligent mixer you can heat up your shower and tap water and your house.
Apparently tankless water heaters draw something crazy like thirty kilowatts, which is more than the rest of your appliances combined and probably requires a landlord to agree to rewiring. Maybe if you have a natural gas supply it's more practical.
This is also why it's rare to fast charge an EV at home—that can top fifty kilowatts.
If you need 10 liters/min of hot water, got an intake temperature of 10C and want to heat it to 40C (hot shower, my gf loves them), it takes about 21kW[1].
If you use a water-saving shower head that only uses 5 liters/min that cuts in half, but still quite a lot of peak energy.
Now you might argue that you only use it for a wee bit while you shower. And that is true. However here in Norway they will soon introduce a peak demand pricing element on the electricity bill, which means you'd likely end up paying a lot for that instant heating.
In contrast, showers are typically taken in the morning, so the water heater tank could get nice and hot during the night when electricity prices are low, and then kept off during the day.
[1]: https://bloglocation.com/art/water-heating-calculator-for-ti... (Water Heating Power Calculator)
Natural gas tankless exists: https://www.homedepot.com/b/Plumbing-Water-Heaters-Tankless-...
I have this in my house.
Electric tankless water heaters draw a massive amount of power. A water heater with a tank is actually quite insulated and efficient. If I am not home and basically use no hot water it can go days without dropping more than a couple degrees. With the heat pump I think I spend about $7 a month on my heat pump water heater.
They take longer for the water to heat up.
And a heat pump is going to beat it hands down.
I don't understand. One of them takes hours the other one produces hot water as soon as you open the tap.
There is a small "spin up" time to get everything hot and hot water pouring out. This is pretty minimal IMHO, and dwarfed by the amount of time it takes to get cold water flushed out of the pipes. Recirculating systems exist to prevent this problem- for tank/tankless systems- but according to some have drawbacks around increasing wear on your pipes.
I have a natural gas tankless system and find this to be a non-issue.