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Comment by Gravityloss | Hacker News Reader

Here, hot tap water is heated by district heating, while most dishwashers take cold tap water and electrically heat it.

District heating is waste heat from coal plants that produce electricity - so, depends on how you count, it can be considered free from CO2 perspective. Or then as purely made by coal, so actually really bad. Depends.

If you use electricity and the coal plant is 30% efficient, that means each kWh of electricity produced 2 kWh of waste heat.

So the calculation might not be so simple energy wise.

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> while most dishwashers take cold tap water and electrically heat it

That's surprising. Here in the US, my dishwasher is connected to the hot water line.

Maybe it does vary from one region to another, but I can't see why it would make sense to connect to the cold water line. Typically, dishwashers are near the sink (so it's easy to load), so hot water should be available in most cases.

In my parents home, their dishwasher and washing machine is connected to both. In both the houses I've lived in in the last 10 years, all the appliances I have had have had their own heaters.

> can't see why it would make sense to connect to the cold water line

My hot water line might be a different temperature to what my dishwasher works at. If it's too high, were wasting energy by heating up the water too much. If it's too low, the dishwasher doesn't work properly. Some people don't have 100% hot water on demand; in the UK and Ireland, having a hot water tank is common (and will likely be common when ground/air source pumps are installed, or if electric boilers are used), and there's no guarantee that the tank will have enough wayer at the right temperature

> If it's too high, were wasting energy by heating up the water too much. If it's too low, the dishwasher doesn't work properly.

Even if it's too high, it's not wasting compared to electrical heater, but possibly it break dish washer. If it's too low (but not cold), then just additionally use electric heater in dish washer. Still efficient than just heating cold water by electric heater.

> Even if it's too high, it's not wasting compared to electrical heater, but possibly it break dish washer

Why would it break the dishwasher? It would need a cold intake aswell, and to measure the water temperature on intake and mix it with cold water to get to the right temperature.

> If it's too low (but not cold), then just additionally use electric heater in dish washer

So now you need a hot water supply, _and_ an extra electric heater? That sounds like a bunch of extra complexity.

> Still efficient than just heating cold water by electric heater.

Electric heaters are remarkably efficient, and a dishwasher is likely well insulated. They're often closed loop systems, reusing the same water for the entire wash. The first dishwasher I found (for £350) on a national retailer advertises using under 10L of water, and 0.75kWh of energy per cycle. That's practically nothing.

Here in Japan, no dish washer have dedicated cold intake because it's nonsense. Usually hot water on kitchen can't be too high because it's pre mixed. It seems that some US model has both. Could faucet on kitchen in US supply too hot like 80C water?

Hot water supply should be available in nearby dish washer because it's on near kitchen sink (but YMMV). So no complexity added compared to cold water supply.

Yes I agree even only electric heater is used, it won't charge much higher electricity bill, but plugging hot water pipe is still efficient.

My guess is spacing issues in non-US homes. When I was growing up in the US, running the dishwasher meant wheeling it over to the kitchen sink, hooking it up to the hot water tap, and then running it.

Friends had their dishwashers built-into the kitchen cabinetry and had a water line directly to it.

I live outside of the US now in eastern europe, and for older apartments, dishwashers aren't really possible unless you do specific renovations to run piping for it. There either just isn't space or isn't piping (or both) to make it work.

Newer apartments it's much more common, but the dishwashers are about half-the size of what I grew up with in the US.

I have solar panels though and don’t use all the energy they produce. So running the dishwasher during the day is free.

Even in a typical domestic setting where hot water is heated (at something like 90%+ efficiency) by natural gas, it's still probably better than using grid electricity.

The problem is that it might not take hot water at all unless you have a water circulation pump installed and running (not sure how that thing is called in the states).

If your pipes are not insulated, the dishwasher might get only room temperature water anyway.

Also, district heating - it depends. In the winter, they run this district heating not only for electricity, but ramp it up for more heat production, in the winter.

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