Comment on Heat-pump water heaters are a winner for the climate — and your wallet
Nollij@sopuli.xyz 10 months agoWhy would that save energy? It’s going to be resistive heat, which at best matches the water heater. Some models in the US include a heating element, but it’s more of a convenience. The vast majority are simply connected to the hot water line, since that’s why it’s there.
Also, let’s talk numbers. From what I could Google, EU dishwashers use 1-1.5KWh/load, while US dishwashers use an average of 1.15KWh/load. Certainly there are more efficient models, but this shows that there isn’t a significant difference in energy usage between them.
Your system - Add cold water to water heater and heat (using energy). Keep at required heat until needed, could be a whole day or more (using energy). Pump hot water through cold pipes to dishwasher, losing energy. Reheat (using energy)
Our system - add cold water to device directly. Heat (using energy)
Your criticism is accurate in terms of thermodynamic efficiency, where we have to account for all energy entering and leaving the system.
But we are talking about economic efficiency, where we only count the energy passing through the wires. In the context of a thread on heat pump water heaters, the distinction is important: heat pumps use a small amount of electrical energy to scavenge a large amount of thermal energy from the environment. Thermodynamically, we have to count that energy; economically, that energy is “free” for the taking.
I have never heard of a “heat pump dishwasher”; the dishwasher is using a resistive heat source, and not the “free” energy from the environment.
When you understand why heat pumps are commonly 200% to 300% economically efficient (without breaking the laws of thermodynamics or becoming perpetual motion machines), you will understand why heating cold water in a dishwasher can be less efficient than heating it in a water heater.
orrk@lemmy.world 10 months ago
your energy calculations are forgetting the energy cost of the pre-heated water, it’s the appliance equivalent of an offshore tax haven!
zalgotext@sh.itjust.works 10 months ago
Do European homes not have hot water heaters?
Liquid_Fire@lemmy.world 10 months ago
They do, they’re just not connected to the dishwasher so don’t need to be factored into its energy usage.