A good mechanical engineer must understand the entire building, not just the HVAC systems. The building enclosure is the separation between the indoor and outdoor environments; it is the first line of defense against weather, heat loss, air leakage, and outdoor contaminants. HVAC systems cannot be designed in a vacuum – each one must be specific to the building that it serves. The designer must understand the building enclosure.
One of the first steps in any HVAC design is the calculation of heating and cooling loads. The loads are fundamental to the design concept and the selection and sizing of systems and equipment. The heating and cooling loads will determine how much capacity the HVAC system needs to keep the space comfortable throughout the year. Using the wrong inputs — key inputs will be explored more below — in the load calculations results in an oversized system, which will short-cycle and create comfort issues, or an undersized system that can’t meet the demands of the building — garbage in, garbage out.
Let’s take a look at a few key inputs:
The wall assembly: A common construction method is to use a combination of continuous exterior insulation, and insulation between the framing (studs). The R-value of the continuous exterior insulation can be input directly (as long as it truly is continuous). The R-value of the insulation between the studs cannot. It must be derated because it’s interrupted by material that has less thermal resistance than the insulation. The material of the studs matters – both wood and metal studs are worse at insulating than the actual insulation, but one of them (metal) has a far more significant effect and must be derated more. Metal is a conductive material. You can demonstrate this to yourself by touching the inside of a metal window frame on cold day. When you put insulation between conductive framing, the elements of the assembly are working contrary to each other with respect to heat transfer.
Infiltration: Infiltration is one of most significant factors in the heating and cooling loads. High performance building standards like Passive House achieve substantial energy reductions with super air tight construction. New energy codes require building envelopes to meet a maximum air leakage rate, which is verified by blower door testing. The International Energy Code (IECC) prescribes a maximum leakage rate of 0.4 CFM/SF of building envelope at 75 Pascals of pressure difference. (Reference: IECC) The newly updated Massachusetts Stretch Code reduces the allowable leakage rate even further. A typical code compliant building of just 10 years ago might have an air leakage rate 2 times higher than the values prescribed by new energy codes. An old building with a leaky envelope may have an air leakage rate 5 times higher. Air barrier design and construction has improved but even a well designed and constructed air barrier will deteriorate over time. Engineers need to understand the enclosure of the building they are designing and have a sense of how it may degrade over the lifespan of the building and mechanical systems.
Thermal bridges: Thermal bridges are localized areas of heat flow through the building envelope (Reference: Building Envelope Thermal Bridging Guide). A thermal bridge is a conductive element that passes through the thermal control layer (insulation). Common examples of thermal bridges are structural framing, window frames, cantilevered elements such as balconies, and fasteners and joints in the wall assembly. When these elements are not thermally isolated, they allow heat to bypass the insulation and flow from the outside of the building to the inside. Due to the nature of heat flow, the result of even a small amount of thermal bridging is a significant increase in energy use. Think of the insulation layer as a dam, and the heat flow as water trying to move from the high side of the dam to the low side. A thermal bridge effectively puts a hole in the dam – even a small hole results in a significant amount of water (heat) getting through. Thermal bridges also decrease the surface temperature inside the building which often leads to condensation on interior surfaces. The new Massachusetts Stretch Energy Code requires taking thermal bridges into account for building energy modeling.
When the mechanical engineer understands building enclosures, the result is a mechanical system designed to fit the building it serves, and a more comfortable, durable, and energy efficient building.
References:
1. Thermal Bridging Design Guide: Report Template (bchydro.com)
2. International Energy Conservation Code: Digital Codes (iccsafe.org)
3. Massachusetts Stretch Code: download (mass.gov)
Written By:
Ted Hetzel
Associate | Sustainability Specialist | Mechanical Group Leader
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