Wicked Tiny House Plumbing System Overview
What's the difference between a tiny house and glorified shed? Not much if you think about it. Most sheds have a door and some windows. Some even have electricity and heat. But I bet you've never seen a shed with running water before... So, for my first installment of "Yes, I built it myself!", let's do an overview of the Wicked Tiny House's plumbing system.
The whole process started with clarifying what my needs were. I knew wanted running hot and cold water, a shower, a sink, and some amount of water storage on-board so I could operate off-grid if needed. From there, I started to figure out what kind of equipment I would need - tanks, pumps, water heater, shower fixtures, sink fixtures, and piping. All pretty straight forward, until you start comparing the amount of space available in the tiny house to the size of most commercially available plumbing fixtures. Most shower stalls are the size of my kitchen. Many hot water heaters are the size of my bathroom. Your typical kitchen sink would probably take up about half of my kitchen counter. It was time to do some research....
Hot Water Heater - First consideration, fuel source. My desire to be off-grid capable immediately took electric hot water heaters off the table. They suck up way too much power for small battery bank I'd use to run the tiny house. That left me with propane. Next, choice on-demand hot water or a hot water tank. Well, even a small tank (5-gallons) takes up quite a bit of space, and a tank that size doesn't get you much hot water. You'd better be good at taking navy showers! On top of that, smaller propane models are hard to find and are relatively expensive. The left on-demand. Next was to figure out my hot water needs. On-demand heaters are rated by how much they can raise the water temperature at a certain flow rate. For example, an on-demand heater might raise the water temperature by 55°F at 4 gallons per minute (GPM) and 70°F at 2.5 GPM. For a larger house, the calculations can be a bit tricky, but for me, I figured I would only be using one hot water source at a time, either the shower (Federal standard is 2.5 GPM max) or sink (typically around 2 GPM).
With this in mind, I started searching for an on-demand propane hot water heater that could heat 2.5 GPM of water from a conservative 40°F (its cold up north!) to 110°F (typical shower temperature is 105°F). I ended up settling on the Eccotemp FVI-12. It has a flow range of 0.93 GPM to 4.8 GPM, with a 45°F temperature rise at 4 GPM and 77°F temperature rise at 2.3 GPM, which fit my needs well. I felt it had a good amount of features including temperature display, manual independent temperature and gas controls, vent kit, and two year warranty (Note: The warranty is only valid if installed by a "professional", whatever that means). It was also fairly sleek and could be flush mounted to the wall, saving space in the already tight bathroom area. The price was on the lower end of the spectrum in the mid $200 range, and although the reviews were not spectacular, I couldn't justify spending double or triple for slightly better reviews.
UPDATE JUNE 2022: The Eccotemp FVI-12 performed flawlessly for five years until I let it freeze, which burst one of the copper tubes inside the unit in a location that could not be repaired. I am replacing with an Eccotemp i12 which is slightly more expensive. I'll leave a review once installed!
After a few months of use, the only issue that I have run into is that I need to have the temperature VERY well dialed in. I have a 2 GPM shower head, but the 2 GPM is calculated at 80 PSI. I only have a 55 PSI pump, and after accounting for losses in the system, I am probably only getting just over 1 GPM at the showerhead, which is fine for actually showering, but the heater has a low flow shutoff at 0.93 GPM. So basically I have very little adjustment at the shower hot water valve - it basically needs to be on 100% hot water, as mixing any amount of cold water will reduce the hot water flow below 0.93 GPM, shutting off the hot water heater. I'll likely get a higher pressure water pump in the near future which will fix this issue. Which is great segway into...
Water Pressure/Water Pumps - The Wicked Tiny House plumbing system was designed to operate both on and off-grid. If there is access to water, via a hose from a "real" house for example, the tiny house can operate without using it's on-board water storage or pump. The water pressure from the "real" house will be enough to operate all the fixtures in the tiny house. In an off-grid situation, water would be supplied from the on-board water storage and would need to be pumped from the tanks to the shower or sink. As I mentioned above, most fixtures are rated for a certain flow rate, which is typically given at a standard water pressure of 80 PSI. So my showerhead that is rated for 2 GPM will only put out 2 GPM at 80 PSI. Less pressure = less flow through the showerhead. I ended up purchasing a SHURflo 4008-101 3.0 Revolution Water Pump which is typically marketed to the RV industry.
Without getting into a hydraulics lesson, the pump is rated for 3 GPM and 55 PSI, but not at the same time. Basically, the 3.0 GPM number is the maximum flow the pump can produce with no restrictions (the pump just spitting out water into the air), and the 55 PSI number is the maximum amount of pressure the pump can produce (but there isn't any flow at this pressure). When I am taking a shower, the pump is probably producing about 45 PSI of pressure and pushing about 1 GPM of water to the showerhead, which is barely enough to get the hot water heater working. The pump has worked great, and has a lot of great features including a simple, maintenance free design, automatically turns on/off on demand, can run dry without damage, is self priming to six feet, and can be mounted in any orientation. You can certainly hear when it is running, but it is in no way obnoxious. This version of the pump is 12 volt (its for RV's), so I purchased a power supply to run the pump. If/when I end up with a solar system and battery bank, the pump can be wired directly to the batteries. Although this pump has worked fine so far, for the reasons mentioned in the hot water heater section, I'll likely upgrade to a bigger pump (higher PSI and flow) eventually Unfortunately, stepping up to a larger pump comes with a pretty hefty price, which is why I started out with this model.
Another part of the system, which is often overlooked, is the pressure accumulator. The pressure accumulator basically acts as a buffer between the pump and the fixtures. For example, if you turn on the sink to get a small flow of water to do the dishes, the water line pressure will quickly drop, the pump will turn on, build the pressure back up, turn off, and the cycle will continue. This isn't healthy for the pump. An accumulator does exactly what it's name implies - it accumulates pressure, reducing the number of cycles on the pump, extending the pumps life. It also acts to reduce water hammer caused by quickly shutting sink or shower valve. It is necessary? No. Do I recommend it? Yes.
Plumbing - PEX! PEX! PEX! If you're planning to use anything but PEX to plumb your house, you've got to be crazy. Do you really want to be working in super tight spaces sweating copper fittings? I didn't think so. PEX is cheap, flexible, durable, easy to work with, and more forgiving if you accidentally allow your pipes to freeze. The only special tools you need are a crimp tool and cutting tool, and you can get both of them for well under $50 total. On top of that, I feel that you can get a larger variety of fittings than copper, especially if you shop online. PEX fittings and valves are readily available online at significant savings to big box stores. I recommend drawing out a plumbing schematic before you start ordering so that you know how many fittings and valves you need. Even though I did this, I ended up making a couple of additional orders as I pieced things together. I used mostly 3/4" PEX throughout the house, mainly because I had a large quantity on-hand from a previous project. 1/2" PEX should be sufficient for most projects, and comes with the added bonus of cheaper fittings and valves.
On the exterior of the house, I have an RV hookup by JR Products, which allows direct connection from a garden hose and a gravity feed to your on-board water storage. I opted not to use the gravity feed, so when I connect the garden hose, I just need to flip two valves to choose if I want to fill my on-board storage, or use pressurized water directly from the "real" house. Water storage is provided by two 21-gallon food grade low density polyethylene tanks that are tucked into the bathroom wall next to the composting toilet. There are a huge variety of shapes and sizes of tanks available, but be careful, as some non-standard sizes are significantly more expensive than standard sizes. Do your research!
One other thing I built into my plumbing system is the ability to easily drain everything. I planned on living in a cold climate and enjoy traveling, but don't enjoy heating an empty space while I'm not there. With that in mind, all of my plumbing is pitched to a low spot that has a valve that allows me to easily drain the water lines without using compressed air. All I need to do to drain the system is pump the tanks dry, open the shower valve, open the sink valves, and open the drain valve. It costs almost nothing to build this feature in, adds very little time, and is a huge bonus. It just takes a little extra planning when running the water lines. The pump, accumulator, and all valves are located in the false floor in the bathroom. Although it is a bit of a rats nest of PEX and valves, everything is concentrated in one place, which makes it easy to troubleshoot any issues.
Fixtures - Now for the visible part of the plumbing system, the fixtures! The largest item in the bathroom was the shower, so that needed to be purchased first in order to determine the width of the bathroom. I ended up buying the smallest and cheapest stall (Durastall Model 68) I could find. I didn't particularly care how nice it was, as long as it was functional, and knew that I could upgrade later if I wanted. I wasn't a huge fan of the valves that came with it, so I upgraded to a more functional set made by Delta and got a better showerhead with an adjustable flow rate. For the kitchen sink, I wanted something stainless steel and simple that was large enough to be functional for washing dishes by hand. I ended up with a Moen sink tub that is 25" x 22" and thus far, it has been just the right size. Large enough to wash dishes but small enough that it doesn't monopolize a huge amount of counter space.
Greywater - The Wicked Tiny House is outfitted with a composting toilet, so blackwater isn't a problem. Greywater from the sink and shower is handled by 1 1/2" PVC. The lines come together in the false floor in the bathroom where two valves allow the greywater to be sent outside to a greywater trench or to a greywater tank (not yet installed).