The short answer
Eleven pieces of household advice that are wrong: soap does not harm cast iron seasoning (it is a bonded polymer, not grease); vinegar damages cement grout, marble and espresso machines; rinsing dishes before the dishwasher makes cleaning worse because enzyme detergents need soil; more detergent leaves residue rather than cleaning better; salt does not set dye; a hot empty cycle does not reach the gasket or drain filter; petroleum jelly degrades rubber seals; external window condensation means good glazing; a thermostat at maximum does not heat faster; steam mops damage floors; and a full freezer uses less energy, not more.
Household advice propagates faster than it gets checked. These eleven are repeated constantly, and each one is either useless or actively damaging — with the reason it spread, and what to do instead.
1. “Never use soap on cast iron”
Wrong. Seasoning is polymerised oil chemically bonded to the iron, not grease sitting on top. Modern dish soap is a mild detergent and removes food, not polymer. The rule dates from when household soap contained lye.
Actually damages it: soaking, the dishwasher, and long acidic cooking. Guide
2. “Vinegar cleans everything”
Wrong, three times over. It dissolves cement grout, etches marble and limestone, and attacks the brass and seals in espresso machines. It is genuinely excellent on limescale — and that success gets over-generalised.
3. “Rinse dishes before loading the dishwasher”
Counterproductive. Modern detergents are enzymatic and need protein and starch to act on. Machines with soil sensors read a rinsed load as clean and select a weaker cycle. Scrape solids off; do not rinse clean.
4. “More detergent means cleaner laundry”
The opposite. Surplus surfactant does not rinse away in a high-efficiency machine. It stays in the fabric, attracts soil, stiffens towels and feeds the biofilm that makes the machine smell. Most households use two to three times too much.
5. “Salt sets the dye in new clothes”
Does nothing. Salt is used during commercial dyeing at concentrations and temperatures no washing machine reproduces. It is harmless and it will not stop a red shirt bleeding.
6. “Run an empty hot cycle to sanitise the washing machine”
Half right. A hot maintenance cycle helps, but it does not reach the door gasket channel or the drain pump filter — which is where the smell actually lives. Cleaning those by hand is the part that works.
7. “Petroleum jelly keeps fridge seals supple”
Shortens their life. It softens the rubber briefly, then degrades it. Warm soapy water and thorough drying is the actual maintenance.
8. “Condensation on the outside of a window means bad glazing”
It means the opposite. Dew on the external pane means the outer glass is nearly as cold as the outside air — very little heat is escaping. It is a sign the glazing is working.
9. “Turn the thermostat to maximum to heat the room faster”
No. A thermostat and a TRV set a target temperature, not a rate. At maximum the valve simply stays open longer before shutting off. The room heats at the same speed and then overshoots.
10. “Steam cleaning is the safest way to clean floors — it’s just water”
It is water plus heat plus pressure, which is the combination that damages wood, laminate and vinyl. Effectively every flooring manufacturer voids the warranty for it.
11. “A full freezer wastes energy”
Backwards. Frozen mass holds cold, so the compressor cycles less after each opening and there is less humid air exchanged. Filling empty space with bottles of water works and costs nothing.
Why these persist
| Pattern | Examples |
|---|---|
| Was true once | Soap on cast iron; rinsing dishes |
| Over-generalised from one success | Vinegar; “natural means safe” |
| Confuses target with rate | Thermostat at maximum |
| Intuition points the wrong way | More detergent; empty freezer; external condensation |
| Sounds like maintenance | Petroleum jelly on seals; salt in the wash |
Common questions
Is soap on cast iron really fine?
Yes. Seasoning is a polymer bonded to the metal, not a layer of grease, and dish soap is not strong enough to break it. The rule is a survival from the era of lye soap. What genuinely strips seasoning is soaking, the dishwasher, and sustained acidic cooking.
If vinegar is so risky, why is it recommended everywhere?
Because it works extremely well on limescale, which is a very common problem. The trouble is that acidity is the mechanism, and the same acidity dissolves cement grout, calcareous stone and espresso machine components. Use it for limescale and nothing else.
Should I really not rinse dishes?
Scrape solids into the bin — bones, peel, rice, anything fibrous. But leave the film. Enzymatic detergent needs something to bind to, and soil sensors read a spotless load as clean and downgrade the cycle.
How do I check advice like this myself?
Ask what the mechanism is. “Vinegar is acidic” tells you immediately that it is wrong for cement and calcareous stone. “Seasoning is a polymer” tells you soap is irrelevant. Advice that comes with a mechanism can be checked; advice that comes as a rule cannot.
Which of these costs the most money if believed?
Steam cleaning floors, because a swollen laminate core or a lifted wood finish means replacement rather than repair. Over-dosing detergent costs the most cumulatively — wasted product, shortened fabric life, and a machine that eventually needs deep cleaning.