Connecting a Wood Stove to a Solar Water Heater
(using a wetback or water jacket)
Solar hot water and wood heaters make a perfect combination. With a correctly sized solar hot water system, the sun will provide at least 70% of a households hot water needs. However, in the colder months solar hot water systems need boosting, and this is the time of year when households with wood heaters will be using them to keep warm. Why not use some of the heat going up the flue to heat the water? It makes good sense to use wood heaters or stoves fitted with water jackets, otherwise known as ‘wet-backs’, to provide hot water boosting.
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’. 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; wetbacks 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! Running the fire with an empty wetback can cause it to burn out very quickly, reducing its lifespan.
There are two options for dealing with this 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 always has water in it. The systems thermosiphon heat transfer must not be impeded by any constrictions. 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 is not impeded.
Connecting a stove wet-back to a low pressure hot water storage tank
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 supplying the house at mains pressure. Wetbacks can be safely connected directly into the low-pressure open vented part of these tanks, so with these systems a separate heat exchanger is not required.
Connecting a stove wet-back to a mains pressure hot water storage tank
Using wood fires to boost mains pressure solar hot water systems works best with the traditional roof mounted ‘close-coupled’ solar tanks, because the solar tank is located above the level of fire. Most solar hot water storage tanks are at mains pressure and therefore need to use a heat exchanger, which is normally installed in the roof space, or on the roof, just below the level of the solar hot water storage tank.
In principle, this is a simple device with one pipe inside another, larger pipe. The outside pipe usually has the water from the wetback flowing through it and is ‘open vented’ at low pressure, whilst the inner pipe transfers heat from the outer pipe into the solar storage tank at mains pressure. 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 water lost due to the wetback reaching boiling point is replenished from this header tank. Heat exchangers and their header tanks are available through solazone. Many solar roof tanks in Australia’s cooler states 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 this fluid, glycol, that the water from the wetback heats in the heat exchanger suspended in the roof space below the solar unit.
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 free hot water all year round. Well-maintained wood fires burn so efficiently that they use 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! Wood is not a fossil fuel and can be harvested from timber planted for wood and firewood.
These pictures show an Edwards LX solar hot water unit connected to a wood-fired stove, using a Tedson coil heat exchanger. Mains pressure can be maintained for the hot water service, whilst the stove circuit is open vented for safety.
Connecting an LX model Edwards Solar is easy using a Tedson Coil heat exchanger. The glycol circuit of the Edwards unit is connected to the heat exchanger, and the stove side of the heat exchanger is maintained at low pressure using the supplied stainless steel top-up tank. The glycol circuit of the Edwards LX must never be connected directly into the wood stove. This would void the warranty and cause a potentially very dangerous situation.
The heat exchanger is usually installed immediately below the solar how water tank, either inside or outside of the roof, with a definite slope on it to assist in heat transfer. The stove water pipes must be installed with a continuous slope downwards towards the stove, with no flat or up-hill sections to cause heat blocks. In general, the slope must at least 1 in 10, and the pipe diameter at least 20 mm. The hot pipe should be well-insulated all the way up, as should the entire heat exchanger.
Other types of Heat Exchangers are also available to suit unusual situations. Feel free to ask what would be best suited to your particular application.
Long, insulated heat exchangers for thermosyphon systems.
Primarily used for pumped systems, connected to ground tanks, or where the height difference is insufficient for thermosyphon to work.
Various sized copper and stainless steel top-up tanks are available, with ball valve for automatic filling.
Stainless steel stove flue heat exchanger fits to the base of any standard 6 inch stove flue, to capture waste heat going up the flue, and use it to heat water.
Heat -Exchanger & top-up tank Prices – March 2016
|Tedson bare copper heat exchanger coil (for in-roof connections only)||$575|
|15 Flat plate heat exchanger||$670|
|20 Flat plate heat exchanger||$850|
|Solar Mio 830mm compact high output heat exchanger (only for pumped systems)||$699|
|Solar Mio internal heat exchanger – fits into electric element port of the tank||$367|
|12 litre Copper expansion tank & ball valve||$275|
|Frost-tec Copper top up tank & ball valve||$260|
|3 litre Insol Stainless steel top up tank & ball valve||$275|
|45 litre Solar Mio expansion tank with swing check valve||$230|
|Automatic filling (ball) valve||$35|
|Solar Mio stove flue hot water heat exchanger||$269|
|Stainless steel stove flue heat exchanger – 150 diam x 650mm long||$275|
Prices do not include delivery from the factory.