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Hi,
I'm currently trying to model a house with a water underfloor heating system. To do this, I'm using type 56 and an active layer. The results I'm getting from the simulation seem very strange.
This is my problem :
-The underfloor heating is supplied by a water mass flow rate with an ON/OFF control (depending on air temperature conditions).
-The inlet water temperature to the active layer is constant (28°C).
When the inlet mass flow rate to the underfloor heating system is set to ON, the outlet water temperature of the active layer rises directly by 4°C over a time step of one hour. It seems that my floor has no inertia... I was expecting a delay between the increase in flow temperature and the increase in return temperature. +4°C is a significant increase too.
Have I forgotten an important parameter for modelling water underfloor heating system?(I've already increased the capacity of the airnodes and change the timestep).
Kind regards
@sv2541 I think that your simulation is just telling you that the time constant for the radiant floor is less than an hour (which doesn't surprise me too much). Try reducing your time step to something like 1 or 5 minutes and you will probably begin to see the thermal capacitance/inertia of your floor.
As a side note, using a shorter timestep is also going to help with your controls. With an hour timestep, the controllers can only make one decision per hour, which is going to result in large temperature swings in your building and probably some convergence problems as well.
kind regards,
David
DavidBradley 01/04/2024 1:45 pm
@sv2541 As I was pressing send, I noticed your comment about having already tried changing timesteps. What kind of timesteps did you try?
Thank you David for your answer. Yes, I have already tried smaller time steps like 5 minutes. The problem is still there : there is no delay between a decrease in flow temperature and the decrease in return temperature (see graphic). I don't understand why in my model there doesn't seem to be any inertia in the underfloor heating system.
Kind regards
@sv2541 It could have something to do with the timebase. Type56 uses something called the Conduction Transfer Function method to represent energy transfer through envelope constructions (walls, roofs, floors, etc.). The CTF method basically assigns some weighting factors to certain driving forces (such as the temperature difference across the construction) stepping backwards in time. In essence the heat transfer now depends on (among other things) the temperature difference now, the temperature difference one timebase ago, two timebases ago, etc. The transfer function is updated once every timebase, which can lead to some stepwise behavior in building air temperature but could also explain why you are seeing temperatures jump immediately.
You might try changing the wall timebase to be maybe 15 minutes in the TRNBuild "outptus" window. If that doesn't solve the problem then please reach out to your distributor and send them the project, the *.b18 file, and any other files they would need in order to run the simulation. If you aren't sure who to contact, you can send me a direct message (techsupport@tess-inc.com)
kind regards,
David
Thank you for your advice. I changed the time base in the TRNBuild outputs to 0.25 but unfortunately I got an error message : "Error creating the wall transfer function coefficients : stability criteria not fullfilled". It seems that the active layer thickness is too large (it's actually only 0.34 m), which is kind of problematic as I would like to work with a 5 minutes trnsys timestep.
Kind regards
