Solar hot water and wood heaters make a perfect combination as in the colder months solar hot water systems usually need boosting, and this is precisely the time of year when households with wood heaters will be using them to keep warm. Why not tap some of the heat going up the flue to heat the water? In this article, we will try to give you a clear explanation of how to safely and easily use wood heaters to boost a range of commonly available solar hot water systems.
For any dwelling where wood fire is the primary source of heating and/or cooking, it makes good sense to use a wood heater or stove fitted with either a ‘wet-back’ or ‘water jacket’ to provide hot water boosting. Using a wood fire on a winter’s day to keep warm, while at the same time boosting the solar hot water system, allows households with this set-up to enjoy virtually free hot water all year round. Modern, well-maintained wood fires fed by properly dried fuel burn so efficiently that they create relatively small quantities of greenhouse gases, using the renewable resource of wood. In fact, they emit less than 20% of the greenhouse gases emitted from open fireplaces! Firewood is not a fossil fuel and can be sourced from timber planted for wood, pulp or firewood production.
Whilst solar hot water system retailers generally have a range of systems available that use either electricity or gas for boosting, few have any detailed knowledge or keep in stock the specialised equipment needed to integrate wood heaters for this purpose, so finding out how to do this is not always straightforward. Hot water boosting from wood fires can be done in two ways, namely: –
A ‘wet-back’ within the firebox – this is generally the preferred option, because of the more efficient heat transfer to the water. ‘Wet-backs’ are available with many wood stoves, ovens and furnaces and also for a small number of ordinary wood heaters. They should not exceed 5 kW in rated water heating capacity if they are being used only for domestic hot water boosting and not also for central heating purposes.
We recommend ‘wet-backs’ be installed only where they are available as a standard option from the fire manufacturer, as they can reduce the operating temperature in the firebox and this can affect the performance of the wood fire and this must be taken into account in its design to ensure the fire burns efficiently and complies with air quality standards.
2. Flue Water Jackets:
A way of converting existing wood fires that have an accessible flue (i.e. not set into a masonry chimney) to also heat water heater is to install a flue ‘water jacket’ that replaces the first section of standard flue. Flue ‘water jackets’ are generally made of stainless steel and can fit a variety of flue sizes.
Why boost at all?
Wherever you live in Australia, solar hot water systems perform well and provide at least 70% of a household’s annual hot water needs. Appropriately sized systems provide all of a households hot water needs in the summer and most of the needs in the autumn and spring, but will still require some form of supplementary heating, particularly in winter in the southern regions. Also, in order to ensure that no dangerous bacteria breed in the hot water system, our health codes require that all solar hot water systems be installed with an additional heat source as an integral part of the system so as to guarantee that the temperature of the stored water is raised to at least 60°C before being used. So the booster not only guarantees the water is hot enough during extended cloudy periods, especially in the winter, it also ensures your hot water is safe to use all year round.
Most people who install ‘wet-back’ or ‘water jacket’ boosters will also have either an electric element or a gas booster connected. This means that if they have not lit the fire on days of low sun radiation, or they have an unusually high hot water demand, they can simply switch on the electric or gas booster.
No Explosions Please!
Electric and gas boosters have thermostats to switch them off when the hot water systems storage tank reaches a predetermined temperature, normally 65°C. Wood heaters, on the other hand, are an ‘uncontrolled heat source’ (as are solar hot water collectors). This means that the input of heat from the firebox to the hot water storage tank cannot be easily and quickly switched on or off, so wet-backs or water jackets will just keep on adding heat whilst the fire is going. We recommend that you never cut off the water flow to the wetback, as this can cause a potentially explosive build-up of steam!
There are two options for dealing with this concern safely. Both options require the hot water storage tank to be located above the wood heater and sufficiently high so as to ensure the wetback or water jacket always has water in it. These boosters utilise the thermosiphon1 principle to transfer heat up to the hot water storage. As the water heats up, it is critical that the flow of hot water up to the storage tank and the return of cooler water back down to the wetback must not be impeded by any flow constrictions. Draining the system and running the fire with an empty wet-back or water jacket is not a good idea either, as this can dramatically reducing its lifespan, causing it to burn out very quickly.
Option 1: Connecting to Mains Pressure Solar HW storage tanks
Using wood fires to boost mains pressure solar hot water systems works very well with the traditional roof mounted ‘close-coupled’2 solar tanks, because the solar tank is located above the level of fire. Most solar hot water storage tanks supply water at mains pressure and therefore require a specially designed heat exchanger, which is installed just below the level of the solar hot water storage tank. The heat exchanger is a simple device, with one copper pipe inside another larger copper pipe. The larger outer pipe usually has the water from the wetback flowing through it and is ‘open vented’3 at low pressure, whilst the inner pipe absorbs heat from the outer pipe and is connected to the mains pressure solar hot water storage tank. The outer pipe has an extension rising up to a small open vented make-up (or ‘header’) tank situated just above the centreline of the main storage tank.
Any boiling first happens at the make-up tank and any water lost to evaporation is replenished at the make-up tank. Many solar roof tanks, particularly in cooler areas, have glycol (a frost resistant heat transfer fluid) going through the solar collector panels and a separate heat exchanger within the solar tank. In these types of systems it is fluid from this glycol circuit that the water from the wetback heats in the heat exchanger suspended below the solar unit.
Option 2: Connecting to Low Pressure Solar HW storage tanks
Some solar hot water systems have the storage tanks ‘open vented’ so that the main body of stored hot water is not under mains pressure. These systems either rely on gravity feed to supply the hot water to the house or they have their own internal heat exchanger coil inside the tank, with the water inside the heat exchanger coil supplying the house at mains pressure. Water jackets and ‘wet-backs’ can be safely connected directly into the low-pressure open vented main body of these tanks, so with these systems a separate heat exchanger is not required.
- Thermosiphon is the process of heat rising and in so doing it sucks cooler water into the bottom of the solar collector or wet-back/water jacket when there is a flow-and-return circuit.
- Close-coupled’ refers to solar hot water systems where the solar storage tank is on the roof above the solar heat collector panels – the heat is transferred to the water in the tank above by Thermosiphon.
- Open-vented’ is a term that describes a hot water storage tank that delivers gravity fed water (i.e. water pressure will be governed by the height of the tank above the water outlet). These tanks are ideal for use where an uncontrolled heat source such as fire or solar is used to heat the water. The header / make-up tank is fitted with a ball cock to replenish the water in the main tank as it is being used.
Watch our video on Woodfire Boosted Solar Hot Water.