Sun, gas and wood powered home heating

In a recent rural project at Mt Barker, Wanaka, we installed a complex yet highly efficient off-grid system that involved utilising power from sun, wood and gas for water and home heating. Here is more about that project and how the various components of the integrated heating system work together for maximum efficiency.

All three sources of energy – sun, wood and gas – are set up to contribute to the heating and hot water for summer and winter:

Summer: solar contributes most, if not all, hot water; gas is backup

Winter: solar contributes what it can; gas and wood fire are backup

The Rotex Sanicube thermal store is the hub for the hot water and heating and can collect the energy from various sources and deliver the heated water to various locations/requirements.

The main source of hot water for the thermal store in winter is a Thermalux Linea 20kW wood fired boiler powered by a Linea wood burner. This wood burner has a glass front as a normal freestanding fire would have, but instead of a standard wetback, the fire has a water jacket that sits on three sides of the fire box and is able to generate up to 20kW of hot water quickly and deliver up to 8kw of heat to the room .

The heated water is delivered to the thermal store by a low energy pump and heats the thermal store directly without a heat exchanger.

The photovoltaic solar panels provide all the energy to power the home. Any excess energy is directed to the Rotex thermal store to heat the water via an electric element, effectively acting as a “battery”.

More about solar energy

There is a Rinnai Infinity gas continuous water heater also connected to the Rotex thermal store. This provides an on-demand boost to the hot water should the solar or other energy sources not provide the hot water or heating demand required.

The Rinnai is connected to heat the body of water within the tank (so not connected to a heat exchanger) and is regulated by a controller to run by a time period on/off setting to allow the other energy sources to provide as much energy as possible. The time period settings will power up the Rinnai and pump at a pre-set time and can be adjusted to suit the highest demand times for domestic hot water and central heating.

The Rinnai will only boost the water temperature up to 60°c in the middle of the Rotex. The boosting will only be adding to the heated water already in the Rotex (assuming the PV solar and wetback have raised the water temperature to say 40°c, the Rinnai will only heat up the water the extra 20°c required). This will happen if a large demand of water has been used and the solar and wetback do not have time to recover.

The Rinnai is a very efficient water heater and, if required, could heat the entire 500L in approximately 30 minutes. An electric element would take approximately 6 hours, which is very slow recovery. A standard central heating boiler can produce 24kw or 37kW of energy, while the Rinnai can produce 56kW for a much smaller and controllable unit. The Rinnai will heat very quickly and keep up with huge demands if required.

The Rinnai makes system integration with the solar and wetback easy by allowing the renewable energy sources to operate first and then boost only if required.

The Rotex thermal store allows for water stratification – meaning that the temperatures can be set to achieve different temperatures within the tank at different levels.

Bottom of the tank set to 30-40 degrees to use for underfloor heating

Middle of the tank at 40-60 degrees for domestic hot water

Top of the tank at 60-80 degrees for radiators

Using the stratification within the tank allows the use of the various heating and hot water requirements to be delivered with maximum efficiency. The Rotex also has 90-100mm of insulation which is double that of the standard domestic hot water cylinder.

The bottom electric heating element will draw energy from the PV solar to heat the water from the bottom of the tank upwards. This allows as much energy to be added to the tank as possible from the solar PV. The top element can be used to heat the top of the tank for domestic hot water requirements in the event of a breakdown of the other systems.

While this is an energy efficient system, like any heating system, it will be worthwhile to monitor power consumption to maximise the times of the highest power usage with the maximum solar generation. The shoulder season and winter will utilise the gas boosting more often because of the heating demand within the home and more limited PV solar generation.

We advise and help our clients to take the time to monitor how best to utilise the heating. The best way to do this is to set the underfloor heating to 18°c. If you find that this is not warm enough for your comfort, add another degree at the wall thermostat and carry on until you find the temperature that is comfortable. The higher the temperature you need for comfort will require a higher energy demand from the Rotex, which then requires more energy to be produced from the energy sources.

The radiators are a quick form of heat for the bedrooms, but due to the higher temperature required to provide heat they will use more energy in a short space of time. Again, it is best to monitor the heating and times required for your comfort and what the ultimate temperature setting the radiator thermostatic valve is to be set at. The radiators can be used to take the chill off the room or toasty warm which will pre-set the demand on the Rotex and energy/heated water to be replaced.

The heating of the underfloor is controlled by Neostat thermostats and can be adjusted by time and temperature. The times can be set to when you want the heating on and off and the same for the radiators. If the fire is on and the room gets to the pre-set temperature on the thermostat (e.g. 19°c) then the floor zone will go off, saving energy and heated water from the Rotex.