Design Journal Entry - Module 1

Journal Entry For
Module 9 - HVAC Systems

Overall HVAC System Strategy

To stick with the goals of maximizing environmental sustainability, the overarching strategy for my HVAC system design is to be highly efficient and fully electric. This means that as the electricity grid is continually “cleaned up” and fossil fueled power is taken off the grid, my building will be using that cleaner electricity rather than burning fossil fuels on-site.

In Somerville, MA, the average temperature throughout the year is about 50 degrees Fahrenheit (see chart below from Climate Consultant). Though heating is a primary concern, since this space will be filled with occupants, cooling will also be an issue.

Psychrometric Chart Showing Comfort Zones - Somerville, MA


As shown above, heating and humidification makes up the majority of the chart above, showing its significance in this climate. To do this properly, I aimed to already maximize the amount of glazing on the south elevation to allow passive solar radiation hit the first and second floor slabs so that it can be absorbed and re-radiated into the space to lighten the heating load during the warming months.

Radiant Floor Heating

To supplement the passive solar, I would like to implement a fluid-based radiant heating system (within the slab on grade and slab on metal deck) to take advantage of the even heating that this would provide. This would ultimately be a hybrid system as it would comprise of radiant floor heating and air cooling/ventilation (as shown in schematic below).


Schematic of hybrid air/water system



Radiant Heating Floor System (and Manifold)

VAV Air System

Since cooling the internal loads will also be a large issue in my building, my overall strategy is to use a package AHU sized for cooling/ventilation for each space, and then have a VAV system to monitor and control how much supply air goes into each of the zones. I am choosing a VAV system specifically since this space will likely have fluctuating amounts of people. With a VAV, I can cut down the amount of conditioned air being supplied to the space to match the internal loads, saving in energy.

Heating Strategy Implementation

Radiant Slab

Overall, implementing the radiant slab for each of the room was tedious but ultimately worked out. For each building and floor, I brought back each radiant loop to a mechanical room where necessary to putt the manifold (pictured above). For Building 3, I realized that there was not a specific mechanical area designated, so I routed the piping to the space behind the restrooms. In future design iterations there will need to be a mechanical room to house the equipment needed.


First Floor




2nd Floor


Cooling Strategy Implementation

Implementing the cooling strategy was difficult since I was unable to run the HVAC loads calculation fully for my model. I required assistance from Glenn to work some magic on the backend to get my cooling/ventilation total supply airflow. That said, with much work I was able to get the outlined table below:


Based on this, I set up my terminal diffuser locations and where my VAV boxes would be located. For ductwork, I mainly followed the following criteria:

  • Elevation of Bottom of structural beam is 12’ 4”, giving me about 3 feet of room for duct above the scheduled diffusers at 9’.
  • Supply main duct would be 24” x 18” generally.
  • Return duct would be 18” by 18” generally.
  • Use hard duct into space and then final connections used with flex duct.

Special HVAC Challenges

The biggest challenge for my project in this module was defining the air flow rates for each of the spaces by running the load calculation. For some reason, EnergyPlus would not even run its model and this set my back quite a bit.

That said, I did run into other challenges and learned a ton of lessons. Below are a few:

  • Building Core: setting up a good, relatively spacious core from the beginning of design will make it easier to set up ductwork, find the “trunk” of the system and then connect the “leaves.” I struggled since my core mechanical spaces and shafts were not set up in the best way.
  • Ceiling Space: I honestly feel like I had enough vertical space for my ductwork, but I was struggling at points to find horizontal space to set my diffusers and especially the large VAV boxes. I think being more aware of how the spaces tie into each other, how zones play, and where diffusers might not be needed could be a good course of action to address this issue.
  • Value of Similar Spaces: I have learned that having similar spaces set up floor to floor can make design of these repetitive systems that much more streamlined and clear.

Overall, I have learned that HVAC design and implementation takes critical thinking in overall strategy, and then disciplined, consistent placement of equipment throughout the space to meet that strategy. It takes time.