Case Study - HVAC

Question 1

What is a HVAC System?

Answer 1

Heating, ventilation and air conditioning system.

Used in buildings to:

• maintain internal air quality
• regulate internal temperature
• regulate internal humidity

Question 2

What does an MEP system do within a building?

Answer 2

Important part of building services and can have many different functions.
Includes mechanical, electrical, and plumbing

Question 3

Talk me through how a HVAC system works

Answer 3

• Usually delivered mechanically (can also be natural)
• Conditions the air to suit requirements, i.e. can heat/cool and control moisture

1. Boilers / chillers (now ASHP) in basement deliver hot and cold water to AHUs (on roof)
2. 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.
3. 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.
4. 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.]
5. Cooling towers located on roof remove heat from chillers and expel into environment.

Question 4

What to consider when selecting MEP system?

Answer 4

• equipment required and sizing
• retention of existing equipment
• sustainability credentials
• warranties and life span
• design intent and aesthetics

Question 5

What is an AHU? And what does it do?`

Answer 5

• 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).

Question 6

What is an air source heat pump? What does it do?

Answer 6

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

Question 7

What is a VRF system? How does it work?

Answer 7

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.

Question 8

How does a heat recovery VRF system work? (i,e. FS)

Answer 8

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).

Question 9

What is a hydronic system?

Answer 9

HVAC system that utilises water as primary fluid to transfer energy around a building i.e. to heat, cool & condition

Question 10

What is an all-air HVAC system?

Answer 10

Conditions by means of air throughout a building - can be single / multizone

Question 11

Pros vs cons of all air system

Answer 11

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

Question 12

Pros vs cons of hydronic system

Answer 12

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.

Question 13

Pros vs cons of VRF system

Answer 13

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

Question 14

Why VRF selected for FS?

Answer 14

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.

Question 15

Embodied carbon analysis undertaken on systems?

Answer 15

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

Question 16

How was the operational carbon analysis (whole life carbon) undertaken?

Answer 16

Electricity consumption x by carbon factor = carbon cost

hydronic system = £27.6k p/a
VRF system = £26.5k p/a

Question 17

What is a refrigerant?

Answer 17

A working fluid used in the refrigeration cycle of AC systems and heat pumps.

Undergo a repeated phase transition from a liquid to a gas and back again.

Question 18

R32 refrigerant?

Answer 18

A refrigerant used in air conditioning, developed primarily to reduce the impact of hydrocarbons on the environment.

Has less envr impact.

Since Kyoto Protocol - global regulations have led to a shift towards refrigerant gases with a low envr impact.

Question 19

What does a condenser do?

Answer 19

Keeps air conditioner performing its best.
Includes:
- rejecting superheated refrigerant gas
- condensation
- subcooling refrigerant once it is transformed back into a liquid state.

Question 20

What is Embodied carbon?

Answer 20

Total GHG emissions produced from the creation of an asset.

• Typically is cradle-to-gate
• EU Framework A1-A5 which includes the production (manufacturing and materials) and Construction (transport and installation)
• Excludes operational emissions
• Expressed as kgCO2/kg of material / product

Question 21

What is operational carbon?

Answer 21

Carbon emissions’ association with energy consumption (operational energy) while the building is occupied.

EU Framework - Stages B (In use - maintenance, operation and refurbishment) and Stage C (End of Life - deconstruction, transport, disposal, waste, reuse/recycle)