THERM and PHPP

[Atis]

Hi, 

I am trying to calculate PHPP Ψ [W/mK] value using THERM for simple two wall outer connection.

Using these two PHPP guides:

A)

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B)

1) Maybe someone know what do they mean with Ai in guide A ? 

2) What value from THERM should I use when calculating L2d ? My guess is Heat flow (W) / Length (m) 

3) Q 2Dim says Heat flow, calculated. Is that just heat flow in Watts ? or W/L ?

For some reason I have two different values when calculating using guides A and B

[Mark Brierley]

Hi Atis,

1) Ai just means A1 or A2. So the 1 dimensional calculation of heat flow is U1.A1.δT + U2.A2..δT. The sum of simple heat flow through each surface. 

2)  the answer you want is in W/mK (ie heat flow per unit length per temperature difference) so you need to find the difference between W/mK for the 2D simulation and W/mK for the two 1D simulations

Therm will give the U factor (W/m^2K). If you multiply this by the length in the 2 D plane, it will give you the heat flow.  

Thermal bridge is heat loss per unit length, but this is not the length you can see in the 2 dimensional drawing. It is coming out of the page towards you. 

If you set the lengths of each wall in Therm to 1 metre, this makes the calculation easy since length =1 and area = 1. It also makes it easier to visualise: imagine you have two bits of wall, each one square metre (A1 and A2). You can simulate the heat flow through each bit of wall separately (Q1 and Q2), then put them together and simulate the heat flow through the assembly (Q2d). The heat flow through the assembly includes the thermal bridge effect. The thermal bridge effect is working on the boundary between the two bits of wall, which is one metre long. So the thermal bridge Psi value is (Q2d - Q1 - Q2)/1. 

3) A and B is the same method so should get the same result. If you are calculating the individual elements rather than using Therm to estimate them, it's possible you are ignoring or using different surface resistances. 

Hope that makes sense.

Mark

[Gergaud Frank]

Hi everyone,

Thanks for your answer. I have a question with the L2D. Is this correct:

The heat flow marked in red in the picture below divided by the difference of temperature (30 deg) should give the L2D? 

I am not sure how to find the L2D after reading your explanation.

Best,

Frank

[yal...@gmail.com]

Frank,

The result in W needs to be divided by the temperature difference delta T and the length of 1meter to get the L2D.  You can find the equations in  ISO 10211.