Unstable behavior of RadiantConvective Baseboard Heaters with an air-water heat pump
Hi everyone,
I am not familiar with posting on a forum so please tell me if more (or less) information is needed, I tried to be as extensive as possible. Also, English is not my first language so if anything is unclear, please let me know.
The model :
I am working with a model that I did not design myself and I only have access to the idf file. I am using EnergyPlus 24.2.
A base model of a single-family house was modified to have different isolation levels (4) and inertia levels (3). In total, I have 8 different “building versions” of the same house.
The general model is a single-family house with 6 heated rooms and 2 not heated. The system used here is an air-water heat pump (HeatPump:PlanLoop:EIR:Heating) connected with baseboard heaters (ZoneHVAC:Baseboard:RadiantConvective:Water). The baseboard heaters are in parallel in the PlantLoop (with Connector:Splitter and Mixer). I have 2 bypass branches (Pipe:Adiabatic), one parallel to the heatpump and one parallel to the heaters. I only have one loop (CommonPipe=None) and the pump is a VariableSpeed at the beginning of the Plant Side. The pressure simulation is disabled.
I am trying to model the use of low-temperature baseboard heaters (inlet temperature 45 to 50 °C) and I found data from manufacturers with a Delta T of 10°C (I put it in Sizing:Plant), with the different Heating Design Capacities autosized and I calculated the corresponding rated water mass flow rate. The thermostat is on the Operative temperature.
In case this is relevant, I am simulating the model from python with a fmu (package fmpy), and I give schedules for heating/cooling setpoints (communication at each timestep) and other things (that are constant for now).
The issue :
When I simulate this (over a year, normal weather conditions), an instability appears after some time (1 to 300 days in). The model is stable and converges fine during the beginning of the simulation but once the instability happened, the model stays unstable until the end. When I say instability, I mean that the mass flow rate in the baseboard heaters oscillates, sometimes between 0 and a fixed value, sometime between two non-zero fixed values, and these values can change during the simulation. This causes the air and the operative temperatures, as well as the power from the heat pump to oscillate. In some of my tries, only one baseboard had an instable behavior, in others, it’s all of them. Often, not all the model are unstable, it’s 3-4 of them, but it’s not always the same ones, it changes depending of my attempts to stabilize the lot.
Therefore, I can fine-tune each of the model by hand to reach stability but it feels like covering up something I don’t understand. Also, I am trying to obtain a general rule for all of the models, because in the future I will use different weather files and I cannot ...
