RV Winterizing Calculator: How Much Antifreeze, and Which Bypass Position

Two to three gallons for most trailers with the water heater bypassed, eight or nine without. The bypass decision is worth more than everything else on the job combined, and no manufacturer publishes a gallons figure at all — which is why every guide quotes the same vague number.

There is a second thing worth knowing before you start, because the standard advice has it backwards: if your heater is tankless, you must not bypass it.

Why nobody will tell you how many gallons

Four documents, no number. Grand Design’s owner’s manual: “Make sure to have enough RV antifreeze to winterize all fresh water lines. Several gallons may be required.” Jayco’s guide: put the hose into “a container of RV antifreeze solution (this quantity should be enough to winterize the RV)” — which defines the quantity as itself. Splash, who make the stuff: “follow the manufacturer’s recommendations.”

The one figure anybody publishes is the cost of a mistake. Grand Design’s winterizing article says that letting antifreeze into the water heater wastes “at least six gallons” — the capacity of the tank it ships.

So that is what the calculator is built on: one term that is a manufacturer’s own number, and one that is our arithmetic on top of one. The tool says which is which in its own output rather than burying it here.

The bypass instruction that is wrong for your heater

Almost every winterizing guide says the same thing. Jayco’s: “Turn the water heater bypass valves to the BYPASS position.” No qualification, no mention of heater type.

Now read Grand Design’s manual for the Imagine, a trailer sold with either a 6-gallon tank heater or a Furrion tankless. For the tank heater it agrees: “it is recommended to bypass the Water Heater and remove the Anode Rod (if equipped), particularly if introducing RV antifreeze into the plumbing system.”

For the tankless, on its own page, in bold: the bypass valves “MUST be kept in the ‘Normal Position’ NOT the ‘Bypass Position.’” Antifreeze is run through the tankless heater, and then drained out of its drain plug afterwards.

If you have a tank heater, bypassing is worth roughly six gallons. Grand Design’s own blog calls the step “technically optional” because antifreeze will not damage the tank — which is true, and it still costs you six gallons to ignore. (Those six gallons are also what every hot shower draws on, and the hot water calculator shows how many minutes they last.)

The number on the jug is not a freeze point

This is the part that changes what you buy.

RV antifreeze is sold by temperature — -50°F, -75°F, -100°F — and almost everyone reads that as the temperature at which it stays liquid. It is not. Splash publishes both a -50°F and a -75°F product, and then states in its own FAQ:

The -50°F and -75°F solution will freeze between +20°F and +16°F.

Both ratings. The same band. Your antifreeze turns to slush at about +18°F whatever you paid for it. The rating is burst protection — the promise that the slush will not split the pipe it is sitting in.

What must never go in

Grand Design’s manual carries this warning twice, on separate pages, which is unusual enough to be worth repeating here:

Both automotive antifreeze (ethylene glycol) and windshield washer antifreeze (methanol) are poisonous. NEVER use these products in your fresh water system. They are harmful and may be fatal if swallowed.

There is a subtler point about what does go in. RV antifreeze comes in two formulations: propylene glycol, and ethanol blends. Splash, which makes an ethanol blend, warns on its own product page that “ethanol-based antifreeze is dangerous in applications that involve heating” — and Grand Design’s manual arrives at the same hazard from the other direction: “DO NOT USE YOUR WATER HEATER WHILE THE SYSTEM IS WINTERIZED.” Two manufacturers, two documents, one rule. Do not run the heater until the system is flushed.

That is why the calculator ranks propylene glycol first at the same rating. Camping World’s antifreeze guide goes further and recommends it for drinking-water systems outright; we rest the ranking on the manufacturer’s warning rather than on a retailer’s preference.

Getting it back out in spring

Winterizing is half the job. Grand Design’s manual gives the other half in its tankless water heater section — the steps are written for that heater, but the flush-then-sanitize order is the part worth copying — and it is short:

1. “Thoroughly FLUSH the water heater and system with clean drinking water through the hot and cold side before using.”
2. “FILL water heater at City Water Connection & DRAIN several times.”
3. “SANITIZE the water system per the recommendations of your coach manufacturer.”

Only after that does the water heater go back on. The same manual’s warning about running it while winterized still applies until the antifreeze is out.

Spring is also when a winter’s worth of damp under the bedding shows itself. If the mattress is coming out to dry or be replaced, measure the platform while it is bare — what actually fits is a different question from what the size label says, and the platform is far easier to measure with nothing on it.

What the antifreezes are rated for

Model	Burst protection	Freeze point	Composition	Container	Sold as	Buy
Star brite Winter Safe -50°F PG	-50°F	Not published on Star brite's product page. A retail listing states a freeze range, but a retailer's figure is not the maker's, so it is not recorded here	Propylene glycol	1 gallon, per the retail listing	Sold by the gallon	Check price(opens in a new tab)
Splash -50°F, 1 gal	-50°F	+20°F to +16°F. Splash's own FAQ states that "the -50°F and -75°F solution will freeze between +20°F and +16°F" — the headline number is burst protection, not the temperature at which the fluid stays liquid	Ethanol blend	1 gallon	Sold 6 × 1 gallon	Check price(opens in a new tab)
Splash -50°F, 2.5 gal	-50°F	+20°F to +16°F, per Splash's FAQ — the same as the -75°F product. The headline number is burst protection, not a freeze point	Ethanol blend	2.5 gallons	Sold 2 × 2.5 gallon	Check price(opens in a new tab)
Splash -75°F, 1 gal	-75°F	+20°F to +16°F — identical to the -50°F product. Splash's FAQ states that "the -50°F and -75°F solution will freeze between +20°F and +16°F", so the extra 25 degrees on the label is burst protection and nothing else	Ethanol blend	1 gallon	Sold 6 × 1 gallon	Check price(opens in a new tab)

Read the Splash rows first. Their burst ratings differ by 25 degrees and their freeze points are identical — that is the whole buying decision in one row, and it is the row no retailer puts on a shelf label. The Star brite row has no freeze point because Star brite does not publish one.

Where this goes wrong

1. Bypassing a tankless heater. The most expensive mistake on this page, because the antifreeze never reaches the heat exchanger it was meant to protect.
2. Buying the coldest rating on the shelf. It does not stay liquid longer.
3. Forgetting the anode rod. Grand Design says to pull it on a tank heater while you are bypassing, and it is easy to skip once the valves are turned.
4. Leaving an aftermarket water filter in line. Grand Design’s manual is explicit that it must be set to bypass before you start.
5. Skipping the drains and the toilet. The same manual says it is “also important to run RV antifreeze through the Toilet and the Sink/Shower Drains” — the per-faucet figure covers the supply lines, so keep some back for those.

Questions worth answering