mechanical 2 Flashcards
What are heating and cooling degree days used for?
To estimate annual energy needs and costs for heating and cooling a building.
What is the base temperature used in degree day calculations?
65°F.
How do heating degree days (HDD) work?
If the average outdoor temperature is below 65°F, HDD = 65 - average temperature.
How are HDD used in heat loss formulas?
They replace ΔT and are multiplied by U, area, and 24 (hours).
What is the purpose of using HDD instead of just peak ΔT?
To estimate overall yearly energy usage and cost, not just worst-case sizing.
Why is worst-case ΔT still important in HVAC design?
It’s used for sizing the system to handle peak load.
What is the benefit of room-by-room HVAC load calculations?
They allow for more accurate system component sizing and efficiency.
What are the three basic components of an HVAC system?
Generation, distribution, and termination.
Why does cooling require more airflow than heating?
Cooling air has a smaller ΔT, so more volume is needed for the same effect.
What is the usual design focus in mixed air-based systems?
Cooling, because it’s harder and requires more airflow.
What are common heating system types?
Hydronic (hot water), steam, air-based, electric, and infrared (IR).
What is an advantage of hydronic heating?
Small pipes carry a lot of heat, making distribution space-efficient.
Why are air-based systems sometimes preferred?
They can handle both heating and cooling with the same infrastructure.
What is a typical zoning strategy in hydronic systems?
Zones based on use type or orientation to balance heat delivery.
Why are radiators and baseboard heaters usually placed at the perimeter?
To combat heat loss at exterior walls and prevent condensation.
What is a baseboard heater and how does it work?
A finned pipe that radiates and convects heat, often placed under windows.
What are pros of in-floor radiant systems?
Comfortable, energy-efficient, and aesthetically unobtrusive.
What is the biggest risk with in-floor radiant systems?
Leaks are difficult and expensive to fix because they’re embedded in concrete.
Why is tubing density higher near perimeter in in-floor heating?
To combat higher heat loss at the building’s edge.
What dictates boiler size in hydronic systems?
Total heat loss in BTU/H with a safety factor.
Why are heating supply lines typically routed to the perimeter?
To address cold infiltration and maintain interior comfort.
