TRNSYS Forum - Recent Topics

Effective convective heat transfer coefficient, identical boundary conditions.

Alex Sz — Thu, 12 Mar 2026 14:11:11 +0000

Dear TRNSYS-Team and -users,

I am currently working with Type 56 and have two questions regarding the treatment of heat transfer in TRNBuild / Type 56.

1)
First, regarding the convective heat transfer coefficients:
In the Construction Types, I specify constant convective heat transfer coefficients for the front and back surfaces,
for example 9.72 and 72 kJ/h·m²·K.
However, in the Type 56 outputs, the reported values for the "effective" convective heat transfer coefficient are different from the input values.

This raises a few questions for me:
What exactly is the difference between the input convective heat transfer coefficient and the output “effective” convective heat transfer coefficient?
Is the effective value directly used in the simulation, or is it only a reported/post-processed output quantity?
Is it recalculated during the simulation depending on surface temperature, air temperature, or other boundary conditions?
-Because when i applied different room air temperatures it did not change. It seems to be purely dependent on the surface Area proportions?
Which physical or scientific model is implemented here?

I would have expected the effective convective coefficient to remain equal to the prescribed input value if a constant coefficient is defined.
Since this is apparently not the case, I would like to better understand the underlying formulation in Type 56.

2)
My second question concerns the definition of the category “Boundary” or “Boundary conditions”, especially when it is set to “identical”.
What exactly does “identical” mean in this context?
Does it mean that the heat fluxes on one side of the construction are mirrored to the adjacent side of the neighboring zone,
i.e. that convective and/or radiative heat exchange is transferred consistently between both zones?
Or does it mean that a temperature boundary condition is imposed on the other side?
I could also not find an unambiguous explanation of this point in the Trnsys-Documentation/ Literature.

Any clarification on either of these topics, or references to documentation or publications, would be greatly appreciated.

Best regards,
Alex Sz

Type 88 connection with equation editor issue

Quentin_Courjeau — Tue, 10 Feb 2026 19:03:42 +0000

For a simple thermal modelisation of a machinery room ventilation I used the Type 88.

I connect as input an equation editor with Type 88 to handle the sensible heat loads.

Unfortunately, those are not assumed by the model - ie Type 88.

When I fill directly the heat loads in type 88 they are properly considered in the model.

When I try to interconnect equation editpr with a type 57 for example, in order make a link with the type 88, the heat loads are neither assumed.

Just to complete the message, I use a, equation editor, because I need to add several inputs from a type 9c

Does anyone face to the same issue ?

Thanks to the community.

Regards

Q.Courjeau

Battery discharge control

HumbertoSantos — Mon, 26 Jan 2026 15:28:29 +0000

Hello everyone,

I have been reading materials in search of a way to control the discharge of battery. In a few words, the system configuration that I am simulating includes a solar panel (type 103b) connected to the inverter (type 48b) and a battery (type 47a). The the electricity produced, stored, and imported from the grid is used to meet the electricity demand, which includes the electricity for the operation of a heat pump.

For some comparison that I am trying to accomplish, I wanted to control the discharge of the battery only when the heat pump is turned on to simulate that the entire stored electricity is used for the heat pump operation, plus additional electricity needed to run it. From my mind arose different possibilities:

1 - through equation - create a signal on_off with output#2 of the battery (fractional state of charge) and multiplying it by the heat pump signal and then connecting it to input #3 (battery fractional state of charge) of the inverter. I have attempted this option but it failed. In fact it operates just the normal setup.

2 - through inverter type 48d, trying to use the input # 11/12 start/stop charging, but it does not have a control for discharging, so this would not have much sense.

3 - this third option, the easiest way, would be to change the current configuration making the PV-INV-BATT entirely to meet the heat pump demand only, however, that would deviate much from my research objective and from the results I already have would mean much more electricity dumped.

Do you think it is possible to control the discharge of a battery? Could you please comment on my thoughts and give some suggestion of how to proceed? Any literature available is much appreciated. Thank you in advance.

Regards

Humberto

TRNFlow and Type 56 Outputs

TS_Team — Mon, 26 Jan 2026 14:49:30 +0000

Dear users,
Just recently a question came up regarding the outputs of Type 56 and TRNFlow, that we would like to share here for clarification:

Question:
I would like to ask a question about output variables when TRNFlow is enabled. When TRNFlow is used, Type56 reports the following outputs:

QVENT
QINF

Could you please briefly explain:

- the concept used to define QVENT and QINF when TRNFlow is enabled, and

- which airflow rates are used as the basis for the calculation and classification of QVENT and QINF.

Answer:
TRNFlow has three categories of "input mass flow rates" of airnodes (see TRNFlow manual Section 3.7. TRNFLOW Outputs; similar to Type 56 itself):

NTYPE 201: FLINZ Outdoor air infiltration flow per airnode from an external node (outdoor) into airnode:

is used to calculate NTYPE 4: QINF = air mass flow * cp_air * (Tamb - Tairnode))

NTYPE 203: FLVSZ Ventilation supply flow per airnode Sum of all flows from auxiliary nodes into airnode

the disaggreated mass flow rate of NTYPE 203 and the related temperature differences are used to calculate NTYPE 5: QVENT

NTYPE 205: FLIZZ flow into thermal airnode from specified thermal airnode Sum of all flows from another specified airnode into the thermal airnode

inflow into a thermal airnode from a specified thermal airnode (coupling)

Question:
FLINZ, FLVSZ, and FLIZZ are not mapped directly to the Link type itself (e.g., Crack/Fan/Duct), but are determined by the types of nodes connected, i.e., where the flow comes from and where it goes.

Therefore, if a fan is connected directly from an external node to an airnode, the flow should be counted as FLINZ and thus contribute to QINF, not FLVSZ/QVENT.
On the other hand, if outdoor air is first taken into an auxiliary node (duct system) and then supplied to an airnode, it is counted as FLVSZ, and the outdoor air fraction of that supply is also reflected in FLTOZ.
Correct?

Answer:
Yes, that is correct.

Viscosity in pumps

ShamsoddinGhiami — Mon, 26 Jan 2026 13:15:04 +0000

Hi,
I am considering a pump component that accounts for the effect of inlet fluid viscosity on the required pumping power. Is there an existing component in TRNSYS or the TESS libraries that models this behaviour?

Free Spots in upcoming virtual TRNSYS18 Training 1 and 2 January / February 2026

TS_Team — Wed, 21 Jan 2026 16:28:06 +0000

There are still free spots left in our upcoming classes end of January / mid of Feburary 2026 (29. - 30.01.26; 11. - 13.02.26) and you can enrol by sending us an e-mail to hotline@transsolar.com including your name, TRNSYS user-ID, company name and the training number.

Attached you find a short overview of the upcoming classes you can participate.

The classes will take place virtually. The training language is preliminary and might be adapted according to agreement. Details regarding the program and organization can be found on our homepage at Training and in our FAQ.

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