this post was submitted on 22 Nov 2023
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I think it's everywhere but in the US the water comes already hot, in Europe the dishwasher heats it up from the regular cold water.
Yeah the hot water way seems better.
It's because our dishwashers are massively more efficient and environmentally friendly.
They use very little water, which they heat up themselves to save energy
The only drawback is they take longer to clean the dishes, up to three hours
But nobody would be so selfish as to demand their dishes be done quickly over preserving our environment, would they?
Why 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 energy calculations are forgetting the energy cost of the pre-heated water, it's the appliance equivalent of an offshore tax haven!
Do European homes not have hot water heaters?
They do, they're just not connected to the dishwasher so don't need to be factored into its energy usage.
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)
I couldn't explain it any clearer so I'll fuck off now
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.
I'm confused, our as in European dishwashers are more efficient and environmentally friendly?
But wouldn't it be even more so if the hot water that was stored and not being used was the feed instead of cold water? That was the temperature increase is minimal and a lot more efficient.
The eco wash is the best wash on my dishwasher.
It has nothing to do with energy efficiency, and everything to do with UK plumbing. A building's hot water system is presumed to be subject to environmental contamination, and not considered potable. Only the cold water supply is considered potable.
It's the same reason why they have separate taps for hot and cold water, while the US uses mixing taps almost everywhere.
The majority of our hot water is on-demand so no. Also, is it more efficient to heat the water, pump it through a potentially cold pipe, only to have to reheat it again? Nope,just heat it where you need it, and with a lower wattage heater
Who is "our"? I don't know where you live dude
everyone, ever. unless you are running some industrial operations that require constant hot water, there is no reason for even a large family to be using water continuously, and hot at that
Heating a volume of water a given number of degrees takes the same amount of energy regardless. Using a lower wattage heater is just going to make it take longer, not save any power.
Edit to explain in detail -
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)
No idea how Americans can't understand that most of the developed world is decades ahead of them environmentally 😂
I understand the difference, I was pointing out the wattage thing doesn't really make sense.
Using gas to heat water is considerably cheaper than using resistive electric. Especially when the electric was historically provided by coal or gas anyway.
Burning gas to heat water into steam to turn a turbine to turn a generator to pump electricity to a resistive element inside a dishwasher is not nearly as efficient as just burning the gas inside a water heater and sending it to the dishwasher. The heat losses incurred while passing the water from a gas heater to the dishwasher are a tiny fraction of the losses incurred in the convoluted processes involved in traditional electrical generation.
No.
The reason your dishwashers use cold water is because your hot water supply is not presumed to be potable water.
EU regulations allows for gravity-fed hot water tanks in certain jurisdictions, open to atmospheric pressure, and thus possible contamination. They did this to prevent the possibility of exploding boilers.
Since the building's hot water supply is presumed unsafe, dishwashers are required to use the safe, cold water supply to generate their own safe, hot water.
US regulations do not allow hot water to be held at atmospheric pressure. We use T&P valves to limit boiler pressures and prevent explosions. Our hot water is not exposed to environmental contaminants, and is presumed potable.
Ha ha ha that's about 50 years out of date bud 😂
It may be 50 years out of date now. It wasn't 50 years out of date when dishwashers became common and the standards were established.
I'm so confused. Whose dishwashers are you talking about? I'm in the US, you're describing every dishwasher I've ever had, except that we always hook it up to the hot water line. Our unit takes very little water, it takes hours to run a load due to efficiency features. It has a heating element inside to take whatever water it gets and keep it hot for the cycle.
I don't really see why it's any less efficient to use the hot water we are already heating with our water heater (which heats much more efficiently than a small electric heater would). The water originally arrives to my house cold, it has to be heated one way or another. My dishwasher is less than 10 feet away from my water heater, water is not losing appreciable heat on the way to the dishwasher.
It depends. Spraying your dishes with water that is prone to legionella doesn’t seem super safe to me. But even assuming that, I have dishwasher programs than run at 70C, which is above what my hot water tank produces.
Besides, isn’t there a heating element in a US dishwasher regardless? Otherwise, it feels like it has got to continually add more hot water to keep the temperature up…
That is the real reason why UK / EU dishwashers use the cold water supply. They don't consider a building's hot water supply to be potable water. Their hot water was once held in atmospheric pressure, gravity-fed tanks, exposed to environmental contamination. Brits treated central hot water as unclean. This is also why they often used separate taps for hot and cold water. If they need clean, hot water, they heat cold water at the point of use.
The US never allowed atmospheric pressure hot water tanks. Our hot water is not exposed to environmental contaminants, and is presumed potable.
As a Dane living in the U.K., I agree. I’d never seen an atmospheric pressure water system growing up but coming here, that seemed the norm. Now, 20 years after, the norm in U.K. new installation is high pressure water systems (so called “system boilers”) so it is changing slowly. But in the U.K. they have an almost mortal fear for high pressure systems, thinking they’ll explode at any moment.
I believe it's mostly due to not having always hot water hookups available where laundry would be or not consistently having hot water always available. (as in on demand, from a large boiler that wouldn't impact the remain of the hot water uses)
Yes in many places, hot water is produced on demand in smaller residences. This means your dishwasher relying on hot water would kill your shower.
I know this seems odd probably to Americans … but many houses in the U.K. doesn’t have the space for a hot water tank.
I'm intrigued- does that mean the UK uses "tankless" water heaters, at least in smaller residences? Are they underspecced that they can't handle a shower at the same time as a (typically) 3.5 gallon dishwasher?
Speaking for The Netherlands here, but I believe our situation is very similar to the UK in this regard. A lot of houses are equipped with a gas-powered tankless heater which supplies both central heating and hot water. These come in various sizes and output capacities, so you can install one which comfortably suits your needs. Using hot water for multiple things at once shouldn't be an issue if you have the right heater installed.
Same in the U.K. Although my experience is that you’ll have to have a gas boiler with some buffer (40L say) to not experience a pressure drop of hot water when somebody else washes their hands. It doesn’t matter how often I’ve heard a plumber say “you won’t feel the pressure drop with this bad boy installed”; you always feel it.
In the UK your dishwasher is typically connected only to the cold water intake, so that's not a problem unless you have multiple showers in your house... that said, water heaters are often limited to either heating or hot water (not both at the same time), but that's not an issue in practice since you're not going to be using the hot water for long periods of time.
That seems way less efficient and more prone to issue than just having a central appliance that’s responsible for making hot water.
It is, but the dishwasher has to have a water heater in it. It has to heat water to a temp that you shouldn't keep you got water tank at and heats throughout the cycle.
Your clothes washer (generally) also has a built in water heater.
My tank is 60° and my washing machine is 40° and dishwasher is 70° at a maximum. A lot more efficient to have a hot water feed to these that have them increase the temperature 30-50°.
In most places in Europe, hot water that’s been stored is treated with some suspicion. Besides, having a heating element is probably the least error prone thing you could make.
That probably means that the hot water tank needs to be larger though. Guess it depends on the heating source though
Do they not have mixing valves where you are?
Genuinely for the longest time you'd only ever see mixer taps at a kitchen sink, they were not allowed anywhere else for fear of stagnant hot water contamination.
Hundreds of engineers and scientists who designed modern dishwashers disagree with you
I don't think you understand how it works if you think there is an opportunity for disagreement here.
It is more efficient to heat hot water to hotter water than it is to heat cold water to the hotter level. If I have a tank of hot water doing nothing why wouldn't it be more efficient to use that instead of cold water?
You misunderstand on-demand hot water
ya no, I think you might be the one overlooking something here, you talk about efficiency, but leave out the energy needed to heat the water to "hot Water" in the first place, all you did was split the work, and ignore everything leading up to the second work step.
you are getting water from the tap, let's say 10°C and need it heated to 70°C, it will always require the same amount of energy input (10°C->40°C->70°C is the same amount of energy as 10°C->70°c), but if you centrally heat part of it you will lose energy while you store it, and during transport.
My air to water heater operates at 4.3:1 efficiency and heats to 60°. It stores it and loses 1° a day. The dishwasher would need to heat the water just 10° more given the tank is in daily usage.
Otherwise the dishwasher would have to do it on in a faster timeframe. This is why electric power showers are more expensive to run than mains showers, they have to heat quicker which takes more energy.
fundamentally, water takes the same amount of energy to heat up, no mater the time frame, most commonly the "mains showers" are cheaper to heat because many run on some combustible
Yes but we are not heating water with 100% efficiency. So it's not the same, it is cheaper and more energy efficient for me to use my air to water heater to heat water and store it than it is to burn coal to generate electricity to heat an element to bring water to the same temperature
ya, but you have energy productions other than coal
Dump a kettle of boiling water in the dishwasher before you start the cycle.
That wouldn't work with every dishwasher I've ever had. They all start the cycle by draining any liquid in the dishwasher before they fill to run the first pre-wash cycle.
Oh. Admitably I'm not an expert, I'm still ony first ever dishwasher. I did that when our water heater died and it seemed to help.