Manual J Question
I am in the beginning stages of designing a house and I am doing a Manual J calculation to help inform the design. I would like to be able to do things like trade off the expense for more foam under the slab compared to triple pane windows, before we finish design and hire HVAC contractors. I am using HVAC the ResLoad – J app, which is listed on the ACCA website as approved for manual check calculations.
I set up four identical walls facing the four cardinal directions and it makes sense. The same heating load for each of them and the north having lower cooling load than the others. When I add windows, I get confused. I put four identical windows, one on each wall. The window on the south has the lowest cooling load, the north the next lowest, and then east and west more than double. I assumed that the window on the south would have the highest cooling load.
I added 4 more walls without windows, and I see that the system had removed some heating and cooling load from the walls with windows, which makes sense.
Am I doing something wrong here? Or do I not understand how this works? Thank you so much for any help,
Scott
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Use beopt. It is what it’s designed for
Second that recommendation.
That looks pretty interesting, I am on a mac, so will need to get a VM set up. I am still pretty curious what is going on with the Manual J though.
I can imagine why the north facing windows would get more solar gain than south facing, but won't say here since it's speculative. The manual j guide and associated tables would be more helpful here and I just don't have it handy.
As for south facing windows having the highest solar gain-- the south windows get a little gain during the peak of the day, but that has to make it's way at an oblique angle through low coated glazing assemblies. So first, gain is decimated by trigonometry, then that's compounded by material science.
At least with north facing windows, you might get a few hours of more direct beam radiation at sunrise and sunset.
What is your location? Is it accurately reflected in the software?