I never understood why Harry takes cold showers
I know he says he does it because he thinks his power will interfere with the technology in a boiler, but it just makes no sense.
He lives in the basement of a building with several tenants. That's where a boiler is likely to be in an older house. The other tenants need heat and hot water. Several of them are elderly and I don't think they're going to be using a wood-burning stove and a fireplace for warmth. And the landlady has to provide heat and hot water by law.
He has a kerosene heater for the sub-basement. A really old school boiler isn't going to be that much more complicated (mine has electronic parts but my grandmother's didn't).
I never understood how this would actually work.
Yes, the boiler could be upstairs, but that doesn't make sense. Most likely there would be a small boiler room in Harry's apartment. Where the magic is.
Also, I don't get why wizards don't have enchanted items that mimic tech to a certain extent. Not cell phones, but lights?