7 Useful Rule of Thumbs in HVAC (Must Know)

In HVAC, rule of thumbs are useful for preliminary design checks and quick estimations. They also can be used to counter-check design results from manual or software calculations. Here are 6 useful rule of thumbs that every HVAC engineer must know.

Btu/hr/sqft

Btu/hr/sqft is the most common, most used rule of thumb in HVAC. It stands for British Thermal Unit per hour per square foot. Btu/hr/sqft is used to estimate the required cooling/heating capacity of a room.

In the early stage of a construction project, many construction materials, types and other details are often not available (yet to be finalized). However, these information are essential for HVAC load calculations.

The material used and the type of wall, windows, doors and ceiling are important details for calculating the building’s external heat gains. What each room will be used for and how many people the room is designed to hold at one time are also critical for internal heat gain calculations.

Unfortunately, many projects don’t have a firm confirmation of these details until later in the design stage or even halfway during the construction stage. Meanwhile, the HVAC design engineer must provide the preliminary design and cost estimation for the HVAC system without these details.

That’s where the btu/hr/sqft rule of thumb becomes a very handy tool.

With just the architectural layout drawings, the HVAC design engineer can measure the room’s floor area and multiply it by an “appropriate value” to estimate the cooling/heating requirement.

The “appropriate value” is the btu/hr/sqft.

The btu/hr/sqft value varies greatly depending on the application. For instance, a typical office space would require 60 btu/hr/sqft of cooling. Meanwhile, the same space but used as a conference hall would need 120 btu/hr/sqft because there are a lot more people in a conference hall at once than a typical office space.

What btu/hr/sqft value to use is mainly based on the experience of the designer but there are also resources we can take reference from. For example:

I sometimes prefer the btu/hr/sqft suggested by local equipment suppliers because they take into account the local weather, design practices and preferences.

Different weather and design conditions result in different cooling/heating requirements. A resource may suggest a btu/hr/sqft value for buildings with insulated walls. But, if you’re designing for buildings without wall insulations, then referring to that resource may cause you to underestimate the cooling/heating requirement.

Overall, btu/hr/sqft is useful for quick estimations. Sometimes, it can be as good as a load calculation for typical applications like residential houses, small office buildings, hotel rooms and etc. Nonetheless, a proper load calculation should always be performed to officially finalize the cooling/heating requirement.

Cfm/sqft

Cfm/sqft is useful for cross-checking and estimating the amount of airflow needed for a given space area. It stands for cubic feet per minute per square foot.

When it comes to airflow estimation, cfm/sqft is used more in residential applications. The most common value is 1.0 cfm/sqft which means for every square footage, the air conditioning unit should provide 1 cfm of airflow. So, if a room has 200 sqft, the supply airflow should be 200 cfm.

The reference cfm/sqft values can also be found in the cooling load check figures table by ASHRAE and equipment suppliers. Similar to btu/hr/sqft, what cfm/sqft value to use depends on the applications, number of people held at once, the designer’s experience, design conditions and etc.

Since airflow is a function of sensible heat, the building’s construction materials and types will also affect how much airflow is needed. If a room has a lot of windows, the sensible heat is expected to be high and therefore, we can expect to have a higher cfm/sqft value.

Conversely, a room located at the center of the building (internal) without any exposure to sunlight, the cfm/sqft value will be lower.

Other than residential applications, cfm/sqft is hardly used to initiate a design but rather as a cross-check value. For airflow design in commercial applications, cfm/ton is more useful (see below).

ΔT

ΔT stands for delta T which means temperature difference. The delta T rule of thumb can be applied in different areas of HVAC. The more common ones are:

  • The temperature across a cooling coil.
  • The difference between supply air temperature and room design temperature.
  • The temperature difference between chilled water supply and return.

The temperature across a cooling coil is calculated as on coil temperature (entering air temperature) minus off coil temperature (leaving air temperature). A typical example is on coil 80°F minus off coil 55°F and the ΔT cross the coil is 25°F.

The ΔT across a cooling coil is useful for troubleshooting cooling related problems. For instance, if the cooling coil is supposed to give a ΔT of 25°F but it is now 20°F, we can suspect it is a refrigerant problem or a chilled water temperature problem.

The ΔT of a cooling coil or basically any heat exchanger (finned-tube, shell-and-tube and etc.) can be used to calculate the current cooling capacity of the coil using the Q=mcΔT formula.

Another use case of the ΔT rule of thumb is deciding the appropriate supply air temperature. Usually, the supply air temperature is 10°C or 18°F above the room design temperature where the room design temperature is typically 24°C (75°F). If so, the supply air temperature should be 14°C (57°F).

In chilled water system, the temperature of the water before entering and after leaving a water coil is the same as the chilled water supply/return ΔT. Typically, the chilled water supply/return temperature is 44/54°F. So, the ΔT is 10°F.

Generally, a higher ΔT is more desirable as often translates to higher capacity and better energy efficiency. For example, 15°F is the design in high delta T chilled water systems.

On the other hand, approach temperature or simply, approach is the opposite of ΔT. While more is better for ΔT, less is preferred for approach as it indicates better heat exchange efficiency.

Gpm/ton

In chilled water systems, gpm/ton is used to estimate the amount of chilled water and condenser water needed for a given chiller and cooling tower capacity.

Gpm/ton stands for gallon per minute per refrigeration tonnage. It means for every ton of cooling capacity expressed in refrigeration ton (1 ton = 12,000 btu/hr), how much water flow rate (expressed in gallon per minute or gpm) is needed.

In a standard 10°F chilled water supply/return ΔT design, the rule of thumb for chilled water flow rate is 2.4 gpm/ton and for condenser water flow rate is 3.0 gpm/ton. In a high 16°F chilled water supply/return ΔT design, the chilled water flow rate can drop to 1.5 gpm/ton and the condenser water flow rate to 2.0 gpm/ton.

The reduced gpm/ton means the pump power is lower which translates to a significant operating cost saving in the long run with the right equipment type and setup.

KW/ton

The energy efficiency of chillers is often expressed in kW/ton which stands for kilowatt per refrigeration tonnage. Because kW is power, kW/ton is a direct measurement of how much power is needed for every ton of cooling which is basically the cost needed to run the cooling system.

KW/ton is a popular measurement unit in commercial buildings that uses a chilled water system. A typical water-cooled centrifugal chiller by itself has about 0.5 kW/ton. Whereas a similar-size air-cooled chiller has about 1.0 kW/ton. On that basis, water-cooled chillers are twice as energy efficient as air-cooled chillers.

However, when assessing the power consumption in a building due to cooling needs, it is more useful to calculate the overall energy efficiency of the system which includes the chiller, cooling tower, pump, AHU and other associated components.

A lower kW/ton means the cooling system is more efficient and therefore, cost less to run.

Knowing the kW/ton rule of thumb allows us to quickly assess whether the efficiency of a chiller or a plant is average, above average or below average. It also allows us to translate into monetary value for design comparison.

Cfm/ton

Cfm/ton is a rule of thumb for AHU airflow check and estimation. It stands for cubic feet per minute per refrigeration tonnage. Cfm/ton is more useful for airflow estimation than cfm/sqft.

In most applications, the golden rule for airflow is 400 cfm/ton. It means for every ton of refrigeration (1 ton = 12,000 btu/hr), the required airflow is 400 cfm. There are several factors as to why 400 cfm/ton exists.

The capacity of an air handling unit (and basically any air conditioner) is comprised of sensible capacity and latent capacity. The sensible portion is responsible for bringing down the air temperature while the latent portion is for moisture removal (dehumidification).

How much airflow (cfm) is needed for an air conditioning unit primarily depends on the sensible portion. However, the refrigeration ton in cfm/ton is the total capacity (sensible+latent). So, if the latent portion increases, the ton is higher but the cfm remains unchanged. Hence, the cfm/ton value decreases.

In short, applications with high latent load see a lower cfm/ton value than applications with low/standard latent load. For example, the AHU of an auditorium may have 300 cfm/ton while the AHU of a data center has 450 cfm/ton.

ACH

ACH stands for air change per hour. It is sometimes written as ACR or air change rate. Regardless, it represents how many times the entire air volume of a room is replaced by new air in one hour. For example, 6 ACR in a 100 sqft room with a ceiling height of 10 ft means 1,000 ft³ of volume is replaced 6 times in an hour, translating to a ventilation fan airflow rate of 100 cfm.

100 cfm x 60 min. = 6000 ft³ per hour (1,000 ft³ x 6 times) = 6 ACR

ACR = (60 x CFM) / VOLUME

ACR is like btu/hr/sqft but for ventilation. A typical bathroom ventilation requirement is at least 4 ACR subject to local building codes and requirements. In Malaysia, the smoke spill system for an enclosed basement carpark floor is 12 ACR during fire mode (and, 6 ACR during normal mode for basic ventilation).

In some applications, ACR supersedes cfm/sqft and cfm/ton for airflow requirements. For instance, residential applications usually use cfm/sqft or the number of occupants as the primary factor for airflow. Whereas hospitals and cleanrooms use ACR for airflow.

Higher ACR means the ventilation fan requires a higher airflow capacity. High ACR is usually needed in places where ventilation is critical (eg: hospital, surgery room, cleanroom, chemical plant and etc.).

ACR is often associated with the introduction of the outdoor air to replace the indoor air. However, well-filtered indoor air (recirculation system) can also be considered as air change.

If you have anything to add (or ask) about this topic, leave a comment down below!

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