dishwashers

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Re: dishwashers

Postby zzsstt » Sat Sep 04, 2010 12:08 pm

35KPa equates to a static head of 3.6m of water, though if the dishwasher connector is 800mm off the ground that comes off the head, meaning the tank water level now needs to be 4.4m above the feet of the dishwasher.

However be aware that static head is not what is important here. Friction in the pipes will reduce the pressure when a valve is opened and the water flows, so make sure that you have sufficient pressure under "flow" conditions. An easy test is to measure the time taken to fill a pot - if it takes more than about 15 seconds to fill a 2 litre pot you may be struggling. There are dishwashers that are designed to run on very low pressure, and that state their suitability for gravity fed water.

I have successfully run dishwashers and clothes washers on gravity fed water, but always with more than 3.5m of static head - 3.5m of "tank height" is marginal to have a decent shower with the shower rose at 2m high, and with pipe and heat exchanger friction!

I have also seen dishwshers state 50KPa or more "minimum pressure", so make sure of the figures for the one you are looking at.
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Re: dishwashers

Postby Smurf1976 » Fri Sep 17, 2010 7:59 pm

A few observations on performance (ie actually washing dishes).

1. Just about everyone seems to either rinse everything first. Those that don't generally complain that the machine doesn't get everything clean. This is especially the case if the machine is in a smaller household (1 - 2 people) and only run once every few days when it's full.

2. My dishwasher is connected to hot water supply. I never ever rinse anything, all I do is remove things like chicken bones and then put things straight in the machine. It is used once every 4 days usually, giving plenty of time for anything on the plates or pans to set. No problems with cleaning performance whatsoever. Since it's sitting almost right above the heat pump water heater, connection to hot should use less energy too (and what energy it does use for heating water is at half the rate on off-peak). It was on hot when I bought the house (dishwasher included).

3. Same at my parents' house. Rather ancient dishwasher not used every day. No rinsing and it gets everything clean. It's on hot water (conventional electric system). Not at all energy efficient, this machine is 29 years old and goes through about 1 kWh per load plus 35 - 40 litres of hot water but it does the job 100% every time. Not really worth replacing with a more modern one since it's only used twice a week. And with both the hot water and the dishwasher itself on off-peak, the cost isn't too much of an issue - works out to $10 a quarter or thereabouts. Needless to say given its age, there's nothing electronic in this dishwasher - all fully mechanical and getting a bit rattly with old age. Hot water connection was recommended for this machine by the manufacturer.

So overall, my preference is for hot water connection based purely on experience that it seems to clean better. I've lived in this house since early 2008 and still haven't opened the bottle of dishwashing liquid for hand washing - literally everything goes in the dishwasher.

All that said, in terms of long term maintenance hot water is more likely to lead to failure of the rubber seals, internal hoses and so on. Cold water is more likely to wear out the pump motor, since it runs for longer whilst the water is heated. If you're in a hurry, hot will be quicker (though the difference is less these days due to the smaller water volumes used in modern dishwashers).
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Re: dishwashers

Postby zzsstt » Sat Sep 18, 2010 7:04 am

Using hot water does have pro's and cons. I believe it was stated in an earlier post, but many dishwashers don't use much water, so in a long pipe run they may actually fill with cold water whilst recharging the pipe with hot water from the tank - very inefficient! If however we assume the pipe run to be short, and the hot water to be "free" (i.e. solar/heat-pump) then clearly there will be an energy saving by avoiding the need for the dishwasher element to heat the water. On the other hand, if the dishwasher is run on a PV system it makes little difference as all the power is "free". Running the dishwasher from the hot water system also removes temperature control from the user - if the machine fills with water at 70C, it makes it difficult to run a "low temperature" wash cycle, should it ever be required.

Our dishwasher runs on pump fed cold rainwater, with the waste going to a Bioseptic and then irrigating some trees. We run it on off-peak power, rarely pre-rinse anything, and normally the "energy save" cycle gives perfectly adequate cleaning.
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Re: dishwashers

Postby greg c » Mon Sep 20, 2010 5:46 pm

We used to have an old Simpson that had a hot and cold connection. We used to hand wash delicate glasses in the sink to just before we turned on the dishwasher, and always after 10pm so we would get the off peak rate. That was 20 years ago and the unit used to get the dishes pretty clean. It was replaced with a Miele, and I think it is slightly better, it is a lot quieter, essential in an open plan home. Interestingly I normally run the Miele on economy which does not have a pre-rinse just gets into the washing straight away. It gets most loads clean, but anything that has been in the oven needs a more intensive program to get things clean.

Good luck finding a non electronic dishwasher, the only solution would be to get an old one and restore it.



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Re: dishwashers

Postby lad » Wed Sep 22, 2010 11:09 am

We am fully off grid. Run gas instant hot water (solar evac tube system on the way), very short run from gas HW to the dishwasher. We feed hot water straight in, tempering valve fitted and set to 60 degrees. Only rinse egg/cheese anything else scraped off for worm farm/pets/poultry. We chose a Bosch and it performs very well. We opted for hot in to conserve battery charge.

LIkewise, one fitting on the washing machine, gone for cold and we select cold setting each wash.

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Re: dishwashers

Postby Sou » Thu Sep 23, 2010 12:58 am

I have a Miele which can run off hot water or cold. I didn't realise until after it was installed or I would have had the SHWS plumbed into it. It wasn't worth the cost of extra plumbing after the event. I haven't yet measured its power usage. This thread has reminded me to do so.

Re water useage - this dishwasher uses much less water than washing and rinsing in a sink (I think it would be about 20% or less).

Re washing before the dishwasher - I don't, except for really any grunged on stuff on pots and pans. Normally I just put all the pots, pans and dishes etc straight in and let it run (now only between 11 pm and 7 am, since getting solar panels). Pre-washing is a waste of water and negates the water saved by using the dishwasher. Never a problem with pots or dishes getting clean - well, almost never.
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Re: dishwashers

Postby Sou » Sat Sep 25, 2010 9:19 pm

Just out of interest, I measured the energy use of our Miele dishwasher on the 'Normal 50" setting and it was exactly as specified in the dishwasher manual, at 1100 watts for one cycle, which takes 1 hour 14 minutes to run. The Economy setting would be 740 Watts a cycle and takes 2 hours 14 minutes.

The cost of the Economy cycle over a year, on the off peak tariff of $0.1502/kWh less 6% discount on the payment plan (TRU Victoria) would be $38/year for one load every day. Or, more realistically in our household, about $27/year for five loads a week. (Approx $6.80 per quarterly bill.)

Water usage is 12 litres a load, which is less than half the capacity of my kitchen sink (about 25 litres when full) :)

Re the original post - this dishwasher can use hotwater hooked up (via a tempering valve if necessary) at 45C to 60C. The energy use would be lower in that case.

PS I notice this is an old thread, but hope the info benefits someone.
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Re: dishwashers

Postby toldminer » Sun Sep 26, 2010 6:41 pm

Thanks for the info Sou. By comparison one of our F&P dishdrawers used 51 kWh/y as measured over 27 days on the eco cycle. We use it about every decond day. The other day we use the second one. They are both connected only to the solar hot water. So total power usage in a year for dishwashing is around 100kWh or $26 as we use them at night on purchased green power (AGL) (so we sell the maximum PV power at $0.52/kWh). Could be a bit of extra cost charged if cost of pumping the water up from the tanks and occasional electric boost of the SHWS is added.
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Re: dishwashers

Postby Newhomeowner09 » Thu Nov 11, 2010 10:11 am

I would strongly suggest that you don't run hot water directly in. If you have a new dishwasher, it will be equipped to heat up the water. We have a dishwasher, but I find myself washing the dishes by hand most of the time anyways. I appreciate it being done right then and knowing they are clean. Chances are that if you are considering getting a DW because of the amount of dishes you have to clean, your family is large and there is a lot to making and cleaning up a dinner. This will make you use more energy anyways. Install a larger sink and I think you will see better results in terms of electric usage and water usage.
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Re: dishwashers

Postby Gordon-Loomberah » Mon Nov 15, 2010 10:21 am

Newhomeowner09 wrote:I would strongly suggest that you don't run hot water directly in. If you have a new dishwasher, it will be equipped to heat up the water.


Sure dishwashers can heat the water themselves, by energy wasting resistive element heating... did you know that resistive heating hot water systems are going to be outlawed because they waste so much electricity?

I use solar heated hot water (sourced from rainwater tanks), and solar/batteries at night power to run mine and it does a good job. By having it connected to the solar hot water, a lot of wasteful resistive heating is eliminated. The ~2kW water heating is reduced to just 1 minute. I'm using water from the regulated side of my tempering valve in the dishwater, which simplified the plumbing, just needing a T from the sink HW pipe, rather than a long pipe run from the hot side of the tempering valve. If unregulated temperature water was used, that heating could be eliminated entirely. The majority of the wash cycle time is running a pump, and near the end the air is heated to speed the drying process.

Here is a profile of current draw from the batteries (before inverter, @~24VDC), but it does have various other loads superimposed- fridge, freezer, computers, TV, lights etc. The water (1min) and air(3min) heating stages are clearly visible though. If cold water was connected to the dishwasher, that hot water heating stage would be greatly extended.
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