The following diagram is the architecture I determined for the water supply side of the house. I was informed by this interesting whitepaper but ultimately decided on a manifold approach vs zoned or trunk and branch due to the relatively short “home runs” I had and the desire to keep a majority of the connections accessible from the inside for maintenance and leak finding. There were also a few items mandated by the ANSI A119.5 code such as the backflow preventer, minimum supply line size, and main water connection fitting type.
I also decided to route the majority of the home runs underneath the trailer to minimize structural impacts to the walls and maintain serviceability in the future. I decided on PEX-A (expansion PEX) because it is cost effective and easy to work with vs. copper or PVC/CPVC and I think it is fundamentally a better system from a first principles standpoint over PEX-B (re-bending of pipe, not relying on crimp rings).
For all components I sourced, I was bent on trying to keep any leaded brass out of the system. For some parts this was unavoidable, but I purchased mostly stainless steel fittings when possible which drove up cost but I think were also generally higher quality than their brass counterparts.
Manifold
I chose the Viega Manabloc system as it has individual shutoffs and I had seen some YouTube videos from builders I watch that praised the product. I got their 6 HOT/8 COLD block as this gave me a few extra ports for any future fixtures/pipes. I had already decided on PEX-A as a piping material, but unfortunately Viega doesn’t sell any PEX-A products so they don’t make an adapter to their manifold for the barb shape PEX-A requires. I had to get a little creative - I used the Uponor 1/2 NPSM adapter that is usually meant for faucets, and wound up machining a hole in the Viega ManaBlock 1/2 port cap so it would work as a nut over the adapter. This is because the Uponor adapter actually has a 1/2” NPSM thread but mates properly to the sealing surface, where the Manabloc has a 5/8 NPSM thread and requires a Viega part to properly mate. I was worried about this interface leaking but was rock solid in leak testing. I also had a leak through one of the valves, but the valve replacement was easy and I think it was because a piece of debris on the O-ring on the valve.
expansion tank
Keeping in the theme of stainless steel over brass, I went with this very-expensive-for-what-it-is expansion tank. I believe a 2 gallon expansion tank was recommended for a ~30 gallon hot water tank. It came pre-charged to 30 psi and has a little Schrader valve to recharge or increase pressure as needed.
Filter
Though we will have a filter specifically for drinking water, I wanted a filter at the main water entrance because I had no idea what our water supply was going to look like. I decided on this filter from Aquasure basically because it was the smallest high-flow filter I could find. It filters sediment with a pleated filter and chlorine with an activated carbon filter. The tricky part about this was making sure to install it in a way that the filters could be serviced.
Thermostatic mixing valve
The thermostatic mixing valve was provided with the hot water heater so I used that one - it is either this one or one very similar in the Resideo Braukmann AM1-Series.
Shower Mixing valve
I used the 1/2” MultiChoice rough in valve from Delta (R10000-PFS-MF) with the PEX-A barbs. I didn’t spend too much time shopping around for this.
backflow preventer
I spent a good chunk of time trying to find an affordable stainless steel body backflow preventer but couldn’t and eventually settled on this Caleffi valve.
General connections
Since I was using PEX-A I used a lot of Uponor plastic fittings, and for the stainless steel side for pipe-to-threaded connections I used Boshart stainless steel expansion fittings. I also used Boshart stainless steel valves for all valves in the system.
Tools
I used the Apollo PEX-A manual expansion tool as I couldn’t stomach investing in Milwaukee auto-expansion tools just for this. It turned out fine but there were a few low clearance issues underneath the trailer where the handle of the tool would hit the ground. It was also a workout especially for the 3/4” pipe! I used a standard pipe cutter to cut the PEX, and had to invest in another set of adjustable wrenches to crank on the threaded fittings.
Before the plumbing could be distributed along the bottom of the trailer, I had to insulate and close up the bottom of the trailer. I used Rockwool ComfortBatt in the R-15 2x4 16” O.C. Wood stud variant and had to trim them lengthwise as the trailer crossmembers are 16” O.C. but are a true 2” wide, taking away 1” of cavity width compared to a 2x4 16” O.C. wall. I then sealed this in with ZIP R-6 sheathing giving the floor a total rating of R-21 (true R value somewhere near R-18 with stud considerations). I fastened the ZIP panels with Simspon Strong Tie TFP1475S wood to metal screws, which need pre-drilled contrary to the product specification.
Now we could start routing pipes. All the 1/2” PEX lines needed a 3/4” hole. We had to drill through the subfloor and bottom plate while avoiding the trailer steel, as all the pipes re-entered the house into the wall.
Next all the pipes were terminated at the manifold and the upstream 3/4” pipe connections were made. It was a fairly tight squeeze into the allocated space especially with the hot water heater, but all-in-all turned out well. There are two drains that exit through the bottom of house - one for the hot water tank PRV and the other for a valved drain to drain the system if needed.
The main supply is available at the rear of the house, and enters through the floor and attaches to the filter.
To get signed off on the supply side, a pressure test needed to be conducted. There are a few options but I decided on the option to pressurize with air between 30 and 35 psi and make sure no pressure loss was recorded in 10 minutes since I didn’t have access to water but had an air compressor. I wound up have to redo a lot of the threaded joints as they leaked when pressurized (I used Snoop to check for leaks), I think because of not enough Teflon tape and the unforgiving nature of stainless threads compared to brass. It’s been quite a few weeks since I ran the test and I still haven’t dropped below 30 psi!
Last up was the connection to the hot water heater heat pump, which unfortunately wasn’t going to fit on the front of the house as originally intended. I decided to make a removable side mount for the heat pump that hangs off the bottom of the trailer. I welded the mount out of steel at the makerspace, paint it black, and clad it in redwood. Conduit for electrical and the hot water lines to the hot water tank were run here. We got it running and it was heating up water!
