All that will happen is that the pipework between the hot water cylinder or boiler and the washing machine will get the most hot water drawn into it and this water will sit in the pipework and go cold. Plus if hot water is supplied via a hot water tank then all the water drawn into the pipework will be replaced by cold water from the storage tank cooling down the water in the hot tank and possibly triggering the boiler to top it back up to temperature.

Washing machines usually impulse on after reaching temperature, if not instantly at least after a set time. If you supply a cold fill machine with hot water I would expect it to reduce wash efficiency. Ultimately these machines are designed to use cold water and they have achieved A wash efficiency ratings by washing slowly and thoroughly using small amounts of cold water heated up slowly.

This issue of using hot water efficiently in the home needs a complete rethink regarding how we heat up water. For one thing, why are none of the internal hot water pipes lagged to slow down the wasteful cooling of water in the pipes?

The blogger at http://www.grumpyoldman.be/batteries-not-included/ seems to like it, but I wonder what you think? Clearly it would still need very short coupling or one would have to run off contents of the feed pipe first. (I’ll be using a combi - and anyway hope to tee-off mains pressure solar hot water before it even reaches the combi).

Also, did I rightly understood the second bullet point in your article, about warm fill undesirably shortening a machine’s cycle? So the washing machine starts timing from the moment the correct temperature is reached?

If you can’t use solar powered hot water in the washing machine and dishwasher it seems a waste. Unless a household runs lots of baths each day it’s hard to see how you would save lots of money considering the amount of initial investment needed in solar heating because most people probably hardly use any hot water if they have a dishwasher and washing machine.

I know that apart from baths and showers, our household hardly uses any hot water from the hot water system, and in fact heating up a massive hot water tank each day just to use several litres of water is a waste. I tend to agree that water should be heated up at source - just the amount needed and nothing more. A large hot water cylinder can hold over 200 litres of water. What’s the point of heating up all that each day to 60 degrees only to use a couple of bowlfuls at most?

However, solar powered hot water is still more environmentally friendly so many people will still want to convert.

It wouldn’t damage your machine to mix in a little hot water in during the initial fill process if you were prepared to go to the trouble. It should increase the initial wash water temperature and in theory the washing machine would use a little less energy. However, it could use more water because most people would need to run off a fair bit before seeing any hot water due to cooling in the pipework. Then all the water you run off plus the water that goes into the washing machine will need replacing in the hot water cylinder (unless you have a combination boiler).

In this scenario this would cool down the hot water in the cylinder as all the water used is replaced by cold water from the tank in the loft. This would trigger the immersion heater to heat this water back up to temperature causing extra energy usage that goes towards cancelling out the lower energy used by the washing machine. Hot and cold water fill washing machines always heat the water. Many people mistakenly believe they don’t but they always still use the heater and it’s all controlled by thermostats so no over heating can occur.

The points I’m making are pretty general. There will be lots of people where this doesn’t apply exactly as some may have shorter pipe runs and experience less cooling in the pipes and some may use combination boilers etc. but the average UK user still uses a gravity fed how water system as described and washing machines are designed to run efficiently with that system.

A further point on this topic is that if you make the initial water intake to hot it can interfere with the efficiency of biological detergents which work best when used in cold water that’s heated slowly. Anything over 40 degrees is bad for biological detergents as it kills off the enzymes.

Some washing machines have 30 minute quick washes still. Economy washes take longer to wash because they use less energy heating but need to spend longer allowing the detergent to work as explained here - Economy settings take much longer - why?

If I do this will it save time as the water won’t have to be heated up, or will the machine automatically heat the water up anyway and cause the machine to overheat?

OK, you can tell from those 2 questions that I haven’t a clue how washing machines work but I’d like to know the answers as I know several people who are wondering the same things. I’d rather not risk finding out by trial and error that I ‘ve written off a new washing machine!

For the record, from the late 80’s early 90s, I used to obtain endless hot water from an experimental solar panel (now defunct) so it always made sense to use a hot and cold fill machine. Running costs were very cheap, and my washing was never damaged or overheated. Even though I mainly used 30 and 40 degree washes, the hot water definitely flowed into the machine. It filled up in a fraction of the time as my new machine does.

I don’t see the point in saving up for a domestic solar panel to heat water if that water can’t be used in washing machines and presumably dishwashers too?? Cold fill only has made them a bit redundant hasn’t it?

My new machine is cold fill only but the quick wash takes at least one and a half hours as opposed to my 14 year old machine’s 40 minute quick wash. I don’t see how my modern machine is saving energy!? The parts will wear out quicker too, considering each wash will generate twice the wear and tear of the old one. It doesn’t look like progress or efficiency to me.

This would mean all rinsing would be done at 30 or 40 degrees which would reduce rinsing efficiency. It would also waste a lot of heated water because only a small amount of warm water is needed for wash and all rinsing needs cold.

If you mean the manufacturers should fit a thermostatic mixing valve and make washing machines hot and cold fill again the problem is that it takes so long for hot water to start running through to the machine that it’s already finished filling up by the time it does. In order to use hot water you’d have to let the hot valve run until it was delivering hot water and monitor the temperature. Then either store the cooler water that cam out first (but there’s no space to store it) or pump it down the drain and waste it. Even then, the simple act of drawing off water from the hot supply means that extra hot water would be drawn into the plumbing pipework which is likely to just cool down and be wasted. Cold water would top up the water in the cistern and cool down the rest of the hot water in the tank which may then need heating up further.

I have to say I can see why manufacturers decided it’s simpler and more economical overall to just take in cold and heat it up. The fact that a small minority of users may have ideal set ups with short pipe runs, solar powered heated water and maybe insulated pipes doesn’t detract from the fact that overall, chances are that the UK will use much less energy using cold fill washing machines than hot and cold. I remain open minded but am mostly convinced of the argument.