Connect a cold fill washing machine to the hot water tap

Do's-and-Don'ts I’ve had several people ask if you can connect a cold fill washing machine to the hot water tap instead of the cold one. They were unhappy about the disappearance of the hot water valve in UK washing machines, and were wondering if they could circumvent the change.

Many people who are using solar powered energy to heat their water, or who had another cheaper environmentally friendly supply felt that the advantages of their environmentally friendly and economical hot water will be wasted by not using their hot water supply.

However, the answer to the question is no, for the following reasons..


Hot water supply is too hot

The incoming water temperature would be too hot (usually at least 60 degrees Centigrade). This can damage delicate laundry and shrink woollens.

There would be no way of controlling the temperature of the water going into the drum and onto the laundry.

A washing machine designed to use hot & cold water will control the temperature of the water in the drum by either filling with a mix of hot and cold – or filling only with cold water on all wash cycles except the really hot wash.

Too hot for most wash cycles

The temperature of household hot water is also too high for most commonly used wash cycles, which only need 30 or 40 degrees.

It is pointless putting on a wash cycle that needs to heat the water up to 30 or 40 degrees if the water is 60 degrees from the start.

So apart from potentially damaging some laundry, the washing machine’s thermostat would close almost immediately and the wash cycle would move on to the rinsing too soon. This would shorten, but compromise the wash quality.


Hot water not good for biological detergent

Biological detergent contains living enzymes. These enzymes are killed off at the temperature of most people’s hot water. So filling with hot water only would again compromise wash efficiency when using biological detergents.

Biological detergent is more effective when starting in cold water with the water gradually heating up.

Rinsing in hot water is very bad

If a washing machine is only connected to the hot water supply then obviously when it comes to rinsing the laundry – the water going in would also be hot.

So the laundry would be rinsed in hot water. This would cause severe creasing, as well as wasting all the hot water and being very energy expensive. Also hot water tends to activate detergent and create suds whereas cold water doesn’t and is therefore more likely to be better suited to rinsing laundry.

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What about connecting both hot and cold hoses to a cold fill washing machine?

Hot-water-plumbing You can’t swap the cold water supply for a hot supply for the reasons given in this article. However, there might be an argument for using a y-connector to connect both hot and water supply at the same time. However, this seems a lot of trouble for little benefit. You would need to manually adjust the water pressure of both supplies to get a balanced, “warm” combination.

But as hot water usually takes a while to run through the pipework, the chances are it wouldn’t use much if any on the wash cycle anyway. This is in fact one of the main arguments for cold fill only machines. It would do all the rinses in warm water, which may (or may not) be better. But it would also use a lot more energy.

A full explanation as to why almost every washing machine is cold fill these days is here – Pros and cons of hot and cold fill verses cold fill washing machines

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57 thoughts on “Connect a cold fill washing machine to the hot water tap”

  1. All of the ‘reasons’ put forward above for not using a thermostatic valve to supply a cold-fill only washing machine just don’t stand up. I’ve had a thermostat feeding my cold-fill washing machine for years, and it must have saved me a few hundred ££s in that time.
    The thermostat is a cheap shower mixer bar, set to its default 38 degrees Celsius. The hot water comes from a multipoint gas boiler that currently costs 4p per KWh to heat the water, while electricity costs about 20p per KWh. Expensive electricity is therefore only used to raise the water temperature by 2 degrees for a 40 degree wash, saving 50p to £1 on each average wash cycle.
    In my opinion, the cold-fill only washing machine was an utterly idiotic idea in the first place. The hot fill inlet could just have been retained, and easily blanked off by those people who like wasting their money!

  2. Andy Trigg (Whitegoodshelp)

    Hello Pete. Thanks for your contribution and for sharing your experience. I don’t have a single “reason” against connecting a cold fill washing machine using a thermostatic valve. My article is only about connecting it directly to the hot water tap, which would be a very bad idea as pointed out. I don’t mention thermostatic valves in the article although I’m not convinced even they are a good idea although obviously you prove it can be done.

    I’m not sure though that if you have thermostatic valve to constantly supply water at around 40° how much money will be saved. There is no need for a washing machine to rinse in water at 38° so arguably it’s a waste unless you turn off the hot water after the washing machine is filled for the wash cycle.

    Also, with this set up unless the boiler is only a few feet away from the washing machine you are likely to suffer the same problem as most of the UK in that because modern cold fill washing machines hardly use any water on the wash cycle (you can’t use even see much water while it’s washing) then by the time the hot water is actually running into the washing machine it has often virtually filled up with all it needs. So therefore can be very difficult to get much hot water into a washing machine for the main wash. I have discussed this problem at length here pros and cons of hot and cold water washing machines versus cold fill only

    Modern washing machines use the overwhelming majority of their water during the rinses. They hardly use any on the wash in comparison. So here we are comparing heating a few litres of water at 20p per kilowatt-hour with heating at the very least several times more water at 4p per kilowatt. Someone would have to do the maths but maybe the savings aren’t as big once you take into account all of the rinse water being heated up. I won’t be surprised to be fair if you are still on the winning side though :-)

  3. Andy Trigg (Whitegoodshelp)

    Hi Pete. I forgot to mention that one of the problems with injecting hot water artificially into a wash is that a cold fill washing machine is designed specifically to get good wash results by starting with cold water. This is particularly true when using biological detergents. It’s designed in such a way that it takes roughly a specific amount of time to heat the water up to 40° and during this time the cleaning is achieved. Once the water reaches 40° the chances are that the wash cycle is complete and the dirty wash water is drained out for rinses to commence.

    If you inject water into the washing machine that is already almost at the correct temperature, this is likely to substantially reduce the wash time and therefore substantially reduce how well the laundry is washed. Again someone would have to test this in a laboratory to prove it 100% but to me the logic makes total sense. It might be interesting to do an experiment by doing two washes, one with a thermostatic valve, and one with the hot water turned off to see if and by how much it affects how long the washing machine washes for.

  4. When I bought my present washing machine about 10 years, I was unhappy that it only had a cold fill. So I set about thinking how I could overcome this, and came up with the idea of fitting two leaver type valves back to back with the leavers bolted to each other so in the first position one valve would be open while the other was closed, then when you moved the leaver to the second position the valve that was closed opened and the valve that was open closed. one of the valves was fed from the hot supply and the other was fed from the cold supply.
    When I wanted to use the washing machine I would first run the hot tap nearest to the washing machine until it had hot water running, that would clear the cold water sitting in the pipework. I would then put the leaver valve in the position that fed hot water into the washing machine.and start the machine:
    When the machine had done its wash cycle after about 1/4 hour, I would then move the leaver valve to the second position which would put cold water into the machine for the rest of the rinse part of the programme:
    Win,Win situation

  5. Andy Trigg (Whitegoodshelp)

    Hello Paul. I think I would also apply my last two comments (in reply to Pete) to your situation. The problem with trying to circumvent a cold water washing machine by deliberately introducing hot water essentially it boils down to one thing, which is that a cold fill washing machine is designed specifically to wash effectively starting with cold water and slowly heating it up. Therefore introducing hot water from the start is highly likely to shorten the wash cycle (and your quote of 15 minutes seems to confirm this) because they are designed to finish washing once they have reached temperature. So introducing water that is already at or close to the wash temperature is likely to impact wash performance.

    It also wastes hot water although how much is likely to be different depending on the systems in individual houses. Hopefully yours and Pete’s individual situation is good and it all works satisfactory for you. But for most people it’s best to feed a modern washing machine with cold water because they are specifically designed to work best that way. These days they hardly use any electricity to heat up a litre or so of wash water. Anyway the complicated argument is in my article pros and cons of cold fill versus hot fill washing machines if interested :)

  6. Love my American top loading washer, currently a Speed Queen. It has both hot & cold connections, washes a full load in a fraction of the time taken by a modern European front loader & clothes come out drier after the wash. When hot wash is selected, the built in thermostatic valve regulates the incoming water temperature & warm rinse can also be selected. Apart from the physical size & cost, I’m not sure why they aren’t as popular here in the UK.

  7. Andy Trigg (Whitegoodshelp)

    Hi Nigel. Yes top loaders are rare in the UK. I think it’s because the majority of UK houses never used to have a wash room so washing machines had to fit under a worktop in the kitchen. Most still do although modern houses always have a utility room now but even so they often come fitted with a sink and small worktop and are quite small.

    Thermostatic valves are part of the potential solution to properly utilising hot water but in the UK most people get their hot water supply from a hot water cylinder in a cupboard on a higher floor than the washing machine – sometimes two floors higher. This means it takes too long for hot water to arrive at the washing machine and as modern washing machines use so little water the washer has finished filling before hot water gets inside for most people. This is demonstrated nicely in my latest article Is a hot and cold fill washing machine more economical?

    Therefore a thermostatic valve would need to monitor water temperature until it detects hot water. Tis could be as much as a washing up bowlful. Then, either pump out or store the water drawn in in. so it can start the cycle in a position where it can totally control the incoming water temperature.

    But then you would also introduce hot water wastage, because all the pipes from the hot water cylinder to the washing machine (which could be several metres) would now be full of piping hot water. In most cases it would just slowly cool down and be wasted. And all the water that had been drawn into the washing machine and all of the pipework would have been replaced in the cylinder with cold water. This could be several litres and
    it will cool down any hot water stored inside which has used energy to heat up and will need more energy to re-heat.

    So essentially when you look at the big picture, for most people it really is a lot cheaper to just heat up the water in the washing machine. Many people though will have different circumstances, and may have a “free” source of hot water with solar panels, or may have the washing machine next to a powerful combination boiler that can deliver hot water to the washing machine in 3 or 4 seconds. In those cases a sophisticated washing machine with thermostatic valve would be ideal.

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