Today, I washed my hands at the kitchen sink while making lunch, and was unexpectedly chastised by my partner because she had just started the dishwasher. Apparently, she has recently come across something that has convinced her that running the hot water in your sink means the water that enters your dishwasher will be hotter (checks out), and thus the dishes will be cleaner. I tried searching startpage to see if this is real, but I couldn’t make it through even one llm-slop-ticle about it before my eyes rolled back into my head, so here I am asking y’all: chat, is this real? Doesn’t my dishwasher have a heating element? Is there any meaningful difference between heating the water with a hot water heater vs a dishwasher, other than I can’t use my sink while the dishwasher is filling and I have to pour a bunch of cool water down the drain?
Yes. For a typical American dishwasher installation, anyway, where the dishwasher is connected to the hot water line which is in turn connected to your storage tank water heater.
First, your water heater is already full of hot water that you’ve already paid the energy cost to heat up. It’s therefore nonsensical to deliberately pump cold water from the line into your dishwasher only for the dishwasher to have to heat it up (using energy/money) and then have your water heater also heat up the displaced water inside it that was just pumped out.
On dumb dishwashers that don’t do anything to sense the internal water temperature, this will also moderately improve your wash performance. (GE calls this feature “Automatic HotStart,” for instance. Other brands call it something different. These machines will pause and run their little internal water heater coil at full blast until their current charge of water is at a temperature their programming considers acceptable to start the cycle. There is no performance detriment in this case, but it will cost you time and more energy/money.)
Modern dishwashers use a surprisingly small amount of water per cycle. Usually somewhere around 3.5 gallons for the entire thing from start to finish. This is contrary to what your parents think, who will insist to you they can totally hand wash an entire sink of dishes with “less water” than that wasteful dishwasher, which they absolutely cannot. Your dishwasher is not consuming water the entire time it is running. It will fill and recirculate that fill’s worth of water inside itself, usually going through 3 or 4 phases of filling and draining throughout the cycle to arrive that that final water usage figure of around three and a half gallons. Therefore it does not readily have access to hot water unless you’ve primed it by running your sink, as what used to be hot water in your pipes rapidly cools off when said water is not flowing. Your dishwasher probably will not draw in enough water in one fill cycle to purge all of the standing cold water that’s in the pipe unless the distance from your water heater to the dishwasher is unrealistically short.
(The latter actually is the case for the dishwasher at my office, which is fed by an undersink “low boy” water heater that’s directly next to it and only separated from the dishwasher by a grand total of about 18 inches of pipe.)
your water heater is already full of hot water that you’ve already paid the energy cost to heat up. It’s therefore nonsensical to deliberately pump cold water from the line into your dishwasher only for the dishwasher to have to heat it up
The hot water will be missing from the tank and has to be replaced with cold water that needs to be heated up again. So shouldn’t the amount of water that needs to be heated be the same in the end?
More than that, you will be pumping more water to flush the cold water from the line, which is more energy. You will use more water overall because that cold water will all be going down the drain. You will then have to heat up enough water to match the cold water you flushed from the line plus the water used to wash your dishes to reheat your water tank. If, from an energy/cost perspective, that is less than what it would cost for the likely less efficient water heater in your dishwasher to heat the water, then you have saved some money. If it isn’t, and your dishes are already coming out clean, then you are neither getting a better nor more efficient cleaning job done.
Technology Connections (youtube channel) says yes run until it’s hot IF you have trouble with dirty dishes, iirc. I think the bigger thing is to clean your debris trap. Also you should be fine washing your hands. The dishwasher will stiill pull as much hot water as it needs, they are typically only hooked up to hot water so it’s not like you having the tap on mixes it up or anything.
The Technology Connections guy on YouTube thinks so
If you live in the US. Dishwashers works very differently in Europe for example.
Ah thanks, I was beginning to question everything I thought I knew about how my dishwasher works.
Reliably having 3 phase 230 V (thus 400 V delta between two phases) makes a big difference from single phase 120 V. Usually rated at 16 A too, available from each phase pair.
Almost 10 kW available at the peak (although almost nothing made for regular homes use it that much, aside from electric cars).
https://everycalculators.com/kilowatt-calculator-3-phase.html
As compared to a US outlet typically limited to 20 A at 120 V in one phase, giving you 2,4 kW (a quarter) or 15 A at 1,8 kW.
Indeed. Both apartments I’ve lived in (in Europe) had the dishwasher hooked up to the cold water feed, so preheating the water would absolutely do nothing at all.
He also said that with modern detergents, you should do colder washes anyway, as the hot water has little benefit.
I noticed an improvement preheating the water. It’s also probably more energy efficient than letting the dishwasher do it, if it can at all.
They both have electric heating elements so identical efficiency. The advantage is that priming hot water means it’s hotter for longer and reaches a higher peak temperature in the dishwasher, which means more effective cleaning.
The heating element in the dishwasher at best maintains the water at a warm temperature. It will not instantly heat the water up for the very short first cycle of prewashing, just like if you suddenly put cold water into a water heater it wouldn’t be suddenly hot. It takes a lot of energy/time to warm water up.
My statement is taken directly from TCs video where he graphed the temps for primed vs unprimed washing. Priming the water leads to hotter (higher peak) and more area under the curve. Not priming leads to lower peak temps, but it’s certainly warmer than at best warm. The element will heat your water up even hotter than it comes out of the primed line, assuming you primed it.
When I open my dishwasher it is absolutely steaming and the dishes are too hot to handle. This temperature is above the setting on the hot water heater, ergo this extra heat can only have come from the heating element. It provides significant extra heat.
You are correct that it will take significantly longer than the prewash cycle’s duration and is not something to be relied on if you want optimal washing.
Not every home heats their water with electric resistance heating. My previous place used natural gas for hot water, and my current home has a heat pump water heater, which has an effective efficiency much greater than a normal electric water heater.
Technically I think he says that IF your dishes don’t already get clean.
Yeah if you aren’t having issues why make the extra step
Yes. I’ve watched all of them in their entirety
It is real.
Here’s the biggest rabbit hole you can go down. Alec has multiple videos just about dishwashers.
You can curse me later when you’ve spent 2 hours learning about Christmas light colours and LEDs.
Just mind that his is a mostly American-centric view. Here in Europe my dishwasher is hooked up to the cold water. Energy usage is negligible.
European dishwashers seem to be designed vastly differently to American ones.
Water efficiency goals specifically; I’ve seen consumption of 7 US gallons (26.5 litres) per cycle called “eco” on American dishwashers, where mine (Miele) claims 12 litres on the heaviest cycle.
The manual for mine calls out connecting to hot water as a way to increase efficiency if you have hot water with a >1 COP, mentioning solar or heat pump cylinders.
A professional I asked noted though that for protein-heavy residues (like egg), hot water could bake it on before the dishwasher starts cleaning it.
A professional I asked noted though that for protein-heavy residues (like egg), hot water could bake it on before the dishwasher starts cleaning it.
And a little detergent in the pre-wash helps a lot with that. (My machine, a Bosch, does not have a pre-wash compartment so I just spill a little when I fill it.)
thank you. I’m supremely annoyed that I’m reduced to posting to fedi to find good sources for a basic adulting question, but I’m equally grateful when someone comes through for me.
Always try to be one of today’s lucky 10,000: https://xkcd.com/1053/
If you read the dishwasher manual you’d find it in there.
It’s my understanding that dishwashers generally do not have a heating element, so the hot water is coming directly from your water heater. I believe you should run the sink water till it gets hot before starting the dishwasher, and not run cold water until the dishwasher stops filling. They generally share the same line. If the dishwasher had a dedicated water line I imagine it wouldn’t make a difference, but most inexpensive dishwasher models and homes share with the sink.
And yeah, the hot water cleans better, especially if you have stuck on food.
Ok edit: your dishwasher DOES have a heating element, but it’s not enough on its own, so the logic still applies. If you want the full deep dive check this out, I love this guy’s videos. https://youtu.be/jHP942Livy0?is=KOvmqlGaLBekpM6Y
Hot and cold absolutely do not use the same line. Dishwashers tap off the hot water pipes to your sink. Dishwashers also commonly have heating elements. Even my $150 landlord special Midea has one.
Running cold water in the sink has no effect on the dishwasher. There’s no cold water connection to the dishwasher.
Yeah, I was wrong about the heating element, but I would still run the water hot. Check out the video I posted
Yeah, I’m echoing my knowledge from that video as well.
Ummm…what are all the USA people in here talking about?
I’ve installed and replaced my own dishwashers 4 or 5 times over the years and helped friends and family members many times. They only last about 5-7 years so it happens often enough for it to be routine for me.
The water supply is connected to the cold water. The dishwasher has it’s own heating element. It heats the water to “sanitizing” levels. There’s even a sanitize button.
That water gets so hot that I can’t open the door mid-cycle with my reading glasses or they’ll get completely fogged up.
The sanitize button is not a new development…
Not USA but North America and we ours is hooked to hot water. Has to do with having a shitty heating element since they wanted it to be plugged into 120 15A circuit.
I checked under my cabinet just recently, and the dishwasher is indisputably connected to the hot water line.
It was like this in the last place, too, and my sibling’s place.
They have 120 volt electricity, it can’t heat quickly enough, so the wash cycle will begin with less than optimal temperature and take a while to get to sanitizing temperature.
So they feed it water from the hot water tank, which isn’t hot enough usually but the heating element can get it up to temp much faster at 120v.
Many modern dishwashers only have a cold water inlet so it literally does not get water from the hot water tank.
120V really doesn’t matter that much as they are commonly on their own 20A breaker. At 1500W of power it can heat 1.5L of water from 60F to 168F in 1 minute.
Where are you getting your information? Can you find an example of a dishwasher where the installation manual says to use cold water?
And the claim that 120v doesn’t matter is baseless.
At 1500W of power it can heat 1.5L of water from 60F to 168F in 1 minute
Lol no it can’t, are you nuts? That’s physically impossible and mathematically incorrect by a factor of more than 4.
My information is based on installing 3 dishwashers in the US that all said to either connect to cold water or give an option for cold water or hot water
Voltage doesn’t matter for heating what matters is overall power. 1500W at 120V and 1500W at 450V both heat at the same rate. The advantage of higher voltage home systems is they usually allow higher power since current is generally the limiting factor but they make higher current breakers for 120V
I was off on my conversions and did calories instead of joules. Either way it takes 4 minutes to get from 60 to 160 which is still trivial on a multi hour cycle
Here is one that allows either connection but emphasizes increased efficiency with hooking up to cold water. I admitted I was wrong on my conversions will you admit you are wrong that they have to hook up to hot water?
Here is one that allows either connection but emphasizes increased efficiency with hooking up to cold water.
"For fastest possible wash times, with slightly higher energy consumption, connect the dishwasher to a hot water source. Hot water supply specifications: Recommended temperature: 120 °F (49 °C), max 140.
will you admit you are wrong that they have to hook up to hot water?
So weird. You want to be right? Okay, you’re right, you don’t have to hookup to a hot water, it’s recommended and commonly done… I don’t think I claimed you have to… The reason it’s recommended to hook to hot water is because it’s slower at 120v. They all allow you to do whatever the hell you want, but American tap water (which doesn’t sit in a rooftop tank that gets warm as it sits around, but that doesn’t apply much in the northern hemisphere) is typically not ideal.
You want your dishwasher to take longer? hook it up to cold water and save some electricity, you do you.
1500W at 120V and 1500W at 450V
Why would I use 1500W at 240V? My dishwasher has a 2100W heating element, which heats 40% faster, that’s not negligible. You can get higher wattage models.
You installed 3 dishwashers? That doesn’t make you an expert on international dishwashers, it just makes you a homeowner. Jesus, you don’t have to just argue for the sake of arguing, just say “I learned something dumb today” and move on with your life.
You said “Many modern dishwashers only have a cold water inlet so it literally does not get water from the hot water tank” and then provided an example that says what I said, if you want a faster wash use hot water.
The reason most of the world uses the cold tap for dishwasher hookup is because they use a higher wattage heating element, which they can operate safely because they use 220V power. I didn’t come up with the 1500W limit, you did, don’t move the goal posts.
They aren’t all designed to connect to your cold water line. In fact, most seem to specifically instruct you to connect to your hot water intake. You are right that most in the US have water heaters, but that doesn’t mean that the machine’s cycles will heat the water effectively for the full duration of the wash.
Maytag, Whirlpool, GE, and LG all recommend you connect to hotwater. Bausch recommends cold. Depending on your machines cycles as well, the heating element may not get the water hot enough to make effective use of the pre-wash cycle. If your machine is connected to the hot water line and you purge the cold water from the line, you will see a more effective pre-wash (and thus more effective wash) than if you let it run cold.
North America vs Europe, North America cannot heat the water fast enough due to the limit of being on a 120V and 15 A circuit.
You probably have higher voltage and more power going to that heating element, so the cold water line is fine. In the US, the heating elements in our dishwasher can’t draw enough power for cold water, so it needs to start with hot water.
Also, 5-7 years is… Pretty bad. It’s not uncommon for dishwashers to last multiple decades here. I’m pretty sure mine is at least 20 years old (and ugly lol)
I’m not sure why you’re being chastised for using water while the dishwasher is filling. That is a non sequitur. But for most people, the answer is probably yes, it moderately improves performance.
I’m just telling you how I discovered his piece of information. It was a very normal household interaction but with a dash of source amnesia.
edit: I misunderstood how you meant non sequitur. I turned the water cooler to wash my hands because I have soft hands and cannot tolerate the water at full heat.
That won’t make a difference. You need to pull the hot water up before you start the dishwasher, but after that you can run the sink with pure cold water and it won’t change the dishwasher temp at all. The dishwasher is connect to the hot water pipe directly
I had a suspicion. Water coming out of pipes is just kinda black magic for many people (not to be negative, irs just not worth learning about). Because the cold and hot water don’t combine until inside the tap, it doesn’t matter for the dishwasher. But pulling hot water out of the tap gets hot water to wherever the dishwasher ties in - in most kitchens, but not always in fancy ones.
Depends on how the dishwasher is connected, but I would assume it’s connected to the cold water. At least that’s the common way in Scandinavia
That’s a good point. I think in the US it’s connected to the hot water line, so the answer to this probably differs by region.
100% does considering my country doesn’t even have hot water lines
You don’t see the giant bent calrod in the bottom?
Doesn’t work for me. My dishwasher water line is a separate line. It has it’s own valve at the manifold.
It’s just to get the hot water flowing in the hot water line. So your dishwasher doesn’t get as much cold water as it would other wise.
For my specific setup, the hot water from the sink is hot immediately if I’ve already been running the dishwasher. So, just by that, I’d assume that clearing out all of the cooler temperature water by pre-running the sink could provide some benefit. As to whether it makes a substantial difference or not, I don’t know. I’d say it’s certainly possible though.
This is the correct answer. Turning on the hot water before the dish washer starts clears out the not-too-hot water that was in the line. They don’t use that much water so it apparently makes a difference.
ETA: but washing your hands (with hot water I presume?) won’t impact the machine.
Yes, hot water cleans better, and results in dryer dishes due to condensation drying.
The heating element in a dishwasher is weak. It can’t heat water up that well since it’s limited to 120v and probably not even 20A. The first rinse cycle will effectively be cold water.
Your tank water heater is 240v and has a dedicated circuit, probably 30A. It can heat a lot of water, fast.
If you’re concerned about wasting water you can get a small tank or tankless hot water heater under the sink. Your water will then be hot nearly always.
Some wash cycles will both purge more water and heat it up longer to compensate.
The water heater tank is also insulated better than your dishwasher because it doesn’t have a big door on it. Even if the heating elements were the same, the water heater would be more efficient at heating water and keeping it hot due to less energy loss.
They are both 100% efficient, although 240v has less loss over the same amount of length.
Insulation doesn’t matter too much if you’re just dumping the water down the drain.
But you’re right a dishwasher doesn’t have nearly as much insulating properties. The heat element in one is basically there to counter act the heat loss.
Different country and all, but both are 240v in my neck of the woods, and since I live on tank water with mains water supply as a back up. Coupled with what I believe may be the most circuitous route from hot water cylinder to kitchen tap on earth (3 meters point to point yet seeming out to the local post office and back based on how long it takes to get hot water out of the tap) we use the Dishwasher to heat our water.
Which is all a very verbose way of saying “It depends on your circumstance…”
US doesn’t get 240v at most outlets unfortunately. My hot water to the kitchen is 50-60 ft as it goes up to the 2nd floor attic, across maybe 45 ft, then back down.
Takes a couple of litres of water before we get hot water. I have filled the kettle twice before the water was too hot to hold my hand under before. That’s roughly 3.5 litres of water just to get to the hot stuff. Luckily boiling water was a key consideration in our voltage level (actually that’s something I made up on the spot 😜) but more power equals good so… Its weird that this is a field that the US apparently just decided to let a large portion of the world be more “powerful” almost expect that if there were parts of the world settling on 240v the US would arbitarily decide they needed 500v.
Are you going to pay to change the entire electrical grid, rewire every house, replace every appliance, etc.
It would be an unimaginably massive undertaking for very little gain.
The US electrical system was well-established when Europe had to rebuild post-WWII.
I always thought the decision was made a lot earlier than that. Wasn’t it largely due to Westinghouse that America settled on 60hz 110v? Where AEG pushed 50hz 220v in Europe? That was late 1800s early 1900s from memory. World war 2 may have resulted in some rebuilding but they rebuilt along lines that were already established, the bump up to 230-240v with time and gained efficiencies is mirrored in the US with the move towards 120v at least that’s what I recall, but I must admit that outside of some Technology Connections videos I haven’t delved too deeply in recent years (and yes I know Alec would say the US technically already sends 240v but that’s not what your general appliances are rated to).
You guys don’t hand wash?
That’s the dumbest thing I ever heard.
It’s an old thing that isn’t necessarily needed anymore. Technology Connections has a video about it.
The dishwasher has a booster that brings to steaming- much hotter than your full-home water heater should be set to.
I wouldn’t worry about it. The difference in efficiency is negligible.
Not in the US.
US dishwashers run on a 15 Amp circuit so don’t have a preheat for the first wash. The heater is too slow.
How does that change anything in any meaningful way?
The difference between the hot tap and the cold tap is peanuts compared to the difference of hot tap to dishwasher’s hot.
It’s not even all that more efficient- unless you ignore the water heater’s energy use. There is, though some reason to hook up to the cold side any ways- water heaters can get nasty and the water in them is soaking in that nastiness.
