How can you determine the right size HVAC system for your building?

Manual J Calculation provides thermal capacity, duct work and HVAC equipment sizing for buildings. Steps required for Manual-J calculations as follows: 1. Find out house surface area in square feet 2. Find out house insulation material installed 3. Find out house space usage - heating & cooling 4. Find out BTU values to be assigned for elements: Here are few standard rules for BTU for Manual J calculation: 4a) 1 person = 100 BTU 4b) 1 window = 1,000 BTU 4c) 1 entry door = 1,000 BTU 5. Find out HVAC load for house space: 5a) Surface area ft2 x height of ceiling ft 5b) Nos. of occupants x 100 BTU 5c) Nos. of exterior doors x 1,000 BTU 5d) Nos. of windows x 1,000 BTU Finally, add all 5's BTU and divide by 12,000 to get tons. MAK

Using rule of thumb to determine the HVAC System might work for preliminary phase, as we might have not received any valid data from the client to calculate all the details. And for other sector / non critical industry, using rule of thumb are quite useful, as detailed documentation might not be required

My common sense is based on the air conditioning rules set by inventor - Willis Carrier. Big thanks to him for great work in HVAC field. On the basic of my 30 years experience in the HVAC field; I hereby put forward rule of thumb as rough set of general guidelines that won't work in all situations: "The average tonnage and airflow values for surface area correspond to residential building in MENA region" with ceiling height of 3 meter (10 feet) is imagined or suggested as: 1. 150 square feet per ton (or 14 square meter per ton), 2. Airflow equal to 1 ft3/min for one m2 area (or 1.7 m3/h for one m2 area) MAK

Applying rule of thumb is a good way to estimate the HVAC system. Be it cooling capacity or system size, it provides a quick way to estiamte capex requirements. However, care is to be taken to where this rule of thumb is taken. Seasonal countries have different HVAC requirement compared to tropical countries. The humidity levels are different as well so designewrs have to take note of that as well. A more precise method is load calculation during detailed design. This will allow more science based evaluation of the number.

It's important to have ENERGY STAR Certified HVAC System for higher seasonal energy efficiency ratio (SEER) and energy efficiency ratio (EER) ratings for less energy consumption than conventional models. These two main sets of units that should be required: (1) EER and (2) SEER to understand that efficiency of AC units in the refrigeration cycle is greatly dependent on the heat rejection provided at the evaporator and condenser. For cooling EER calculations, all manufacturers' equipment are tested at 95F DB outdoors and 80 F/50% RH indoors values. Conversion from EER to kW/ton: 1 EER = 1 Ton/12kW. SEER: The SEER is similar to EER, except manufacturers' equipment are tested at multiple conditions for various seasons. MAK

Building cooling load can be calculated using either a spreadsheet or a BIM model. Through the cooling load required by each component and the summation of all these with some margin, the building cooling load can be determined. Modern calculations can be done using a model where the environment is modelled and the cooling load observed. This will provide more visual evaluation for stakeholders as they can be assured that the sizing is done correctly.