Sanitize the Fresh Tank Calculator

How much bleach to sanitize an RV water tank?

The target is about 50 parts per million of chlorine across the whole system, and "the whole system" means the tank, the water heater and the lines, not the tank alone. For a 50 gallon tank with a 6 gallon heater and a 30-foot trailer's plumbing that's 56.8 gallons of water, and at 6% bleach the dose is ¾ cup (6.1 fl oz), about 12 tablespoons, mixed into a gallon of water first. The common tank sizes, heater and lines included:

50 ppm unless stated; 6 gal water heater in the loop; lines for a 28 ft rig (0.8 gal). Cups rounded to the nearest eighth, so measure with a measuring cup.

If the heater is bypassed and stays out of the loop, the same 50 gallon rig drops to 50.8 gallons and ⅝ cup (5.4 fl oz). The heater still needs doing separately, and it's the part that smells first in spring.

What does "¼ cup per 15 gallons" of bleach mean?

Two fluid ounces of 5.25% sodium hypochlorite in 15 gallons (1,920 fluid ounces) is about 55 ppm of chlorine, so the rule is a concentration written as a measure. The Winnebago manual that NRVTA quotes gives the rule as ¼ cup in a gallon of water for every 15 gallons of tank, calls the result "approximately 50 parts per million," and asks for at least four hours of contact; Jayco's manual says the same. The rule dates from when household bleach was 5.25%. The CDC now puts household bleach at 5 to 9% sodium hypochlorite: most bottles are 6%, some 7.5%, and "concentrated" is 8.25%. Same quarter cup, different result. The common rules, converted:

ppm = fl oz × strength ÷ (gal × 128) × 1,000,000. Across these strengths the rules land between 51 and 129 ppm, so all of them clear 50 ppm; the bottle decides by how much.

None of this is dangerous. A tank at 100 ppm is a fraction of the 500 ppm solution in Clorox's own water-tank procedure (⅓ cup per 3 gallons, left for five minutes, then a 10 ppm rinse), and the flush takes it out. It matters for a duller reason: a cup of bleach poured into a 30-gallon tank out of caution makes 125 ppm at 6% and 172 at 8.25%, which means extra rinse cycles and a tank that tastes of pool for a week. Set the strength on your bottle above and the dose comes out at the concentration the manual meant.

How long do you leave bleach in the RV water tank?

At 50 ppm, 4 hours is the minimum the RV manuals give, and overnight (12 hours or so) is the easy way to be sure, because the point is contact time on every surface. That includes the top of the tank and the inside of the lines, which only get wet if the tank is full and every faucet has been run until it smells of chlorine. Driving during the soak helps by sloshing the solution over the tank walls. At 100 ppm, 4 hours does the job. Longer than a day buys nothing and starts to be unkind to rubber seals and the water heater's anode. One rule has no exception: the water heater's element stays off from the first drain to the final refill, because an electric element switched on with air in the tank burns out almost at once.

Do you sanitize the water heater too?

Set the bypass to normal and the heater fills with solution along with everything else, which is the point: it holds water all season and needs the same contact time. Run the hot side of each faucet until it smells of chlorine, and drain the heater along with the fresh tank at the end: plug or anode out, relief valve open, then a hose in the drain to flush the sediment that's been cooking in the bottom. A tankless heater has no tank to fill; the solution just passes through its heat exchanger on the hot side. If you keep the heater bypassed (some owners prefer a separate vinegar flush for scale, done on its own with no bleach in the tank), untick it above. The dose drops, and the checklist says plainly that the heater was left out. The water heater calculator covers what the tank does once it's back in service.

How do you flush the bleach out?

Drain everything, fill with fresh water, run every faucet (hot and cold, toilet, outdoor shower) until the smell is gone, drain again, and do it once more: 2 cycles, which for the 50 gallon rig above is about 114 gallons of water, so do it at a spigot and not from your jugs. Don't reach for vinegar to speed it up. Vinegar doesn't neutralize chlorine, and France's health agency ANSES warns that bleach mixed with vinegar or another acid releases chlorine gas.

Hydrogen peroxide does neutralize it, at 0.479 mg per mg of chlorine (the factor in the Town of Fort Frances dechlorination procedure). That's worth knowing when the solution has to drain somewhere sensitive, like a septic field or a ditch. The 56.8 gallons above at 50 ppm hold 10.8 g of chlorine, which takes 5.2 g of peroxide: about 172 mL, or 5.8 fl oz, of drugstore 3% stirred into the full tank before draining. A capful is a small fraction of that. Then the new filter cartridge goes in (never before, since bleach destroys carbon filters), the ice maker valve reopens (dump the first two bins), and only after the heater is full does its element go back on.

When should you sanitize an RV fresh water tank?

Every spring after storage: the pink antifreeze goes out and the bleach goes in, which is why this page pairs with the winterizing calculator and is a step in the de-winterizing checklist. Then every six months of use, after any fill from a source you didn't trust, after the rig has sat a month or more with water in the tank, when the water starts to smell or taste of anything, and once on a rig you've just bought, because you have no idea what the previous owner put in it. Between sanitizings, the thing that keeps a tank clean is turnover: drain and refill before a trip rather than topping up water that's been sitting since the last one, and run the tank low before you park it. The Boondocking Days calculator works out how long each fill lasts your crew.