Case Study - HVAC Flashcards
What is a HVAC System?
Heating, ventilation and air conditioning system.
Used in buildings to:
- maintain internal air quality
- regulate internal temperature
- regulate internal humidity
What does an MEP system do within a building?
Important part of building services and can have many different functions.
Includes mechanical, electrical, and plumbing
Talk me through how a HVAC system works
- Usually delivered mechanically (can also be natural)
- Conditions the air to suit requirements, i.e. can heat/cool and control moisture
- Boilers / chillers (now ASHP) in basement deliver hot and cold water to AHUs (on roof)
- AHU’s draw in air from external envr. The air passes over heating and cooling coils to heat/cool air. If moisture is too low, the humidifier will add moisture into the air. Supply fans located in the AHUs then push the conditioned air through primary ductwork which distributes air throughout the building.
- Then the air is fed into secondary ductwork (or VRF pipework) which is fed into FCU’s. The FCUs condition the air a final time before the air enters the space you want conditioning.
- Stale air is extracted from the building, via extract ductwork, this then enters the AHU, whereby some exhaust air is recycled and heat recovered and rest is put back into atmosphere.]
- Cooling towers located on roof remove heat from chillers and expel into environment.
What to consider when selecting MEP system?
- equipment required and sizing
- retention of existing equipment
- sustainability credentials
- warranties and life span
- design intent and aesthetics
What is an AHU? And what does it do?`
- forms part of HVAC system
- conditions and distributes air within a building
- it takes fresh air from outside, cleans it, heats / cools it (over coils), maybe humidifies it and then forces it through ductwork around the building (through fans).
What is an air source heat pump? What does it do?
use external air (or energy) as a heat source to provide heat for buildings.
they work by running low temperature, low pressure refrigerant fluid in heat exchange coils through the external air.
fluid absorbs heat and boils
gas is then compressed to increase temp further
gas is then passed into heat exchanger coils, where it condenses releasing heat and process restarts.
- can be air to water system or air to air
- can reduce energy consumption, fuel bills and carbon emissions –> becoming popular for move towards net zero
What is a VRF system? How does it work?
HVAC system based on flow of refrigerant between external condensing unit and multiple internal evaporators (typically FCUs).
- doesn’t have any ducts and works at high capacity.
Either 2 / 3 pipe system. With a 2 pipe system, the whole system is either cooling or heating. But with a 3 pipe system, there can be both heating and cooling in different zones. Generally, the 3 pipe system is the most popular because it gives greater control.
How does a heat recovery VRF system work? (i,e. FS)
Each outdoor air-cooled condenser is connected via 3 pipes to an indoor heat recovery unit; a high pressure gas refrigerant line (for heating), a high pressure liquid refrigerant line (for cooling), and a low pressure gas suction line (for return to the outdoor unit).
What is a hydronic system?
HVAC system that utilises water as primary fluid to transfer energy around a building i.e. to heat, cool & condition
What is an all-air HVAC system?
Conditions by means of air throughout a building - can be single / multizone
Pros vs cons of all air system
PROS - nothing exposed on the floorplate, can provide simultaneous heating & cooling on the floorplate with precise controls across occupied areas.
CONS - operationally requires a lot more energy (due to fans pushing air throughout building), requires more kit (AHUs on roof significantly larger c.500mm) and has a lot more on the floor plate, 2.5x more than other 2 options as 200mm underfloor and for AHU ideally 300mm
Pros vs cons of hydronic system
PROS - good for control as the FCU units on the ceiling control the ventilation, operationally it is sustainable (uses water, but can use wide range of energy sources), efficient & effective climate control and long lifespan / reliable kit.
CONS - kit comes from multiple suppliers with different warranties (increases embodied carbon factor), more ductwork required than VRF and initial cost is expensive.
Pros vs cons of VRF system
PROS - energy efficiency, single source of installation/ warranty for equipment, consistent comfort and zonal operation is good, minimum noise (no AC sound), reduced space take of pipework in risers
CONS - suited to buildings of 10 storeys or less due to manufacturing limitations of refrigerant pipework (long pipe lengths can jeopardize performance), risk of leakages is dangerous
Why VRF selected for FS?
GF-L6: A VRF system is used for heating and cooling of the office floor instead of the existing All air heating and cooling system. This enables zonal control with better control accuracies (due to smaller zoning) and reduces energy consumption by increasing efficiencies, adapting to zonal demands and through specification of more efficient heating, cooling and ventilation kit.
LGF café area – underfloor heating (heat pumps) and VRF for cooling. These systems are sized solely for the café area therefore reducing waste heating or cooling to the space.
Embodied carbon analysis undertaken on systems?
MEP consultant requests manufacturing information i.e. how everything is made and assembled
e.g. the carbon footprint of FCU, how many units etc and carry out evaluation
also considers replacement carbon too - i.e. 60 year lifespan with 1 replacement
Hydronic system = 406 tonnes of CO2
VRF = 382 tonnes of CO2
