Effectiveness of DW pre-rinses/pre-washes

[henene4]

So I was wondering once more...

How effective are pre-rinses in DW?
So how much work do main washes have to actually do?

I was thinking as my Miele seems to have exactly one kind of pre-wash for any occasion: 20min cold.

And having added some dishes into the main wash the water seemed rather clean.

Have seen many pre-washes far more intensive than this (heated and longer) and the Mieles results are good already.

I mean I did switch to using some detergent in the pre-wash, but even without not much can be left for the main wash.

Unless something is actually burned on, with a total wash time of an hour, I can't really see how anything beyond a quick hot-ish wash can be needed.

Temperature and detergent are mainly needed for degreasing and some time is needed to rehydrate dried on stuff.

Unless burned on, it really shouldn't be hard to build a quick effective cycle.

Actually, over here, 90min seem to give the best balance of efficency and results.

90min gives you 10min for a cold pre-rinse, a quick main wash up to 50-65C (120-150F), a rinse and still about 30-45min for final rinse and dry.

Usually heating rates are about 2C per min (that is 4F) or higher and with a 20C fill (like 60F or so) you only need 15-25min to reach top temp.

Usage data for such cycles often go no higher than 15l (that is like 5gal or so) and 1.2kWh.

With a hot fill and less heating power that should still - more or less - adf up the same.
Maybe some higher usage (like 6gal or so) and to split the one pre-wash into 2 shorter once to get more of a hot water line purge effect.
Heck, maybe even do that sensor based (cold first fill means 2 pre washes, warm first fill means 1 pre-wash).

If you don't mind some more time (2h) a 4gal cycle with extended heating in the main wash should be doable and be verry competitive with other machines.

Anyway, wanted to know your opinions.
Maybe share you post-pre-wash pictures.

Happy to hear from yall!

 

[Marky_Mark]

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: 5/15/2020-17:11]

 

[launderess]

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.

 

[warmsecondrinse]

"With a hot fill, this can help warm everything up, reducing the dishwasher's electricity consumption -- can be useful or not, it depends."

This can be important as the water heater could be a good distance away. Skipping a pre-wash could lead to luke warm water being used for the main wash unless a water heater is used. That'd increase cycle time and electricity used.

GE Hot Starts use an unheated 'hot' fill for pre-wash(es). The heater turns on for the main wash if the temp isn't high enough.

 

[rolls_rapide]

I prefer the idea of a cool prerinse or a lukewarm prewash - one which actually drains away before the main wash. I think you get a better result.

I'm not too fussed on the idea of a greasy prewash being 'sensed' by the machine on its auto-sensor programme as being okay (i.e. kept) for the main wash. Tomato sauce stains are a nuisance.

 

[jeb]

My 55 kitchen aide has a purge cycle at the beginning of the wash. sprays water in and pumps it out for about 2 minutes, then valve closes and wash,rinse,rinse, dry all done with-in 1/2 hour and dishes are very clean. The purge clears the cold water from the pipes while spraying it onto the dishes wetting everything. It also flushes out the bottom were everything has slide off the dishes. With the short wash and rinse times you are only using tap hot water and a lot of it but you can do 2 full loads in under an hour and they will be clean.

 

[launderess]

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.

 

[iheartmaytag]

We had a GE Mobile Maid

Purchased in 1971. It was our first dishwasher, a great little machine. As our dear Laundress stated it used a lot of water, 11 gallons for normal wash, and heated the final rinse to 180 degrees.

 

[Marky_Mark]

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.

 

[henene4]

Most modern EU DW kind of actually discourage hot water connections.

Thing is that most people use a cold connection anyway and that with puny fills of maximum 1gal it often only makes sense in verry slim circumstances to even use a hot fill.

And most cycles don't actually follow the heat-wash philosophy (first heat, then timed wash) but more a heat&wash aproach (timed main wash that happens to heat as needed) since detergents get active at basicly any temperature.
So time savings are slim.

Honestly you all have to be lucky.
I got a pretty cheap tarif and am still paying 0.23€ per kWh averaged over a year.
A cubic meter of water in total runs just shy of 4€.

My mum back down south pays about 0.30€ per kWh and by now about 5.50€ per cubic meter (waste water treatment is like 3€ there).

Really have to calculate if more efficent machines actually make financial sense.

 

[sudsmaster]

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.

 

[foraloysius]

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.

 

[henene4]

Been doing the same with the liquid Finish I grabbed 4 bottles of Amazon a year ago.
Crappy detergent on its own, good for prewash purposes though I guess.

Main wash I am currently using up the Somat tabs I have.
Got some 80 Finish tabs as well as almost 180 Miele Ultra tabs waiting.
All bought because of low prices.
On the regular I go with the cheap detergent only DM (a german drug store chain) ones which are loaded with bleach.

I do have to say while the new Miele filter system is rather smart and seemes to be verry good filtering wise, it suffers the same self-cleaning-issues the current BSH machines seem to have.
Around the top of the fine filter (the plastic casing holding the screens more or less) yibbles just get stuck and don't leave.

I checked the filter today after yesterday's cycle (QuickPowerWash with extra clean) and it was surprisingly clean.

Usage was 16l and 1.4kWh, so about 2-4l and 0.4kWh more than the same Auto cycle.

Though I do have to say that the cycle cheates a lot time wise by basically not doing more than the base minimum of drying in official cycle time and just pushes another 20min or so of active drying after the cycle end.

This cycle really benefits from a pre-rinses.
Doing this trickery they could have just as well added a 5min high pressure warmed pre-rinse.

 

[rolls_rapide]

Lidl tabs

I have to say, I find the Lidl W5 tablets rather good - both the multi-function one and the basic tablet. Both seem to be more effective at cleaning than Finish, especially tea stains; although the Finish Quantum floppy gel-pac things do appear to create a superb shine across the whole load.

 

[mrboilwash]

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.

 

[henene4]

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.

 

[sudsmaster]

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

 

[lakewebsterkid]

Pre-washes

I do not like the lack of a pre-wash using the Normal Cycle on our Bosch. It can do pre-washes, but generally negates them. The pre-wash does quite a bit, and keeps the filter from getting junked up during the wash. The purges that the Auto cycle does after the pre-wash, and after the main wash certainly cleans the filter and sump out quite well.

 

[henene4]

EU equivalent

Our Eco cycle which assumes the role of your Normal cycle tricks it's way around even more on BSH machines.

It says it does a prewash.
Thing is it runs the cold pre-rinse stage at medium pressure with a normal fill level - then pauses for a couple of seconds - drains for a couple of seconds (timed depending on fill amount and load size sensed) and starts its main wash at low pressure.
Idea is that all heavy soils are settled at the bottom of the sump and pumped out with that drain of maybe 2-3 glases if water.

Is that better then Mieles approach of just running just the main wash but with more water in return?
You judge!