RV RV Maintenance and Repair

RV Winterization with small air compressors and the Nautilus P4

Greetings RV enthusiasts.  Its that time of year again – for those of us who don’t use our RVs over the winter, that is.  Today’s topic is utilizing small, portable air compressors to winterize coaches equipped with Nautilus P4 water management system.  Small air compressors require a little special attention, primarily because they can’t move the volume of air that a larger compressor can.  For this reason, the procedure I have developed “conserves” air, directing it to the right lines at the right time instead of blasting all of them at the same time.

Just to be clear — I’m not talking about the toy air compressors sold at Walmart or your local auto parts stores. Those devices are intended for occasional and rare use — for inflating small passenger car tires.  These units are barely capable of inflating my truck tires and won’t even operate at the pressures required of my trailer tires!  For this post,  I’m talking “serious” portable air compressors from the likes of Viair, which I will describe later.

I should point out that some RV dealers provide a winterization service at no cost for their customers.   Bishs RV in Meridian, ID, for example, suspends all regular service work for two days, dedicating 100% of their service  bays and technicians to this task.   No appointments are necessary or taken — owners simply pull in and stay with their vehicles while the service team swarms around the coach to perform the necessary steps.  This is quite an event – RVs line up in great numbers to take advantage of this service, often times taking up a considerable length of the nearby pubic roadway!

For time commitment reasons, however, this type of service isn’t feasible for some of us, which is why I decided to winterize my own coach while it is parked along side my house.   I use the same method that Bish’s does, only with a smaller air compressor:  Commonly called the “blow-out method”, the technique removes water from the lines with compressed air and fills interior drains (P-traps)  with anti-freeze.   There are a number of advantages to this method, not the least of which is that starting things back up in the spring does not involve removing anti-freeze from the pipes!

For this post, I will describe the winterization method I have developed  by consolidating information from multiple sources, including the P4 manual, documentation provided by Viair, my own experimentation, and an impromptu conversation with the Nautilus manufacturer (B and B Molding) themselves. I’ll start with a brief description of the Nautilus P4 itself , so that RV owners without this handy piece of equipment can apply the same principles to their own coaches.  I’ll then describe the Viair  400p compressor that I use, before going into more winterization detail.

The Nautilus P4

Found on a number of late model RV’s,  the Nautilus P4 is a follow-on to the popular “P1”.    These two units do exactly the same thing, except that the P4 does it with fewer control handles — that means fewer hoses and clamps (i.e. lower manufacturing cost) for the RV manufacturer and clearer instructions for the customer.  In short, the Nautilus systems allow you to quickly configure for different modes of operation, either for camping or maintenance without becoming a contortionist or a plumber.  No more crawling into tight spaces just to find the “hot water heater bypass” valve, for example:  The Nautilus does it all.


Here’s a photo of my P4 in the “Dry Camping” mode.  I’ve been using the fresh water inlet port, which is why the cap is off.  The black tank rinse port, on the bottom right, has the cap installed.  This is a completely separate plumbing system with no possible connections elsewhere in the coach —  your fresh water hose should NEVER be connected to this port.


Here are the five essential P4 modes:

  • Dry Camping mode:  Allows the on-board water pump to draw water from the fresh water tank, filling the hot water heater and pressurizing interior fixtures
  • City Water mode:  Fills the hot water tank and pressurizes interior fixtures via an external water source such as the campground spigot, bypassing the on-board fresh water tank.
  • PowerFill tank mode:  Directs water from the external water source directly to the on-board fresh water tank.
  • Sanitize Mode:  Utilizes the on-board water pump to siphon sanitizing solution directly from a bucket into the fresh water tank.
  • Winterize Mode:  Uses the on-board pump to siphon antifreeze directly from a bucket to interior fixtures.

The Viair 400p air compressor

I have described the 400p air compressor in another post, so to keep things brief  I’ll simply say that this compressor provides the right amount of air volume at the right pressure to maintain my truck and RV tires.  It will deliver about 2 cubic feet per minute at the winterizing pressures involved (20-30 psi), which means it is sufficient but requires a bit more patience and knowledge of how the winterization process works.

At the heart of the “blow out” method is a mechanism to inject air into the RV water system, via the City Water inlet port on the Nautilus.   Often purchased separately, Viair provides an adapter in their winterization kit just for this purpose.  As is the case with water, high pressure compressed air is also dangerous to plumbing, so the kit also includes a pressure regulator, which is adjusted to approximately 30 psi.  Below is a photo of the Viair system:  Their standard inflator gun (far left in the photo), connects to the air pressure source (the compressor, not shown) normally.  The inflator sends air into the the pressure regulator (center) which is connected to the water inlet adapter.  You can see that this adapter has a 3/4″ Garden Hose end, for this purpose.


The Viair winterizing kit, consisting of a water inlet adapter (far right) and pressure regulator (center), which connects to a standard inflator (left).




The procedure

The steps below assume that everything is ready in your coach — for example, the pump and hot water heater are both turned off, and the water system itself has been de-pressurized.  To tart things off,  make sure that all interior fixtures are closed –we’ll open them one at at time to conserve the available air volume from the Viair.   Ok, here we go:

  • Water tank and heater drain
    • Open low-point drains
    • Open fresh water tank drain
    • place P4 in “PowerFill” mode.  This insures that the lines to/from the water tank can drain
    • Remove the water heater anode (assuming Suburban) and store.  This will start draining the heater
    • (optional:  At this point, I put some PVC pipe and fittings into the anode port, to re-direct the water that is about to come gushing from the water heater anode port when the pressure relief valve is opened)
    • Open the hot water heater pressure relief valve and let the gushing begin
    • Flush/clean the water heater as required
    • let things drain.
    • Close water tank drain
    • Close low-point drains (with a large air compressor we could keep these open, but to direct more airflow to the lines of interest, we close these for now.  We’ll come back to the low-point drains again, later.




My PVC pipe and fittings directing water from the hot water heater drain away.  These fittings are not cemented together and easily assembled ‘on the spot’.





This Anode, removed from the hot water heater, has only a few camping trips under its belt, and won’t be replaced come next year.




My two low-point drains,  partially opened here for photographic reasons.  You’ll want to open them up all the way, for a complete drain




  • P4 and water heater lines
    • Place P4 handles at 45 degrees (in between settings, something you would never do while camping).  This insures that all P4 lines are open
    • make sure all interior fixtures are closed
    • make sure water heater anode is removed and port is open
    • Connect the Viair system (described above) to the P4 water inlet
    • Blow out the lines to and from the P4 and water heater.   All air escapes through the anode port)



Here is my P4 with the control handles in a 45-degree “V” formation, which opens up all lines in all directions.



  • Pump circuit and external shower
    • Place the P4 to “Winterize” mode.  Instead of siphoning anti-freeze into the water lines,  this opens up the pump circuit to the P4 water inlet so that we can blow out the pump and associated lines.
    • Open the P4 external shower port (for pressure relief)
    • blow out the pump and external shower circuit. Be sure to open up any auxiliary shower ports as well
    • close the external shower port(s)
  • Main water system blow-out
    • Place P4 into “City Water” mode. This directs air from the water inlet to the interior fixtures
    • Plug the water heater anode port with a 3/4″ NPT plug.  this is to allow pressure to build in the water heater, so that you can blow out the hot water lines.
    • make sure the water heater pressure relief valve is closed
    • blow out the interior fixtures one at a time, both hot and cold, starting with the highest one.  Don’t forget the toilet and any appliances you might have (dishwasher, clothes washer).
    • Finish with all interior fixtures closed.

My homemade PVC plug that installs in place of the magnesium anode.  If pressure equalization is important to you, simply avoid tightening the threads so that the port is closed off to critters but still allows air ingress and egress .


  • Low-point drains. (with small air compressors, small amounts of water can still accumulate during the above steps)
    • Open low-point drains again
    • Open the outside shower port so that the low-point drains will gravity drain.
    • Place P4 handles to 45 degree “V” formation again
    • Close shower port
    • Blow out the low-point drains. Leave open for now in case more water accumulates via gravity.
  • Black Tank Flush line
    • move compressor to the black tank flush inlet
    • blow out black tank flush line
  • Finish
    • Cap both the fresh and black tank inlets on the P4
    • close low-point drains
    • place P4 in dry camping mode for storage
    • Pour anti-freeze into interior drain P-traps
    • grease toilet seal and all sewer hose seals


One thought on “RV Winterization with small air compressors and the Nautilus P4

  1. I purchased a fifth wheel this year with this system. Have always winterized with air. Had never seen this system previously. This is a great easy step by step guide. Also appreciated explanations of why each step is done.
    Thank you

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