Here is a link to my amazon webstore that we have organized alot of Tiny House hot water options that we think would work in Tiny house or Off Grid applications, the first page includes a Coleman camp shower bag for $15
Here is a link to the coleman camp shower bag kit [$15]
If you want a bit more of a shower than a bag, but don't want to hook into the house or dont have house systems, for $300 you can get the Coleman camp shower on demand kit. It runs off a 1lb propane tank and a rechargeable battery for the pump. You can take your shower and fold it up and put it under the sink when you aren't using it, then just pull it out and take a shower! This system has its own built in pressure pump so you don't have to worry about it.
Here is the Heater itself: [$225]
Here is the on demand shower head attachment: [$20]
And here is the link to the bulk tank adapter [$20]
If you already have a gravity fed water tank you can get an on demand unit that doesn't require minimum water pressure like this one [$120]:
All you have to do is attach your water input and propane tank, then vent the exhaust [or have the heater mounted outside and feeding inside, but this may be hard in the winter when outside temperatures drop.
Most on demand hot water systems require 20-50psi of minimum pressure to operate. You can get around this with pressure pumps in line with your gravity tank. Here are a few pumps that will fit the bill and just install in line with your current water line. Each one has their own perks and drawbacks, the main difference is pressure vs power consumption. Make sure to match the correct pump to your application:
Flojet 03526 [$55]
FloJet LF122202A [$75]
SHURflo 2088-422-444 [$80]
The following tanks, like most, need a minimum pressure to operate. If you don't have pressurized water you need to install a pressure pump in your water line. However the benefits of these systems include a nice powerful hot shower as opposed to a more natural rain like shower that you would get with a gravity fed unit.
The main downfall with these systems is that the higher the flow, the lower the temperature will be, meaning you will need more energy to bring the water to a higher temperature.
If you have pressurized water these are some great options:
Rheem RTE 13 Electric Tankless Water Heater, 4 GPM [$185]
If you don't like the idea of on demand systems you can always just use a mini tank. This is a 6 gallon electric system, If you only have one bathroom [like most tiny houses] 6 gallons is plenty of water to take a shower (when mixed with room temperature water). Another perk about tank systems is that they don't need water pressure to function.
Electric 6 gallon Tank Heater
Reliance 6 6 SOMS K 6 Gallon Compact Electric Water Heater [$200]
Lastly is a unique system which is basically a stripped on demand system. You put the coil on a heat source [fire, stove, incinerator, solar concentrator etc...] and run water through it. As the water moves through the pipe it acts like a heat exchange and heats the water, giving you hotter water on the other end.
The above example does not come with the tanks pictured so I believe that it is grossly overpriced, especially since the amount of heat exchange pipe is so small. If you want to build one of these systems all you need is 20 feet of flexible copper pipe, and two "shark bite" style attachments that attach to your copper pipe on one end and 3/4 inch [one male and one female] on the other end. Total cost will be around $35 for the same thing [only it will work better because it will have more heat exchange area.] as opposed to the $50 [plus shipping] that is advertised.
Now I like on demand systems, they are more energy efficient if you don't use alot of water [because nothing is being heated until it is needed.] but we didn't like the thought of generating heat just for making hot water. Heat is the least efficient use of energy, it is a product of inefficiency. Using it for one pupose is a complete waste in our opinion. Also we didn't like the fact that these systems only ran on one type of fuel. When you are off grid and miles away from civilization and you run out of propane it may be a week or more before you can get more. we sat down and designed a system that can use any burnable material to generate heat, this heat can be used to incinerate waste [my favorite method of waste disposal due to the sterile ash produced.] heat the house, heat the shower [because if your environment is hotter you will be comfortable with less hot water, ] and even heat water. The system will be heavily insulated to prevent heat loss anywhere except into the heat exchangers. There will be a loading/ignition hatch at the bottom to load solid fuels [wood chips, coal, pellets, dehydrated waste, etc...] and to ignite liquid fuels [lards, fats, oils, petroleum products etc.] To maintain liquid fuels, a fuel line runs away from the burn chamber to a fuel tank with a drip valve attachment to regulate the flow of fuel and prevent unnecessary risk of a house fire. [because when your house is as big as a large closet, once it starts, it goes, and there isn't much you can do to stop it.]
directly above the burn chamber is the chamber where waste is incinerated. This chamber can also be used to distill and evaporate grey water so you don't have to treat it or dispose of it.
The exhaust [shown in yellow as heat] flows up the side of the incineration chamber to prevent heat loss in the incineration chamber. On it's way up, it also passes around several coils of copper water pipe winding down, and back up the side of the chamber, spaced away from the side of the chamber to allow air flow on all sides of the copper water pipe to allow optimum heat exchange with the water.
The reason the copper water pipe goes back up the side of the chamber is that we decided to have a gravity fed tank in the loft, and the shower below. Having the coil go back up the chamber will allow us to have a gravity fed hot water shower. We all enjoy rain style shower heads and, well....rain is gravity fed. Water will always find its level, so as long as the output of the water is lower than the input from the tank, you will have water flow.
The main water tank will also have a cold water outlet to give you control over the temperature of the water.
Below is a picture of the proposed system, feel free to use it, and if you build one, document it and we will feature you in a post!