Following on from last weeks blog, Living Off-Grid with Solar Power, this post will give you the run down on our hot and cold water systems. The water system you choose will largely depend on your location, rainfall, gradient of your land, budget and personal preferences, but again I hope that publishing the details of our system may help others planning their off-grid home, or those looking to be more self-sufficient in terms of water provisions. We are lucky, or unlucky depending on how you look at it, that we live in a seriously rainy area. The rain gauge on one of the mountains we look out onto just happens to be the wettest meteorological station in Australia, with an average annual rainfall of 8,312 mm. As a result, unlike many off-grid Australians, we rarely have to worry about running out of water, if anything, most of the year we have an over abundance of it. Rainwater collection was therefore the obvious solution for all our water needs, but if you don’t have high rainfall in your area, a system with a hydraulic ram pump or a bore hole may work better for you.
Specifications of our system
Our off-grid water system is made up of the following elements:
– 220 ²m colorbond roof (rainfall catchment area)
– 2 x 22,500 litre poly tanks
– First flush system
– Gutter guards
– Grundfos CMB 3-4 pressure pump (P2 of 0.5 kw ad 3.1 Amp draw), with filter and pump cover
– Rayburn 345 W – wood burning wet back stove.
– 300 litre evacuated solar-tube hot-water heater
Note – we also have an additional concrete tank and small water pump on the shed that was installed when we arrived. This is used for watering the garden and is not included in the household system.
Modern rainwater tanks come in all kinds of shapes, sizes and materials. When choosing a tank, cost is generally a major factor. Bear in mind not just the cost of the initial outlay, but how many times it will need replacing over the years. For example, galvanised steel tanks may be cheap, but if you have to buy them 5 times more often than poly, they are not cost effective in the long run. Rebates may be offered on particular tanks so be sure to check available offers to reduce your initial outlay. It also makes sense to buy larger tanks at the outset, rather than gradually add small tanks, as cost per litre reduces with size of tank. If you care about the environment, embodied energy should be a consideration i.e. the energy from mining/sourcing of materials, the manufacturing process and transport. Regardless of the type of tank you install, water collection is a great step towards being environmentally friendly and self-sufficient.
Rainwater Tank Material Options
There are 5 main choices of material for tanks, here are some of the advantages and disadvantages of each. Non-combustible tanks (concrete or metal) are vital in bushfire zones, but are not a problem here in the rainforest, which gives us a bit more scope.
Advantages – Cheap. UV stabilised and food grade. Resistant to damage by saline water, so good for properties located near the coast. Lightweight and easy to move around/transport. Does not need a concrete slab, can be placed on sand, gravel or partially buried. Flexible material so can be squeezed into tight spots where needed (we nudged ours under the deck using a bobcat!). 25 year warranty.
Disadvantages – Not suitable for bush-fire prone areas. Can impart a plastic taste to the water, but it is rarely noticeable in large tanks, and placing the tank in a shady spot reduces this effect. Whilst the tanks are advertised as being fully recyclable, there are concerns that this may not be the case long term due to sunlight exposure rendering the tanks useless. As this type of tank is relatively new, only time will tell. High embodied energy and greenhouse gas emissions during the manufacturing process as made from non-renewable fossil fuels. Embodied water (the amount of water used to make them) however, is relatively low compared to the other tanks choices.
Advantages – Steel tanks are the most environmentally friendly option as they have low embodied energy, greenhouse gas emissions, embodied water and nitrous oxide emissions during manufacture. They generally last at least 20 years and can be recycled at the end of their useful life. They require little maintenance and do not rust or corrode.
Disadvantages – Expensive compared to poly tanks and fibreglass. Should be placed on a concrete base or reinforced pavers minimum 100 mm thick with steel reinforcing, or else you may void the warranty. They should not be fitted in-ground. Bushfires may destroy the seal on the tank. Less environmentally friendly if not manufactured locally and need to be shipped long distances.
Steel (e.g. Galvanized steel, Zincalume and Colorbond)
Advantages – Cheapest type of tank available.
Disadvantages – Steel rusts and corrodes, and the zinc can leak into your water leaving your stored water tasting metallic. Need to be flushed before use, which can be difficult if you are off-grid and remote. Many steel rainwater tanks now come with liners made of food-grade polyethylene…which defeats the object of avoiding poly tanks. They break down over time and can require repair. Whilst galvanised steel tanks may last less than 5 years, a zincalume tank will last 10-15 years and a colorbond tank may have a 20 year warranty. Unfortunately, any damage to the coatings used for protection will seriously reduce lifespan. Requires concrete footings.
Advantages- Greater heat resistance than poly tanks. Light, rust and UV resistant. Can mount on compacted sand.
Disadvantages – Brittle and Prone to cracking. Allows more light into the tanks, which can lead to algal growth. Not recyclable and will likely end up as landfill.
Advantages- Wind, vermin, rust and fire proof. They are the only type of tank truly resistant to bushfires. Warranty of 20-30 years and a good quality tank can be repaired. Concrete tanks keep the water cooler than the other options and therefore limit bacterial growth. They can be buried.
Disadvantages – Highest embodied water, energy and greenhouse gas emissions of all tanks, but most parts recyclable at end of useful life. Ugly. Lime can leach into water and increase the ph. Tend to sweat. Pre-cast versions must be positioned using a crane and are very heavy to transport.
Note**Make sure to check the regulations pertaining to your area, it may sound crazy, but depending on where you live you may actually need a special permit to harvest rainwater!
In the end, our choice came down to a combination of cost, the limitations posed by the very difficult logistics associated with our off-grid site (very narrow rainforest dirt road and no running water) and the fact that we did not want to put down a concrete slab. We also wanted to squeeze the tanks under the deck, which required flexibility and partially submerging them. The main disadvantage for us on the poly tanks we chose was our concern about recycling. It is possible however, to extend the life of a poly tank by coating them with masonry. This provides complete shade and sunscreen and dramatically extends the life of the tanks. This is a project we have planned for the future. If money and footings were not an issue, we would likely have opted for stainless steel. In terms of taste and safety of water, we have consumed water from our tanks since December 2012 and have never once detected any plastic taste or become sick from our water supply. We do not filter our water or treat it in any way.
Rainwater collection system
The rain water collection system includes gutter guards; fine mesh grills over the rain water roof gutters to filter out leaves and larger debris.
It has a “First Flush” filter system that uses vertical pipes with a floating ball inside. The first rush of rain water fills the vertical pipe with water and dirt from the roof as the ball rises. When that pipe section is full, the floating ball closes off that route and diverts rain water to the storage tanks. There is a drip feed valve and a filter at the bottom of the vertical filter pipe so that when it is not raining the water level and ball falls ready for the next time it rains. The filter is easily removed and needs to be cleaned at regular intervals.
The amount of water used to flush the roof is a factor of surface area of the roof and the amount of pollutant on the roof and gutters. As a general rule, divert a minimum of 0.5 l water per m². For a roof that is prone to leaf cover, bird faeces, dead insects etc. divert 2l of water per m². The more water that you divert, the cleaner the water in the tank will be.
Tank size is a function of the size of the catchment area (i.e. the size of your roof), rainfall and household consumption. I used an online tank calculator (tankulator.ata.org.au) that takes these factors into consideration and the tanks we bought have been more than adequate. It is worth checking the calculators under normal conditions and then under dry conditions. I incorporated considerable redundancy into our system and included driest possible scenarios, which worked out well this year when we experienced the worst drought in 50 years in 2014! Our tank ulster result under 5 normal years and under the driest conditions are shown below.
We have a wet-back range to heat our rainwater. A ‘range’ is a combined stove-top and oven. There are three types of cooking range available, only one is suitable for providing domestic hot water:
Dry range – Cooking only
Semi-dry range – Cooking and central heating
Wet-range (also known as wet-back) – Cooking, central heating and domestic hot water.
There are a number of manufacturers of wet-back ranges on the market including:
When choosing a wet back range you need to consider:
- Overall physical size of range and how this will fit in your kitchen.
- Size and number of ovens (only the Esse 990 has three ovens)
- Size and number of hot plates
- Flue requirements
- Air intake (some models require a hole in the flue, which is not possible in my elevated house).
- Fuel supply available – Ranges are available in electric, gas, oil, pellet and wood models.
- Foundations, many wet-back ranges are seriously heavy (ours weighs 380 kg) and as a result they need sturdy foundations. A suspended floor is rarely suitable, but we got round the problem by building a reinforced concrete plinth beneath the stove. This added $800 to the cost of the house build.
After a lot of consideration, we chose a Rayburn 345W. I wanted a cast-iron cooker that would last a lifetime and was impressed by the Rayburn build quality. I was very interested in the Esse 990 as it has great specs and 3 ovens rather than two, but there were many green living forums on the web at the time of purchase with heated complaints about the Esse, which made me lean towards the Rayburn. I’m sure my decision was also partially subjective, I have always admired Rayburns and basically wanted my own. I like the design, the look and the history and I thought the colour (British Racing Green) would work well in my kitchen. As a range becomes the main feature in your kitchen, at the end of the day, it is important, regardless of any other consideration, that you are happy with it. If you are interested in the Rayburn 345w more information can be found here and installation instructions here. I also wrote a blog about our initial experiences with ours, which you can read here.
If power is an issue, it’s best to place your tanks on higher ground so that water can gravity feed and an electric pump is not needed at all. Whilst it’s not necessary for the tanks to be near the house (as the water can be transported by pipes), we built our home on the highest point of our property to take advantage of the view, so gravity feed was just not an option for us. We considered installing a header tank near the house so that we could reduce power by pumping infrequently to the tank then relying on gravity, but given our exposed site and the fact that we are in a cyclone zone the cost of such a structure was prohibitive. In retrospect, the decision to use a pressure pump was the right one. We don’t have to look out onto tanks, our pump in reality uses very little power and we are able to store the tanks under the deck where they are protected from sunlight, which will increase their lifespan. If you are looking at a pump to supply water to your house, here are the main points to consider:
– Wattage -always choose the lowest wattage pump for the job required
– Number of taps you would like to use at any one time.
– Noise- whilst our pump is fairly quiet, it still wakes me up (it’s located under our bedroom) if a tap is not turned off properly during the night
– Maximum Head height – i.e the maximum height to which the pump can raise water. At this height the water would stand still and not flow. For outlets/taps below this level water will flow at greater rates at lower outlet/tap positions. That is, there is a trade-off between required head of water to the outlet and the flow rate available from it. This relationship can be read off from the graphs supplied by the pump manufacturers by drawing straight lines from one axis to touch the curve of the graph and from there running at 90 degrees to the other axis. E.g. starting from an estimate of the flow rate required would lead to the maximum head available or starting from the height of the outlets, (required head), the achievable flow rate available can be read off.
– Flow rate – in cubic metres /hour (m3 /h )
– Reputation – Pumps differ considerably in quality and reputation, make sure to check reviews to avoid buying a lemon…
We chose a self-priming grundfos CMB pump as they are known for their quality and reliability. We opted for the latest (at the time) CMB model as it is very energy efficient and has a generator friendly pressure manager. Model choice came down to the CMB 1-4 or the CMB 3-4 and we went for the latter, more expensive model, as we wanted to be able to have a few taps on whilst showering without having to worry about it. The pump is installed at the base of the tanks. As part of the offer, we received a 4 year warranty and pump cover.
Evacuated tube solar heating tank
The Rayburn specified the need for a 190 litre hot tank. These boilers cannot be operated dry and must be plumbed into a water filled heated water circulation system. The hot water cylinder needs to be raised higher than the Rayburn so that heated water can circulate between the boiler and the hot water cylinder by natural convection. As we have open ceilings in the kitchen and the tank needs to be directly above the cooker, this made the use of a conventional header tank too difficult. The fact that any gravity fed water tower would need to be cyclone rated meant this was not a consideration for us. After searching for possible alternatives and solutions we discovered that there were solar water heating tank systems available and certified for roof mounting even in cyclone regions like ours. Furthermore, some of the available roof mounted evacuated tube solar water heating systems had added connections for a “Wetback” stove boiler. This opened up the possibility of using the solar water heater tank to fulfill the plumbing requirements of the Rayburn, that is, the hot water storage cylinder together with a means of maintaining its water level.
This is achieved in the Solar Wetback tank by reversing the roles of the main tank and internal indirect heat exchange coil normally found in conventional domestic water heating systems. Indirect cylinders have a heat exchange coil of copper pipe immersed in the cylinder and connected to the boiler. The heat from the boiler heats the water in the internal coil and it in turn heats the water in the cylinder for domestic use. In the Wetback system the boiler heats the main body of water in the tank. The tank water heats the water inside the immersed heat exchange coiled copper pipe. When a hot tap is opened cold water either gravity fed from an elevated storage tank or pump fed, (as in our installation), is heated as it flows through the coil.
Unlike in the normal indirectly heated type of installation the tank, water does not flow to hot taps but some unwanted evaporation does occur and it needs the water top up valve arrangement to replace it. The solar roof tank must be mounted so that there is a continuous rise in the pipework between boiler and tank, with no valves, inline taps or other restrictions that could lead to a build-up of steam pressure. The tanks come in several sizes typically around 300 litre capacity which is higher than the minimum specification for the Rayburn alone. However when the fire is lit and the sun is shining there are two sources of heat and the water in the tank can start to boil off more quickly than normal. Some kind of automatic or accessible manual means of shading the solar tubes whilst running the Rayburn to cook would be a worthwhile addition and is a future project.
Top Up Tank
The automatic water top up valve can be a typical ball cock valve or a modern solar powered solenoid valve with a high level sensor and a low level cut off sensor used to detect the possibility of a broken glass solar tube, a situation which would drain the tank rapidly and cause the water top up to run continuously in vain. The solar powered type runs directly from its own solar panel. It does not have any battery storage so only provides power in sunlight. This can be a problem over a period of prolonged low light level due to poor weather conditions coupled with the Rayburn heat source. This problem occurs when the heat from the Rayburn boils over or evaporates water from the tank causing the level to fall below the low level sensor during long dark periods. At this point the tank filling solenoid is locked out even if strong sunlight returns. In this situation the tank needs to be topped up by some other means for example manually with a hose pipe.
It is possible to run a 2 core cable/flex to a push switch at ground level, as we have, to bypass the limit switches in order to force the tank to fill when the sunshine returns. A less dependent more permanent solution would be to replace the solar panel solenoid supply with a mains powered, (from our solar electric power system), 230 volt to 12 volt 1-2 Amp DC power supply, (seen on ebay etc. for $10-$20), some are available with terminal block adapter plugin ends making it easier to get the polarity (positive and negative) correct. It would require a 2 core cable/flex from the roof to the power supply unit indoors.
One simple, but very useful, addition to the system was to connect a hot water run off feature via a tap or inline wheel valve back to the massive cold water storage tanks under the house. This is used to cool down the water in the evacuated solar tank when it is boiling and rumbling during intensive Rayburn cooking periods and brilliant sunshine. It saves the precious collected rainwater and avoids the alternative of running unnecessary baths and washing clothes to use up hot water which would end up down the plughole. Note that it is the heat exchanged to the immersed coil which is run off and which cools the tank water in the process. It only takes about 5 – 10 minutes to provide sufficient cooling at the peak of the heat generation.
How we minimise our water usage
In Australia, the best way to work out whether a fitting is water efficient is to check the Wels rating. You can read more about Wels ratings and how it may apply to you here. Here are some of the steps we took to ensure wise use of water:
- Shower head with water efficient >3 Wels rating
- Dual flush toilets with 4* wells rating. Replacing single flush with dual flush can save 90 litres of water a day
- Using mulch in the garden to retain moisture and reduce watering
- Native plants used for decoration in garden rather than exotic ornamentals. Natives do not require so much water
- Energy efficient top loading washing machine (we already owned a Wels 3 washer, but when it needs replacing we will opt for an even higher rating)
- 3 & 4 star wels rated taps and other water fittings.
- Water tank hack to minimise evaporation of water
- Use of all grey water in orchard
If you have any questions, on any aspect of our system please feel free to drop a comment below and I will get back to you.