The wood-fired furnace is not controlled actively by the controller but rather by the amount of wood set on fire. The purpose of the control system is to efficiently transfer as much of the thermal energy generated by the furnace into the buffer as possible. This is done by feeding cool water from the buffer into the furnace circuit and hot water from this circuit into the buffer at the same time using a mixer.
The furnace and connecting lines circulate hot water so that the built-in heat exchanger is operated at high temperature. This is to avoid soot agglomeration on its surface which would hamper heat transfer from the smoke gas to the water in the heat exchanger thus diminishing its efficiency.
The furnace circuit is activated if the Comparison KaminAktiv detects a temperature rise in the heat exchanger S11 above a fixed threshold of 60°C. This activates the circulation pump A6 in the circuit with constant pump speed and the Mixer controller MischKamin. In the initial phase of operation the water in the circuit is circulated in a closed loop (mixer closed) and heated by the heat exchanger. To feed the water into the buffer at least the temperature THeizungsunterstützung has to be reached in the circuit. The Analog function MAX(An)4 thereby implies a lower limit on the target temperature in the circuit of about 70°C and then sets the resulting temperature as an input for the Mixer controller MischKamin. This controller function regulates the water temperature fed to the heat exchanger by opening or closing the mixer A10/A11. It opens the mixer if the temperature within the circuit rises thus mixing cool water from the buffer into the circuit and feeding hot water into the buffer. The thermal energy meter S14/S15 measures the thermal power and energy transferred from the furnace to the buffer. In contrast to classic mixer opertion e.g. in the floor heating circuit the mixer here operates in an inverted mode because here the furnace acts as the heat source and the buffer acts as the heat sink.
The temperature required to support the floor heating system using other heat sources has already been referenced several times in this documentation. It is calculated by the controller using the following algorithm:
According to this the THeizungsunterstützung=Min(Tbuffer-mid,Max(Tbuffer-bottom,Tfloor-heating-flow-target)) which has the following effect on the control algorithms:
The control systems for the gas-fired furnace, the solar thermal system and the wood-fired furnace have been modelled independently and the UVR1611 controller runs them concurrently. However, the individual control subsystems are indirectly coupled by the temperatures and water flow paths of the installation. Some controller inputs or outputs are used multiple times in different parts of the control algorithms as control factors or command variables. In practice some situations occur where this indirect coupling of the subsystems requires some optimization:
To handle these special cases the individual control algorithms for the subsystems have to be connected using additional logic circuits which control the activation of the floor heating system and the gas-fired furnace. These circuits may stop the operation of the gas-fired furnace under certain circumstances but will not influence the operation of the solar thermal system or the wood-fired furnace. This is because we want to collect as much thermal energy as possible and store it in the buffer, and we cannot really control the operation of the wood-fired furnace anyway because wood is fed to the furnace manually.
Whereas the first few bullets can be handled by simple comparisons the sun-cloud problem requires an analysis of the duty cycle of the solar thermal system because we want to run the gas-fired furnace on a cloudy day continously, and we want to switch it off on a sunny day even if there are a few clouds.
In order to analyze the duty factor we calculate the average speed of the pump on the secondary solar circuit using an Analog function FILTER1. The pump speed setting output of the corresponding PID controller will be 0 if the solar thermal system is switched off and will be larger than 0 otherwise. Its temporal average therefore is an indicator for the total thermal energy collected in the chosen time interval (20 - 30 min). We compare the averaged pump speed with a fixed threshold value using Comparison VERGL1. If it is larger than the threshold we assume a sunny day with just a few clouds, if it is less we assume a cloudy day with just a few short periods of sunshine. We carry out this computation for the floor heating mode of opertaion of the solar thermal system and for the tap water heating mode separately.
It might be interesting to note that the averaging function doesn't just cosider the on/off-times but also takes into account the average solar power (via the speed setting of the pump). To just average a digital (on/off) signal is not possible with the filter function module of the UVR1611 due to a confirmed bug in the firmware of the controller (as of 21.12.12).
The other special cases mentioned above will just require a few logic functions: