Modeling shading elements

[GCaruanaSmith]

Hi,

I have prepared a very simple model of an extension to a house and would like to perform a thermal analysis check (comfort periods) with various shading elements. I have drawn in vertical shading but it doesnt look like it is working properly.

1. I am not sure whether the vertical shading should be modeled as a separate non thermal zone - when I do this the overheating hours increase which is the opposite of what I am expecting.

2. I have also carried out the analysis with the shading on the same zone as the extension to the house (on a thermal zone) - when i do this the results are more realistic but I get an error saying I have modeling discrepencies.

I am afraid that I am modeling the shading incorrectly but not sure what the correct way is.

Would appreciate some guidance.

Thanks,

Greta

[Pennetier1]

Hello Greta, 

Your are correct, any shading devices should be on their own, non-thermal zone.  The reason for this is that if they were on the same thermal zone as your zone to analyze, they would be picking up much more heat gains, being typically exposed to more sun/heat that the interior of the zone, increaing the surface area of your current thermal zone.

So something else is not quite right with your model.

Prior to running the thermal analysis, have you used the "see from sun position" feature to check that your shading devices are actually doing their job?

You can also do some investigating work using the Hourly Heat Gains/Losses function of the Thermal Analysis, as well as the Passive Gains Breakdown.

You also want to make sure Ecotect is performing the overshadowing calculations correctly.

Are you using the Monthly Loads/ Discomfort calculation? Oftentime I like to assign a Full Air Conditioning in my thermal zone, just to assess how much heat gains or loss my design is contributing; the results from the comfort calculation (no AC) can be a little bit trickier to interpret.

Feel free to send me your .eco file if you want me to have a quick look at it for any obvious no-nos.

Cheers,

[GCaruanaSmith]

Hi Olivier,

Thanks for your help - I checked the model as you have suggested and looks like the shading is working well once I place it on a non-thermal zone. I think I just expected more of a change in the discomfort hours or possibly this analysis is not that accurate? Once I carried out the other analysis I could observe expected changes

I would apprecitate if you could have a quick look at the model since I havent used Ecotect much yet. Where can I send it? Looks like I cannot attach .eco files here.

Thanks a lot.

Greta

[Pennetier1]

Hi Greta, 

Yes, the comfort analysis is based on a few comfort algorithms,and might not be the most straightforward insights to your analysis.

Personally, I feel that the Degree Hours option is more indicative than the % of Time or Total Hours option, as it gives more of an indication as to how much hotter or cooler the space was.  These calculations are based on the thermostat range settings for your particular zones.

You can .zip your .eco file, or email it to me at olivier@symphysis.net

Cheers,

[Pennetier1 · Accepted solution]

Hi Greta, 

I've had a look at your model, and all looks well.

The reason you see HVAC cooling loads higher than HVAC heating loads even though your comfort calculations show higher "Too Cool" degree hours than "Too Hot" degree hours is due to the way thermal engine Ecotect uses; this is a good question and I'll try to explain a bit here.

When the interior temperatures fall below the thermostat range, the heating should turn on to keep the interior temperature within the settings of the thermostat range.  You can see this happening mainly during the night time.  However, as soon as the sun rises and is able to shine through any apertures, it creates an instanteneous heat gain within the zone.  Because Ecotect uses Dry Resultant Temperatures rather than typical Dry Bulb Temperatures (it is a better gauge for comfort and assumes the radiative process of the internal materials), as soon as the sun heat gains come inside the zone and Ecotect sees that it brings the DRT within the comfort range, it stops the HVAC heating - even though the interior Dry Bulb Temperature is below the lower comfort range.

Obviously, this cannot happen on the HVAC cooling side as solar gains can only be gains.

Thus, even though the comfort calculations shows the heatring degree hours being higher than cooling degree hours - based on dry bulb temperatures - the HVAC loads will be different because of the effect of solar gains "warming" up the occupants by radiation, even though the interior temperatures are cooler than the thermostat lower setting.

Your model shows a very large window to the East, so the HVAC heating system will be working hard at night, but as soon as the sun rises, it will warm up your occupants and the HVAC system will slow down its heating.

By the way, be careful when setting your glazing thermal properties; a little known fact is that you must not assign the transmittance AND the SHGC to that of the manufacturer when performing thermal calculations: if your SHGC is 0.75, then set that under the SHGC, and set the Transmittance to 1.0, otherwise, you will multiply the Transmittance by the SHGC and the results will not be correct.  For example, if your SHGC is 0.5 and your Transmittance is 0.5 and both are used in the thermal properties, it will result in a total SHGC of .25, as they both do the same things - this is not the case when performing daylighting analyses.

I hope this makes some sense.

Let me know if you have more questions on this topic, otherwise please accept as a solution so that others can benefit from this information.

Cheers,