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Is it a good practice to connect the dish washer and laundry machine to the hot water line, instead of the cold water line?

My house has heated water, so this would save energy and time; I wonder if there are any risks or caveats.

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    Most laundry machines require a hot and cold line so your question does not make sense. As for the dish washer, you should consult the manual and see if there are any caveats about using hot water. Many dish washers will heat the incoming water as required. – Barry Dec 30 '16 at 23:57
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    Where in the world are you located? – Tyson Dec 31 '16 at 0:03
  • @Barry My laundry machine has only one incoming water line. – Adam Matan Dec 31 '16 at 5:37
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    If this dish washer is in a residence, then a hot water line is required. The heating of the water in the dish washer is only for raising the water temp. to a required point for that part of the cycle. – d.george Dec 31 '16 at 11:27
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    @d.george when making such generalisations you should state where you are. The opposite is true in much of not all of western Europe. – Chris H Dec 31 '16 at 12:41
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The problems are twofold: too hot for some things and too cold to save much.

A modern washing machine uses surprisingly little water (though top-loaders still use quite a lot). By the time the hot has come through, most front loaders will already be full. At this point the water just cools down in the pipes wasting heat, while the machine's heater has to run. Many programs use cold water for at least the first rinse. If you connect only a hot line you waste more heat at this step. By the final rinse the water will be quite hot. But when you want a cool wash for delicate clothes, the final rinse will still be hot (even if all previous steps aren't) thus ruining expensive clothes.

Some machines may also get confused and refuse to run if the incoming water (expected to be cold) is above the program's setpoint.

Cold feed only is the norm in Europe, specifically to save energy. The only exception would be if hot water is plentiful but electricity is expensive. This is rare.

The situation is similar for dishwashers.

To answer your title: no, or at least probably not given that it's cold feed only

  • "By the time the hot has come through, most front loaders will already be full": I'm not sure that's right. Even with a very long and large pipe (say 30 meters, 3/4" diameter) that's 7.5 gallons. I've read an efficient laundry machine requires 20 gallons (front loader, or top loader with a horizontal axis rotating drum). Also you are likely to feed too hot water, so these first few cold gallons might compensate for that. All in all, even with this very long pipe, you might get enough hot water. Warm pipes contribute to house heating, so it's not completely lost. – youen Sep 9 '17 at 21:00
  • Remade my calculation with a more standard pipe: 10 meters long with 1/2" inner diameter, you are under 1 gallon of cold water. In my house I've measured 2 liters (0.4 gallon) at the farthest point, before I get warm water. – youen Sep 9 '17 at 21:08
  • @youen my front loader doesn't use anything like 20 gallons for the full cycle, and the water in the pipes can cool down between the wash and the rinse. Heating the hose is only desirable sometimes – Chris H Sep 10 '17 at 6:18
  • I'm interested to know how much water it uses if you have this figure. I'm dependent on online sources for this. I could also get the output of my machine in a big container and measure that... – youen Sep 10 '17 at 7:03
  • @youen the newer version of mine has a larger capacity - 8kg. It uses 50 litres, so 11 gallons, according to the manufacturer. – Chris H Sep 10 '17 at 18:37
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It depends how you heat your domestic hot water.

Short answer

You won't save a lot. But there are a few net savings that can be made if your house and appliances are designed for this from the beginning.

Switching from cold water to a new hot water feeding system is a questionable choice, as it will pay off only after a long time.

Sum up per energy type

Here is a sum up for each energy for Dish Washer (DW) and Laundry Machine (LM). See below for calculation details, this is not meant to be accurate but to give an idea of how little is at stakes. Numbers are savings, in dollars per year, and in Primary Energy (PE) percent, compared to electricity from a fossil fuel power plant (i.e. higher numbers are better).

Energy       | DW ($) | LM* ($) | PE %
-------------|--------|---------|------
Electric     | $0     | $0      | 0%
Heat pump    | $7     | $13     | 66%
Propane      | $2     | $4      | 54%
Solar        | $11**  | $20**   | 100%**
Cogeneration | ??***  | ??***   | 100%

* These numbers assume your laundry machine has two water inputs, as they use cold water for rinsing ; providing hot water instead is likely to waste more energy and money than you can save.

** Assuming all your water use can take advantage of your solar heating. This clearly needs refinement as your solar heated water might not be warmed enough, or not available depending on when you use it. Also what you use for laundry isn't usable for something else, so you might end up consuming more backup heat power.

*** I haven't found any price information for cogeneration heat.

Technical limitations

Dish washer

A dish washer uses only hot water. Providing it from a better heating system is beneficial. However you will loose a little bit in the pipe between your central boiler and the dish washer.

The question I don't know how to answer is: can you do that with a dish washer that's not designed for it? I don't know. I don't see why not (it should use a thermostat, so it shouldn't heat again already hot water), but I'm not sure.

Laundry machine

A laundry machine designed for hot water feeding uses both hot and cold water. Rinsing is made with cold water to save energy. Providing only hot water (to a machine that is not designed for it) is likely to waste energy, even if you efficiently heat it.

I've heard of devices with two inputs and one output, that you place before you laundry machine input, that will mix cold and hot water and use a timer to feed hot water, so that you rinse with cold water. It would allow upgrading an existing installation. I have no feedback on the effectiveness of such devices. In any case, the additional cost will take a lot of time to pay off.

Replacing a laundry machine only to save energy by feeding it with hot water is unlikely to ever pay off. The ecological impact is very questionable as well.

Calculation details

I do not account for heat losses during the cycle, because those are the same whether you feed it with cold or hot water. Same reasoning for other electric costs, such as pumps, drum rotation, etc. So you dish and laundry actually costs more than the figures here, but you won't make savings on these additional costs by feeding hot water instead of cold water.

What I'm calculating below is the cost for heating input cold water with the built-in electric heater of your device. The actual savings you can make depend on the fraction that you can save for each liter of hot water (see the table at the top of this answer), by using a more efficient central boiler.

Dish washer

One cycle uses around 15L, of which 1.5L comes cold in all cases, so 13.5kg. You need 4.18KJ/kg/K * 13.5kg * 40K = 2257KJ = 627Wh. If you need 150 cycles per year, that's 94kWh per year, about $11 per year.

Laundry machine

Same calculation, but for 50L-1.5L of water at 40°C: 4.18KJ/kg/K * 48.5kg * 30K = 6082KJ = 1689Wh. If you need 100 cycles per year, that's 169kWh per year, about $20 per year.

Numbers used and sources

  • Water used for each dish washer complete cycle: 15L
  • Water used for each laundry machine complete cycle: 100L, of which I make a wild guess that 50% is hot, 50% cold for rinsing
  • Water temperature for dish washer: 50°C
  • Water temperature for laundry: 40°C
  • Average cold water temperature: 10°C
  • Water specific heat capacity: 4.18KJ/kg/K (note that when calculating temperature differences, 1 Kelvin is the same as 1 Celsius degree)
  • Electric heater efficiency: I assume 100%
  • Propane cost in the US: $2.50/gallon
  • Usable heat energy per gallon of propane: 85% of 12.8kWh/kg, which gives 25kWh/gallon for 0.5kg/L at 60°F. This gives a usable heat cost of $0.10 per kWh.
  • Electricity cost: $0.12 per kWh
  • Electric power plant efficiency relative to primary energy content, for fossil fuel plants: 35% (1kWh of electricity is 3412Btu)
  • Losses on power lines between the power plant and your house : 12%
  • heat pump Seasonal Coefficient of Performances: 3 (meaning for 1kWh of electricity your generate 3kWh of usable heat). This varies depending on the heat pump on your climate, but recent equipment should revolve around 3 in temperate climates. I'm speaking about air-water heat pumps here, you can get a little more with geothermal ones.
  • I'm neglecting the fuel spent to deliver propane at your house, compared to what it takes to deliver it at a centralized power plant
  • The pipe between the central boiler and your appliance contains about 1.5L of water (give or take, largely depends on your house, this has been measured in my house for ~10m distance)
  • Warm water that remain in the pipe is not completely lost, about 50% will be reused (house heating in winter, usage in another appliance or tap water on the same pipe). This is a personal estimation, not meant to be accurate. Anyway, I'm neglecting it in above calculations.
  • Capital cost of various heating system is not accounted for, based on the assumption that you already have chosen your heating system for other reasons than laundry savings
  • This doesn't take into account HPWHs (which are weird) and propane (which is $$$) – ThreePhaseEel Sep 10 '17 at 0:58
  • @ThreePhaseEel I did mention heat pumps, you can save about 2/3 of the electric consumption with those. But 2/3 of $10 per year is still little. It's better for laundry machines, but then it should be designed for the task. Not sure what you mean about propane (mentioned as "gas" in my answer), in this case you'll save primary energy (bypassing losses in the cables between the power plant and your house, and poor efficiency for electric production), but as you say, not much on money. I'll add a table with gain sum up for different energies. – youen Sep 10 '17 at 7:00

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