Tuesday, 25 March 2014

Advice on dishwasher connection running hot and cold

When we were planning the kitchen, we were advised to plumb cold water into the dishwasher. I think the reason given was that if the dishwasher is in control of its own water heating, then it will be at the right temperature. Domestic hot water was going to start off cold until water from the boiler got through the pipes, and this would upset the wash.

Meanwhile, the Japanese manual we have for the dishwasher announces that you can save energy by connecting hot water rather than cold water, since the dishwasher uses electricity to heat water, and electricity is a very expensive way of producing heat.

At the one (and a half) year inspection last year, one of our questions was about the dishwasher plumbing, and the builder came out with the same story we'd heard at the beginning: that the domestic hot water may be cold when it is first turned on as it goes through the pipes.

Further discussion with the Japanese suppliers of the German-made dishwasher confirmed this. Also, rinsing with cold water first is apparently better for removing proteins, like egg, which can be baked on with hot water and become more difficult to remove. So plumbing cold water allows the dishwasher to use both hot and cold water, but if you plumb in hot water, it can only use hot water.

There is a range of forum discussions online discussing whether to plumb dishwashers with hot or cold water, which seem to veer towards the side of cold water and leaving the dishwasher to deal with the heating, unless you're getting your hot water from a solar thermal system, in which case the hot water is effectively free.

But our hot water is not solar thermal, and even though the domestic hot water would save some electricity, and some cost since it's using night time rates rather than evening rates, so it doesn't represent free heat. The water is still using some electricity, probably more efficiently, but by the time it's lost heat waiting in the boiler to be used, lost heat in the pipes on the way to the dishwasher, been used when the dishwasher would rather have been using cold water, and possibly been further heated, then the saving is really marginal.

So once again, we're counting the number of angels that can dance on a pin head.