TL;DR
Hot water is the whole method -- buttercream is fat plus sugar, and water hot enough to melt the fat clears a piping tip in seconds while cold water just packs it tighter. Scrape the bag, run hot water through the tip from the wide end, brush the opening, and dry the bag inside out so it does not sour in storage.
Why cold water makes it worse
Frosting is mostly fat and sugar. Sugar dissolves in water at any temperature. Fat does not.
Under a cold tap, the sugar washes away and the fat sets into a waxy plug that fills the narrow opening of the tip. That plug then resists everything you do next.
Hot water reverses it. Butter, shortening, and cream cheese frostings all soften well below the temperature of a hot tap, and once the fat is liquid the whole residue simply flows out.
That is why the first move on any greasy kitchen tool is heat rather than force, a principle covered throughout the guide to cleaning kitchen tools without damaging them.
How do you clean piping bags and tips?
Scrape the leftover frosting out with a spatula, remove the tip and coupler, and run water as hot as the tap allows through the tip from the wide end so residue exits the way it went in. Brush the opening with a small brush, wash everything in hot soapy water, rinse, and dry the bag inside out.
The full routine
1. Scrape first. A small spatula gets most of the frosting out of the bag and back into the bowl. Do this before water is anywhere near it.
2. Separate every part. Bag, coupler ring, coupler base, and tip. A coupler assembly holds frosting in the threads where nothing else reaches.
3. Flush the tip from the wide end. Hold it under a hot tap so water enters the wide opening and exits the shaped one. Flushing backward pushes residue into the narrowest part.
4. Brush the tip inside and out. A tip brush, a straw brush, or a stiff toothbrush. Pay attention to the shaped opening, where star points and petal slots hold buttercream.
5. Clear a stubborn plug carefully. A toothpick or a bamboo skewer. Never a metal pin or a knife tip, which bends the thin metal of a shaped tip and changes the pattern it pipes forever.
6. Wash the bag in hot soapy water. Turn a reusable bag inside out and wash both surfaces. The inside is where the fat film sits.
7. Rinse hot and dry inside out. Hang the bag over a bottle, a jar, or a tall glass so air circulates through it.
Reusable bags versus disposable
Disposable plastic bags are single use. They are thin, they crease, and there is no reliable way to get fat film out of the folds. Discard them.
Reusable coated cloth or silicone bags are washable but demand real drying. A damp bag folded into a drawer develops a stale smell quickly, and the smell transfers to the next batch of frosting.
Silicone bags hold a greasy film more stubbornly than coated cloth. Wash them in very hot water with a degreasing dish soap, and use a baking soda paste if they still feel slick afterward.
Any bag that has gone permanently discolored inside, feels tacky after a proper hot wash, or smells stale after drying should be replaced. They are inexpensive and a compromised bag is not worth the trouble.
Cleaning the tips properly
Piping tips are the part that actually determines your results, and a tip with dried frosting in one star point pipes a lopsided rosette.
Soak tips in hot soapy water for ten minutes if anything has dried. Hot water plus time beats scrubbing every time.
Then brush inside and out, and hold each tip up to a light to look through it. Any shadow in the opening is residue still in place.
Most metal tips are dishwasher safe, but they are small enough to fall through a rack, so use a closed basket or a mesh bag. Chrome plated tips can lose their plating in a dishwasher over time and are safer by hand.
Dry tips completely before storing. Cheap plated tips rust in exactly the way tinplate baking gear does, which is described in how to clean cookie cutters.
Couplers, and the part everyone forgets
A coupler is two pieces: a base that sits inside the bag and a threaded ring that holds the tip on the outside.
Frosting works into those threads and hardens there. A coupler with residue in the threads binds, and the ring becomes hard to turn.
Unscrew it fully every wash, brush both threaded surfaces, and rinse hot. Ten seconds now saves a seized coupler later.
Getting a stale smell out of a bag
A stale smell in a reusable bag comes from oxidized butterfat left in the fabric or in the folds.
Wash the bag again in water as hot as your hands can comfortably stand with a degreasing dish soap, inside and out. Then either scrub it with a baking soda paste or soak it for thirty minutes in one part white vinegar to three parts water -- one or the other, never both together, and never combined with any other cleaner.
Rinse thoroughly with hot water and dry the bag fully inside out before storing. Storing a bag rolled up while still slightly damp is the usual cause.
Grease held in fabric and fibers behaves identically in brush bristles, which is covered in how to clean a pastry or basting brush.
Cleaning equipment thoroughly reduces residue and rancid flavor transfer. It is not a food safety program on its own.
Source: USDA Food Safety and Inspection Service guidance on cleaning food contact utensils
FAQ
How do you unclog a piping tip?
Run water as hot as the tap allows through the tip from the wide end, so the plug exits the way the frosting entered. Brush inside and out with a small stiff brush. For anything remaining, use a wooden toothpick, never a metal pin, which bends the shaped opening permanently.
Can you reuse disposable piping bags?
No. Thin disposable bags crease and hold fat film in the folds, and there is no way to wash the inside reliably. Use reusable coated cloth or silicone bags if you want something washable, and turn them inside out to clean and dry them.
Can piping tips go in the dishwasher?
Most solid metal tips can, in a closed basket or mesh bag so they do not fall through the rack. Chrome plated tips lose their plating over time in a dishwasher and are safer washed by hand. Dry all tips completely before storing so they do not rust.
Why does my piping bag smell stale?
Oxidized butterfat left in the fabric, usually because the bag was stored before it was fully dry. Rewash it inside and out in very hot water with a degreasing dish soap, then dry it hung over a bottle so air circulates through. Replace a bag that still smells after that.
Should you use hot or cold water on frosting?
Hot. Frosting is fat plus sugar, and cold water washes away the sugar while setting the fat into a waxy plug in the narrow part of the tip. Hot water melts the fat so the whole residue flows out at once.
How do you clean a piping coupler?
Unscrew the ring from the base completely and brush both threaded surfaces under hot running water. Frosting hardens in the threads and eventually makes the ring bind. It takes ten seconds during the wash and prevents a coupler that will not turn later.
Decorating gear cleans up fast with hot water. The kitchen after a baking day takes rather longer. If you would rather hand that off, get a quote or read more about house cleaning services.