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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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
It depends how you heat your domestic hot water.
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.
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.
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.
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.
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.
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.
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.