Your Design Journal entries for this module should highlight your design thinking and analysis results that influenced your decisions about:
- your overall HVAC system strategy
- your heating strategy and how you implemented it in your design
- your cooling strategy and how you implemented it
- any special HVAC system challenges that you encountered
My building is located in Jasper Ridge Biological Preserve in California, so it has a relatively mild climate. This was very helpful for finding passive strategies to efficiently design my HVAC system. used the psychrometric chart from the Climate Consultant tool to figure out how I can best design my system (check out a screenshot of the tool below). Originally, already 7.8% of the time people are comfortable without anything. I chose to implement sun shading of windows (since my building has a lot of glass, obviously people wouldn't want there to be bright sunlight all the time i.e. in offices / conferences rooms when there are presentations or anything like that) and that will help with some of the cooling issues. Fan-forced ventilation and natural ventilation were two big ones in terms of requirements for making people feel comfortable in my building. Since my building is mostly glass and a large monolithic structure that's sealed off — there's a definite need to convert some of the curtain panels so that they can be open air and let in some fresh air. The HVAC will supplement where natural ventilation cannot reach. Luckily, there aren't too many days that are too hot so that's not too big of an issue. Internal heat gain (heat from the occupants) would be extremely helpful for me, supplying 50% of the heating issues that I had previously. To supplement that, I am selecting passive solar heat gain from all the sun that manages to get into the building from the glass panels. This will heat up the concrete floor and hopefully retain the heat so that the occupants don't have to deal with the cold. I decided to use radiant floors to make sure that the occupants are warm enough.
Based on my Insight model from the building envelope module, I knew it was important for me to have a good and efficient HVAC system to make my building energy efficient. That's why I chose a combination of radiant floors for the heating and just forced ventilation for cooling. I used a museum for my building and modeled all the spaces with central heating as the building service. Based on my energy model, my peak cooling load is about 370,000 Btu/hr while the peak heating load is about 180,000 Btu/hr. Some of the big changes that I made to get this to be lower than the expected values of other students is that I lowered the number of occupants in the building. For some reason based on Revit's assumptions of some spaces, I had over 1200 occupants originally in the building. Even just walking through the building in 3D, I couldn't imagine over several hundred people in the building. But I guess it's always better to over-model than under-model. So for my assumptions, I set the number of occupants in the building to about 600 which seems like a generous estimate. I used the space schedule to figure out how much ventilation I needed to put in each area. Something that I also changed was the flow rate of the air terminals. All of them were at 500 cubic ft of air per minute which was quite a lot. For the restrooms to have ventilation, I figured they don't need that much air for a smaller space. This way, I could better place the air terminals to meet the ventilation needs and be more accurate with placement.
Check out my HVAC [ceiling] ventilation system below (blue is supply of fresh / recycled air while pink is returned air). The way I decided to lay out my system was a central system that split the building into north side and south side. This made the most sense given my building shape and the abundance of shared open space. I decided that there's not enough of a need to have zoned air controls especially given that I'm not heating or cooling the air that I pump out. I figured decentralized systems were also going to be hard especially as I want my space to be as open as possible without units running between floors and in random areas / mechanical rooms.
By far the biggest challenge for me was making sure that the ducts lined up with the structure of the wall. As you can see from my building, the edges slope inwards as you move from higher to lower floors. It is especially problematic because I had to move the duct connections with each floor. The connections were very finnicky. The elevations of the ducts were also challenging and even a little bit problematic. To prevent clashes, I had to move some of the pink ducts so that their elevations would be below that of the blue duct. However, I also wanted to maximize floor to ceiling space. For the third floor where there wasn't as much open space, I made the ducts run so that they would lower only when they cross another pipe. This can be seen in the little ridges across the horizontal pink ducts as shown in the third image below. Another issue I realized I had only after modeling the HVAC system was that it would stick out like a sore thumb if a vertical element wasn't hidden behind a wall. So for example the fourth image — you can see that the pink duct runs outside the room and is next to the stairs. I designed the space so that people could come out the stairs and stand by the railing and look downwards towards the center open space. However, this duct runs right in front of that space. On top of all of these challenges, I had to make sure that my ducts weren't intersecting the structural elements either.
Some of the successes I had were learning how to move duct systems after they were all branched out — after I realized some of my ducts were intersecting structural beams, I had to change the entire branch. You have to move them in a very specific way at the connection points to affect the rest of the branch.
I also made a basic model of the radiant floor piping system so that I could understand how this system works in Revit. I decided to only put it on the first floor (the concrete slab on the first floor is thicker and has higher thermal mass that will retain the heat provided from the pipes. As heat also rises up, it'll warm up the rest of the floors (at least based on my experience of radiant floor heating in various residential homes — the upper floors are always warm from the heating provided by lower floors). Since the space is largely open, I decided to stay consistent with my philosophy of not separating the heating into separate zones.