Why is my tap suddenly pouring out soapy white water and chemicals?

That's almost certainly **excess flux** left in the pipework after the soldering was done. When we join copper pipes, we use flux to help the solder flow, and if a plumber uses way too much—wich happens a lot—it sits inside the pipe. Water-soluble flux is basially a detergent-based paste, so when the water finally flows thru it, it foams up exactly like soap and carries a heavy chemical scent.

The white grains you're seeing are likely flux salts or calcium deposits that got dislodged. When the water was turned off and the pipes were drained, air got into the system. When they turned it back on, the rush of water and air acts like a pressure washer on the inside of the pipes and that under-sink heater. It knocks loose all the sediment that’s been sitting there for years. The plumber saying it’s a mains issue is a total cop-out. If the rest of the building is fine, the contamination is clearly in the specific lines he worked on. You shouldn't just take his word that it's fine after a five-minute flush. Flux is acidic and definitely not something you want to be making tea with. The soapy texture is a dead giveaway that there's still surfactant in the line. What you need to do is unscrew the aerator (the little mesh bit) from the end of the tap. It’s probly full of those white grains, which will keep leaching chemicals into the flow if you don't clean them out. Once that’s off, run both the hot and cold taps full blast for at least 20 to 30 minutes. If the smell or the bubbles come back after that, then the under-sink heater might need a proper drain and flush, because the flux could have settled in the bottom of its tank. Stick to bottled water until that chemical smell is completly gone.

I’m a total novice here, but could that stuff actualy damage your appliances? I’d be worried about those grains getting stuck in a dishwasher filter or ruining a fancy coffee machine. My cousin had a similar "soapy" issue and it ended up messing with the seals in her mixer tap. If it’s flux, does it eventually stop being slimy on its own, or does it just keep coating everything until it's manually scrubbed out?

To answer the novice's question—yes, that gunk can definatly wreck seals. Those white grains are often a mix of flux residue and bits of PTFE tape or even plastic burrs if he used push-fit. If they get into a ceramic disc cartridge in your tap, they'll scratch the hell out of it, and you'll have a permanent drip within weeks.

The fact that it's worse on the hot side suggests your under-sink heater is acting as a settlement tank for all the debris. A 5-minute flush is a joke. If I were you, I'd insist on a proper system flush. Most of these guys are in such a rush they skip the basic commissioning steps required by the regs. Don't let them brush you off.

That plumber is definitely taking the mickey. If this is a commercial studio, there are actually strict UK water regs regarding how new installs are flushed and commissioned. Just running a tap for five minutes isn't a proper flush-out.

If those white grains are still showing up, you might want to check the internal filter on that under-sink heater too. Those small units have tiny baffles that trap debris perfectly. If it's still tasting funky after a long flush, the manager shold relly be getting a water sample tested. Don't let them bully you into drinking 'soapy' water just because they're too lazy to do a proper system purge.

If you want to be 100% sure before you start drinking it again, grab a pack of cheap pH test strips from a hardware shop or a garden centre. Flux is usally quite acidic, and if your local water is typically neutral or slightly alkaline, those strips will show you instantly if there's still residue lingering. I did this after a bathroom refit when I suspected the plumber used way too much Everflux. Even after the water looked clear to the eye, the pH stayed low for a good while. It's a solid, low-cost way to prove to that manager that the water isn't actualy 'fine' yet regardless of how it looks in a bowl.