We control our heating system using the programmable universal controller UVR1611 from the company Technische Alternative. This controller offers many options and ways of programming and tempts the developer to constantly optimize the control algorithms and to search for even better solutions for the control strategy of heating systems, solar collector system and many more. In our system we have implemented the following features.
The control optimizes the energy transfer from the available heat sources like solar thermal system, wood-fired furnace with a water heat exchanger and gas-fired condensing furnace to the loads like the floor heating system and the heating of tap water. Thermal energy currently unused is stored in a buffer for later use.
The water buffer stores in its upper zone hot water for the on-demand heating of tap water. The remaining storage capacity in its lower zone stores thermal energy to be used for the floor heating system.
The pump speeds in the solar heating circuit are controlled such that the incoming solar power and the demand of thermal energy for floor heating and tap water heating are matched in an optimal way at all times. This is achieved by an optimal control (in the sense of maximal useable energy transfer) of the feed temperature from the solar collector system to the buffer.
In case of enough solar power available for tap water heating the solar collector system is operated in tap water heating mode. If the available solar power drops below a dynamic limit the operation mode of the solar collector system is switched to floor heating support as long as the available solar power is still sufficient. Even if the solar power available is above the switch-point the system is still operated in floor heating mode if enough thermal energy for on-demand tap water heating has already been stored in the buffer. This means that available thermal energy is not stored for tap water heating beyond the actual demand but is used or stored for the floor heating system instead.
The optimal temperature to support the floor heating system is dynamically calculated from the required feed temperature of the floor heating system and the thermal energy stored in the buffer. This calculated optimal temperature is used as a target temperature tu control the pumps in the solar circuit.
The stability of sun radiation is measured and used as an input for the control of the gas-fired furnace and the thermal solar collector system. The gas-fired furnace is stopped in case of constant sunshine. Minor drops in the available solar power caused by clouds will not immediately trigger the start of the gas-fired furnace even if the total thermal energy available at a certain point in time cannot fulfill the demand. On the other hand short periods of sunshine on a cloudy day will not immediately stop the gas-fired furnace. Instead excess thermal energy is stored in the buffer thus avoiding frequent starts and stops of the gas-fired furnace. A similar algorithms is implemented for tap water heating.
The wood-fired furnace is operated from time to time only. If in operation it produces enough heat to warm the house and enough hot water for tap water heating. The gas-fired furnace is then stopped in any case.
The water flow within the hydraulic system can be switched and mixed in order to have optimal conditions for the different operating modes of the overall system. The switching process takes some time, up to several minutes depending on valve and mixer type. In order to avoid unwanted water flows in the different parts of the system the control takes into account the mechanical switching times via electronic timers.
The thermal power of the gas-fired furnace is controlled in both operating modes of the system (floor heating and tap water heating). Thereby water temperatures produced by the furnace will never exceed the temperature currently required to operate the system in the automatically selected mode of operation.
I'm publishing my control software and all related documentation under the GNU General Public License. Please note the exclusion of warranty and the limitation of liability described therein. Readers subject to German law may also have a look at the Disclaimer/Impressum.