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  Hot Fill Washing Machines

Hot fill washing machines are a waste of time for most people, we explain why


We get asked all the time about connecting a washing machine to a hot water supply, usually this is for purposes related to some sort of renewable energy source that heats the water and stores it in a tank such as solar heated water or geo-thermal systems. Essentially people want the environmental benefits of using heated water from a renewable source and, more often, to save money by doing so.

This is a laudable goal but, can it be achieved in the real world?

Bear in mind that modern washing machines will calculate water load based on conditions and load so the amount of water required per load will vary and will almost never be the same twice. Most will fill with about three or four liters of water to start a wash cycle.

   Experiments To Try At Home

These little experiments are not designed to be 100% accurate or truly scientific in nature. What they are intended to do is demonstrate the reality of delivering "hot" water from a solar system or hot water storage tank in particular, to a washing machine is not as simple as most people think. It's not a case of connect to the hot supply pipe and you're done, I'll explain why that is.

A hot water connection for a washing machineWashing Machine Hot Water Fill Experiment 1

The first thing to do is calculate how much of a pipe run that you have from the hot water source, usually a storage tank in the loft in the case of solar heated water, to the delivery point, i.e. the point at which the washing machine's fill hose is connected.

Now, most UK homes (most EU probably) use 15mm copper pipe for internal water pipes and this holds a volume of approximately 0.15 litres per meter of pipe.

So, simple calculation, multiply the meters of pipe run by 0.15 and you have the volume of water, in liters, that is "standing" inside the pipe. It is important to remember that pipes will not run directly from point to point most often, they will follow a path around rooms, other pipes and so on so you will have to account for this.

Then don't forget to add about .3 litres for the water in the standard 1.5m fill hose and the water left in the washing machine.

Most homes will have a around a 6m or so pipe run, in a good number of installations many will have much longer runs. What this means is that, at best, you will have about 1-2 litres of "standing" cold water in the pipe before you even get a whiff of hot water.

If you have a combination boiler then you need to also consider the standing cold water in the system there, if it isn't running and then the time that it takes to deliver hot water, see more below.

So, we just shown that a good bit of the of the "hot" water you just filled with is, in fact, stone cold or, at best at ambient temperature. In practical terms for most UK homes and, for most people, more than half the "hot" water that you fill your washing machine with will be at room temperature. At best.

Washing Machine Hot Water Fill Experiment 2

Take one empty plastic soft drinks bottle that you can have a "reasonably" accurate measurement of the volume with. A favorite of ours when we demo this is a 1.5l soft drinks bottle.

Now, put the bottle under the hot tap in your kitchen or utility room beside the washing machine or, as close as you can get. Open the tap and fill the bottle. Close the tap then fill it again. Later, if you like and just for fun, you can do this once more.

In the meantime take the first bottle and in the interests of fairness pour it into a plastic basin or suchlike for two reasons, plastic will retain the heat better and most cheap washing machines these days have plastic tanks. Now, add the second bottle.

You will then have a plastic basin with about three liters of what your washing machine would get as being "hot" water. You'll notice one very important thing, it isn't hot. At best, it's luke warm usually.

Meanwhile, what you have done is fill the pipework with hot water from the tank.

Washing Machine Hot Water Fill Experiment 3

There's another little issue with hot water that we can demonstrate although, you've probably seen this and just never known why it happens, most people just accept it.

Asides from the fact that a tank fed system has lower water pressure than mains (virtually always) there's a little physics trick that hot water displays and, this is going to sound a bit odd at first but, hot water weighs less than cold water.

What this means is that, as the water gets warmer the gravitational force on it reduces and so, in a gravity fed tanks system, the flow slows down. Granted it's only slight but, it does.

Just open the hot water tap in your kitchen and let it run. Assuming it's fed by a storage tank the flow will reduce as the water gets warmer but only very slightly. What this does is extend the fill times and, after you've filled with luke warm water (which many people think is hot) then it's hot water going onto cold water, slowly. This cools the incoming hot water more than is ideal.

Washing Machine Hot Water Fill Experiment 4

This is a good one to prove the case. You may need an assistant for this one depending on how far apart your washing machine and normal taps are and, it can be tricky to get right but it is worth it.

Most modern machines will fill with cold water only on a normal 40˚C cotton wash, make sure that this is the case.

Now, put a typical load in the washing machine just as you would normally and get ready to start the machine.

Put a basin under the tap in your kitchen or utility room.

Try to press start on the washing machine and open the tap on the hot supply at the same time.

You will hear the water filling into the machine and, when it stops, close the tap.

The machine will generally then start to agitate, stop briefly, then fill again a little, you need to mimic that fill with the hot tap as before.

Once this is done you have, in your basin, what is a reasonable representation of how a hot fill would go from your own installation circumstances and it is probably fair to say that the water and the water temperature that you have in the basin is not what you expected. Probably pretty far removed from what you expected.

  Hot Water Experiment Conclusions

Most probably you've gotten by now what we are saying here (again), hot fill is, for most people, a total and utter waste of time and effort and certainly not a reason to buy or not to buy a particular model.

Of course most that are available with a hot fill only do so at one point in the program and, unless you want to stand over the machine and run off perfectly good cold water before it happens to decide to fill on hot, then it's not going to save you a bean or, use any stored hot water really. It might fill up your pipes with hot water but that's about all it would do.

What we've given you above should be more than enough to prove beyond all doubt that, for most "normal" UK installations, you cannot get hot water into a washing machine as easily as you may think you can.

Please consider all the above before you buy a new washing machine.

  Can A Hot Fill Washing Machine Be Done

Short answer, yes.


Hot and cold water fill valves on an ISE W288eco washing machineIn order to get hot fill working and, working correctly, you need to meet certain installation requirements and you need a machine that will give a true "intelligent" mixed fill. Like the machine that ISE introduced that is sadly no longer available since the demise of ISE.

On installation you need to have the pipework insulated from storage tank to delivery point in accordance with Passivhaus or AECB standards, a simple internet search will give you all the gory details on both but, don't expect light bedtime reading!

Pipe insulation to prevent heat loss as much as possible is absolutely essential in order to deliver water that is as hot as possible. If you don't insulate the pipework, you go back to hot fill being a complete waste of time and energy. This will apply to any storage or hot water delivery system whether it is a solar, geo-thermal, wind generated or communal heating system as, it's all very well producing your hot water needs from renewable sources but not so bright if you just let all that energy seep into the atmosphere.

Then we hit a slight roadblock, for the UK at least.

There are no washing machines at the time of writing available on the UK market that can make use of such a feature or indeed many (any?) hot fill washing machines at all.


Simple, no demand for it and, even if there were, most people wouldn't understand it or the installation requirements to actually achieve the (up to) 70% reduction in energy use when you use a renewable resource for the supply of hot water! It's too complicated, it's expensive and so, nobody does it... yet.

Any other machine that offers a "hot fill" will be a dumb hot fill, usually it only draws hot water on the first fill after the pre-wash on a boil wash. If in doubt ask the manufacturer and, if they are either unwilling or unable to tell you what it does then assume it's a dumb fill and not an intelligent one.

For us at least, these are a bit of a con. They are designed to make people think that the machines are filling with hot water but the reality is, they don't. They normally only take hot on the first fill after prewash on a full 90˚ program only and will not take hot water at any other time.

So yes it can be done but, don't expect it to be a simple "plug and play" solution, don't expect it not to involve a bit of DIY effort or contractor work and don't expect that any machine boasting a "hot fill" function will satisfy what most people think that actually means. And, don't expect the machine to be even slightly cheap.

In the meantime, insulating your hot pipes properly will not do any harm and will in fact save you money and wasted energy, it's a long payback period but, it will eventually.

Eveart Boniface
Cold fill washers
Since 1964 we've had a number different makes of washers all with hot and cold fill except for the most recent which is cold fill only. It's a good machine but it takes three times longer to do the same job; thus it destroys any economic advantage over a hot and cold fill machine as well as being a pain because it is so slow. Now seeking a reasonably priced hot and cold fill machine . EB
Nicholas Wade
Incomplete info
Shame, I wish you had substantiated your claims about why hot water is not as efficient by including relative costs. ie. cost of filling with hot water from a combi gas boiler as against the cost of heating water from electricity. I don\'t like these new machines which use minimal water as I personally don\'t think that they do a good wash and the wash times are extended to counter this, which then uses more electricity. So I have a hose connected to the hot tap and run that to get a \'good\' measure of water sloshing around and general set the machine for a 30 minute wash.
I have a combi boiler that has a small hot water reservoir within the boiler casing to give instant hot water. The washing machine is within 2m of the boiler (one cupboard away). So in my case none of the above assumptions and calculations apply. As electricity here is 4 times the cost of gas per Kwhr it would be a no brainer to have a hot water fill. It\'s quite common for kitchen combi boilers to have hot water reservoirs and to be in kitchens, close to a washing machine location.
That may well be so but, you still have to consider the fill level on a modern washing machine is very low, merely a few litres. With the standing water as well as what\'s in the machine already, the effect is largely just filling the pipework and firing the boiler for little real world benefit.

That\'s how you get the A+++ or whatever rating, just read the other articles on here in relation to that and you will understand far better.

Many people in the industry gets that most people probably do not understand this but in 99.99% of UK homes, hot fill is going to cost users more and save them little if any time on the wash cycle.

So for manufacturers under massive pressure to save energy, not waste it, the goal is to reduce the energy use therefore hot fill is almost completely ignored as irrelevant given the odds of it being of any benefit other than in very unique circumstances. Even those that do have a purported \"hot fill\" usually are not what buyers assume it to be, not by a long shot.

Nobody you speak to in the industry sees that altering anytime soon as about the only places there\'s any demand at all for it of any significance is the Nordic region and a low demand in the UK. So no real market.

Consumers have bought the cheapest possible for decades, there\'s now hardly any actual manufacturers and very, very few regional ones due to that and, even where there are they struggle to compete so the chances of a small player stating up and making a success on one point like this, highly unlikely.

No market, no demand, no-one willing to pay for it. Guess what, no products.

Ebac say they are making a hot and cold fill machine (maybe, if it ever sees production) but we do not yet know how that will work, if it\'s smart, if it just opens the taps blindly or if it is like all the other \"hot fill\" machines and only fills with hot on specific cycles. But at the price points mooted, don\'t expect it to be too clever.


Alistair Wilson
All a big con just like the EU
Cost to run using internal water heating £51.61 per year = 134.45 kgCO2
With Hot Water feed connected £28.11 = 44.82 kgCO2
Who in the UK White Goods team are in the Energy companies back pocket.
Its a no brainer - if you have an abundance of Hot Water - Why go for a cold fill washing machine or dish washer, we are being brain washed by the energy companies and their goofy clowns in the UK white goods

Ehm, excuse me!

What sort of buffoon comes on here with not a clue and spouts allegations like that?

We are not in anyone’s pocket in any way, shape or form. We have never received a single penny from any energy company and, your figures assume an absolutely ideal installation, of which none we have ever seen are or could be made to be. So, you are fundamentally wrong sir.

Please take you nonsensical conspiracy theories and your tin foil hat elsewhere and accuse other people falsely.

If you want to accuse people of stuff, do so with knowledge and evidence and here, you clearly lack both.

George R Taylor
I bought a Whirlpool 3RLSQ8033 American washer 6 years ago, the only hot fill machine I could find. This form of machine is relatively simple, takes an enormous load, is very common in USA, is a top loader and doesn't even have a heating element. It has now gone wrong, prompting me to surf the Web.I see ISE, who made a hot fill front loader, have gone out of business. Anyway, for some reason (probably calculated on the basis of assumptions which do not necessarily apply in every case, and most probably not in mine) that machine apparently was too 'intelligent' for me: it would only enable the hot fill for washes over 30 degC.My house has solar thermal, and recirculates hot water, so as to keep it near the draw-offs when I run the pump. My better half would prefer a new, fast-spin washer. Can anyone find a hot & cold fill machine which allows the user to use only the hot feed for pre-wash and/or main wash (unless cold wash is selected)?
jayne dough
May have to look at replacing mine soon.Slimline Top loader with hot and cold filler-could be a problem-mind you i could connect to hot only in the summer when my solar water heating panel is going full blast, may not be recommended I suppose.I just wish manufacturers would tell us the truth-less of the 'its for your own good...they make more profit on cold fill only so tough to those of us who want something different
Joan Dutton
On the other hand most households use gas for heating water and heat from gas is roughly one third of the cost of that from electricity.This means that even if the washing machine only takes in a thimble full of hot water it is going to be cheaper than a cold water only fill.If you only do a few cool washes and your washing machine is a long way from the boiler then, I agree, you aren't going to save much.If you do a lot of hot washes and your washing machine is a close to your the boiler then the cost of having cold fill only are going to be significant.So why don't we have a choice? Because a cold fill only washing machine is cheaper to manufacture.
Florin N
there is truth in this, but consider that the hot water comes through mostly the same pipework that goes to your shower, tub & sink, so on a regular day, when you use that hot water for washing hands a.s.o., there wouldn't be that much stagnant lukewarm water in the pipes. Obviously, the "intelligent mix" of hot & cold is a must, but i built my installation to do the mix myself: it feeds mixed water to the machine's hose, from a mixer tap i control manually. Not an automation at all, but it cuts the heating time. At the cost of the hardware. At the end of the day, it doesn't feel like a waste.

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