Is hot water needed for dishwasher, or does it heat cold water?

julie94062March 27, 2009

I have a 40 gallon water heater in a closet in my kitchen which only supplies water to the kitchen area (there is another water heater for the bathrooms, laundry). In doing my kitchen remodel, we are considering a few different options...tankless in the same location, tankless in a different location, smaller tank, etc.

I hadn't thought of it, but recently read something about dishwahers heating water and was there a hot water line to the dishwasher, or does it heat cold water??

If it heats cold water, my hot water needs are obviously much less and that would affect my decision :-)

Anyone know? Thanks!

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most use hot water, though some higher end models can heat cold. check teh manual for your DW, but honestly i would plan on being able to supply it with hot.

    Bookmark   March 27, 2009 at 8:03PM
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Many units that can heat cold water are still recommended by the manufacturer for connection to a hot water supply, generally for consumer convenience in not having extended cycle lengths due to the heating time. In some cases the only way to know is by checking service documentation, technical manuals, or just being familiar in detail with how a particular dishwasher works.

The user instruction manual can sometimes tell the story, if it outlines specific target temperatures for the various cycles. In some cases it may advise, in regards to a "sanitizing" cycle for example, that the unit may not necessarily reach the target, which means the machine can heat but only within allowed time limits.

One that I know for sure can work with cold water is the Fisher & Paykel DishDrawer. Some Miele units can, and probably Bosch.

    Bookmark   March 28, 2009 at 1:42AM
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Thank you both for your replies. I'll look into the dishwashers mentioned and check the manuals.


    Bookmark   March 28, 2009 at 1:55AM
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"If it heats cold water, my hot water needs are obviously much less and that would affect my decision"

A DW only uses a small amount of hot water when it is actually filling (a couple times per cycle).

If you limit hot water use at the sink when the DW is running you could size the instant unit for the larger of the two.

    Bookmark   March 28, 2009 at 4:18PM
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A DW only uses a small amount of hot water when it is actually filling (a couple times per cycle).Dishwashers fill more than for just one wash and one rinse per load of dishes. Typically four to six water changes depending on the selected cycle. Some of the newer HE models may get by with three on a light cycle. My DishDrawer does 7 water changes on the heaviest cycle (approx 0.8 gal per fill).

    Bookmark   March 29, 2009 at 9:38AM
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Someone answering the phone at Miele told me last year that connecting their dishwashers to cold water was good for the Crystal wash cycle, as that wash option needs lukewarm water, not hot hot water. However, he said he advised mixing both cold and hot (combining the pipes, mixer) so that incoming water was neither too hot nor too cold, but just right.

Depending on your house plumbing, some hot water pipes give too hot water.
Depending on your cold water temperature (in winter it can be very very cold in some places), the temperature of your cold water pipe input can be SO cold that the dishwasher has to heat it a long time. He didn't say whether their DW had a timer that prevented the heating process from continuing a long time when incoming water was very cold.

Now, I didn't get a Miele, and I don't have to wash much Crystal any more.

So, I'm considering using IHW and cold mixed together. Both hot and cold, combined into one input... This would then help provide enough water of this just right (warm-hot or hot-warm) temperature setting, to not drain the entire IHW tank and to not push the DW heater to its limit.

In the long term I could foresee kitchen product manufacturers calibrating Instant Hot IHW with dishwashers' needs so that they can sell a more integrated suite of products. And then homeowners and remodelers wouldn't need to concern themselves with the Hot water tank far away near their shower. It might even become unnecessary to have a hot water line from that big tank to the kitchen.


    Bookmark   March 29, 2009 at 11:14AM
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"My DishDrawer does 7 water changes on the heaviest cycle (approx 0.8 gal per fill)."

Sounds like a reason to stay away from them.
Maybe they omitted the filter.

    Bookmark   March 29, 2009 at 2:20PM
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All dishwashers have a heating element that will help to maintain the temp of the water - some of the more expensive ones will actually pause and boost the temp of the water since hot water is critical for proper performance.

For those that do boost the temp of the water, they usually take considerable extra time to do that. And as folks have pointed out, they fill multiple times.

Most dishwashers state that water temp is important for proper cleaning - that they require at 120 degree water (I think they used to say 130, but scalding was a problem)

In general practice, I've never heard of connecting a dishwasher to anything but hot water.

I have heard that very hot water combined with harsh dish soaps have caused etching on crystal. I think that can be eliminated by reducing the amount of soap and using the Crystal setting.

    Bookmark   March 29, 2009 at 4:51PM
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My DishDrawer does 7 water changes on the heaviest cycle (approx 0.8 gal per fill).

Even with that much, that's a very low burden on a hot water heater--that's about as much hot water as you'd use to wash your hands (that is, each .8 gal).

Put it this way--whatever you get for a dishwasher will not need a huge hot water heater.

    Bookmark   March 30, 2009 at 10:30AM
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About a year and a half ago I used a Kill-a-Watt meter to check my DishDrawer's power consumption. I set my household tankless water heater at 70°F to insure the dishwasher received a "tap cold" fill and had to heat the water fully to the target temps.

0.4 KWH - Normal cycle with Eco modifier - 4 water changes (3.2 gal), 125°F main wash and final rinse.

0.58 KWH - Normal cycle (non-Eco) - 5 water changes (4.0 gal), 140°F main wash, 150°F final rinse.

0.68 KWH - Heavy cycle (non-Eco) - 7 water changes (5.6 gal), 150°F main wash, 163°F final rinse.

I don't know what is the current national average cost-per-KWH for electric power, but taking $0.15/KWH as an example and not including municipal water/sewer costs:
- Normal Eco - $0.06
- Normal non-Eco - $0.087
- Heavy - $0.102

    Bookmark   March 30, 2009 at 6:05PM
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Hey Dadoes:

I ran the numbers and mine match up exactly with yours. That is, how much electricity does it take to heat 0.8 gal 50 degrees * 7 fills * cost of electricity.

So if you are running the DW 5 times a week you pay about 2.00 per month in electricity to run it. Not bad.

Your mileage may vary.

    Bookmark   March 30, 2009 at 7:48PM
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Every DW manual I've ever read says to run the water hot at the associated tap before turning DW on. Huge water waste to begin with. At my place, that's 2 1/2 gallons down the drain before serious hot water even begins to come out. And that original fill drops by 40 degrees or more within moments of beginning circulation over the surfaces of the ambient-temp dishes in the machine.

Then there are the drains/refills....after the water in the hot water line has cooled waaaay down again. That's what the machine gets for fills for all of the rest of the cycles. Basically, the whole thing is a joke unless the machine can, in fact, heat its own water.

There are a few machines that hook up to 220vac and heat their own water efficiently. The rest of them -- like my two Whirlpool Gold 1500's -- just do what they can with 110. Both of these machines wash very well. They both take two hours or more to do it.

    Bookmark   March 30, 2009 at 11:18PM
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Jake, not all the wash or rinse periods are heated to a target temp. The element may be on, but only the main wash and final rinse are extended as needed to reach the target. I rarely run the machine 5 times per week. Usually can get by with two or three loads.

asolo, my DishDrawer instruction manual does not advise the sink tap is to be run before starting. The troubleshooting section for cleaning problems and such does not reference running the tap as a potential solution. It does say that the initial displayed cycle times are estimated based on 120°F incoming water and that cooler water will result in longer times.

    Bookmark   March 30, 2009 at 11:41PM
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I just did the math on the electricity needed for water heating... It didn't include the motor and such. That's probably why it came out the same.


Dude, you should really insulate your hot water pipes.
Part of the reason that the dishwashers go through a couple of rinses at the start - in addition to getting the a lot of the food off -- is to get the water hot and start heating the dishes so that by the time it gets to the wash cycle, everything is hot. If your pipes aren't insulated and it's a long run, you may have significant heat loss.

Also, 110v and 220v are equally efficient at heating water. It's just that the 220v will do it faster given the same current draw.

The bottom line remains that the less expensive dishwashers have a heating element that helps to maintain water temp. These may have more fill cycles in order to get the water and dishes hot.

The more expensive ones can actually pause the cycle to bring the water up to proper temp. Since temp is critical to proper cleaning, this can be very helpful and can save water if you don't have to run the water at the sink.

In any case, you connect the dishwasher to the hot water line unless your manual says not to.

    Bookmark   March 31, 2009 at 8:38AM
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A little off topic, but has anyone done the comparative figures for just washing them in the sink the old fashioned way and using a dish rack, rather than using a dishwasher?

I find dishwashers annoying (wait for it to be filled before running, bending over too many times, applying some sort of load strategy that others in the house will disagree with, wet cup bottoms afterward, etc. I prefer wash and get them done in the sink.

Just wondering if I'm the only one out there.

    Bookmark   April 8, 2009 at 11:14PM
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Let me answer you by taking a moment, on behalf of all males living alone, to praise the inventor of the dishwasher... aka the place to store dirty dishes other than the oven.

Dishwashers have vastly improved the sanitation habits and mate attracting abilities for thousands of men who would otherwise be denied an opportunity for lasting companionship because of the condition of their kitchen.

    Bookmark   April 9, 2009 at 8:53AM
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if you are really frugal with water and soap you might outdo a modern dishwasher. Otherwise they win in terms of efficiency.

    Bookmark   April 9, 2009 at 9:24AM
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You obliterated my argument with one fell swing. I forgot to consider that - a major oversight!

    Bookmark   April 11, 2009 at 8:16AM
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Yes, you should hook up your dishwasher to hot water. 120ú F is recommended. Any hotter and you are wasting energy and risk serious scalding in your home. Any colder and your dishwasher is not working as efficiently. Using a tankless water heater near your dishwasher will reduce the waste of heat in pipes and in water left in those pipes.

I'd like to clear up some common misunderstandings about dishwashers, some were repeated in the forum.

Dishwashers heat water. Most cycles take in 120úF water and heat it to between 140úF to 145úF. With NSF certified sanitary cycles they can heat water to around 155úF to 160úF; any of those temperatures can cause 3rd-degree burns. Those temperatures also disinfect dishes. Bacteria levels on hand-washed dishes have been measured as high as 16,000 bacteria per plate, where dishes washed in a dishwasher usually measure around 1 per plate. Holy food poisoning, Bat Man!

Dishwashers use less water than hand washing. Really, 16,000 bacteria can't be wrong. If that doesn't convince you, fill four 1-gallon jugs full of water (water used on Normal cycle on most modern dishwashers). Split them up for washing and rinsing then go at it in your sink. Look at all those nasty food soils in the water. Ugly isn't it? That's because you're not filtering heating filtered water like your dishwasher is doing; what you're doing is culturing bacteria.

Do not prewash your dishes; simply scrape and load. It wastes water. In fact prewashing dishes in the USA wastes the equivalent of 11 days flow of water over Niagara Falls. So if you're prewashing then shame on you. Think you can prewash better? Look at that lonely sink sprayer you're using to prewash and compare it to all the sprayers in your dishwasher. Think about it then say it after me, "Duh!"

Use the Dishwasher Sensor! These things are amazing. The most advanced ones measure load size and the dirtiness of the dish load. To measure food soils, the sensor sends light through water circulating through the pump system and the more light the sensor sees, the cleaner the load. Some sensors measure load size and to do that they measure heat loss in the first cycle. The more heat loss the bigger the load. The dishwasher uses these measurements to determine the number of cycles to use and water temperature that will clean your dishes effectively. Smart huh?

Dishwashers save on breakage. In fact that is why dishwashers were invented. Look up the story of Josephine Cochrane, the inventor of the dishwasher. She is my hero. I have a picture of her in my office! Studies, too, have shown that you break less glassware and dishes using a dishwasher.

There you have it. Today's modern dishwasher is energy efficient, water efficient and very effective at washing dishes. That makes it one mean, green cleaning machine.

    Bookmark   March 9, 2012 at 11:36AM
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