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.
Your dishwasher uses hot water so if you run your sink until hot water is coming out before running your dishwasher it means it’s immediately getting hot water. It’s about clearing the cold water out of your pipe.
Whether or not you can use the sink while the dishes are being washed depends on your water heater. If you can wash your hands while someone is taking a shower without blasting them with cold water then you should be fine to use your sink while the dishes are being washed.
In the United States and other countries with 120V electricity
It has nothing to do with 120v electricity. We used to have dishwashers in the USA that could heat water from cold to 140ºF without issues.
Some dishwashers have a hot and cold water intake, but it’s pretty rare, abd only makes sense if you heat your water with some alternative energy source. Letting the cold water bleed out makes sti zero sense either way.
In the US, you need to run hot water before starting the machine, but once the machine is running there is no reason you shouldn’t be able to use the sink as normal.
The federal government put restrictions on how much energy a dishwasher can use in a cycle, which has significantly restricted how much time they can spend on heating water and drying the dishes at the end of a cycle. In order for dish detergent to work properly, the water needs to be at least 135ºF, most machines aim to reach 140ºF. Most manufacturers will also state incoming water needs to be 115-120º minimum in order for them to be able to work (which is a common-high setting for a hot water heater - 120ºF is hot enough to scald you if you are not careful). If you do not get the cold water out of the pipes between the hot water heater and the kitchen, then the water starting temperature won’t be high enough and the dishwasher won’t work as well (specifically the detergent won’t work as well).
Not every dishwasher has a heating element. My dishwasher does, and the manual still says to run the hot tap to make the dishwasher more effective.
Also, just because your dishwasher might have a heating element, that doesn’t mean you always wanna use it. You know what the “sanitize” setting does if you accidentally leave some oily residue or food on your dishes? Everything gets some junk baked onto it. And then you (actually, me) have to scrub at hard, dried crud that you should have handled before loading the dishwasher the first time, but you were lazy and let your wife wash the dishes, and you didn’t know at the time that she doesn’t inspect things before baking them in the soapy dish-steamer. (Rant over)
As for washing your hands while the dish washer is running, I’ve never heard of this issue. Most residential plumbing is capable of carrying somewhere around 6 GPM (Gallons Per Minute), and your faucet is probably limited to 1.5 - 2.2 GPM, but if you wash your hands at that pressure, your shirt is probably gonna be soaked from splashing or spray. Your dishwasher only takes a minute or two, maximum, to fill, and the remaining 3.8 GPM is more than enough. Once your dishwasher fills, the pump (sometimes heated, sometimes not) cycles that water through the jets, and then eventually pumps the water out the drain hose.
If she’s upset because you don’t like using scalding-hot water to wash your hands, then that’s also a non-issue. Your hot water lines are connected to your water heater, and the hot water lines will deliver water from your heater whether you want them to or not. Your cold water and hot water lines shouldn’t interact behind the faucet. You can run all the cold water you want at your faucet and the only difference is that not only will the hot water supply line be hot, the cold water supply line will be cold. Separately.Now, something you shouldn’t do is run your disposal at the same time as your dishwasher. When you install your dishwasher, it specifies that you should install a “high loop.” This just means to make your dishwasher’s drain hose go as high as possible under the cabinet before reaching the sink drain. It doesn’t have to be a literal loop, but I like making an actual loop because I like the way it looks. This “high loop” acts as a vacuum break to reduce the chances of your dishwasher sucking dirty water back into it. If you recall how to make a siphon, the two things you need are an unbroken hose with no air pockets to disturb the water flow, and for the source to be higher than the destination. Your dishwasher’s drain is at the bottom, close to the floor, and the hose intersects the sink’s tail piece around knee-height. The high loop makes it more difficult for backflow water to fill the cavity of the drain hose, so you’d pretty much have to clog your sink drain past the p-trap, and then fill the sink to nearly overflowing for you to move water from the sink drain to the dishwasher. BUT! This isn’t true with a disposal. Disposals are capable of generating enough pressure that they can actually push some water up the dishwasher’s drain hose. So, if your dishwasher is draining while you use the disposal, and it stops draining while you’re still using the disposal, you might actually be able to get some of the ground up food from your disposal past the high loop. Even worse, your disposal can often let through food particles that are bigger than the jet holes in your dishwasher. Since your dishwasher fills and then pumps the water already inside it to spray your dishes, you can end up putting food particles through your pump, breaking it, or through the jets, clogging them. Also, you’re taking that dank smell from your disposal and using it to clean your food.
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.
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.
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.
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.
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
He also said that with modern detergents, you should do colder washes anyway, as the hot water has little benefit.
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.)
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.
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 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
Also why US homes tend to have the immersion heater setup for hot water with a big tank that gets heated over time, compared to the common euro system of on-demand instantaneous hot water.
The water heaters are usually gas here though, so electric system doesn’t really matter. Tankless heaters have become a lot more common over past decade or two, both gas and electric. For electric ones you just use a 240V circuit. Most electric appliances that need a lot of power run on 240V here, 30A and even 50A or 60A circuits are common and provide enough power for household needs.
I gotta love how the US is really advanced for the most part but then occasionally you come across one area in which it operates like 1980s Estonia.
the whole gas thing is always funny to me because most of sweden just went directly from wood heating and candles to electricity, so it manages to sort of feel more outdated than basically anything short of a viking longhouse.
The newest construction near me puts tankless in townhomes and full-size in single-family homes. (as long as gas is available)
They’re super efficient when you don’t use tons of hot water because your waste on tank heaters is heat loss through the pipes and heater itself.
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.
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.
Thanks, everyone else was missing this point and I was about to write up something similar.
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 😅
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
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.
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.
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.
