Don’t Oversize: The Importance of right sizing HVAC EquipmentBy Mohammed Nader Alamirah

Presentation TitleThe Importance of right sizing HVAC Equipment

• Can’t be too safe, right?• Wrong! At least when sizing HVAC equipment• Causes adverse effects, even on comfort!

Topics◦ How and why:- Mistakes at design stage- Added mistakes during construction◦ Importance of maintaining true SHR◦ Effects of Oversizing HVAC equipment:- Effect on the DC plants◦ What do we do? Recommendations

So, why do we do it and how?1- Mistakes at design stage2- Added mistakes during Construction

Mistakes at Design Stage◦ Rules of thumb calculations

Committing to crazy deadlines

◦ Communication breakdownOverly Conservative assumptions

◦ High safety marginsDon’t add safety margins, or follow AHJ

◦ Poor SelectionSelection for higher SHR

Added mistakes during construction◦ Contractor’s fault?

Not paying attention to supplier’s SHR selection

◦ Consultant is also to blame!Worried Inspectors-Not engineers.

Importance of maintaining Design SHR

𝑆𝑆𝑆𝑆𝑆𝑆 =𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝑆𝑆𝐶𝐶𝐶𝐶𝑇𝑇𝑇𝑇𝐶𝐶𝐶𝐶𝑆𝑆 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝑆𝑆𝐶𝐶𝐶𝐶

◦ It varies based on application (0.5 all the way to 1.0)◦ Very important in part-load (Which is most of the year)◦ Selecting units with higher than design SHR will cause discomfort: Either

overcooling or sweating due to high humidity.

Importance of maintaining Design SHR

Source: Trane Engineers Newsletter ENEWS 29-04

Effects of oversizing HVAC equipment◦ Higher capital costs:

Equipment, ductwork, cables, installation, etc.◦ Higher operating costs:

Electrical & Energy bills, maintenance & replacement, shorter DX unit life, DC provider penalty.

◦ Discomfort: Overcooling/poor humidity control, Thermostat wars.

The bigger picture: DC Plants◦ Oversizing is one of the causes of “Low Delta-T Syndrome”

𝑄𝑄 =𝐿𝐿𝑆𝑆

× 𝐶𝐶𝑝𝑝 × ∆𝑇𝑇

◦ Other reasons include: poor balancing, using 3-way valves, poor control valves, dirty filters, etc.

◦ Remember: 𝑃𝑃𝑤𝑤 ∝ (𝐿𝐿𝑠𝑠)3

◦ Causes pump & chillers to work in full capacities, may starve some terminals too!◦ DC providers enforce penalties on owners because of low Delta-T from their buildings.

So, what do we do?◦ Design Stage:Communicate with the Architect & the Client (Integrated design):

Reasonable deadlines, design inputsUse proper assumptions: refer to ASHRAE handbooks/standards,

coordinate with lighting engineer, Take credit for treated OA supply.Use reasonable Safety factors: from AHJProper SHR selection

o Construction Stage:Again, proper SHR selection. Educate the supervision team about oversizing risksAccept some compromise on Sensible and Airflow selection

capacities.

THANK YOUFOR ATTENDING THE PRESENTATION

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