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Hi,
I would like to model a heating circuit in a building, with a single-speed pump (type 114) and a valve that is connected to a PID-controller. The valve should modulate the flow rate in the circuit, based on the control signal from the PID-controller. What is the easiest way to model this? I have not found a valve type appropriate for this purpose. Is it best to use a valve+single-speed pump, or is it easier to use a variable-speed pump instead? If I use the single-speed pump, will the flow rate be affected by a valve, or will it always keep the same, constant flow rate?
Best regards,
Johan
You can definitely model the single-speed pump with the PID controller in TRNSYS! The PID controller outputs a control signal value relative to the minimum and maximum control signal values (inputs 4 and 5). Set the minimum control signal to 0 and the maximum control signal equal to the flow rate (kg/h) of your pump. Then send the output control signal to a Type11i Flow Diverter - desired mass flow mode. I would recommend hooking up just the pump, diverter, and controller and make sure you get the controller working properly before connecting it all into the rest of your heating circuit.
Depending on what you are trying to accomplish with your heating system, a different controller may be more appropriate, such as a tempering valve controller (Type115 or 953 in the TESS controllers library) or an aquastat (Type 106 or 1502 in the TESS controllers library), but the same modeling technique of a control signal connected to a Type 11 flow diverter applies. Depending on the format of the control signal output, you can choose between Type 11f which takes a control signal 0-1 and sends a fraction of the input flow rate to outlet 2, or 11i which takes a control signal 0-inf and sends the desired flow rate to outlet 2 and the remainder to outlet 1.
Important to keep in mind - Be sure to have some mass throughout the system (i.e. pipes and tanks), particularly between the return to the pump and the pump. And once you have the controller working the way you want and connect it to the rest of your system, check that the flow returning to the pump is equal to the flow leaving the pump.
jolind Topic starter
15/04/2026 8:11 am
@rosesears Thanks a lot for your answer. The problem with using a single-speed pump + a flow diverter + a pipe that passes by the radiator is that then I will get a constant flow rate in the circuit, but a lower flow rate through the radiator (depending on the PID-control). But in the real system, the valve at the radiator constitutes a resistance that creates a pressure drop, resulting in a lower flow rate in the entire circuit, with the same flow rate through both the pump and the radiator. Is it then more appropriate to use a variable-speed pump instead, even if such a pump is actually not used in the real system?
RoseSears 15/04/2026 1:05 pm
@jolind If I understand your system correctly, the flow rate through the pump in the actual system is variable based on the pressure drop from the radiator - in that case, yes, I think better to use a variable-speed pump, and you'll need to set up a control function or controller device to modulate the flow for the pump, in which case you may not need a valve at all. If the actual pump has constant flow rate at the pump, then there's a diverter valve where some of the flow goes to the radiator and some of the flow bypasses the radiator, then you could model it with a constant-speed pump and the controller connected to a diverter valve determines the amount of flow that goes to the radiator and how much bypasses the radiator.
jolind Topic starter
15/04/2026 1:35 pm
@rosesears Thanks for your answer. The pump in the actual system has a constant pressure head, and the actual flow rate in the circuit depends on the pressure head of the pump and the pressure drops of pipes and the radiator valve. I think I will go for the variable-speed pump without valve. However, perhaps also the type 747 pump would be an option, if I would like to model pressure drops/head?
