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?
Counter-question: Why do you ask this online rather than just testing and seeing if it makes a difference?
Also, does it even give bad results anyways? Because i have never once in my life encountered a dishwasher that didn’t get things squeaky clean almost without even needing detergent. I suspect many people worry about the cleanliness of their dishes for absolutely no reason.
The Technology Connections guy on YouTube thinks so
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.
If you live in the US. Dishwashers works very differently in Europe for example.
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.
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.
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
He also said that with modern detergents, you should do colder washes anyway, as the hot water has little benefit.
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.)
Alec recommends powder detergent, and simply pouring some outside the compartment if there’s not a dedicated one for pre-wash
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.
Yes, as many others have said, there is an explanation for running the hot water before turning on the dishwasher.
However, your partner is wrong. The purpose of running hot water is to prime the hot water pipes. Once that’s done you can go ahead and wash your hands with cold water, because the hot water pipe is already full of hot water.
EDIT: just to elaborate on the above comment:
You can run a mix of hot and cold water all you want, if the hot water is on, it still primes the hot water pipes with hot water; they don’t mix until they join in the faucet itself.
Running the hot water at the faucet will fill the hot water pipe with hot water (hasn’t had time to sit and cool down) so your dishwasher’s initial pre wash cycle has a higher temperature water to begin with as it draws from that same pipe; you’re not starting with slightly cooler water and waiting for the heating element to bring it up to temp (at least not as long, usually dishwasher water is slightly hotter than water heater temp).
Running cold water at the sink does not cool the dishwasher’s intake water down. Cold water isn’t mixed with hot until it’s already in the faucet (after the valves you control at the faucet itself). As long as you run the hot until hot water makes it up to the sink itself, you’re golden no matter what else is going on.
It’s correct if in a country where the dishwasher lacks its own water heater.
Dishwashers in the U.S don’t have water heaters and therefore it’s beneficial to purge lukewarm water from the hot water line before starting the dishwasher, such that the water in the dishwasher is on average hotter and as such more effective at washing the dishes.
Dishwashers in Europe have water heaters and as such it’s entirely irrelevant to do it there
Dishwashers in the U.S don’t have water heaters
Crazy. Mine only takes in cold water. Never even knew ones that take in hot water existed.
You’ll be surprised to hear that U.S washing machines have the same setup, and this is why they talk about “wash cold” or “wash hot” instead of having specific temperatures on their washing programs like 30 C, 40 C, 60 C etc.
How do you do a 90°C wash? Surely the tap water doesn’t get that hot.
The washing machine heats the water, many go up to 95°C
I wouldn’t know unfortunately, I’m in Europe.
As a European that carefully checks the temperature levels indicated on the clothing before washing, I feel like I did it wrong - or at least overly complicated - my whole life now 😅
It’s because the US electrical system sucks and they refused to upgrade it, although reasonably speaking it would be kind of impossible at this point. In order to be able to actually run a heating element you would have to have a special circuit fitted with a decent voltage.
it’s pretty trivial to combine two hots to get back up to 240. It’s common for industrial and some consumer appliances
Ah that makes sense. Cheers.
I live in the US and every dishwasher I ever owned had a heating element, and was capable of heating its own water as needed, depending on what cycle was selected.
Same with my washing machines, my current one even has a steam cycle.
I feel like if dishwashers without heating elements were ever a common thing here, it must have been many decades ago.
Its not so much about that, as it is about the fact that they almost always do an initial “clean-off-the-dirtiest-bits” cycle for around 10 minutes which pulls in (mostly) cold water if your line is not purged like the OP describes. And they don’t use the heating element for this initial cycle.
After the first 10 minutes they flush out the super dirty water with the dirtiest food bits and sauce and whatnot, fill in brand new water, and then run for the duration of the washing cycle (1hr plus) along with the heating element.
So flushing the cold water out of your kitchen water lines AND adding pre-wash detergent (such as some extra powder right in the door, outside of the main closable detergent compartment) helps a ton with this initial 10ish minute cycle.
AND adding pre-wash detergent (such as some extra powder right in the door, outside of the main closable detergent compartment) helps a ton with this initial 10ish minute cycle.
Yup, and that’s why some dishwashers include PreWash text somewhere on the side of the door. Sometimes it’s even on that little flap where you put the pod
I was so sure the answer was going to be no. TIL
So do dishwashers in North America they are just less powerful because of the 120 volt.
The technology connections guy says it helps assuming your dishwasher water connection is at the sink and that the heating element in your washer isn’t very good or efficient.
Detergent packs are kinda wishy-washy (Dishwashers Explained)
Provided your dishwasher is even connected to the warm water line. Typical dishwashers with internal heaters are not. A family member of mine has the dishwasher connected to the warm water line to save on electricity for heating the water (has solarthermal elements on the roof for heating water). Though that can alao mean, that on hot days the warm water line gets too hot for most plastics (like 70°C). A bit inconvenient.
This is primarily a North American concern. A 15amp 120V circuit does not heat water fast enough for the pre-wash cycle to actually have hot water in it. The wash cycle will eventually get hot water but not until at least halfway through, so for half of it you’re washing with cold/warm water at best. Depending on what you’re washing, that may be okay, but the dishwasher will definitely wash better if it is connected to the hot water supply and the hot water pipe is pre-filled with hot water by running the tap at the sink. European dishwashers are often connected to cold, because they have 220V+ heating elements that can deliver a lot more heat energy and they are designed for this. North American dishwashers are not.
The drain does not matter, you can pour whatever you want down the drain. The concern is on the supply-side only, and only applies if the dishwasher is in fact connected to the hot water line (which it should be, in North American installations, but sometimes it is installed wrong)
you can pour whatever you want down the drain
Oh good to know! I was looking for a convenient way to dispose of all the battery acid from my dead 12V batteries 😅
I believe one of the technology connections videos talks about making sure the sink water is hot so the dishwasher can have hot water and not clean with cold water
It might be this video
https://www.youtube.com/watch?v=jHP942Livy0came to post this. note: this works if your dw is on same line feed as sink. if for some reason it’s not, you can’t purge the cool water away.
One thing I just want to point out that’s stated in the video is the hot water is for the pre treat cycle. It always heats up the water for the main wash cycle.
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.)
https://m.youtube.com/watch?v=jHP942Livy0
Here is a video from someone explaining why you should run the water hot before turning on the dishwasher, along with a few other tips.
Upvote for Technology Connections
Is this an American thing? Dishwashers run off the cold water and heat it up themselves over here.
Plumbing code in the UK (and parts of Europe?) allowed/allows hot water tanks to be open to the atmosphere, to prevent boiler explosions. The specific tanks in question are filled via float valve, much like a toilet tank. Being potentially exposed to airborne bacteria, fungi, and other biological material, water out of these tanks is not considered potable. If you have one of these atmospheric pressure heaters, you shouldn’t consume the water, or clean cookware, dishes, and utensils with it. Because they might be installed in buildings with atmospheric pressure hot water, dishwashers in areas with atmospheric pressure hot water tanks are designed to be connected to the potable, cold water supply, rather than a potentially contaminated hot water supply.
US does not allow atmospheric pressure hot water tanks. US tanks are under positive pressure, using T&P valves to protect against excess pressure and boiler explosion. The water in US hot water tanks is considered potable. Dishwashers in the US do have heating elements, but they are designed to be connected to a hot water supply.
This is also why UK taps typically have two spouts. The cold water side is pressurized; the hot water can be gravity fed. This causes two problems: 1. If there is any restriction in the output, cold water can backfeed the hot supply and overfill the hot water tank. 2. non-potable hot water can contaminate the cold water in a mixing valve, rendering the cold water non-potable as well.
It has nothing to do with that. It is about the electrical system in the US not being able to supply water heaters in dishwashers and washing machines. Also the system you are describing is certainly a UK-centric one.
It is about the electrical system in the US not being able to supply water heaters in dishwashers and washing machines.
That is a myth arising from US typical residential circuit standard being 20A @ 120V, or 2400W.
Major appliances are not limited to this typical standard. US/Canada have code for and commonly install dedicated 30A@120V, (commonly used for large through-window air conditioners) as well as 240V circuits rated from 20A to 80A. The North American electrical system typically provides 200A @ 240V to the residence. That’s sufficient for heating hot tubs and in-ground pools.
No, the decision to use hot water instead of cold water arises primarily from plumbing factors, not electrical.
It’s because I have never seen a dishwasher in the US designed for a high current circuit.
The standard is a single dedicated 15amp circuit for the dishwasher, because the standard dishwasher is designed to use the house hot water, so doesn’t need a larger circuit for a water heat cycle at the start.
Since US homes have a built-in, stable hot water supply, why would you build further pre-heat into a dishwasher?
It has fuck all to do with the water supply,
It’s because I have never seen a dishwasher in the US designed for a high current circuit.
Right. But, if they did make a 5000W dishwasher, it would not be difficult to power it in the US. Even though typical NEMA 5-15 and 5-20 outlets can only handle 1800/2400W, US electric code provides for 50A@120V, 20A@240V, and much, much larger circuits for ranges, water heaters, hot tubs, pools, arc welders, air compressors, etc. There is no electrical limitation preventing adoption of cold-water dishwashers.
Sounds like a legacy thing. The vast majority of houses now have combi boilers so no hot water tank. Hot water is on demand. Which would also be why separate taps have pretty much disappeared from UK homes since at least the 2010s.
Though I do think it’s very likely that a big chunk of it is as the other commenter said, an electricity thing.
With the price of gas what it is over here at the moment I’m rather glad my dishwasher is electrically heating as I can run it even on a dull day and my pannels will provide a good chunk of the power.
Sounds like a legacy thing.
Exactly. UK and European legacy standards required cold water dishwashers; US legacy standards called for hot water dishwashers. Once that expectation took hold, it’s somewhat difficult to alter it. Builders install plumbing for their respective systems; consumers buy appliances to match their plumbing. It’s a vicious circle with little compelling reason to break it. Neither is objectively better; neither is objectively worse.
Though I do think it’s very likely that a big chunk of it is as the other commenter said, an electricity thing.
The overwhelming majority of households had gas-fired water heaters, because for decades, it was significantly cheaper than electric, and those gas-fired systems remain in place in a majority of homes. In that sense, it’s an electric thing.
In the sense that US circuits simply can’t handle the current requirements of a permanently installed heating appliance, that is a complete myth. Trust me, we know how to
wasteuse electricity.
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
Yes. Foolishly most of the less expensive dishwashers don’t heat the water themselves, and the first cycle is more effective if it doesn’t have a period of using cool water from the pipes until the hot water arrives.
I was skeptical at first too but it truly works.
But it may be worth clarifying to your partner that once the water is coming out of the tap hot, the water in the hot pipes is already hot, so turning the tap to cool at that point no longer negatively impacts the dishwasher.
The important part is getting the cool water in the hot water pipes that has been sitting there flushed out before starting the dishwasher. If that’s already achieved, then what you do with the tap doesn’t matter anymore
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).
The US uses many smaller circuits. I have two appliance circuits, a lighting circuit, a dedicated refrigerator circuit, and a 240V range circuit. Five separate electrical circuits, just for my kitchen.
Rebuilding after WWII, the UK standard was to run a single ring circuit for each floor of the home. They ran as few household circuits as possible, because they needed the copper.
If you look at the amperage/wattage ratings of UK circuits, they are considerably higher than other 220v/240v countries, because their circuits are intended to be shared among many more devices.
You know how they warn you not to plug in too many devices into a single circuit? The UK electrical system took the exact opposite approach, because they needed to. This is why they have a fancy plug that nobody else in the world uses: it is an essential safety component that serves the same purpose that the household circuit breaker performs for the rest of the world.
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 still 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.
