A tankless gas hot water heater might be a good choice for your off the grid home. If you are not familiar with the tankless hot water heater they are a box shaped unit that heats water instantaneously as water passes through it rather than heating water slowly and storing it in a large tank.
They are also known as demand, on demand or instantaneous water heaters.
The benefit of the demand hot water heater is they can be more efficient than the standard tank style water heater. The only source of heat loss is through the vent, chimney or flue. There is no tank that constantly loses heat thru it sides and top.
However, if you look at the venting of the instantaneous water heater you will be able to judge its efficiency. A water heater that requires a double wall steel vent or full blown chimney is probably not very efficient. On the other hand if your water heater is vented with plastic pipe, it is probably very efficient.
Different Types of Gas Demand Water Heaters
There is a lot a variation in how the flame of the gas tankless water heater is ignited. In the beginning, all of them were ignited by a pilot flame.
Pilot Ignition – The pilot was on 24 hours per day just waiting for you to open a hot water tap. When a tap was opened, a valve opened allowing a huge amount of propane to be lit by the pilot. The huge flame heated the water instantly as it passed through the heater. The benefits of this system was the fact they would operate without electricity and were very reliable. The downside of course is the propane/natural gas wasted by the pilot. Depending on the size of the pilot, up to 30 liters of propane can be consumed each month.
DC Electric Ignition – Another very cool system for igniting your gas instantaneous water heater was the use of a battery. Either a D cell or two D cells were used to produce a spark similar to your barbeque electric ignition that would light a pilot when hot water was required. The pilot was then used to light the large flame that would heat the water. We have used many of these and they work great. Battery life is usually about a year and can be more. This system worked well but seemed to fall out of favor quite quickly due to relatively low demand. Let’s face it, how many people are out there that are off the grid…not many. The availability of cheap grid power lead to the success of the next style of ignition.
AC Electric Ignition – As the demand water heater manufacturers realized their largest customer base were grid tied, they started designing their heaters to operate from 120/240 volts AC. This is not good news for solar homeowners. While the DC ignition units used tiny amounts of energy, the AC counterparts seem to use massive amounts of electricity. Instead of a basic high voltage ignition system, they can now have power drawing glow plugs or heating elements as well complicated computer systems that monitor everything from input water temperature to the exhaust temperature. Simple mechanical safety systems were replaced by electricity consuming sensors and computers. It would be easy now to purchase a hot water heater that uses more electricity than your refrigerator. Be very careful when purchasing a gas demand water heater that plugs into the wall. Check the specifications first.
Microhydro Ignition – Aquastar/Bosch designed a very unique ignition system (Model 125HX) that consisted of a small water turbine that generated the electricity required to ignite the gas flame. It is by far the best setup for the solar home as it requires no electricity and does not waste and gas operating a pilot. The biggest issue is they are hard to find.
Disadvantages of a Gas Tankless Water Heater
Limited supply – The biggest disadvantage of any tankless water heater is the fact they are limited in how much water they can supply. While they are advertised as providing an endless supply of hot water, most can only make 2–5 gallons (7.6–15.2 liters) a minute. They are not ideal for situations where there could be two showers and a dishwasher running at the same time. They are designed to provide hot water at a specific temperature. They regulate the temperature by throttling down how much water is allowed to pass through them. The more hot water demanded, the lower the pressure.
Requires behavior changes – The standard hot water heater is easy to operate. You want hot water, you open a tap. If you want a lot of hot water you can open the tap fully. If you only want a little hot water (as in when you are rinsing soap off the dishes) you can open the tap a little. A large amount of water (over 1/2 gpm) must be flowing for an on demand hot water heater to operate. If you only open the tap a little, the heater will not function and the cold water will pass through the heater. This results in cold water going to your sink or shower. Even opening and closing your tap many times as when doing dishes results in cooler water.
Requires high water pressure – Most of the on demand water heaters require at least 20 psi for operation. Many off the grid homes use gravity fed water systems and do not have enough pressure for the tankless heater. An easy fix in an inexpensive booster pump on your cold water supply. This works but adds another electric load that you would rather not have.
Most will not work with radiant heat – The majority of instant water heaters need a lot of pressure drop to function. In a radiant heat or hot water baseboard heating application there is no pressure drop.
Most will not interface with solar hot water – A tankless water heater is designed to take cold water and turn it into hot water. It cannot be supplied with hot water. If you supply it with hot water it will still ignite and possibly boil the water (or just waste energy). If you plan on using a solar hot water system to subsidize your domestic hot water heater, a tankless is not your best choice. Instead buy a high efficiency tank style heater.
Will not work in an open direct heating system – We only mention this as the open direct is our favorite heating system. Learn about open direct heating systems.
Require chimney or double wall steel vent – The manual venting gas models require a full blown chimney just like a wood stove. The power venting models all use double wall steel vent piping which is expensive. There are other water heating options that only use plastic piping for venting which is much less expensive.
How to Select the Right Tankless Water for You
Fuel type and availability – The first consideration is the availability and cost of fuel. Your options are propane or natural gas. Most off the grid homes are remote and do not have access to natural gas. Do not buy a natural gas model thinking you can convert it to propane later or vice versa.
Tankless water heaters cannot be converted from one gas to another easily like a gas range or barbecue.
Size – How many gallons per minute will you require? Do you need to operate two showers at the same time or can you get away with one at a time?
Electricity requirements – Do you have electricity to spare? Are you willing to purchase more solar modules so you can operate a computerized water heater. Some models can vent up a chimney and require no electricity. Other models use a high tech fan system to vent through the wall or ceiling. Look seriously at the power requirements.
Energy efficiency – Does it matter to you if you use less propane for water heating? Are you limited in how much you can spend on natural gas? If operating cost is important try to find the most efficient pilot-less model you can.
Cost – Usually the more money you spend, the better the unit will be but that it not true for the tankless water heater. You can purchase a very reliable Bosch/Aquastar 125B for about $600-800 or a highly computerized (electricity hog) and far less efficient model for $2000 from other manufacturers.