Hey offgridders and those considering it, welcome back! This post will be all about hot water without mains electricity or natural gas. It’s pretty important and in fact in most parts of Aus you won’t be able to get an occupancy certificate with out provision for it.
So what are the common options? There’s a wetback for a wood heater, LPG storage or instantaneous, solar thermal or a heat pump system.
Wetback systems
A wetback is an insert in a wood heater that has water running it through it, which in turn heats the water for use in the house. Great if you already have a wood heater that can accept a water jacket.
Negatives for wood heated wetbacks:
- Unless you have an insulated storage vessel you are going to have lag time between starting a fire and getting hot water (maybe 30 min we are advised from people who use this type of system).
- You don’t really have much control over the temperature of the water and wetbacks will easily achieve near 100 deg C if unrestricted – potentially quite dangerous without careful design of the system.
- You have to have wood on hand, chopped and ready to go
- You’re not going to be running your wood heater in summer – so no hot water! People often combine this type of water heating with solar thermal to overcome that issue.
- Inefficient wood heaters are really bad for your health.
Positives for wood heated wetbacks:
- If you already have a wood heater that can accept a jacket then the extra cost is not very high
- In winter if you are running your heater a lot anyway you’ll have abundant hot water for no extra input
LPG Systems
Instantaneous LPG (Propane) systems are quite efficient now and only heat water as you need it. LPG storage systems are less efficient, but still an option. The storage systems are less expensive than a high efficiency instantaneous system but neither are prohibitively expensive.
Negatives for LPG water heating:
- Are you off grid if you are dependent on having a tank of pressurised fossil fuel refilled a few times a year? You guys can argue the toss on that one among yourselves, but we’d say probably not.
- You’re still burning a fossil fuel and you’re dependent on all the infrastructure to get that fuel to you. Albeit it’s not a lot of climate screwing carbon you’ll be emitting, but it’s still significant
Positives for LPG water heating:
- It’s probably going to be the cheapest form of water heating for you to install.
- It’s not dependent on weather or dry wood
- The combustion happens outside so there is no heat added to your internal house atmosphere.
Solar Thermal Systems
This type of system uses direct radiant energy from the sun to heat water. If you are old enough to remember the burn from a metal playground slide in summer then you’ll also appreciate the power of the sun to provide heating.
There are a couple of common types;
- The flat panel type which are essentially a flat, thin metal tank with a black coating that heats up in the sun and has water shunted through it. This type is not that efficient and you may need several units depending on your needs and quite a lot roof space to accommodate them. They are however, the least expensive solar thermal option.
- The evacuated tube type. These are very efficient and require much less area than than the flat panel type, they are also 50% + more expensive for the same output
Negatives with Solar Thermal water heating:
- The obvious one – you need sun! Not easy to come by in southern winters. Evacuated tube generally do better in these circumstances.
- A good system is fairly expensive by comparison to wood or LPG
- System performance is highly variable with the quality of the install and the expertise of the designer. Some people wish they’d never bothered others report amazing performance – we’ve heard both! It all comes down to a well designed and spec’d system.
- Some systems will need a pump and you’ll need to factor this into your electricity budget.
- Some systems come with an electric boost and these are not generally suited to off grid installation because the electrical draw is just too high ( see Off Grid Solar Part 1)
Positives with Solar Thermal water heating:
- It’s the sun baby! The simplicity is pretty attractive.
- If the system has no pump then it has virtually no moving parts to wear out or fail
- You can claim RECs in Australia to offset the installation.
Heat Pump systems
These are basically an air conditioner in reverse. They draw heat from the atmosphere to heat water and are extraordinarily efficient. This is the type of water heating we’ve opted for so we are going to spend quite a lot of time and detail on heat pump systems.
Negatives for Heat Pump water heating:
- They’re expensive. Equal or in most cases more expensive than solar thermal water heating.
- They are a machine and they have moving parts and will eventually wear out.
- Some systems use refrigerants which are not environmentally harmless, more expensive units use carbon dioxide.
- Operating noise is a consideration
- Cheaper units require an electrical booster to cope with freezing at very low temperatures and some units will need to run continuously at low temperatures, You need to factor this into your energy budget as the likely times the anti-freezing systems will need to run is when you are most dependent on battery power
Positives for Heat Pump water heating:
- If you are installing a solar system then it makes good sense just to add some extra solar capacity and run a heat pump
- They are so efficient that they can generate up to 7 times more energy in hot water than they took in electrical energy to do so.
- They are a no brainier installation
- Many have control systems that will only run the unit at times when you have solar power (See above for freezing exception)
- You can claim RECs in Australia to offset the installation.
Heat Pumps in more detail
There is huge variation in the performance and price (and quality) of heat pump systems. We’ve produced a spreadsheet for you which details our research and we’ll use that to explain our choice as we go. There are a lot decisions to be made in selecting a heat pump. Anything with electric boost is a no go, as is anything without a timer function. Generally the bigger the tank the greater the power consumption of the unit will be, but there are a whole heap of variables here. A larger volume of water will stay warmer for longer, but will require more energy to heat. The extra energy will not be an issue if you have plenty of solar power on tap during the day. Smaller units will recover faster and use less energy to do so than larger units (generally), but not stay hot for as long, so think careful about what your usage patterns are likely to be and if they can be adapted to best utlilise a particular system.
About the spreadsheet
Where there are models of different tank capacities they are separated with a backslash and the specs for each model are similarly separated in the same order.
Wattage draw is the power in Watts that the unit draws when actively heating water.
CoP is the ratio of hot water energy produced to electrical energy consumed. i.e. How efficient the unit is, The higher the number the better.
Compressor refers to whether like a split air conditioning system the compressor and fan sits in a separate housing or whether it sits on top of the hot water system. Generally split systems are quieter.
Warranty the first figure is the duration of the warranty in years for the mechanical components of the the system and the second is for the tank.
Control Unit is whether the system has some means for you to control how and when it runs
Noise refers to what sound volume the unit produces when running, Here is a table to help you figure out what the decibel figures mean. Just be aware though that the decibel scale is not linear, so for example 100dB is more than 300 times louder than 50dB.
| dB Rating | Sound |
| 13 | Hum from a light bulb (switched on) |
| 15 | Pin drop on solid floor from 1 meter |
| 30 | Nighttime in rural area |
| 35 | Main hall of a library |
| 40 | Human whispering |
| 45 | Hum from a refrigerator |
| 50 | Corporate office environment |
| 55 | Rainfall; light traffic (closed windows) |
| 60 | Average conversation between adults |
| 65 | Acoustic piano; moderate traffic |
| 70 | Busy restaurant |
| 80 | Vacuum cleaner; hairdryer |
Refrigerant refers to what type of gas is used in the system. R744 is Carbon Dioxide, R290 is Propane, R410a is mixture of difluoromethane and pentafluoroethane and R134a is Tetrofluoroethane.
Booster Element refers to whether the unit has a resistive electrical element (like an electric toaster or old bar heater). These are problematic for off grid systems.
Reheat Time refers to the amount of time to reheat a volume of water from 20 Deg C to hot water temperature (generally < 60 Deg C). Not all units publish this figure.
Operating Range refers to the temperatures in C from and to which the system will still produce hot water.
Hard Water means using bore water in the heat pump system
Price Range is just approximate and does not include the RECs claimed for the system install (which are substantial!). This is a work in progress.
Rating is from productreview.com.au, the first figure is the average rating out of 5 and the second figure is the number of consumers who have rated the product.
So here is the spreadsheet –
After you’ve had a bit of a look we’ll come back in part 2 and look at the comparison in detail.
See you then!
Oh and the spreadsheet is being updated as we go so some areas like price range are not complete and there are a few models yet to add, primarily the Quantum range.