Winterising Water Systems: The Failure Points That Cost the Most

Almost every winterising guide is a flat list of twenty steps in plumbing order, presented as though step four and step seventeen carry the same weight. They do not. One of them is a P-trap that will cost you nothing if you get it wrong, and another is a water heater that will cost you $900.

So here is the same job ranked by consequence. Do it in this order and the expensive failures are behind you within the first thirty minutes.

The underlying physics is simple and worth holding onto: water expands roughly 9% when it freezes, and it does not care what is in its way. Anything that traps water in a closed or semi-closed volume — a pump head, a valve body, a filter housing, a heat exchanger, a length of line between two closed points — is a candidate for splitting. Anything open at both ends usually survives.

Ranked by what a failure actually costs

That last row is the reason to take the job seriously rather than the individual components. A $12 elbow that splits in January and drips into a laminated floor until March is the single most expensive thing on this list, and freeze damage arising from failure to winterise is a routine exclusion in vehicle and RV policies — the coverage question is worth reading before you need the answer, particularly on non-standard and converted vehicles where standard policies fail in ways owners discover at claim time.

Which method: the honest answer is both

Blow-out. Compressed air pushed through the system at 30–40 psi, never above about 50, until only air comes from each fixture. Fast, cheap, no chemicals, nothing to flush out in spring. Its weakness is that it cannot reliably clear low spots, sagging PEX runs, pump internals or valve bodies, and it leaves a film of water that is still enough to do damage in a hard freeze.

Antifreeze. Two to four gallons of pink propylene glycol RV antifreeze pumped through the system to displace residual water. Freeze protection typically rated to around −50°F, and unlike water it does not solidify — it goes slushy, so it does not expand hard enough to split fittings. Its weakness is that it costs more, needs flushing and sanitising in spring, and gets wasted in large volumes if the water heater is not bypassed.

Hybrid — the position this site takes. Blow the lines out first, then draw a smaller volume of antifreeze through to protect the pump, the traps, the valve bodies and the low points that air cannot reach. It is the combination that covers both failure modes, and it uses roughly half the antifreeze of the wet method alone.

Two chemical rules that are not negotiable. Never use automotive antifreeze — that is ethylene glycol, it is toxic, and it does not belong anywhere near a potable water system. And prefer propylene glycol over the cheaper alcohol-based RV antifreeze: alcohol evaporates faster, smells worse and is harder on rubber seals and appliance solenoid diaphragms.

The sequence

Allow ninety minutes the first time. Label the valves as you go and it becomes a forty-minute job every year after.

1. Dump black, then grey, at a proper dump station. Black first so the grey water rinses the hose. Add a small amount of antifreeze to each tank afterwards to protect the dump valve seals.
2. Drain the fresh tank completely. Open its drain, then run the pump briefly to pull the last of it through — and switch the pump off before it runs dry for any length of time.
3. Kill the water heater. Switch off the electric element and the gas at the appliance. If it has been on, let it cool for at least two hours before touching the drain.
4. Drain the water heater and then bypass it. In that order. Release the pressure at the relief valve first, then pull the drain plug or anode rod. A six-gallon tank takes a few minutes to empty. Only once it is empty do you set the bypass valves, so the tank stays empty rather than filling with antifreeze. If you have a tankless unit, follow the manufacturer’s procedure — several specifically say do not bypass, and the heat exchanger is the expensive part.
5. Open every low-point drain. Hot and cold, both sides. Leave them open while the next step runs.
6. Blow the lines out. Fit a blow-out plug at the city water inlet, set the regulator to 30–40 psi, and work fixture by fixture from the furthest point back — hot side first, then cold, at each one. Include the shower, the outside shower, the toilet, and any ice maker or dishwasher line. Continue at each fixture until only air comes out.
7. Close the low-point drains and disconnect the air supply.
8. Draw antifreeze through the pump. Use the winterising siphon valve if fitted, or disconnect the pump inlet and feed it from a jug. Run each fixture — hot side first, then cold — until pink shows, then close it. This is the step that protects the pump head, and the pump head is the second most expensive item on the failure table.
9. Do the fixtures everyone forgets. Outside shower. Toilet flush valve and vacuum breaker — flush until pink. Washing machine, if fitted, per its own procedure. Ice maker line. Wet bath sprayer.
10. Pour antifreeze into every P-trap — galley, bathroom sink, shower — a cup or so each, plus a little down the toilet.
11. Remove or drain the water filters. Take the cartridges out, drain the housings, leave them loose. Antifreeze ruins carbon cartridges and a full housing is a reliable split. Note the replacement date while the housing is in your hand; filter intervals apply by time as well as by volume.
12. Empty the pump strainer bowl and check the city water inlet check valve by pressing it in with a screwdriver to release any trapped water behind it.

The twenty-minute version, when a freeze is coming tonight

Sometimes the forecast changes and you have no compressor, no antifreeze and no time. This is triage, not winterising, and it will get most rigs through an unexpected single-night freeze.

In order, working straight down the cost table:

1. Drain the water heater. Kill the power and gas, release the pressure valve, pull the plug. This is the most expensive component and the one with the most water in it.
2. Open every low-point drain and every tap, hot and cold, and leave them open. Gravity does more than people expect once the system is not under pressure.
3. Open the pump strainer bowl or disconnect the pump inlet so the head can empty.
4. Pull the filter cartridges and drain the housings. Thirty seconds, and it prevents a flood rather than a repair.
5. Drain the outside shower — open both taps and pull the hose down so it empties.
6. Pour whatever antifreeze you have into the P-traps and the toilet. If you have none, a couple of inches of standard vodka or windscreen washer fluid in a trap is better than water, though neither belongs in the supply lines.
7. If the rig has power and you can heat it, run a small heater and open the cabinet doors and bay access panels so warm air reaches the plumbing runs. Heat is the cheapest protection you have when you already have shore power.

Do the full job properly at the next opportunity. This buys a night, not a season.

What else matters while the rig is standing

Batteries. Lead-acid batteries freeze when discharged and lithium has its own set of cold-weather rules — charging a lithium cell below freezing without a heater or a BMS that blocks it causes permanent damage, and the whole picture is set out in lithium batteries below freezing.

Moisture. A sealed vehicle with no airflow grows mould. Leave roof vents cracked with covers over them, use desiccant tubs or a low-wattage dehumidifier if you have power, prop cabinet doors and the fridge open, and never use a solid plastic tarp as a cover — it traps moisture against the shell.

The rest of the storage question. Tyres flat-spot, seals dry out and rodents move in. Storage is also one of the three ownership costs buyers routinely underestimate, and that piece covers the cost side properly.

Dumping before storage. Tanks should go into winter empty — sitting waste is both a freeze risk and a seal-degradation risk. Tank sizing and management covers the operating side of the same system.

Spring: dewinterising properly

Flush the antifreeze through with fresh water, hot and cold at every fixture, until it runs clear and stops foaming. Refit the filter cartridges only after the antifreeze is fully gone, or you will ruin a new set immediately.

Then sanitise the system. Roughly a quarter cup of unscented household bleach per fifteen gallons of tank capacity, filled, pumped through until you smell chlorine at every tap, left for at least four hours, then drained and flushed until the smell is gone. Do this every spring. A fresh tank that has been standing damp for five months is exactly the environment biofilm likes.

Then restore the water heater: close the bypass, refit the drain plug or anode, fill the tank, and purge the air from the hot lines before switching the element on. Firing a dry element is a fast way to destroy it. Inspect the anode rod while it is out — replace it if more than about half has gone.

Finally, pressurise and walk the rig with a torch. Check under the sinks, at the pump, behind the panel where the filter housings live, and at the outside shower. A fitting that split in January leaks the moment you pressurise in April, and finding it with a torch is much cheaper than finding it with a floor.

FAQ

Is the blow-out method good enough on its own? In mild climates and on simple plumbing, often. In sustained hard freezes, or on any rig with a washing machine, ice maker, complex manifold or long sagging runs, it leaves water in places air cannot reach. The hybrid approach costs an extra two gallons of antifreeze and removes the argument.

How much RV antifreeze do I need? Two to four gallons for the full wet method on a typical trailer or motorhome; one to two if you have blown the lines out first. Larger rigs with washing machines and long runs need more.

Can I use automotive antifreeze? No. Automotive antifreeze is ethylene glycol and is toxic. Only propylene glycol RV and marine antifreeze goes into a potable water system, and even that is flushed and sanitised out before use.

Do I have to bypass the water heater? You should drain it and bypass it. Filling a six-gallon tank with antifreeze wastes several gallons of product and achieves nothing that draining does not. Check the manufacturer’s guidance on tankless units, several of which should not be bypassed.

At what temperature does damage start? Damage risk begins once the water inside a component actually freezes, which for a shaded, unheated rig means a sustained spell below about 28°F rather than a single frosty night. Wind, duration and where the plumbing runs matter more than the overnight low on the forecast.

What if I need the rig usable through winter? Then you are not winterising, you are cold-weather camping — heated tanks and bays, heat tape on exposed lines, insulated skirting, and consistent heat. That is a different job with different failure modes, and it is not what the antifreeze method is for.