The Importance of Fan Watts per CFM in Good Ventilation System Design

Ventilation systems are getting a lot of press time recently as increased ventilation air is one of the methods recommended to reduce the spread of COVID -19.  Even when we are not living through a pandemic, ventilation air is critical to good indoor air quality and wellness in the built environment.

Ventilation systems deliver outdoor air to the occupied space to reduce concentration of pollutants (and viruses). They filter the air and for many applications employ energy recovery devices to greatly reduce the energy (and carbon) cost of ventilation air. The down side is they use fans to move the air and thus consume electricity.

The energy recovery component of an energy recovery ventilation unit (ERV) has the biggest impact on reducing the energy cost for ventilation systems in most climates. ASHRAE Standard 90.1 – Energy Standard for Buildings except Low-Rise Residential Buildings and 2015 National Energy Code of Canada for Buildings (NECB) both require at least 50% energy recovery for most climate zones based on hours of operation and the amount of outdoor air.

As the Annual energy consumption for a Ventilation System pie chart above shows, the supply and return fan annual energy use exceeds the remaining heating and cooling load experienced by the chiller and boiler plants. This model includes an 80% efficiency enthalpy rotor.   

It is time to look at the energy used by the fans. The section shown below comes from ASHRAE Standard 90.1-2019. 

6.5.3.1 Fan System Power and Efficiency

6.5.3.1.1

Each HVAC system having a total fan system motor nameplate horsepower exceeding 5 hp at fan system design conditions shall not exceed the allowable fan system motor nameplate horsepower (Option 1) or fan system bhp (Option 2) as shown in Table 6.5.3.1-1. This includes supply fans, return/relief fans, exhaust fans, and fan-powered terminal units associated with systems providing heating or cooling capability that operate at fan system design conditions. Single-zone VAV systems shall comply with the constant-volume fan power limitation.

Using Option 2 for constant Volume systems and 1 cfm, the allowable BHP/cfm is 0.00094 BHP/cfm or 0.70 W/cfm.

ASHRAE Standard 90.1 allows more fan power usage when energy recovery devices are used or filtration efficiency is increased above minimum. For example, add 80% effective energy recovery and increase filter performance to MERV 13.

A =  [2 x (2.2 x 0.80)-0.50) + 0.9] x 1 cfm/4131

A = 0.00083 BHP/cfm or 0.62 W/cfm

To meet the requirements of ASHRAE Std 90.1-2019, the ERV with 80% effective energy recovery and MERV 13 filters is allowed 0.7 W/cfm + 0.62 W/cfm = 1.32 W/cfm.

ASHRAE Standard 90.1-2019 is minimum compliance.  What happens when you want to push the envelope? Consider the new Washington State Energy Code 2018 WSEC_C 2^nd Print  Section C403 covers minimum compliance for Mechanical Systems;

C403.3.5.1 Energy recovery ventilation with DOAS. The DOAS shall include energy recovery ventilation. The energy recovery system shall have a 60 percent enthalpy recovery effectiveness in accordance with Section C403.7.6. For DOAS having a total fan system motor nameplate hp less than 5 hp, total combined fan power shall not exceed 1 W/cfm of outdoor air. For DOAS having a total fan system motor hp greater than or equal to 5 hp, refer to fan power limitations of Section C403.8.1. This fan power restriction applies to each dedicated outdoor air unit in the permitted project, but does not include the fan power associated with the zonal heating/cooling equipment. The airflow rate thresholds for energy recovery requirements in Tables C403.7.6.1(1) and C403.7.6.1(2) do not apply.

For ERVs less than 5 total BHP fan power the fan power is 1W/cfm. Above that the fan power requirements are the same as ASHRAE Std 90.1.

In addition to minimum compliance, the Washington State Energy Code includes requirements to achieve 8 credits by picking a range of energy efficiency packages. One of the packages is for ventilation systems (they refer to it as Dedicated Outdoor Air System or DOAS). 

C406.7 High performance dedicated outdoor air system (DOAS).

A whole building, building addition or tenant space which includes a DOAS complying with Section C406.6 shall also provide minimum sensible effectiveness of heat recovery of 80 percent and DOAS total combined fan power less than 0.5 W/cfm of outdoor air. For the purposes of this section, total combined fan power includes all supply, exhaust, recirculation and other fans utilized for the purpose of ventilation.

Roughly speaking, to achieve high performance, the WSEC expects the ventilation system to go from 60% enthalpy to 80% sensible energy recovery and use half the fan power. 

Fan Power Limitations and System Design

While the energy saving potential is clear, what does this mean when designing the system to achieve the fan energy savings?  Let’s switch from fan energy to airflow and static pressure.

FAN POWER_(bhp) = [AIRFLOW _(cfm) x TSP _{(in w.c.)}]/[6343 x FAN EFF]

Given:

0.5 W/cfm or 0.00067 BHP/cfm

70% efficient Fan

How much static do we have to work with per cfm supply air?

TSP = 0.00067 bhp x 6343 x 0.70]/[1 cfm]

TSP = 3.0 inches w.c.

This is all the static pressure available to push outdoor air through the ventilation unit and duct work to the occupied space, return it to the ventilation unit and exhaust it.

TSP is Total Static Pressure and it is the sum of Internal Static Pressure (the pressure available to push air through the components in the ventilation unit) and External Static Pressure (the pressure available to push air through ductwork, diffusers, grilles, louvers etc).  To achieve this level of performance, it is necessary to change how you would normally design the ventilation unit (manufacturer’s challenge) and design the building air distribution system (design engineer’s challenge). 

Standard vs. High Performance  Example

Let’s design a 2400 cfm ventilation system to meet the Washington State Energy Code section  C403.3.5.1 (standard performance system) and then compare it a ventilation system that meets code section C406.7 (high performance system).

Standard System

The design engineer sizes the air distribution system so that it requires 1 in w.c. ESP on the supply air path and 0.75 in w.c. ESP on the return air path.

A Swegon GOLD RX 12 ventilation will deliver the following;

2400 cfm outdoor air with 1.0 in w.c. ESP

2400 cfm exhaust air with 0.75 in w.c. ESP

82% efficient sensible energy recovery (winter)

79.5% efficient latent energy recovery (winter)

MERV 13 filters on the supply air return air paths  (High efficiency filters on the return air path is important if you are considering the return air may have pathogens)

0.82 W/cfm supply air with clean filters

0.9 W/cfm supply air with dirty filters

This selection exceeds the standard energy code requirements.  The image shows where the static pressure is being consumed.

High Performance System

The design engineer now increases the duct sizing to lower the air velocity and thus lower the required external static pressure.  The higher performance distribution system now requires 0.7 in w.c. ESP on the supply air path and 0.5 in w.c. ESP on the return air path.

To lower the Internal Static Pressure, the ventilation unit needs to increase the filter area and the rotor size to reduce the pressure drops.

A Swegon GOLD RX 14 ventilation unit will deliver the following;

2400 cfm outdoor air with 0.7 in w.c. ESP

2400 cfm exhaust air with 0.5 in w.c. ESP

85% efficient sensible energy recovery (winter)

82.5% efficient latent energy recovery (winter)

MERV 13 filters on the supply air return air paths 

0.50 W/cfm supply air with clean filters

0.54 W/cfm supply air with dirty filters

The figure shows how changes to the entire ventilation system were made to achieve the high performance ventilation standard.

Summary

While achieving 0.5 W/cfm is challenging, it can be done when the whole system design is considered. Recalling that fan power is significant part of operating a ventilation system the high performance goal of 0.5 W/cfm supply air is at least 25% reduction in the operating cost of the ventilation system for the life of the building. Managing fan power is clear path to striving for low EUI, high energy efficient (low carbon) buildings.