There are a range of compelling options for heating water in a small space- whether it be a small urban apartment closet or a tiny home on wheels.
Electric mini-tank – these units are 2.5-10 gallons, essentially miniature versions of the standard 40-60 gallon water heaters in a typical home. They are easy to install (no venting required). These heaters continually draw power to keep water to temperature, even when the water is not being used, and can eventually run out of hot water. Practically, a 4 gallon Ariston water heater in the Minim House can provide a generous 8 min shower using a low flow Bricor showerhead. It can also be run off the solar system if turned on/off just before use- 10 minutes on heats water from 60 degrees to 102. While it uses 1500 watts, the short amount of time it is on makes it compatible with an off-grid solar PV system. Highly recommended for micro-homes of all types. Top brands: Bosch/Ariston, Reliance. [On display in Minim House]
Tankless (propane or electric) – tankless or “on demand” water heaters only heat water when it is needed. Because they are not restricted to the size of a water tank, they provide a limitless supply of hot water. They are more energy efficient and cost effective because they are not constantly heating a tank of water when not in use. While tankless water heaters do have a higher upfront cost, they can last up to 20 years- longer than the lifespan of tank water heaters (8-10 years). When choosing a unit, it is important to take into consideration the temperature rise required, the max GPM flow rate and the minimum BTU requirement.
When deciding between an electric or a propane tankless water heater, take into consideration:
Propane: off-grid living compatible, high upfront costs, low operating costs, 80-85% efficient, not as compact as some small electric models, requires through the wall ventilation system, requires a propane tank/filling, and gas connection in the interior of the house. These models also require constant high water and gas pressure to function, and minimum water flow rates- i.e. a low flow shower head of 1gpm may not be adequate to keep the water heater on- a known and significant issue for these types of heaters. Be sure to purchase one rated for -indoor- use (there are many ‘camping’ models that are made for only outdoor use and do not have proper venting for interior installations). Top brands: Eco-Temp, Tagaki, Bosch, Rinnai.
Electric: Electric tankless water heaters are extremely compact, modestly priced, do not require ventilation system, are easy to install, and require minimal maintenance. However electric instant water heaters are almost always 220V, a high demand power appliance (there are 110V models but these are generally reviewed as inadequate). Therefore electric instant water heaters may be unrealistic for off-grid living, or even on-grid micro house living if a 220V power connection is not available. Top brands: EcoSmart, Rheem, Stiebel-Eltron.
RV Water Heaters: these are available in tankless or standard 6-10 gallon options. They are run on propane or both propane and electric using a 120v AC power supply. If they use both, an on/off switch is used to switch between propane and electric mode – this allows you to use whatever power source is most convenient at the time. Note: electric mode requires a substantial amount of electricity – 12 amps (1400 watts).
Gas heaters come in two different variations: pilot or DSI (direct spark ignition system). Most modern RV water heaters are the DSI (Direct Spark Ignition) type, which does not require you to manually light it. Venting is easy – the water heaters are installed in the RV wall cavity, which allows ventilation to the outside. While this makes venting easy, it does expose the heater to the cold and may not be suitable for year round use in extreme climates. These units must also be built in to the wall cavity, a significant installation issue if one does not already exist. When ordering an RV model, take into consideration that all models do not come with the outer venting door included (they’re often sold separately). Top brands: Suburban, Atwood.
Condensing Combi Water Heaters– these high efficiency units are similar to tankless water heaters, but provide both domestic hot water and hot water for traditional water filled radiators or in-floor radiant heat. These heaters are remarkable, as they can replace a large traditional boiler + 40 gallon hot water tank with a unit the size of a modest suitcase, effectively freeing up an entire closet. Ideal for apartment or residential use. Install cost (unit+labor) approximately $9-12K. A Navien unit was installed in Brian’s DC rowhouse in Feb 2015, excellent performance thus far. Top brands: Navien, Triangle Tube.
I. Water efficiency: note that water heating requirements can be substantially lessened through use of low flow showerheads and faucet aerators. This Bricor 1 gpm low-flow model is one of the most water efficient showerheads one we’ve found (there is even a .55 gpm model). 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. [All on display in the Minim House]
II. How to determine an appropriately sized tankless water heater based on your needs:
Tankless water heaters produce unlimited hot water but the heater’s specified output limits the flow rate. (This means if you have an undersized water heater, it will produce constant hot water, but the water pressure may be underwhelming.) To avoid buying the wrong size water heater, calculate the temperature rise, max GPM flow rate, and the minimum BTU requirement.
How to calculate temperature rise:
- Determine the temperature of the incoming water temperature. (If you are unsure, assume the incoming water temperature is 50ºF)
- Determine the desired output temperature. (In most cases you’ll want your water heated to 120ºF)
- Subtract the desired output temperature from the incoming water temperature and that is your temperature rise.
Temperature rise = (desired output temperature) – (incoming water temperature)
Be sure to consider the region’s climate that your home will reside in. In a cold climate, incoming water temperatures will be lower and will require more energy to heat to the desired output temperature.
III. How to determine the max GPM flow rate:
- List the number of water devices that could potentially be used at the same time.
- Determine the GPM (gallons per minute) flow rate of each of the devices listed in step one.
- Add together the GPM flow rates of each device. This is your max GPM flow rate.
Standard GPM flow rates are listed in the chart on the right to help determine an estimate max GPM flow rate. (Note: it is best to use the exact GPM flow rate of the devices if those are readily available)
IV. How to determine the minimum BTU (British Thermal Unit) requirement:
- Multiply the max GPM flow rate by the temperature rise.
- Multiply that number by 500. This is your min BTU requirement.
min BTU/hr = (max flow rate in GPM) X (temperature rise) X 500
A BTU calculator is available online which does the math for you.