No Stupid Questions

Yes. Foolishly most of the less expensive dishwashers don't heat the water themselves, and the first cycle is more effective if it doesn't have a period of using cool water from the pipes until the hot water arrives.

I was skeptical at first too but it truly works.

But it may be worth clarifying to your partner that once the water is coming out of the tap hot, the water in the hot pipes is already hot, so turning the tap to cool at that point no longer negatively impacts the dishwasher.

The important part is getting the cool water in the hot water pipes that has been sitting there flushed out before starting the dishwasher. If that's already achieved, then what you do with the tap doesn't matter anymore

Is this an American thing? Dishwashers run off the cold water and heat it up themselves over here.

Why did your wife chastise you over having cleaner dishes?

Yes. For a typical American dishwasher installation, anyway, where the dishwasher is connected to the hot water line which is in turn connected to your storage tank water heater.

First, your water heater is already full of hot water that you've already paid the energy cost to heat up. It's therefore nonsensical to deliberately pump cold water from the line into your dishwasher only for the dishwasher to have to heat it up (using energy/money) and then have your water heater also heat up the displaced water inside it that was just pumped out.

On dumb dishwashers that don't do anything to sense the internal water temperature, this will also moderately improve your wash performance. (GE calls this feature "Automatic HotStart," for instance. Other brands call it something different. These machines will pause and run their little internal water heater coil at full blast until their current charge of water is at a temperature their programming considers acceptable to start the cycle. There is no performance detriment in this case, but it will cost you time and more energy/money.)

Modern dishwashers use a surprisingly small amount of water per cycle. Usually somewhere around 3.5 gallons for the entire thing from start to finish. This is contrary to what your parents think, who will insist to you they can totally hand wash an entire sink of dishes with "less water" than that wasteful dishwasher, which they absolutely cannot. Your dishwasher is not consuming water the entire time it is running. It will fill and recirculate that fill's worth of water inside itself, usually going through 3 or 4 phases of filling and draining throughout the cycle to arrive that that final water usage figure of around three and a half gallons. Therefore it does not readily have access to hot water unless you've primed it by running your sink, as what used to be hot water in your pipes rapidly cools off when said water is not flowing. Your dishwasher probably will not draw in enough water in one fill cycle to purge all of the standing cold water that's in the pipe unless the distance from your water heater to the dishwasher is unrealistically short.

(The latter actually is the case for the dishwasher at my office, which is fed by an undersink "low boy" water heater that's directly next to it and only separated from the dishwasher by a grand total of about 18 inches of pipe.)

I feel like this is only a concern if you keep having poorly washed dishes. And if that’s the case, I would opt for a new dishwasher.

Depends on how the dishwasher is connected, but I would assume it's connected to the cold water. At least that's the common way in Scandinavia

Maybe take a look under your sink and check if the dishwasher is connected to the cold or warm water line. It only matters if it is connected to the warm line.

Yes, hot water cleans better, and results in dryer dishes due to condensation drying.

The heating element in a dishwasher is weak. It can’t heat water up that well since it’s limited to 120v and probably not even 20A. The first rinse cycle will effectively be cold water.

Your tank water heater is 240v and has a dedicated circuit, probably 30A. It can heat a lot of water, fast.

If you’re concerned about wasting water you can get a small tank or tankless hot water heater under the sink. Your water will then be hot nearly always.

Some wash cycles will both purge more water and heat it up longer to compensate.

Technology Connections (youtube channel) says yes run until it's hot IF you have trouble with dirty dishes, iirc. I think the bigger thing is to clean your debris trap. Also you should be fine washing your hands. The dishwasher will still pull as much hot water as it needs, they are typically only hooked up to hot water so it's not like you having the tap on mixes it up or anything.

You want to make sure the water in the line is hot by running the sin to flush the old settled hot water that has cooled to house temp in the pipe. , once the cold water in the line has been run and replaced by fresh hot water the dishwasher can take hot water and not have to heat it from cold to lukewarm.

The dishwasher has a heating element inside and it's using maybe 4-5L max for the wash cycle. It heats up pretty fast.

I’m not sure why you’re being chastised for using water while the dishwasher is filling. That is a non sequitur. But for most people, the answer is probably yes, it moderately improves performance.