What percentage of a washing machine's energy is used purely to heat hot water?

Sterlingtimes
Sterlingtimes Posts: 2,358
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edited 31 December 2023 at 5:19PM in Energy
A washing machine heats water, runs a motor, runs pumps and runs controls, but what percentage is used in heating water? Bing AI suggests 80-90%.

I have just ordered a washing machine with mixed hot and cold flow. The price difference between the mix flow and a cold flow machine is a mere £30.

I note that when I run a hot tap close to the washing machine, I have to waste 3 litres to get the temperature up to 45 degrees. 
I have osteoarthritis in my hands so I speak my messages into a microphone using Dragon. Some people make "typos" but I often make "speakos".
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  • 80-90 seems about right,  you of course are wasting 3 litres of water and the heat needed to heat the water in the pipe which will then dissipate with not much effect to the overall household warmth. Let the machine do the mixing and heating…that’s my take on moneysaving, the hot pipes will normally be warmer than the cold flow anyway.

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  • QrizB
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    A washing machine heats water, runs a motor, runs pumps and runs controls, but what percentage is used in heating water? Bing AI suggests 80-90%.
    It all depends on what temperature of wash cycle you choose.
    A typical machine might only fill ten litres for the wash. Heating ten litres of water by 10C takes 0.12kWh. If you fill your washing machine with 10C water and heat to 30C, it will use 0.24kWh to do so.
    A typical 30C cycle on a modern machine might use 1kWh total, so 24% will be for water heating.
    I have just ordered a washing machine with mixed hot and cold flow. The price difference between the mix flow and a cold flow machine is a mere £30.
    £30 would but you about 110kWh at current prices. If you have averted all the water heating costs, it'll pay for itself after 440 cycles.
    This ignores the ~2p/cycle cost of using gas-heated water instead. Offsetting that cost will add another 150 cycles or so. Call it 600 cycles to break even.

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  • JSHarris said:
    I've measured this.  I ran a cool wash (which does use a bit of heating - my machine does this at 20°C), then ran the same cycle as a hot wash (60°C).  The cold wash used 0.27kWh, the hot wash used 0.74kWh.  Based on those data I'd say that your guess (or rather Bing AI's guess) is probably about right.  For the past couple of years I've only used the cool wash.  Takes a bit longer, but definitely uses less energy and I haven't noticed any difference at all in the cleanliness of anything.
    That is an interesting answer, JS. Thank you. I had not thought of measuring it that way. 

    We are heavy washing machine users, getting through a measured 539 kWh in 12 months, which would be 539 * £0.28123 = £151. At 80% of 539 kWh, the hot water cost is 431 kWh in 12 months.

    I surmise that there is a prospective saving to be made in using gas hot water heating and solar hot water heating in the summer. 
    I have osteoarthritis in my hands so I speak my messages into a microphone using Dragon. Some people make "typos" but I often make "speakos".
  • 80-90 seems about right,  you of cours
    Thank you for indicating that 80 to 90% is not an unreasonable guess.
    I have osteoarthritis in my hands so I speak my messages into a microphone using Dragon. Some people make "typos" but I often make "speakos".
  • JSHarris said:
    I've measured this.  I ran a cool wash (which does use a bit of heating - my machine does this at 20°C), then ran the same cycle as a hot wash (60°C).  The cold wash used 0.27kWh, the hot wash used 0.74kWh.  Based on those data I'd say that your guess (or rather Bing AI's guess) is probably about right.  For the past couple of years I've only used the cool wash.  Takes a bit longer, but definitely uses less energy and I haven't noticed any difference at all in the cleanliness of anything.
    That is an interesting answer, JS. Thank you. I had not thought of measuring it that way. 

    We are heavy washing machine users, getting through a measured 539 kWh in 12 months, which would be 539 * £0.28123 = £151. At 80% of 539 kWh, the hot water cost is 431 kWh in 12 months.

    I surmise that there is a prospective saving to be made in using gas hot water heating and solar hot water heating in the summer. 
    You could of course use TOU tariffs too, to load shift your washing cycles and save more.
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  • JSHarris
    JSHarris Posts: 374
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    edited 31 December 2023 at 6:08PM
    I'd definitely try a cool wash and see how you get on.  Surprised me just how well modern detergents work at low temperature.  Most of them are very effective at 20°C now, and running a wash at that temperature massively reduces the energy use over running a hot cycle.
    Years ago I was a confirmed believer that a hot cycle was the only way to get things really clean.  Not any more, the cool cycle is pretty much the only setting we use.  Only downside is that it takes a lot longer, but the machine has a built in timer so it always goes on overnight at the cheap rate, which knocks about 60% to 70% off the day time cost.  Doesn't matter if the machine takes twice as long at 20°C if we're not waiting for it to finish.
  • debitcardmayhem
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    edited 31 December 2023 at 6:22PM
    JSHarris said:
    I'd definitely try a cool wash and see how you get on.  Surprised me just how well modern detergents work at low temperature.  Most of them are very effective at 20°C now, and running a wash at that temperature massively reduces the energy use over running a hot cycle.
    Years ago I was a confirmed believer that a hot cycle was the only way to get things really clean.  Not any more, the cool cycle is pretty much the only setting we use.  Only downside is that it takes a lot longer, but the machine has a built in timer so it always goes on overnight at the cheap rate, which knocks about 60% to 70% off the day time cost.  Doesn't matter if the machine takes twice as long at 20°C if we're not waiting for it to finish.
    I remember when we had our old washer, mrs DCM always used setting 3 which was 30°C , but only. if you set the temp dial to 30….oops always used no heat but the undies always came out clean.
    New machine setting is now 4 which is always 30° …. 

    OH and the dishwasher eco setting uses more kWh than the auto/chef setting, I guess it uses less water?
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  • JSHarris
    JSHarris Posts: 374
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    JSHarris said:
    I'd definitely try a cool wash and see how you get on.  Surprised me just how well modern detergents work at low temperature.  Most of them are very effective at 20°C now, and running a wash at that temperature massively reduces the energy use over running a hot cycle.
    Years ago I was a confirmed believer that a hot cycle was the only way to get things really clean.  Not any more, the cool cycle is pretty much the only setting we use.  Only downside is that it takes a lot longer, but the machine has a built in timer so it always goes on overnight at the cheap rate, which knocks about 60% to 70% off the day time cost.  Doesn't matter if the machine takes twice as long at 20°C if we're not waiting for it to finish.
    I remember when we had our old washer, mrs DCM always used setting 3 which was 30°C , but only. if you set the temp dial to 30….oops always used no heat but the undies always came out clean.
    New machine setting is now 4 which is always 30° …. 

    OH and the dishwasher eco setting uses more kWh than the auto/chef setting, I guess it uses less water?

    You may well be right that the "eco" thing relates to water use, explains a lot.  Water is virtually free for me, as we don't have mains water, I drilled a well in the garden to get water here (far, far cheaper than getting mains water in).  All I pay for water now is the electricity to run the well pump and UV disinfection unit, plus a bit put by to pay for replacement UV lamps every year and a replacement well pump maybe every 15 to 20 years.  I worked out that our water (and I'm profligate with it as the only restriction I have is to not use more than 20,000 litres per day) costs about £200 per year all in (energy cost plus replacement parts fund cost).  I do struggle to understand why mains water costs so much more, given that they have a big economy of scale compared to my well system.
  • I’ve not used the WM above 30°C for regular clothing cycles for years - and now have switched that down to 20° as it occurred to me that the difference in temperature was so insignificant that it was highly unlikely to make any difference! Bedding and towels go through at 40°C as a rule - down from 50° a few years ago. On a routine wash, dropping the temperature saves a surprising amount of energy but also knocks 5 minutes off the wash length - accounting for it taking less time to heat the water. 
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