Negative Opaque Conduction Value

Hello -

Does anyone know why I might be obtaining negative opaque construction loads?
I have to make a negative value result so it looks consistent. I don’t have any reasons to think they should be negative, so I am thinking it may be a bug.

any thoughts?

I am adding the hbjason file in case you need it.
CLZ_2_A-WRD-Wood_Frame_adjusted_CLZ_1_2.hbjson (94.9 KB)
2023.0620 - Opaque Conduction (41.4 KB)

I’m a bit rusty with all the energy simulation topics and there are better people on the forum to answer this but if I have to guess, this can happen if the inside of the room is hot and the outside is cold so the heat flow through the opaque construction is from inside to outside. So you are losing heat through the opaque construction which makes the value show as negative.

Thanks, @mostapha . What you noted partly explains what is going on here.

A cooling design day profile with negative opaque conduction is very common for mid-to-high latitude climates like New York or Boston, particularly during nighttime hours of the cooling design day. This is because, even on the warmest days of the year, you can still get the outdoor air temperature dropping below the indoor room air temperature, meaning that heat is going to conduct through the walls from inside to outside, resulting in a negative conduction term. This is what Mostapha was referring to.

However, what you have going on here, @Yure, is different because you are running this simulation in Houston Texas. Also, the fact that the infiltration term is always positive is a dead giveaway that the outdoor air temperature is not dropping below the indoor air temperature. Let me first say that this is not a bug but taking a look at the boundary conditions of your model, I notice something important - all of your Rooms are in direct contact with the ground:

And the R-Value that you have for your slab on grade is not very high:

This means that most of that big negative conduction term actually represents is a whole bunch of thermal energy conducting from the inside of your building to the Ground, which far outweighs the conduction through other opaque Faces. So this is just EnergyPlus showing us exactly what we put into it. If you instead want to see a Cooling Design Day balance where the Opaque conduction is only the part through the Walls and Roof, then you can set the Floors of your model to be Adiabatic. This is what I did here and you can see that it’s probably closer to what you expect:

2023.0620 - Opaque Conduction (44.5 KB)

Now, there’s a big question about how realistic it is for the ground to help out so much on the Cooling Design Day or whether it really is better to be conservative and assume the Ground is Adiabatic. Certainly, we’re going to get some help from all of the Ground mass and it’s definitely going to lower the peak cooling compared to a case where we did not have a slab on grade. But the way EnergyPlus models the ground by default is very simple and may not be appropriate for your case here. That is, EnergyPlus assumes the Ground is at a constant 18C year round. This is usually not too far from the mark given that the Ground temperature on the other side of your floor will usually be somewhere between the conditioned Room air temperature and the average Meteoroligcal temperature of Houston, which I’m willing to guess is probably still less than 18C. But, if we’re being honest, the temperature of the ground for a case like this is probably more like 21C-23C and not as low as 18C. So, if you really wanted to produce a load profile close to reality, you can probably do this by bumping up the insulation and heat capacity of in your slab on grade construction to mimic the fact that you won’t get as much heat loss through the slab.

Or you can just take the conservative approach with the adiabatic ground. Honestly, I bet that is what a lot of MEP engineers would do in order to reduce their risk and liability.

Thanks @chris and @mostapha. I was not sure if this was an issue from my end, but it is great to know that it is not an issue of the model.

@chris I simplified the model to share the file here, but I did use Jerboa to identify ground temperature. Then, I added the string in my original model. I still get negative opaque conduction regardless, but it is probably for what you said about the infiltration playing a big role here.

Due to the type of study I am doing, I do need to have the slab as a non-adiabatic, but I liked the option to have as adiabatic to check.

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Hi @Yure ,

Just to be clear about this:

If you are still running your simulation in Houston, then the negative opaque conduction is entirely because of the ground and nothing else. The screenshot above shows you what the peak cooling loads look like when you remove the impact of the ground and all terms are positive, including infiltration.

The Jeroba plugin looks good and I would recommend it for production level energy simulations but, given that the “Peak Loads” component doesn’t currently take additional strings for Jeroba’s output, then setting the ground to Adiabatic using HB Adiabatic by Type is going to be much closer to what the Jeroba simulation will show you than using the default Ground boundary condition.