The Micro Showcase has had several years designing, testing and then perfecting small rainwater capture and treatment systems (currently on version 3.0). Currently on the lot is the Minim House off grid water system, which consists of a gutter system that fills a 250 gallon flexible cistern, an on-board 40 gallon tank, and a series of pumps and filters to make the water completely potable for sink/shower use.

Here I share two years of experience designing (and redesigning) rainwater collection and treatment systems- the water that feeds the original Minim House, as well as RV-like water systems for microhomes.  The good news here is that in most climates it is entirely realistic to collect, treat, and use as much potable water as you will need for happy micro house living. (1)

I. Off-grid water collection systems. These typically consist of the following elements:

a) Rain catchment surface. Typically this is the micro house roof, but could equally be a different structure. While rainwater will be filtered, ideally a rainwater will not flow over petrochemical products that leach contaminants into the water- metal is perhaps the best choice.  Also be sure to avoid wood shingles, metal flashing or roof treatments that contain lead.  Calculate the amount of water the roof may harvest by getting your local monthly rainfall, and using the following formula:

For example, for a 11×22 micro house in Washington DC (avg 3” rain/month), collection could be up to approximately 500 gallons for an average month.

b) Rainwater transport. When rain falls on the roof, it should flow through gutters and piping that allow a high water flow to the collection tank- I recommend at least 3-4” pipes for a 250 ft2 roof.  As we know when it rains it often really pours, but if the pipe system can’t handle the times the rain really lets loose, you’ll be losing a high % of your monthly water collection to spillage.  Also, ideally water should simply gravity fall through pipes directly into your cistern (collection tank), without the need for pumping, switches, or active maintenance- having tried an automatic rain-activated pumping system, I’ve found it is far simpler to have a direct gravity ‘roof-to-tank’ system.  Early settlers who built rain collection systems heartily agreed.

c) Collection tank/cistern. The tank is where the bulk of the rainwater is stored. In cold climates it must be insulated or placed underground to prevent water from freezing. In slightly more temperate climates, a bit of electric heat tape under the tank and around the pipes can keep enough water flowing during the coldest months.  Sizing should be based on both rainfall patterns and expected water use.  Just as for electricity, it is best to first minimize water use (rather than invest in larger collection/storage systems) through low flow faucets, shower heads, and foot pedal faucets.  Just as a rule of thumb, with efficient fixtures I estimate 30 gal/week/person with regular washing, showers, etc.  When the collection tank is full, there should be a basic overflow mechanism that gutters the water far away from the house/tires. Tanks can be rigid plastic or flexible- the key design trait being a large enough inlet to accept high water flow.  I personally favor the flexible water pillow tanks (far cheaper to ship than the large rigid tanks, easy to unroll and hide under a trailer). (2)

d) Water pumping.  From the collection tank water is then pressurized to move through a potable water hose to the garden, or through the same hose to an onboard holding tank and water filter.  A simple 12 or 120 volt Shurflo pump and switch will do the trick (I hide mine in the trailer hitch compartment, and make sure the pump is disconnected during freezing weather to avoid cracking- it could also live inside).  For potable water, water could be pumped directly from the collection tank through an on-board filter if freezing is never an issue.  I prefer an on-board RV water tank within the micro house that stays warm, then pump/filter from this tank with a secondary water pump. (3)

e) Water filtration. For potable shower and sink water, a quality water filter is essential. The filter takes water from the collection tank/cistern or on-board tank and makes it drinkable. There are many kinds available. The requirements here are water efficiency (no reverse-osmosis), energy efficiency (no UV filters that require electric UV lights on 24/7), maintenance simplicity, filtration efficacy (on bacteria/virus/chemicals), and cost.  After extensive research the ceramic Doulton RIF-10 with the additional sediment pre-filter was the tool of choice- I’ve used for over a year, with infrequent cleanings, and excellent water quality.  This model is originally designed for freshwater boaters, so taking water from your cistern ‘rainwater lake’ also works well. The ceramic filter has been proven for hundreds of years across the former British Empire, and is easily cleaned up to 50x with a scouring pad.  Note the filter is plumbed in after the second (on-board) water pump, so water is pushed through it. Once filtered, water flows to the plumbed micro house water system, very simple.(4)

So to recap a now thoroughly tested/proven off grid water collection and treatment system, version 3.0:

II. Standard RV water systems. If you have a water source and/or don’t want to do an off-grid system, here are the basics of building micro homes with RV-like tanks and water systems.  In the latter case, we can also design the system to allow hook ups to pressurized water if/when that is available. The essential components of this modified RV system include:

• 40 gallon RV fresh water tank (available in many sizes from places such as the tank-depot). This should ideally be mounted within the building envelope (insulated area) to keep water from freezing- as should all piping. If this is not possible, there are a variety of electric RV water tank heaters available. It should also be mounted securely, as it will weigh over 320 lbs when full.
• A greywater tank to store used shower and sink water.
• A fresh water fill inlet to fill up the tank (unpressurized)
• A fresh water fill inlet to fill run the system without the water tank (i.e. when pressurized water is available)
• A water pump (we’re using this standard SHURflo 2088-422-444 2.8 Classic Series Potable Water Pump). Note that this is a pump designed to run off of 12 volts, but you can easily substitute a 120 VAC model.
• An accumulator tank to reduce pump cycling and smooth water flow (such as this SHURflo 182-200 Pre-Pressurized Accumulator Tank)
• A simple water strainer to pre-filter the water (such as the SHURflo 255-313 Classic Series Twist-On Strainer 1/2″ FPT x 1/2″)
• Some one-way valves to allow city water to plug into the system without any manual switching of valves.
• A hot water heater.  This can be a) a traditional small electric heater, such as the 2.5-10 gal Aristons (highly recommended), b) RV-specific (and pricey) tankless water heaters such as the PrecisionTemp RV-500, or c) residential tankless heaters such as the wall-mounted Eccotemp FVI-12-LP (note that this also requires 120 VAC to run).

Design: The system can be plumbed according to a traditional RV schematic, below, with a few caveats: 

• No blackwater tank: I use an Incinolet incinerator toilets, which just uses an electric connection to dispose of waste.  So we don’t plan to have any water going to the toilet (and consequently no toilet plumbing, no blackwater to dispose of, and no blackwater tank to take up valuable space). However, a traditional RV setup would include a blackwater tank.

• No hot/cold water mixing for shower.  Almost all small water heaters have temperature settings on them which obviates the need for a separate cold mixing valve- simply lining in the hot water directly to the shower is simpler and works great, just adjust the temp at the tank.  Also note that one may run an electric water heater for 5-10 minutes prior to a shower, and leave the heater off the remaining time (this is quite efficient, and allows one to run an electric water heater in an off-grid electrical system, such is done in Minim House).

When plumbing the system, consider designing to to be a) easy to drain, for when the house sits empty during winter, and b) keep pipes outside of the walls, tastefully exposed, so if they fail, they do not fail disastrously, and can be more easily repaired should any freezing ever rupture them.

III. Water Efficiency: water efficiency is always important, but becomes much more important when not connected to pressurized water.  This Bricor 1 gpm low-flow model is one of the most water efficient showerheads one I’ve found (there is even a .55 gpm model, but at a rate less than .96 gpm, the tankless water heater does not click on- a widely noted tradeoff of tankless water heaters- always check minimum flow rates).  Bricor will even ‘tune’ the showerhead to match the water pressure your pump generates (in this case, 45 psi).  Bricor also seems to make the most water efficient faucet aerator on the market (.375 gpm).  At this rate, a 40 gallon fresh water tank would give 30 minutes of shower +  26.6 minutes of sink time.  It’s worth noting that this water efficiency is far superior to any RV on the market today, as they all use flush toilets and typically less efficient showerheads/aerators.  Also note that one of the most water saving devices we’ve come across are the foot pedal water valves, available at restaurant supply stores. These valves are incredibly convenient, more sanitary, and much more water efficient than standard faucets- highly recommended.

IV. Water Heating: see further information on the water heating page.

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(1) For foundation built houses, be sure to check local codes on rainwater harvesting for potable use.

(2) Note that rainwater collection experts will argue a pre-filter on the collection tank is necessary to filter out pollen, dust, etc from roof water. This is clearly ideal.  Though to me this seems more necessary when using under ground storage- above ground storage tanks can be more easily emptied of any sediments as needed.  I simply use a basic leaf screen in line in the gutter.

(3) Note that when filling an on-board sealed water tank, air must be exhausted as water rises. While there are a number of simple RV air release products, I settled on this Hayward air release valve plumbed at the top of the water tank. It has the benefit of releasing air, but sealing automatically to prevent water overflow when the tank is full.

(4) Note RV’ers may find it unpalatable to store ‘unfiltered’ water in an on board storage tank, but even potable water RV water tanks have to be sanitized regularly. I find it preferable to actively filter water at the time of use, though this does mean that water flow rates are a bit less than pumping directly from a clean water tank- the water filter slows the water flow rate down a bit (more if dirty).  For this reason I advisenot attempting to use on-demand (‘tankless’) water heaters if actively filtering water (have tried and failed).  These water heaters just demand too high a water flow rate to remain activated.

Cistern first flush + overflow plumbing

First flush system for cistern

Pre-filter for gutter rain water

MetalRainTanks.com rain tank

Cistern water pump (Shurflo)

Studio Shed roof feeds the cistern

• dan

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  April 5, 2016 at 10:26 PM

  Thank you so much for writing an article on off grid water systems, especially when it came to filtration, pumping, and the accumulator.
  I’ve been searching all over the place, and this is the best article i’ve found yet. If you have any other posts you’ve come across on water systems while doing your research, I’d love to see!

  Thank you so much for writing these articles, it’s appreciated!

• John O'Rourke

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  July 3, 2016 at 7:51 PM

  Wow. Super clear explanation of entire process. Thank you so much. We are just starting to investigate building our own home and this info is a terrific start.
  It’s great to learn from your experience. We really appreciate it!
  Excellent design on your home, by the way.

• Audrée Lacerte

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  October 14, 2016 at 9:25 AM

  MERCI!!!
  I’m am planning my tiny house on wheel, and i’ll go off grid, in Québec…you brough me hope.
  Your article is really helpfull to me. thanks again!