Welcome to the TRNSYS Users Forum.
The forum is a place where people can interact and have discussions about different topics involving the use of the TRNSYS software package. Here you can post topics for discussion or questions on using TRNSYS and get advice from other users or TRNSYS experts. This forum is not intended for detailed technical support. Users should contact their distributor’s hotline for such assistance.
Some tips for success on using the forum:
- Follow the Forum Rules posted in Forum Administration.
- There are categories for different types of topics and questions. Post your topic or question into the proper category.
- Before posting a topic or question search the existing topics (and the TRNSYS Users listserv archive or Post archive) to see if a similar topic or question has already been answered.
- Use a descriptive topic name. Don’t use attention getting subjects, they don’t get attention and only annoy people.
- State the version of TRNSYS and which add-ons your are using.
- Include enough specific details for your topic of question to be answered. Just posting “Why am I getting an error?” without describing the specific error and what you are trying to do when you get the error will not receive a response that fixes your issue.
- Remember when people help you, they are doing you a favor. Be patient, help people out by posting good descriptions of what you need help with, and be polite even if a response does not solve your issue.
- Moderators may edit your post for clarity or move your topic to a more appropriate category.
I am getting a lot of warnings that my simulation did not converge. What should I do?
This is a common problem with many possible causes. Usually, however, the solution comes down to two things. 1. there isn't enough capacitance (thermal, electrical, etc.) in your simulation. 2. your time step is too long.
Imagine the real world situation in which you have a device that heats up a liquid as it flows through. The heater in the device can either be on or off. You connect the heater to a pump and then through a short tube right back to the inlet of the heater. You put a thermostat into the tube and you try to control the temperature of the liquid in the tube. It will be nearly impossible to control the temperature accurately; if the heater is on, the liquid will probably be too hot and if the heater is off, the liquid will be too cold.
The situation is exactly the same in Trnsys. Often times we ignore the pipes in a flow loop and we connect a pump to a heater (or a chiller), to a heat exchanger, and right back to the pump. We decide (often quite correctly) that the thermal losses from the piping is negligeable overall. The problem is that in assuming away the pipes, we also assume away the thermal capacitance of the volume of liquid that they contain. All the rest of the components in the loop are quasi steady state models that do not account for the time it takes for them to heat up or cool down.
Part 1 of the solution is to add back thermal capacitance to the system. Check the documentation on the models that you are using and see if they account for thermal capacitance. If none in a loop do then you can add pipe models or a buffer tank to the flow loops. Make sure that there is at least as much volume in the loop as the loop's pump can transfer in one timestep.
Part 2 of the solution is to shorten the timestep. Trnsys controllers can only make one control decision per time step. If you specify a 1-hour timestep then your heaters, pumps, chillers, etc. can only decide to be on or off once per hour. Often a controller won't be able to decide whether on or off is the better idea and so you get a non convergent timestep. Even if your controller can decide, chances are that your system will not maintain comfort or temperature very close to the setpoint. Imagine what would happen in your residence if you had the opportunity to turn on a heater once per hour and then had to leave it on (or off) until the next hour. When you have a Trnsys simulation that involves control decisions, chances are your time step should not be any longer than 5 minutes.
kind regards,
the Trnsys support team
