From what I know, English plumbing is so bad because:
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Water pressure is much lower than in North America or the rest of Europe. The supply line from the water main to a house is also smaller in diameter in the UK.
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Hot water tanks are small (20 to 30 gallons/80 to 120 liters in the UK, versus 40 or 50 gallons/160 or 200 liters in the US)
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The goofy cistern system, where an open tank of water is kept in the attic, supposedly to provide an emergency water supply in case the French decide to attack and shut off municipal water supplies.
North American residential plumbing looks something like this:
- Municipal water supply
- Backflow prevention valve
3a) Some water is piped to the hot water tank, then to all faucets/taps
3b) Some water is piped directly to all faucets/taps - Hot water and cold water emerge from the faucets/taps under the same pressure.
British plumbing, AFAIK, runs something like this:
- Municipal water supply
2a) Some water is piped to the attic cistern, which is usually open to the air, and then to hot water tanks. I think cistern water supplies toilets, too.
2b) Some water is piped to cold water taps.
3a) Hot water emerges from the hot tap at the pressure provided by gravity from the cistern.
3b) Cold water emerges from the cold tap at the pressure provided by the municipal water system.
The taps are kept separate, because there’s no backflow prevention valve. Potentially contanimated hot water could mix with cold water, and thus find its way back into the municipal water supply.