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Thread Number: 82995
/ Tag: Modern Dishwashers
Effectiveness of DW pre-rinses/pre-washes |
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Post# 1072685 , Reply# 1 5/15/2020 at 15:44 (1,440 days old) by marky_mark  (From Liverpool. Now living in Palm Springs and Dublin) |
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 I imagine it's a complex issue with many variables. It would be interesting to know what would happen if the prewashes were eliminated and the main wash extended instead, perhaps with additional heat and water that would have been used for the prewash. I believe that some Miele machines didn't do a pre-rinse on Intensive and just went into the main wash. While other Miele dishwashers (later and earlier models) did one or two prewashes, some heated, some not.
Some possible pros: I think quick pre-rinses are good for getting rid of tomato soils that can stain plastics. They remove some food soils so that the main wash detergent solution and filtering system can be effective and not overwhelmed. This can prevent foaming and means that some residues can be flushed away before the main wash. With a hot fill, this can help warm everything up, reducing the dishwasher's electricity consumption -- can be useful or not, it depends.
Possible cons: Without enough food soils in the main wash, some machines/detergents/soils/water conditions etc. can foam up too much and can reduce wash results. Insufficient food soils could mean etching and/or other degradation of the machine or the items being washed.
Incidentally, in Spain my Miele G550 dishwasher from 1975 has a 3 kW heating element and uses 10 litres per fill and heats at a rate of 3 ℃ per minute. My Siemens from 2007 has an element of around 2 kW and uses about 3 litres per fill and also heats at a rate of 3 ℃ per minute. I haven't timed this on my American machines, but they will of course heat more slowly.
This post was last edited 05/15/2020 at 17:11 | ||
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Post# 1072690 , Reply# 2 5/15/2020 at 16:43 (1,440 days old) by Launderess (Quiet Please, There´s a Lady on Stage) |
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Much would depend on several variables
First off bat would be extent and type of soils. Households that pre-rise and or scrape nearly every morsel of food or whatever off dishware before it goes into machine likely could get away with a less intense or even skipping pre-wash. OTOH those that just remove bones or large chunks and otherwise pop everything into washer might want or need a pre-wash.
Before advent of enzyme dw detergents you wanted a cool or cold pre-wash to help remove protein based soils (egg, milk, etc...) before really not water that was further heated cooked things on. For American dishwashers that largely were piped to hot water and only heated for main wash; pre-wash is going to be cooler than tap hot (even if line is purged) because hot water will hit cooler interior of machine and the dishes themselves. So by default you're going to get somewhat of a cooler pre-wash which is fine. When machine fills for main wash interior will be warmer and thus less cooling will take place. Have always felt as well things also come down to how effectively machine filters water and keeps yibbles from being redeposited back on dishes. A pre-wash will send a bulk of yibbles down drain which keeps them from redepositing onto dishes in main wash. IIRC many modern dishwashers with sensors "look" at first wash water to determine soiling. If machine believes only one wash is needed that is what happens. Know American dishwashers with mechanical timers or whatever often advised to skip pre-wash if machine was lightly loaded, and or say when doing a load of freshly soiled dishes. Some take an entire day or even week to fill their dishwasher. In such situations it likely is going to take more than one wash to loosen and lift caked and burnt on soils that have hardened into a glue like substance. Of course dishwashers of old used far more water and had such strong pumping action they could scour paint off walls. My Mobile Maid will do a full load of dishes that have sat sitting > two weeks (don't ask) and everything comes out spotless. Mind you it uses an obscene amount of water with enough force to cause machine to shimmy doing so, but there you are. | ||
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Post# 1072727 , Reply# 6 5/15/2020 at 21:55 (1,440 days old) by Launderess (Quiet Please, There´s a Lady on Stage) |
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On my GE Mobile Maid the first wash is rather short but never the less is a full fill. Then comes a "rinse" cycle that proceeds main wash. Two more rinses and then off to drying. Whole thing done in about 30 minutes, but again am quite sure greenies would have me if they but knew how much water and energy is consumed.
The old Kenmore (Frigidaire) on Normal did purge, pre-wash, purge, wash then rinse. First purge cleared sump and lines of stale cool water and primed hot water tap. Also helped wash down sump/drain any yibbles or other bits at bottom of tub. Purge after pre-rinse again helped get rid of muck at bottom of tub.... Think at some point dw design moved to purges instead of full tub fill and cycle as a water saving feature. It does seem wasteful to use an entire fill of hot water just to "rinse" dishes for a few minutes. | ||
| Post# 1072768 , Reply# 7 5/16/2020 at 02:06 (1,439 days old) by iheartmaytag (Wichita, Kansas) | ||
We had a GE Mobile Maid | ||
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Post# 1072788 , Reply# 8 5/16/2020 at 06:30 (1,439 days old) by marky_mark (From Liverpool. Now living in Palm Springs and Dublin) |
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Hi Jim/warmsecondrinse
In Palm Springs my hot water is heated by gas which is far far cheaper in Palm Springs than electricity, which is very expensive. Water is also extremely cheap in PS. I have a hot water circulation pump, so hot water is always immediately available at all taps including the dishwasher. So a couple of quick pre-rinses will certainly be cheaper and faster than having the dishwasher heat up cold water.
In Spain my hot water is heated by an electric water heater using a traditional resistance element and there is no circulation pump. The dishwasher has a 3000-Watt heater. So using a hot fill will save a little time but cost more. I have noticed that some dishwashers will wash for the exact same length of time, regardless of how hot the incoming water is (such as my Siemens) it will just do less heating during the wash. Although it does save time on the final rinse. Other dishwashers, such as my vintage Miele, will reduce wash time if the incoming water is hot. So I think some machines will perform better with a hot fill while others will not perform as well. | ||
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Post# 1072798 , Reply# 10 5/16/2020 at 08:44 (1,439 days old) by SudsMaster (SF Bay Area, California) |
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The Bosch seems to do multiple binge-purges in the starting pre-rinse cycle. Seems to work well for it. As probably stated, the relatively cool water of the pre-rinse(s) would help to solubilize food residues on the items rather than cook them on with very hot heated water. It probably also helps purge the hot water line so that when the real wash comes along with release of the main (or only) detergent dosage it will be most effective, with a minimum of energy expended trying to heat cold water.
I don't argue with the Bosch program. It seems to have great results. | ||
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Post# 1072801 , Reply# 11 5/16/2020 at 09:13 (1,439 days old) by foraloysius (Leeuwarden, Friesland, the Netherlands) |
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I guess cold pre-rinses on European dishwashers are mainly there to get the loose stuff off. I think that makes a difference in the main wash, so the detergent doesn't have to work on stuff that came off anyway by a cold pre-rinse only. I never did it before but lately I have added just a bit of (Sun) powder to the pre-rinse and it does seem to make a difference (my dishwasher is a Miele G4210SC). The filter stays cleaner and somehow the inside of the machine is a bit more shiny. I bought two containers of Sun powder on sale, so I'm set for a long time. I use a multi tab from Sun or from Lidl (W5) in the main wash. The Lidl tabs are cheap but work very well. They have been tested often as the best tabs by the Dutch consumer organisation.
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Post# 1072811 , Reply# 14 5/16/2020 at 11:52 (1,439 days old) by mrboilwash (Munich,Germany) |
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It is indeed impressive to see how much of even dried on stuff comes off in a 20 min cold prerinse without any detergent.
Wonder what the "don`t prerinse because the detergent needs food to chew on otherwise it will chew on the dishes" fraction has to say on this on. Maybe someone can explain why it`s OK for the dishwasher to do it but not for the person who loads the dishwasher? Lidl W5 Multitabs are my favorites too, found the Quantum perform better on very greasy loads, but that`s the only rare occasion where I prefer these. | ||
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Post# 1072816 , Reply# 15 5/16/2020 at 12:43 (1,439 days old) by henene4 (Heidenheim a.d. Brenz (Germany)) |
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Never bought the whole thing with that. If that were true, tabs would just be a no-go. Only reason you should always let the DW do the pre-rinsing: Even if you pre-rinse only 1/4th of the dishes in a load, you will still use more water than the machine would use for the entire load. | ||
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Post# 1072831 , Reply# 16 5/16/2020 at 14:52 (1,439 days old) by SudsMaster (SF Bay Area, California) |
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I agree with Henrik and others. Don't worry about the machine doing pre-rinses. If your dishware is so fragile it cannot stand a hot wash with modern DW detergents, then just hand wash it. Same for glass crystal, which I understand is more fragile than soda or borate glass. But the machines are designed to get the rest clean while conserving energy and water. Just let them do their thing.
If the result is less than expected, check the DW for faults, such as clogged filter, or filter not installed properly, or filter warped (like Frigidaire) and not even trying to filter. Or there may be a pump failure/blockage. Or a clogged drain line (not unusual with air gaps). My Bosch is a bit notorious for having a more difficult than usual filter to install. I remember before I got mine, my cousin had one and asked me to figure it out. I did, and when I got my own Bosch DW I realized why he was having problems. It's not exactly intuitive. But it is, with a little care, to install the filter so it seats securely and does its job. Not rocket science, either. YMMV | ||
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Post# 1073086 , Reply# 20 5/17/2020 at 23:56 (1,437 days old) by SudsMaster (SF Bay Area, California) |
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Post# 1073118 , Reply# 21 5/18/2020 at 07:21 (1,437 days old) by Logixx (Germany) |
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I haven't run a prewash in years.
My 1993 Bosch countertop dishwasher used enough water, heat and pressure to probably qualify as a Hurricane in a Box. A 20-minute wash got everything clean. Plus, it ran three rinses. My current, modern dishwasher... I think it prewashed once. Besides, I tend to use a cycle without prewash anyway and add an extra rinse. | ||
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