Happened twice this week in the same hotel. Shower is fine, then a distant flush, then about two seconds later it goes genuinely painful and I'm pressed against the tile. It settles back down after twenty seconds or so. My shower at home has never once done this. What is different about the plumbing, and why is the delay always about the same?
Old buildings make it worse in a second way: long runs of narrow pipe shared between many rooms. One flush on a 15mm branch feeding six bathrooms is a much bigger event than one flush on your house's supply.
Fun bit of history: the scalding version is the reason a lot of building codes eventually required compensating valves in new work. It wasn't comfort, it was the number of burns, particularly with children and elderly guests who can't get out of the way fast enough.
You can sometimes feel which side dropped. If the water goes hot and the flow stays the same, you lost cold. If it goes hot and weak, something is compensating and partially succeeding. Not useful information in the moment when you're plastered against the wall, but satisfying afterwards.
The reverse happens too. Someone in the next room starts a shower, you lose hot, and you get thirty seconds of cold. Less dangerous, equally rude.
@drip_dana · last mo. · 3 replies
The valve in that hotel is a plain mixer with no compensation, and the flush is stealing cold water from the same branch.
A basic mixing valve doesn't control temperature. It controls two openings, one on the hot side and one on the cold, and you set them so that the blend is comfortable at the pressures that exist right now. When a toilet somewhere fills, cold pressure on that branch drops. The openings haven't moved, but less cold is getting through them, so the ratio arriving at your face shifts towards hot. Nothing has got hotter, you've just lost cold.
The delay is the water already in the pipe between the valve and the shower head reaching you. Same reason it takes a few seconds to warm up when you first turn it on.
Your shower at home almost certainly has a pressure balancing cartridge, which is basically a little spool that slides when one side drops and throttles the other side to match. It's been standard in a lot of places for decades. The nicer version is a thermostatic valve, which measures actual temperature and holds it.
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@drip_dana · last mo.
Genuinely better. Pressure balance protects you from scalding by cutting the hot down, which means your shower gets weaker during the flush. Thermostatic holds the temperature and the flow, and it also handles the case where the water heater itself changes temperature, which pressure balance can't see at all.
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@blistermaps · last mo.
So the fix is a valve that either throttles the hot or measures the temperature. Is a thermostatic one noticeably better in practice or is that a marketing tier?
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