Is a hot and cold fill washing machine more economical?
Many people assume that a washing machine with a hot water valve must be more economical – after all, using pre-heated water seems like it should save electricity. But the reality of how UK homes deliver hot water means this assumption is usually wrong. This guide explains why, using a simple practical experiment to make the point clearly.
In most UK homes, a cold-fill washing machine is more economical than one that can draw on both hot and cold water. The reason is straightforward: modern washing machines use very little water, and hot water takes too long to travel from its source to be useful. By the time genuinely hot water arrives, the machine has often already filled. The cold water wasted running the hot tap in advance can exceed the entire wash water volume.
Why Hot Fill Sounds Sensible – but Usually Isn’t
The logic behind hot and cold fill machines is appealing. If your boiler or hot water cylinder has already heated water, using that pre-heated supply should mean the washing machine’s heating element does less work – saving electricity.
This reasoning is sound in principle. But it rests on an assumption that is rarely true in UK homes: that hot water arrives at the machine quickly enough to be useful.
Your hot water cylinder or boiler is located very close to the washing machine. Hot water arrives within a few seconds of turning on the tap. The machine fills slowly enough that a meaningful volume of hot water enters the drum before the fill level is reached.
Hot water tanks are usually located upstairs. Combi boilers may take 30 seconds or more to deliver genuinely hot water. Modern washing machines fill quickly and use far less water than older models. By the time hot water arrives at the machine, filling is often already complete.
The Practical Experiment: How Much Water Is Actually Used?
Our engineers carried out a simple experiment to demonstrate the issue in concrete terms. The results are instructive.
How much cold water runs before the hot tap delivers hot water?
We ran the hot tap into a washing up bowl and measured how much water had flowed before it became genuinely hot. The result: approximately one full washing up bowl of cold water ran from the hot tap before any hot water arrived.
That cold water is wasted. It goes down the drain before hot water can reach the machine – and in many UK homes the situation is worse, particularly where the hot water cylinder is far from the kitchen or utility room.
How much water does a washing machine actually use in the main wash?
We then measured how much water a washing machine used during a complete main wash cycle on a Cottons 40°C programme. The result: approximately the same volume as the bowl of cold water wasted running the hot tap.
In the experiment, the volume of cold water wasted waiting for the hot tap was almost identical to the entire wash water volume. A hot and cold fill machine would need to fill almost entirely from the hot supply just to get a small amount of hot water entering at the very end. In most households, the practical contribution of hot fill to the wash temperature is negligible.
The Hidden Inefficiencies of Hot Fill
Beyond the basic problem of delayed delivery, hot fill washing introduces additional inefficiencies that are rarely discussed.
Every time the hot tap runs, cold water sitting in the pipe must be displaced before hot water can flow. This wasted water has to be heated again the next time the tap is used, consuming energy for no benefit at the machine.
When hot water is drawn from a storage cylinder, cold water enters to replace it, cooling the stored volume. The cylinder then has to reheat to restore temperature – using energy that was not needed if the washing machine had simply heated cold water itself.
Modern washing machines are not designed to compensate for variable incoming water temperature. If hot water arrives partway through a fill at an unpredictable temperature, the machine cannot reliably control the final wash temperature. Cold-fill machines heat from a known baseline, giving consistent results.
Modern machines use significantly less water than older designs – sometimes as little as 6-8 litres for a main wash cycle. This small volume means the window for hot water to arrive and make a meaningful contribution is extremely narrow.
When Might Hot Fill Actually Help?
There are circumstances where a hot and cold fill machine could deliver a genuine efficiency benefit – but they are uncommon in typical UK homes.
- The washing machine is in a utility room directly adjacent to a hot water cylinder, with a very short pipe run
- Hot water arrives at the tap within five to ten seconds of opening the valve
- The machine’s fill cycle is slow enough that hot water genuinely enters the drum in meaningful volume
- The machine is used for high-temperature washes (60°C or above) regularly, where the energy saving from preheated water is larger
If your kitchen or utility room is far from the hot water source, or you have a combi boiler where hot water delivery takes time, hot fill is unlikely to offer any practical benefit.
What About the Common Belief That Hot Fill Saves Money?
The belief that using hot water is inherently more economical comes from an older era of appliance design. Earlier washing machines used much larger volumes of water and heated it slowly using lower-power elements. In that context, drawing on already-heated water from a cylinder made genuine sense.
Modern machines are fundamentally different. They heat water quickly with high-power elements, use far less water, and fill in a short time. The opportunity for pre-heated water to contribute meaningfully has largely disappeared.
Washing machines are not sophisticated enough to handle the wide variation in how hot water is delivered across different UK homes – especially when the delay before hot water arrives can range from seconds to over a minute. Cold-fill machines remove this variable entirely, heating from a known cold baseline and delivering consistent wash temperatures regardless of your plumbing setup.
Our guides cover everything from cold fill versus hot and cold fill to energy ratings, reliability, and what really matters when choosing a machine.
Frequently Asked Questions
Is a hot and cold fill washing machine more economical than a cold fill?
In most UK homes, no. The potential saving from using pre-heated water is undermined by the fact that modern machines use very little water and fill quickly. Cold water wasted waiting for the hot tap often exceeds the entire wash water volume. Unless hot water arrives at your machine within a few seconds, a cold-fill machine is almost certainly more economical for typical use.
Why don’t modern washing machines have hot water valves?
Most modern UK washing machines are cold-fill only because, for the majority of homes, a cold-fill machine is more practical and often more economical. Manufacturers moved away from dual-fill designs as water usage per cycle fell and heating element technology improved. A cold-fill machine provides consistent, controllable wash temperatures regardless of the home’s plumbing configuration. See our guide: cold fill washing machines.
How much water does a modern washing machine use in the main wash?
Modern washing machines typically use between 6 and 15 litres of water for a main wash cycle, depending on the programme and load size. This is significantly less than older machines, which could use 40 litres or more. The small volume is one of the key reasons hot fill is rarely effective – there is simply not enough water filling time for meaningful hot water to arrive from a distant source.
Can I connect a cold-fill machine to my hot water supply?
Cold-fill machines are designed to be connected to the cold water supply only. Connecting them to hot water can cause problems – including filling with water that is too hot at the start of a cycle, which can damage fabrics and affect the machine’s internal components. Some people do connect cold-fill machines to hot taps to raise wash temperatures, but this is not recommended and can void the manufacturer’s guarantee. See our guide: connecting a cold fill machine to a hot tap.
Does using a cold-fill machine cost more to run than using a hot water supply?
Not in most cases. A cold-fill machine heats exactly the water it needs, from a cold baseline, using a built-in element. A hot and cold fill machine may save electricity on the heating element but typically wastes water and energy in the pipe run before hot water arrives, and causes the hot water cylinder to reheat more frequently. For most households the overall running cost is similar or lower with cold fill.
Cold fill only washing machines are fine, if you have an inefficient Combi boiler. Given that more and more people are replacing Combi boilers with Heatpumps, and hot water can be stored close to the washing machine, why are so few manufacturers not looking to ‘cash-in’ on the growing number of people who will have near instant hot water available? I have a small tank under the sink for my hot water tap, which I also want to use for my washing machine. It re-heats for free so, why wouldn’t I use?
Great article on why it’s a myth to think hot water supply works. It is easily going to increase energy bills because you’re going to have another pipe with heated hot water, just sat there
Changing from hot and cold to just cold. What is the best way? Just turn off the hot supply or some other way?
Hi Richard. I have an article on the subject here what to do with the old hot water tap
I was musing on this exact thing when I woke up and this article explained it very clearly. I would just suggest a further “bowl test” with your combo boiler to show that difference.
Interesting your point about wash time (before rinse) being governed by the time it takes for the cold water to become hot, and not just a fixed time. Are you sure? Also is the rinse in hot or cold water?
I am contemplating installing a cylinder powered by solar (not PV) to preheat water for the combi, and to store solar heat as well as an immersion heater for excess PV/wind electricity. That might change the equation.
But the other point you make about”design of detergents” to work cold is interesting. The manufacturers don’t seem to emphasise incompatibility with “hot fill” machines – perhaps because, for the reasons you say, they are a myth.
Hello Tony. Thanks for your comment and your interesting points. With my combination boiler, it takes around the same time for any water that is remotely warm to enter the bowl under the test. So with either a hot water cylinder system, or a combination boiler, it would be utterly pointless for me to bother with a hot and cold fill washing machine.
Having said that, if my washing machine was in the kitchen, and the combination boiler was in the kitchen, then there would be some hot water getting in during the wash but still with a delay. But with a washing machine only using less than a bowl full of water, it is pointless being concerned with having a hot water supply – even if you mostly use 60° plus wash cycles. Most people used 30 or 40° washes, in which case hot water supply would be of no use. And in fact using hot and cold fill washing machine could be very counter-productive because a lot of hot water gets heated up but not used as it is drawn into the pipework only to just cool down and be wasted.
All washing machines rinse in cold water. I’ve heard an argument that using warmer water can be slightly more effective but I’m not sure if this is actually true. I think warmer water is more likely to excite detergent and create extra suds. This would give the impression that more detergent is being removed. But this could be totally false. Anyway, all washing machines I have ever known have always rinsed totally in cold water, and it seems to work fine if done correctly, with the right amount of water.
Wash times have always been governed by the time it takes for the water to heat up. The temperature of the water is constantly monitored. If the software detects that the temperature of the water is not increasing, it will abort with an error. So a washing machine is aware of the temperature at all times.
Back in the day, if I was testing a washing machine’s wash cycle (when they used to be hot and cold fill) I would turn off the cold water tap during the initial fill. This would get the wash water temperature off to a good head start, which cut the wash time down dramatically.
Washing machines historically have filled up with water and started to wash. They then continue to wash until the thermostat closes at the designated temperature. At that time, the washing machine would either instantly move onto the rinses, or it may be that they just wash for a short time longer.
If washing machines just used a set time period, the results would be pretty uneven and unpredictable. Modern washing machine detergents are designed to work at specific temperatures. So for example, if someone’s washing machine heating element was caked in limescale, their washing machine may never even reach 40° on a 40° wash cycle due to reduced heating efficiency. Also, in winter, the temperature of the cold water entering the washing machine would be many degrees lower than it is during summer. So you would get different wash results in summer to winter. It makes much more sense to ensure that the water temperature has reached the required level before moving onto the next stage. This is why I believe adding hot water to a cold fill washing machine can detrimentally affect its washing efficiency.
It’s always possible that they have changed how they work for some reason that I have not been aware of. But I can’t see of any advantage at all with washing for a set time period, with no knowledge of the temperature of the water.
You’re wrong when you state,”the majority of households in the UK get their hot water supply from a hot water tank upstairs” the vast majority have combo boilers. Almost 55% giving almost instant hot water.
Thanks Clive. It was correct at the time of writing, and more importantly at the time that manufacturers made the decision to switch over to cold fill. I will rephrase that. However, having a combination boiler makes no difference to most people in relation to this issue, because they usually still take a long time to produce hot water, by which time the washing machine will have finished taking in water. If a combination boiler is very close to the washing machine, then this may not be such a problem.
As it happens, in every house I have ever lived in, there has been a hot water cylinder storing hot water. But after 17 years in my current house, we have just had the boiler replaced by a combination boiler. The boiler is in our loft, which admittedly is further away than most, but it actually takes slightly longer for hot water to run through our taps with the new combination boiler than it did with the old hot water cylinder.
It would be nice if manufacturers could state the maximum temp that their machine could take in on the COLD inlet – as I’m thinking of having an under-sink type boiler that pre-heats the water to 30C from my solar panel over say 15-40 mins.
The boiler would be on a solar diverter circuit, i.e. only heating with pure solar electric.
My washing machine would be on a slight delay to allow the water in the boiler to heat up.
Hi James. Tbh I think preheating the water has questionable value. If it’s free to heat the water up that’s not so bad. But modern washing machines and detergents are designed to work best when water is cold and slowly heats up. This is especially the case for biological detergents. So biological detergents in particular could have reduced effectiveness.
Also, washing machines tend to wash until the required temperature is reached, and then pump out the water, and move on to rinsing. If water is already 30 degrees when it starts, then any 30 degree wash cycle is likely to substantially reduce the wash time. 40 degree wash cycles could also cut the wash time substantially. This would be likely to adversely affect how well it washes the laundry.