Designing the water system
Fresh, hot, drinking, and grey water for two people and a dog, living full time between Baja and the Pacific Northwest. The design is settled and the big parts are bought. This entry is the why, the diagram, and the interactive parts list that gets it installed.
1Two constraints shape everything
Salt air and washboard. Salt rules out zinc-plated hardware and bare steel, so fittings are stainless, marine grade, or nylon at the tank. Washboard kills rigid runs, unsupported spans, and anything that relies on standing water, which is why the traps are waterless and the pump hangs on flex lines.
A third constraint is local to this build: the walls are getting microcement over Baltic birch, so a leaking joint behind a finished panel means taking the wall off. The answer is to keep every joint reachable. One hot and one cold trunk run from the utility bay to the fixtures and branch with tees at the sink, the shower and the outdoor box, and each of those tees sits inside a cabinet or a serviceable bay, never behind a finished panel.
2How to use this
The rest of this page is the build, in the order you do it. Each phase has one drawing and one list, and they are the same parts: every badge on a drawing is a row in the list under it. Hover a badge to find its row, tap it to jump there. Tick ord when you order a part and dlvd when it lands; ticks are saved in this browser, so the phone and the laptop each keep their own.
Drawings show what each part IS and which way round it goes, not where it physically sits in the bay. Threads are hatched, barbs are ridged: whichever end threads into something faces that something. Runs are drawn flat for legibility, so a leg drawn sideways may stand vertical in the truck, and each drawing says so where it matters.
Fit the vent before the pump ever runs. Pulling 3 GPM out of a sealed tank puts it
under vacuum, and a 39 x 16 flat face in 7/32 in polyethylene collapses inward. It does not
have to be the finished vent through the floor: T14 in the port, hose up and over, screened,
ending in the bay is fine. But it has to exist first. This is the only way to wreck the
tank this week.
3Fittings on the tank
Everything that threads into the three live ports. All of it is NPT and all of it is poly, and both of those words matter when you order: never G or BSP thread, and plastic at the tank, never PVC.
Not on the drawing, on purpose: T17 drawn as the three clamp bands on the vent hose · M1 tape goes on every metal joint, not in one place · M3 clamps are drawn as bands ON the hoses, not as parts · F13 the fill hose is loose kit, and there is no inlet yet.
Parts for this phase 24 rows
| id | part | where | qty | est | source | ord | dlvd | |
|---|---|---|---|---|---|---|---|---|
| T2 | 1-1/2 in nylon hex nipple · top of the gravity leg, under the capSPARE. It was the top of the gravity leg, and with the leg stored rather than installed it has no fitted job. Keep it with T3 and T4: the backup needs it if the leg ever goes in | tank · fill port | 1 | $7 ea | McMaster-Carr46825K61NIP-112on order 0730JBEREZNY | buy | ✓ | |
| T6 | 1-1/2 in nylon hex nipple · between the gravity valve and the teeSame part as T2, in its own place. Between the gravity valve and the tee. | at the tank | 1 | $7 ea | McMaster-Carr46825K61NIP-112on order 0730JBEREZNY | buy | ✓ | |
| T8 | 1-1/2 in nylon hex nipple · into N1, the fill portSame part as T2, in its own place. Into n1, the fill port. | at the tank | 1 | $7 ea | McMaster-Carr46825K61NIP-112on order 0730JBEREZNY | buy | ✓ | |
| T7 | 1.5 in nylon tee, FNPTThe splitter. Gravity leg up, pressure leg across. | tank · fill port | 1 | $11 | McMaster-Carr46885K156on order 0730JBEREZNY | buy | ✓ | |
| T4 | 1.5 in MNPT x 1.5 in hose barb, nylonSTORED, NOT INSTALLED. Threads into the tee's up branch in place of the T1 cap if the transfer pump ever fails. Keep it bagged with T3 somewhere you will find it in the dark | tank · fill port | 1 | $9 | McMaster-Carr5218K723on order 0730JBEREZNY | buy | ✓ | |
| T3 | 1.5 in reinforced hose, gravity legSTORED, NOT INSTALLED, bagged with T4 as the gravity backup. Already bought, so keeping the capability costs nothing but a bag. If it is ever needed for real, clamp it to T4 and pour in at bench-lid height | tank · fill port | 2 ft | $18 | McMaster-Carr52375K39on order 0730JBEREZNY | buy | ✓ | |
| T1 | 1.5 in cap for the gravity legThe plug for the tee's up branch, and the reason the gravity leg needs no bay space. Jeff, 2026-08-02: with the fill module built, gravity fill is a backup that should never be used, so the leg is NOT INSTALLED. This cap on T6 closes the port instead. If the pump ever dies, unscrew it, thread in T4, and the leg is live in two minutes, or just pour into the open 1-1/2 in port through a funnel | tank · fill port | 1 | $7 | McMaster-Carr46885K206on order 0730JBEREZNY | buy | ✓ | |
| T9 | 1.5 in x 3/4 in reducing bushingScrews into the tee's straight-through port, the one facing away from the tank, and steps it down to the 3/4 in fill line. This is NOT a restriction: the tank port is oversized for this duty and the real ceiling is the garden hose, which is 3/4 in THREAD but only 5/8 in BORE. At 3/4 in the leg already passes 92 percent of everything that hose can deliver; running 1.5 in the whole way would buy about 8 percent, roughly ten seconds on a fill, for a bigger hole through the floor and a size nothing else in the build uses | tank · fill port | 1 | $4 | McMaster-Carr46885K265on order 0730JBEREZNY | buy | ✓ | |
| T10 | 3/4 in poly hex nippleJoins the T9 bushing to the T11 valve. Female bushing exit meeting female valve inlet: the joint the schedule missed until Jeff asked how the valve attaches | tank · fill port | 1 | $2 | McMaster-Carr46825K31NIP-34on order 0730JBEREZNY | buy | ✓ | |
| T11 | 3/4 in NPT ball valve, brass · fill isolationAt the tank end, inside the bay. NORMALLY OPEN, because F7 outside does the daily work. This one isolates the tank for work on the fill line and for lay-up. | tank · fill port | 1 | $16 | Home Depot4190K42BV-34-NPT | buy | ||
| T14 | 1/2 in MNPT x 3/4 in hose barb, nylonInto N2, the top-right port. THE VENT IS ALSO THE OVERFLOW, Jeff 2026-08-02, and that is what fixes the size. On air alone 1/2 in would do: 3 GPM of displaced air is nothing. But as an overflow it has to swallow the whole fill rate without backing up, 5 to 8 GPM off a garden hose, so 3/4 falling by gravity is the size, not a luxury. It also has to never hold a slug of water, because a blocked vent is how the tank collapses, and small bores hold slugs by surface tension. Three reasons for 3/4, one of them structural. | tank · vent port | 1 | $5 | McMaster-Carr5218K711on order 0730JBEREZNY | buy | ✓ | |
| T15 | 3/4 in hose, vent · same coil as the fill lineSame hose and same size as the fill leg F1, so one length covers both. Up first, then continuous downhill, no sags and no traps: nylon reinforcement is what holds that line where plain braided vinyl would sag between supports. SUPPORT IT EVERY 12 IN on the falling run, not the 24 in D8 specifies for pressurised PEX. The vent's whole job is continuous fall, and a sag between supports makes a low point whose far side is a trap. Support interval, not material, is what buys rigidity here: PEX was considered and loses on bend radius, since 3/4 in PEX needs about 7 in of radius to turn and there are only 4 in between the tank top and the bench lid, so it would need an elbow, and an elbow in a vent is a restriction and a debris catcher. One caveat, and it is the only spot it matters: the last section below the floor is exposed, and 162 is clear PVC, so sleeve it or use the opaque 161 for that piece | tank · vent port | 6 ft | $28 | specialist52375K36HOSE-34on order 0730JBEREZNY | buy | ✓ | |
| T16 | Stainless mesh screen and clampScreens the vent outlet, pointing down. Mud daubers nest in open tubes. | tank · vent port | 1 | $10 | Amazon | buy | ||
| T17 | Cushioned P-clamps, stainless, with T-nutsThree ~1 in ones pin the vent hose to the 8020 bay framing: above the barb, at the top of the arc, and holding the screened end rigid and aimed down. Kills the flop without giving up the flexible run. One more on the gravity leg above T5: not structural any more now that the valve is PVC, but a long vertical leg still wants a hand. SEARCH LINK, pick stainless rubber-lined loop clamps sized for 1 in and 2 in OD | tank · vent port | 3 | $10 | Amazon | buy | ||
| T18 | 1/2 in nylon MNPT x 1/2 in barb · into the tank portCHANGED 2026-09-09, Jeff: there is no clearance to turn a valve onto the tank, so the run leaves N3 on a BARB and everything downstream assembles on the bench. Threads in on its own, which is one short hex and easy; after that nothing has to rotate. NYLON, because it is the one fitting touching the tank's moulded threads and the vendor is explicit that metal damages them. The second reason is better than the first: a flexible section keeps the weight of the valve, tee and drain leg OFF those threads, which are the weakest in the system, instead of hanging it all on them over washboard | tank · outlet + drain | 1 | $4 | Home Depot46825K21BARB-12-NYLon order 0730JBEREZNY | buy | ✓ | |
| T20 | 1/2 in brass hex nipple · main valve into the outlet teeTWO: this one and T22. Was nylon and shared with T18; T18 is now a barb, and these two never touch the tank so brass is both fine and easier to buy. Male-male stub. Both thread on with nothing attached to the far end, so neither is trapped | at the tank | 2 | $4 ea | Home Depot46825K21NIP-12on order 0730JBEREZNY | buy | ✓ | |
| T32 | 1/2 in potable reinforced hose, 6 in · tank barb to the valveKEEP IT SHORT. This is the ONLY part of the system that cannot be shut off: it sits upstream of T19, so a failure here empties 41 gal into the bay with no handle to stop it. Six inches, not two feet. DOUBLE CLAMP both barbs with M3. FDA reinforced, same family as F1 one size down; not fuel line and not plain vinyl | at the tank | 6 in | $6 | Home Depot | buy | ||
| T33 | 1/2 in MIP x 1/2 in barb, brass · into the valve inletScrewed into the inlet of T19 ON THE BENCH, in a vice, along with everything else downstream. Turns the whole valve-tee-drain group into one unit that slides onto T32 and takes one clamp in the bay. Brass is fine here: it is nowhere near the tank threads | at the tank | 1 | $4 | Home Depot | buy | ||
| T19 | 1/2 in NPT ball valve, brass · MAIN tank isolationJeff's call, 2026-07-29. Sits immediately at N3 on the first T18 nipple, before the drain tee, so ONE handle kills everything downstream: pump, drain, trunks, the lot. That position is deliberate rather than convenient. Put it after the tee instead and a leaking drain valve could not be isolated, which is the failure it most needs to cover. Normally OPEN. THE CONSEQUENCE, so it is not a surprise at the wrong moment: the drain hangs off the tee DOWNSTREAM of this valve, so draining the tank needs this one open and T23 open. Shutting this does not stop the tank leaving through a failed drain, it stops the tank leaving through anything at all | at the tank | 1 | $16 | Home Depot4190K41BV-12-NPT | buy | ||
| T21 | 1/2 in tee, water outDownstream of T19, so the main valve covers it. Run straight on to the strainer. CHANGED 2026-09-11: the branch no longer feeds a drain, because the drain leg was deleted for want of space. CAP THE THIRD PORT with T35 and leave it. It costs one plug, it keeps the option open, and reinstating a drain later is undoing a cap rather than cutting into a finished run. BRASS, not nylon: it is a three-way junction on washboard and nylon threads strip. | tank · outlet + drain | 1 | $10 | Home Depot46885K152on order 0730JBEREZNY | buy | ✓ | |
| T35 | 1/2 in NPT plug, brass · caps the spare tee portADDED 2026-09-11 as the drain leg came out. The third port of T21 is left capped rather than the tee swapped for a coupling, deliberately: a plug is a dollar, and it means adding a drain later is undoing a cap in an accessible bay rather than cutting into a finished run. Brass to match the tee | at the tank | 1 | $4 | Home Depot | buy | ||
| M1 | PTFE thread tape, potable ratedPotable rated. For tapered threads only, never on a gasketed joint. | every threaded joint | 1 | $10 | Home Depot6802K12on order 0730JBEREZNY | buy | ✓ | |
| M3 | Stainless hose clamps, assortedFor the vent hose and the grey line. Stainless, because plated clamps rust out in a season here. | every barb | 1 set | $15 | Home Depot45945K64 + 45945K68on order 0730JBEREZNY | buy | ✓ | |
| F13 | Drinking-water hose, 25 ft, NSF-61Potable hose, NSF-61 and lead-free, white or blue. A garden hose leaches and the tank tastes of it. Lives in the bin with F11 and F12. | the fill hose itself | 1 | $35 | Home Depot | buy |
4Core: strainer, pump, accumulator
The pump and accumulator are already owned. This phase is the connectors between them and the tank outlet, plus the things that stop the pump shaking the bed frame.
Parts for this phase 11 rows
| id | part | where | qty | est | source | ord | dlvd | |
|---|---|---|---|---|---|---|---|---|
| P1 | 1/2 in MNPT x PEX adapter · outlet tee into the suction runThe bridge between the threaded world and the PEX world: NPT male threads into any female port, PEX pushes over the barb, a copper crimp ring locks it. First one goes in the run port of T21 to start the suction line; the rest wherever threads meet tube | everywhere threads meet PEX | 1 | $5 ea | Lowe's | buy | ||
| P2 | 1/2 in PEX-B tubing · the short suction run, cut off the blue rollOne roll each so hot and cold stay obvious at a glance. | the whole trunk | 6 ft | $40/roll | Home Depot | buy | ||
| P3 | Shurflo 255-313 strainerUpstream of the pump, where it protects it. Downstream it does nothing. Not fitted yet. | pump group | 1 | $12 | Amazon | buy | ||
| P4 | Flex hose, from the silencing kit · pump inletFlex hose on both pump ports, so vibration does not travel into the rigid runs. | pump group | 1 kit | $30 | Amazon | buy | ||
| P7 | Flex hose, from the silencing kit · pump outletSame part as P4, in its own place. Pump outlet. One purchase covers both. | pump group | same kit | $30 | Amazon | buy | ||
| P5 | Wingnut swivel fitting · pump inletBoth pump ports. Hand-tight and reusable, so the pump comes out without cutting anything. | pump group | 1 pr | $7 ea | buy | |||
| P8 | Wingnut swivel fitting · pump outletSame part as P5, in its own place. Pump outlet. One purchase covers both. | pump group | same pair | $7 ea | buy | |||
| P6 | Rubber isolation mountsUnder the pump feet. With the flex hoses, this is what keeps pump buzz out of the bay panels. | pump group | 1 set | $10 | McMaster-Carr9376K22 | buy | ||
| P9 | 3/4 in x 1/2 in reducing bushingopenOPEN: only needed if the accumulator port is 3/4 in. Check the fitting on the unit before ordering | pump group | 1 | $6 | Home Depot | buy | ||
| P10 | 1/2 in NPT ball valve, brass · pump group isolationJeff's question, 2026-07-29: should the pump be isolated on both sides. Yes, and this is the missing half. T19 at the tank already stops the supply, but the trunks beyond stay full and pressurised, so pulling the pump without this drains the whole distribution back through it. Shut both, undo the P5 swivels, lift the group out, and everything downstream stays sealed. GOES AFTER THE ACCUMULATOR, not between it and the pump: the accumulator has to stay on the pump side or it cannot absorb pump pulses. Pull the pump fuse before shutting both. A diaphragm pump deadheads safely on its 55 psi switch, so that is habit rather than rescue, but a pump cycling against two shut valves is a noise you do not want to diagnose twice | throughout | 1 | $16 | Home Depot4190K41BV-12-NPT | buy | ||
| P11 | Accumulator mounting strap and backingMISSING UNTIL THE 2026-07-29 AUDIT. P6 mounts the pump and nothing mounted the accumulator, which stacks ABOVE it and weighs about 2 lb charged. Unmounted it hangs on its own plumbing, and on washboard that load goes straight into P9 and the pump port. One stainless strap or a pair of cushioned clamps into the 8020, sized to the bottle | throughout | 1 | $14 | Amazon | buy |
5Dry-fit the Isotemp
Dry fitting costs nothing. These are what it takes to actually plumb it, and the two swivels are also what make it a LIFT-OUT, which the 4 in of headroom above the tank demands.
Parts for this phase 8 rows
| id | part | where | qty | est | source | ord | dlvd | |
|---|---|---|---|---|---|---|---|---|
| H2 | 1/2 in FNPT swivel to PEX union · cold inIsotemp ports. The gasket seals better than the threads do. | at the heater | 1 | $7 ea | Home Depot | buy | ||
| H3 | 1/2 in FNPT swivel to PEX union · hot outSame part as H2, in its own place. Hot out. NOTE 2026-09-09: Jeff's unit HAS the factory thermostat mixer, confirmed from the Temp knob in the bay photos, so per Fig. 6 this port is 1/2 in BSP OUTSIDE thread, not NPT. A swivel union seals on its GASKET rather than its threads, which is why an NPT swivel is what everyone fits here and why it works. Check thread engagement when you do it up; if it feels loose or will not pull down on the gasket, get a BSP swivel | at the heater | 1 | $7 ea | Home Depot | buy | ||
| H14 | 1/2 in FNPT swivel to PEX union · cold INTO the mixerMISSING UNTIL 2026-09-11, found when Jeff asked why the heater has two cold ports. It has two because the factory thermostat mixer needs cold to blend with, and the sheet drew that blend leg as a grey line without ever listing a fitting for it. Same part as H2 and H3, so THREE not two. Tee the cold AFTER H1, so one valve kills cold to the whole heater assembly when it comes out on its unions. BRASS, not poly: the manual says brass or stainless at the heater because of the heat, and the poly one bought on 2026-09-11 goes back. CHECK THE THREAD: warm-out is BSP on the mixer variant and the mixer cold port may be too | at the heater | 1 | $7 ea | Home Depot | buy | ||
| H1 | 1/2 in PEX ball valve, brass · heater · cold IN to the heaterTHREE, and together they are the answer to whether hot needs a valve too: cold IN to the heater, hot OUT of it, and the bypass bridge between them. Shut all three and the heater lifts out on its H2 unions while cold water still reaches the galley through the bypass. Trapped water expanding in an isolated hot heater is covered by the heater's own safety valve, which is what H7 drips to the floor | at the heater | 1 | $11 ea | Home DepotBV-12-PEX | buy | ||
| H4 | 1/2 in PEX ball valve, brass · heater · hot OUT of the heaterSame part as H1, in its own place. Hot out of the heater. | at the heater | 1 | $11 ea | Home DepotBV-12-PEX | buy | ||
| H5 | 1/2 in thermostatic mixing valve, lead-free brassLIKELY REDUNDANT, 2026-09-09: Jeff's unit has the FACTORY thermostat mixer valve, visible as the Temp knob in the bay photos, and that is the same anti-scald function this part was added for. CONFIRM before buying, and do not simply delete it: verify the factory mixer is a thermostatic mixing valve and not just a manual blend. MISSING UNTIL THE 2026-07-29 AUDIT, and it is the one omission with a safety consequence. The Isotemp is heated by ENGINE COOLANT, which runs near 90 C, so unlike a 60 C electric-only heater this tank can deliver water hot enough to scald in about a second. A mixing valve sits on the hot outlet immediately after H4, blends cold back in, and holds delivery at a set temperature, typically 49 C. It also buys capacity: storing hot and blending down means 4.2 gal of tank water goes further than 4.2 gal of shower-temperature water. Isotemp expect one on a coolant-loop install, and the coolant loop is the whole reason for that heater, so this is not optional dressing | at the heater | 1 | $70 | Amazon | buy | ||
| H6 | 1/2 in PEX ball valve, brass · heater · the bypass bridgeSame part as H1, in its own place. The bypass bridge. | at the heater | 1 | $11 ea | Home DepotBV-12-PEX | buy | ||
| H7 | 3/8 in hose for the safety valve outletIsotemp spec: 3/8 in (10 mm) hose to a FREE outlet at a lower point, gravity only. No valve and no fitting in it, ever, or the safety valve cannot do its job. It weeps a little on every heat-up cycle, which is normal, so it cannot just drip into the bay. VERIFY THE VALVE PRESSURE off the unit: the two Isotemp manuals say 85 psi and 80 psi and this repo has been carrying 70 | at the heater | 1 | $18 | Home Depot9776T11 | buy |
6Trunk to the galley, and the RO branch
A day of PEX on its own, and it depends on the galley being ready. Worth starting clean rather than half-running it.
Not on the drawing, on purpose: D7 clamps are drawn as bands; the tool is a tool · D8 clips repeat every 24 in along both trunks · D9 a tool, not a fitting · M2 paste goes on every plastic joint · D10 labels go on the finished lines.
Parts for this phase 11 rows
| id | part | where | qty | est | source | ord | dlvd | |
|---|---|---|---|---|---|---|---|---|
| D1 | 1/2 in PEX-B tubing · RED, the hot trunkSame part as P2, in its own place. Red, the hot trunk. | throughout | 1 roll | $40/roll | Home Depot | buy | ||
| D2 | 1/2 in PEX-B tubing · BLUE, the cold trunkSame part as P2, in its own place. Blue, the cold trunk. | throughout | 1 roll | $40/roll | Home Depot | buy | ||
| D3 | 1/2 in PEX teesEvery branch in the in-line design. | every branch | 10 | $6 ea | Home Depot | buy | ||
| D4 | 1/2 in PEX 90 elbowsDirection changes on both trunks. | every direction change | 12 | $5 ea | Home Depot | buy | ||
| D5 | 1/2 in PEX couplingsJoining runs, and repairs. | joins and repairs | 8 | $4 ea | Home Depot | buy | ||
| D6 | 1/2 in MNPT x PEX adapter · everywhere else threads meet PEXSame part as P1, in its own place. Everywhere else threads meet pex. | throughout | 5 | $5 ea | Lowe's | buy | ||
| D7 | 1/2 in copper crimp rings, plus a go/no-go gaugeCHANGED 2026-09-11, Jeff: CRIMP, not cinch. He already owns a crimp tool, and a working tool in hand beats the one-sided access a crimp tool would have bought. Rings are copper and go on with that tool; nothing here is a stainless pinch clamp, and the two systems must not be mixed because neither tool closes the other's fastener. COUNT: a tee takes 3 rings, an elbow or coupling 2, every adapter, PEX valve end, union and faucet connector 1 more. The padded fitting counts come to about 130, so buy two 100-jars rather than one. GET THE GAUGE if the tool did not come with one: a crimp that looks fine and is a few thou open weeps months later behind a panel, and the gauge is the only way to know. The fill and vent legs are hose on barbs and take M3 worm clamps instead, which is a third system again | every PEX joint | 200 | $61 | Home Depot | buy | ||
| D8 | Tubing support clampsWithin 6 in of every fitting, then every 24 in. | every 24 in of run | bulk | $20 | Home Depot | buy | ||
| D9 | PEX tubing cutterSquare cuts. A rough cut scores the O-ring. | tool | 1 | $15 | Home Depot | buy | ||
| M2 | Thread sealant paste, potable ratedPotable rated. On threads that must not weep and will not be disturbed. | every threaded joint | 1 | $9 | Home Depot5477K112on order 0730JBEREZNY | buy | ✓ | |
| D10 | Line and valve labelsLabel both trunks and every valve. | both trunks, every valve | 1 set | $9 | Amazon | buy |
7Connect the sink
Supply side only. There is no drain until the floor is drilled, so what comes out of the tap lands in a bucket.
Parts for this phase 8 rows
| id | part | where | qty | est | source | ord | dlvd | |
|---|---|---|---|---|---|---|---|---|
| G3 | Faucet connectors, 1/2 in PEX to the faucet shanksMISSING UNTIL THE 2026-07-29 AUDIT. The trunks arrive as 1/2 in PEX and G4 terminates in its own shanks, and nothing in the schedule joined the two. What you need depends on the faucet you unbox: RV faucets come as 1/2 in barbed shanks, 1/2 in male NPT, or a pair of captive flexible tails. CHECK THE FAUCET FIRST, then buy either two PEX-to-1/2 FNPT adapters or two braided flexible connectors with the right end. Buying blind here is how a build stops six inches short of the tap | at the sink | 2 | $12 ea | Home Depot | buy | ||
| G1 | 1/2 in PEX ball valve, brass · galley · coldTWO, one per trunk, where the branch leaves for the sink. Shut them and you can pull the faucet apart with the rest of the truck still under pressure, which matters because the galley is the fitting most likely to want attention. Part of the isolation set Jeff specified 2026-07-29: T19 at the tank, P10 at the pump, H1 around the heater, these at the galley, S2 at the indoor shower, S8 outside, R1 at the RO | at the sink | 1 | $11 ea | Home DepotBV-12-PEX | buy | ||
| G2 | 1/2 in PEX ball valve, brass · galley · hotSame part as G1, in its own place. Hot. | at the sink | 1 | $11 ea | Home DepotBV-12-PEX | buy | ||
| G4 | Kitchen faucet, hot and coldSingle lever, hot and cold. Deck-mounted at the galley. | at the sink | 1 | $70 | Amazon | buy | ||
| G5 | Sink strainer basketMandatory. Grease congeals in a cold underfloor line. | at the sink | 1 | $15 | Amazon | buy | ||
| G6 | HepvO waterless trap, sinkWaterless trap, not a U-bend: a water seal sloshes empty on washboard and vents grey air into the cabin. | at the sink | 1 | $40 | Amazon | buy | ||
| R2 | RO faucetopenOPEN: probably in the iSpring box already. Open the box before ordering one | at the sink | 1 | to decide | ||||
| R1 | 1/2 in PEX ball valve, brass · RO feedThe RO taps cold and runs continuously whenever it makes water, so it is the one branch you most want to be able to kill on its own: a membrane change, a leak at the unit, or simply parking it for a season should not mean shutting the whole truck down. One valve, on the cold feed where the branch leaves the trunk | at the sink | 1 | $11 ea | Home DepotBV-12-PEX | buy |
8The fill module
One enclosure under the floor holding both ways of filling the tank, so only one line crosses the floor. This is the phase that makes filling in Mexico pleasant rather than a chore: park, drop the wand in a garrafon, flip a switch, and never open the bench or lift 20 kg. It goes in with the floor penetrations because it depends on them, and the vent has to go outside in the same job.
Parts for this phase 12 rows
| id | part | where | qty | est | source | ord | dlvd | |
|---|---|---|---|---|---|---|---|---|
| F17 | Bulkhead quick-connect for the wand, cam-lockThe suction port on the outside face of the box, capped when not in use. DELIBERATELY NOT GARDEN HOSE THREAD: the bibb F7 is GHT, and if this were too you could connect a pressurised garden hose to the pump suction, which is the one genuinely wrong move this design allows. Different fitting families make that physically impossible | at the tank | 1 | $18 | Amazon | buy | ||
| F14 | Suction wand, 1/2 in PEX, about 20 inReaches the bottom of a garrafon. A 20 L jug is roughly 19 to 20 in inside and its neck is about 55 mm, so there is room. Must be RIGID or it curls and floats instead of staying down. Cut from PEX offcuts: food grade, stiff enough, free. BEVEL THE BOTTOM END at 45 degrees, or a flat cut seals against the jug bottom and the pump loses prime with two litres still in there. Stand it a little off the bottom so it is not drawing the sediment layer every garrafon has | at the tank | 1 | offcut | Home Depot | buy | ||
| F15 | Foot valve with strainer, 1/2 in, potableHolds the suction line full between jugs so the pump re-primes the last few inches instead of six feet of air. Without it every jug swap is a full re-prime, and eight of those per fill turns a good system into an irritating one. HONEST CAVEAT: it lives on the end of a wand you put down on the ground, so it will get grit in it and eventually weep. Rinse it, treat it as a wear item, and know that a weeping one just means slower priming, not failure | at the tank | 1 | $14 | Amazon | buy | ||
| F16 | Suction hose, 5/8 or 3/4 in ID, 6 ft, potableWand to the pump. DELIBERATELY BIGGER THAN THE PUMP PORTS. Restricting the discharge side costs flow; restricting the suction side costs PRIME, which is the thing in short supply on a lift. Going up a size on suction is the cheapest margin available here | at the tank | 6 ft | $22 | Amazon | buy | ||
| F18 | Transfer pump, Shurflo 4008-101-A65IDENTICAL to the main pump on purpose, and that is most of the argument for buying it. This is not really a fill pump, it is a PROVEN SPARE MAIN PUMP that earns its keep filling the tank. A spare in a box for two years is a gamble; a spare that runs every few days is known good. Same service kit, same strainer, same swivels, and if the main pump dies in Baja this swaps in on F19 in ten minutes. Mounted in the underfloor utility box beside the fill: 1 to 2 ft of lift against a 6 ft rating, so it primes fast and never struggles at the end of a jug | at the tank | 1 | $110 | Amazon | buy | ||
| F19 | Wingnut swivel fittings, transfer pumpThe same tool-free swivels the main pump uses, and for the same reason twice over: servicing this pump, and swapping it into the main pump's place if that one fails | at the tank | 1 pr | $18 | specialist | buy | ||
| F20 | Rubber isolation mounts, transfer pumpSame as the main pump's. It runs for fifteen minutes at a time bolted to a box under the floor, and an unmounted diaphragm pump makes the whole box a drum | at the tank | 1 set | $12 | Amazon | buy | ||
| F21 | 3/4 in tee, the fill mergeWhere the two fill paths join, INSIDE the box and below the floor, which is the whole point: the bibb and the pump both live down here, so they merge before the penetration and only ONE line crosses the floor. Fewer holes is fewer places to get potting and sealing right | at the tank | 1 | $12 | Home Depot | buy | ||
| F22 | Check valve, 3/4 in, potableOn the BIBB branch. Without it the only thing stopping pump output going back out the bibb is F7 being shut, which is a soft interlock that depends on you remembering while standing in the dark swapping jugs. Ten dollars makes it physical. The pump's own internal check valve already covers the other direction | at the tank | 1 | $14 | Home Depot | buy | ||
| F23 | Drain valve at the low point of the boxWinterising, and it is not optional: Portland freezes, this box is under the floor, and the pump body holds water. A cracked pump head is the cost of skipping it. ONE drain at the box's low corner empties the whole module, pump included, which is the quiet advantage of putting everything in one enclosure. Also give the box itself a drain hole: it is splash resistant, not sealed, so assume water gets in and make sure it gets out | at the tank | 1 | $9 | Home Depot | buy | ||
| F24 | Switch, weatherproof, at the fill pointWhere you can SEE the jug. You are standing outside swapping garrafones, so a switch inside the truck means walking around for every one of the eight. It also means you are present when the jug runs dry, which is the only dry-run protection this needs | at the tank | 1 | $16 | Amazon | buy | ||
| F25 | Fused feed for the transfer pump, 14 AWGAbout 7 A, so 14 AWG and a 15 A fuse at the source. Fused AT THE BATTERY END, not at the pump: the fuse protects the wire, and the wire is what runs under the floor | at the tank | 1 | $24 | Amazon | buy |
9The engine coolant loop
The heat source, and until 2026-09-09 it was missing from the build entirely: H8 to H13 sat in a BOM group with a deferral reason on it, in no phase and on no sheet, while phase 3 said the loop was 'a separate job' and never pointed at one. Tap both cab heater hoses at the firewall, run 5/8 the length of the truck, two floor penetrations, into the heater's engine-water ports. Those ports are 1/2 NPT, not BSP, which is Fig. 6 of the manual and the opposite of what this repo carried until this morning.
Not on the drawing, on purpose: H12 a gallon of coolant goes into the engine, not at a place on a sheet.
Parts for this phase 6 rows
| id | part | where | qty | est | source | ord | dlvd | |
|---|---|---|---|---|---|---|---|---|
| H9 | Brass barb reducing tees, 3/4 x 3/4 x 5/8 inIN THE VANLIFE OUTFITTERS KIT, confirmed from the packing list 2026-09-11. Do not buy it. Two, and the packing list calls them 3/4 x 5/8 barb reducing tees. TWO. Cut BOTH cab heater hoses at the firewall and splice these in: one becomes the supply to the heater, the other the return. Brass or bronze only. Isotemp is explicit that most plastic fittings will not take the temperature | at the heater | 2 | $0 | Amazon | buy | ||
| H10 | 5/8 in ball valves, brassIN THE VANLIFE OUTFITTERS KIT, confirmed from the packing list 2026-09-11. Do not buy it. Two 5/8 brass barb ball valves. TWO, one in the supply and one in the return. Shut both and the van is back to its factory cooling circuit, which is what you want for a fault, a leak, or hot weather when you do not want engine heat in the tank | at the heater | 2 | $0 | Amazon | buy | ||
| H8 | 5/8 in heater hose, 210 F, antifreeze ratedIN THE VANLIFE OUTFITTERS KIT, confirmed from the packing list 2026-09-11. Do not buy it. 50 ft, plus 15 ft in the extra-long kit and 1 ft of 3/4 for the Transit. The coolant loop, specced off the Isotemp manual and the Vanlife Outfitters build. 5/8 in (16 mm) is the size Isotemp calls for and the size the cab heater circuit already uses, so nothing restricts the flow. Must be rated 210 F / 100 C, antifreeze resistant and 70 psi. Isotemp wants at least 0.5 GPM through the exchanger. Buy 50 ft for a 35 ft round trip | at the heater | 50 ft | $0 | Amazon | buy | ||
| H11 | 1/2 in NPT male to 5/8 in barb adapters, brassIN THE VANLIFE OUTFITTERS KIT, confirmed from the packing list 2026-09-11. Do not buy it. Two, and the list says 5/8 barb to 1/2 FEMALE NPT, which independently confirms the Fig. 6 correction from BSPP to NPT. TWO, at the heater. CORRECTED 2026-09-09 from the manual: the Slim Square engine-water connections are 1/2 in NPT OUTSIDE thread, not BSPP. Fig. 6 labels them NPT on both the plain and the thermostat-mixer variant. The earlier BSPP note was wrong and it was the expensive kind of wrong, since the two thread forms will start and then leak. The ONLY BSP on this unit is the FRESH WARM WATER OUT, and only on the factory-mixer variant; ours takes a separate mixing valve at H5, so check the unit before assuming. Seal these with Loctite 577 or Bondline T777, which is the manual's own spec | at the heater | 2 | $0 | Amazon | buy | ||
| H13 | Hose insulation for the coolant runIN THE VANLIFE OUTFITTERS KIT, confirmed from the packing list 2026-09-11. Do not buy it. One roll of foam pipe insulation. CHECK THE LENGTH against the 35 ft this line asked for; the packing list does not say how much. The loop runs through unconditioned space to a heater in the bay. Uninsulated, it sheds the heat you are trying to move and Isotemp warns long runs lose it anyway. Closed-cell, same family as S13 | at the heater | 35 ft | $0 | Home Depot | buy | ||
| H12 | Engine coolant, Ford specThe loop ADDS volume, so the header tank needs topping up after the first run or it will pull air. One gallon covered a ProMaster; a 35 ft 5/8 in run is about a gallon of hose volume alone. Use the coolant the 2025 Transit book specifies, do not mix types | at the heater | 1 gal | $25 | Amazon | buy |
10Not in these phases, and why
Nothing is dropped silently. Every remaining part is here with a reason, and the generator fails if one goes missing. Most of it waits on the same thing: no hole through the floor yet.
| id | part | why it waits |
|---|---|---|
| F1 | 3/4 in Trident 162 potable hose, fill line | Fill line from the inlet. There is no inlet yet. |
| F10 | 3/4 in floor bulkhead fitting | Vent floor bulkhead. The vent terminates INSIDE THE BAY for now. |
| F11 | Adjustable RV pressure regulator with gauge | Regulator for the PRESSURE fill. A gravity pour makes no pressure. |
| F12 | Hose-end sediment filter, reusable steel mesh | Sediment filter, same. Cheap and worth having when the inlet exists. |
| F2 | 1-1/4 in ID rigid sleeve, potted into the floor | Sleeve through the floor. No floor penetration yet. |
| F3 | 3/4 in NPT bulkhead, 316 stainless or lead-free brass | Inlet bulkhead. Under the floor. |
| F4 | Isolating kit for the box bulkhead, nylon washers and backing plate | Inlet plate. Under the floor. |
| F5 | Cold galvanising primer for the cut edges | 5200 for bonding the inlet plate. Under the floor. |
| F6 | 3/4 in FNPT x 3/4 in barb, 90 degree, brass | Elbow turning the fill line up. Under the floor. |
| F7 | Quarter-turn boiler drain valve, 3/4 in MNPT x 3/4 in GHT male | Fill valve. Under the floor. |
| F8 | GHT cap with lanyard | Cap for the fill valve. Under the floor. |
| F9 | Skid guard, no longer a part | Skid guard for the inlet and vent terminus. Under the floor. |
| R3 | RO reject tee into the grey line | RO reject tee. Goes to grey, and grey is deferred. |
| T12 | 3/4 in NPT street elbow, lead-free brass | Elbow turning the fill line down. Only needed once the inlet exists. |
| T13 | 3/4 in MNPT x 3/4 in hose barb, brass | Barb for the fill line. Only needed once the inlet exists. |
| T22 | Nipple, tee down into the drain valve, DELETED | Drain leg deleted: no room for it. Pump empties through the taps. |
| T23 | Tank drain valve, DELETED | Drain leg deleted: no room for it. Pump empties through the taps. |
| T24 | GHT adapter below the floor, DELETED | Drain leg deleted: no room for it. Pump empties through the taps. |
| T25 | GHT cap, DELETED | Drain leg deleted: no room for it. Pump empties through the taps. |
| T26 | KUS SSS reed float sender, 15 in, 240-33 ohm | Level sender. Not picked yet. |
| T27 | Flange gasket and screws for the sender | Gland for the sender. Size follows the sender. |
| T28 | Sensor pad preparation, sanded flat | Closes the drilled pad. Blocked on looking at the pad. |
| T29 | 3/4 in barb x 3/4 in barb x 1/2 in FNPT tee, nylon | Tees the relief into the vent. Only live with a pressure fill. |
| T30 | Low-pressure relief valve, 2 psi, 1/2 in NPT | 2 psi relief. Only live with a pressure fill. Still an open question. |
| T31 | Nipple through the floor, DELETED | Drain leg deleted: the floor penetration could not be placed. |
| T34 | Street elbow turning the drain down, DELETED | Drain leg deleted the same day this elbow was added to save it. |
| T5 | 1-1/2 in NPT ball valve, PVC or poly · gravity leg shutoff | Shuts the gravity leg, and the leg is stored rather than installed. Only needed if you ever fit it. |
| W1, W2, W3, W4, W5, W6, W7 | grey | Grey system, deferred with the floor penetrations. |
| S10, S11, S12, S13, S7, S8, S9 | outdoor | Outdoor shower. Same. |
| M4, M5, M6, M7 | penetrations | Every floor penetration is deferred. |
| S1, S2, S3, S4, S5, S6 | shower | Indoor shower. The trunk is being built to extend to it. |
| D17, M10, M11, M12, M13, M14, M8, M9 | spares | Road spares. Buy before travelling, not before commissioning. |
Already owned
Class A Customs T-4100 fresh tank
Shurflo 4008-101-A65 pump
Shurflo 182-200 accumulator
Isotemp Slim Square 16 heater
iSpring RO500AK-ORB
11Terms
Plumbing words used on this page
Terms
The sheets and the parts list are written in fittings language. This is the key to it.
- annulus
- The ring-shaped gap between one thing and another thing around it. Here it is the space between the outside of the hose and the inside of the sleeve it passes through.
- barb
- A ridged spigot that a hose slides ONTO and a clamp holds on. No threads. Sized by the hose it fits, so a 3/4 barb takes 3/4 hose.
- bulkhead
- A fitting made to pass through a wall or a panel and seal against it, tightened by a nut on the far side. It is how you get a pipe through something without gluing it to it.
- nipple
- A short pipe stub threaded on BOTH ends, for joining two female fittings. A CLOSE nipple is threaded end to end with no flats, so the only way to hold it is to grip the threads. A HEX nipple has a band in the middle for a spanner, which is what soft nylon needs.
- FNPT
- Female National Pipe Taper: the internal side of the standard tapered pipe thread. MNPT is the male side. The taper is what seals, so these need tape or paste and they are not meant to be undone often.
- GHT
- Garden Hose Thread, 3/4 in and straight rather than tapered. It seals on a rubber washer, not on the thread, which is why it undoes by hand. The bore of a hose behind it is usually only 5/8 in.
- BSPP
- British Standard Pipe Parallel. Looks like NPT and is NOT: the thread angle differs and forcing one into the other will leak. The Isotemp's engine-water ports are BSPP.
- potting
- Drilling a hole oversize, raking out the foam core, filling the void with thickened epoxy and letting it cure. The fitting then passes through solid epoxy instead of foam, so nothing crushes under clamping load and nothing can wick sideways into the core.
- crimp
- A copper ring squeezed all round onto a PEX joint with a dedicated tool. The standard way of making PEX joints here, as against push-fit.
- dead leg
- A length of pipe that holds water but never flows. It goes stale and it is the first thing to freeze, which is why the design keeps asking whether a leg drains.
- weep
- A very slow leak: a seep rather than a drip. Dangerous precisely because it is slow, since it can wet a floor core for months before anything looks wrong. A relief valve weeping is normal; a joint weeping is not.
- slosh
- Water moving inside the tank on rough road. It reaches ports that sit well above the static water line, which is why some legs hold water you would not expect them to.
- siphon
- Flow that keeps going uphill once started, because the outlet is lower than the source. Broken by an air gap, which is why both fill paths end above max water level.
- full port
- A valve whose opening is the same bore as the pipe, so it does not restrict flow. The opposite is a reduced port, which is cheaper and chokes the line.
- head
- Pressure expressed as a height of water. 16 in of head is 0.58 psi, which is why the level sensor needs a low-range unit.
12Open items
Roughly ordered by what unblocks the most. The first two need a tape measure and a phone, and the first one gates fourteen line items above.
- Is the Isotemp 70 psi safety valve a combination check and relief? If not, a separate check valve goes on the cold inlet.
- Confirm BSP to NPT adapters are fitted on the Isotemp before cutting PEX.
- Does the Isotemp mixing valve need its own cold feed? If so it takes cold port 06.
- RO minimum feed pressure from the manual, against the pump's 35-55 psi window. Likely fine, the RO has an internal booster.
- RO power brick voltage and draw, for the record.
- Does the inlet plate land inside the bay footprint, so the backing-plate nuts are reachable through the bench lid? If not, through-bolting is out and it becomes a bonded plate plus screws into wood.
- Where the grey vent terminates. W-5 draws it crossing the floor with no penetration marked and the atmosphere symbol above the floor, inside the van. One of those is wrong.
- Whether a strainer came bundled with the pump.
- Accumulator pre-charge against the pump's actual cut-in pressure, tuned at install.
13Settled, and why
- The vent is the pressure protection. Air leaves as fast as water enters, so the tank never sees line pressure. Everything else is margin.
- Both fills share one top port through a tee, and both terminate above max water level, so no siphon can form and nothing relies on a check valve.
- Nylon at the tank, metal downstream. Metal fittings damage the moulded tank threads.
- In line, not home run. Two trunks branch at the fixtures instead of every fixture running its own line back to a manifold. Far less pipe for a compact layout, and the manifolds come out of the build entirely. The rule that survives is that no joint hides behind microcement.
- Build with cinch rings, carry push-fit spares. Push-fit's no-tool, wet-hands advantage is a repair property, not an install property.
- Grey defaults to ground, diverts to tank for compliance stints. Fail-safe direction is ground. One habit that follows: if power drops while in a campground, no water until the valve is checked, because a dead actuator reverts to ground discharge.
- 2:1 RO recovery was the deciding spec. A gallon of drinking water costs 1.5 gallons of tank. A conventional 4:1 unit would cost 5 and be unusable boondocking.
- Freezing kills the RO membrane permanently. The exposure is Portland winter storage, not Baja. Drain, pull the membrane, or keep the van above freezing.