Dishwasher supply (hot vs cold), pearls of wisdom?

Sillychuckie

Hello.
I see that for the most part (80-90% of the market), dishwashers act similarly to washing machines in that they take a cold water supply and heat it themselves to whatever temperature they need for the cycle they are running.

However, there are some on the market that can take a hot supply also.
I thought that this would be better, as I can make hot water in my plant room (gas, or potentially in future, solar, heat pump etc) more efficiently than I can heat it at the dishwasher (electric).

I understand that in this scenario, the dishwasher still has a heating element and still heats the water, but probably has to do it less if you are providing water which is already warm to hot.
Sounded like a no brainer to me.

However, they all have fairly low energy ratings and I'm starting to wonder why.

I have a theory - which is that a dishwasher can do *some* of its cycle, using cold water (e.g. a pre-rinse, or a final rinse... etc). I have never owned one before, so don't know. This might explain why using a hot supply is a bad idea and could end up being less cost effective.

Does anyone have any pearls of wisdom here?
I think I'll stick to the 80% crowd rather than the 20% - but, as more people move over to renewable sources for their heating needs, perhaps there is value in this.

In an ideal world, a dishwasher would take 2 supply inlets. Cold, and  Hot.
It would use Cold for the pre-rinse cycle and any time it definitely doesn't need hot...  then it uses the hot supply (which may or may not be hot, depending on the weather/boiler/cylinder/whether my wife just had a bath etc...), and tops it up with its own internal heating element as it needs to.

However, I have not found 1 Dishwasher on the market that takes 2 supplies (hot and cold) and switches between them during the cycle.

Does anyone know if something like this exists?
Thanks. 

SC

grumbler

They use relatively small amount of water for each cycle. Unless a boiler or a hot water tank is very close to a dishwasher, in practice very small amount of hot water reaches it. I think this is the main reason why most in UK have no hot water inlet nowadays

Section62

Sillychuckie said:

I thought that this would be better, as I can make hot water in my plant room (gas, or potentially in future, solar, heat pump etc) more efficiently than I can heat it at the dishwasher (electric).

The reason why most dishwashers and washing machines are cold fill only is because you won't be making hot water more efficiently elsewhere.

It may be cheaper to heat hot water using another method, but direct electric heating of water (e.g. by a heating element) is as close to 100% efficient as you are likely to get in a domestic setting.  Why your approach will be less efficient is that you aren't allowing for the wasted hot water which will remain in the pipes between your plant room and the dishwasher.

Each time the dishwasher (or washing machine) draws some hot water some of it will remain in the pipework and cool down, wasting the heat energy.  And if the water has cooled down to near cold then you are effectively having a partial cold fill anyway, and the dishwasher will need to use more electricity to reheat the water.

Sillychuckie said:

I have a theory - which is that a dishwasher can do *some* of its cycle, using cold water (e.g. a pre-rinse, or a final rinse... etc). I have never owned one before, so don't know. This might explain why using a hot supply is a bad idea and could end up being less cost effective.

The main reason is as above - directly heating the small amount of water needed is more efficient than having hot fill.

Some makes/models also improve efficiency by operating a heat exchange system - the water for the next part of the cycle is partially heated by the water already in the washing chamber before it is pumped out to the drain, thus reducing the amount of electrical heating required in the next part of the cycle.

Hot fill is mainly an advantage where time is the critical factor (e.g. in a commercial kitchen) - if the machine is filled with preheated water then you don't need to allow (so much) time in the cycle for the water to be heated up to the required temperature.  In that environment having the machine operating at a high level of efficiency is less important than having the plates/cutlery/glasses clean and ready for the next use.

Veteransaver

Modern dishwashers only use about 10litres of water in a whole cycle. So generally they simply won't get enough hot water anyway, and it's actually less efficient as hot water from the tank or boiler will be sitting in the pipes after filling the dishwasher with cold/lukewarm water.

ka7e

You need hot water for the final rinse as it's the residual heat that dries the dishes.

Sillychuckie

Ok, thank you all. Appreciate all the responses and great information.
You are right of course that where I wrote 'efficient', I really meant 'more cheaply'....  but your point on heat loss with the pipe flow, is noted.

I'll go for a cold fill and be thankful for the efficiency improvements etc. Hopefully it wont cost too much to run as a result. Thanks all.

Postik

Seems like a lot of dishwashers in the USA have a hot feed.  I see a lot of people over there recommending to run a hot tap before turning the dishwasher on.

Also, unlike in Europe, they generally don't have water softeners in their dishwashers.

I wonder why there is such a difference?  You would expect that hot/cold feeds and hard water would have the same issues/benefits regardless of where they are in the world.