[TRNSYS-users] Help needed with Type-56 thermal mass thermal time constatnt problems

Mohamed Alhamahmy malhamahmy at masdar.ac.ae
Sun Mar 23 03:32:07 PDT 2014

Hello, Joylan.

I don’t know why the walls of your model do not seem to be demonstrating the expected capacitance effect, but I can answer your question about the capacitance value in the Air node regime data. This value is just an average value of the capacitance of all the zone contents. The default value is set to 1.2 times the volume because an approximate value of the heat capacity of air in standard conditions is 1 kj/(Kg.k), and an approximate value of the density is 1.2 Kg/m3. The assumption type 56 is making is that your zone only contains air. This value of capacitance will be higher the more furniture, etc. you have in your zone.

So no, you should not be increasing this value to account for wall capacitances.

Hope this helps.


Mohamed Al-Hamahmy.
Research Engineer.

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United Arab Emirates
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Email    malhamahmy at masdar.ac.ae<mailto:malhamahmy at masdar.ac.ae>
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From: trnsys-users-bounces at cae.wisc.edu [mailto:trnsys-users-bounces at cae.wisc.edu] On Behalf Of Joynal Abedin
Sent: Sunday, March 23, 2014 1:36 PM
To: trnsys-users at cae.wisc.edu
Subject: [TRNSYS-users] Help needed with Type-56 thermal mass thermal time constatnt problems

I’m simulating a domestic building using Type-56 with three thermal zones. All the building fabrics such as the external walls are massive with the correctly defined conductivity, heat capacity and density. However, when  I simulate the building the thermal response of the rooms appear to have very low thermal time constant i.e. it changes very rapidly and it seems that the massive walls with specific heat capacity are not having any heat storage impact. The capacity value in the Air Zone Regime gets automatically to 1.2 times the volume of the room. If I manually increase this value the thermal time constant does increase, but should I be increasing this value? The main problem I think is that the thermally massive walls are not having any thermal mass effects on the temperature response of the rooms. What am I doing wrong? Why is the heat capacity of the air zone set to 1.2 times the zone volume? One building in TRNSYS17 Example projects have this set to 3 times the Volume.
If anybody as any ideas or suggestions please please email me. Your help will be very much appreciated.

Thanking you in advance.

Kind regards,

Joynal Abedin.
Loughborough University, UK.
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