Why Install an Energy Recovery Ventilator (ERV)?

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As an indoor environment and energy service contractor, the most obvious reason to install an energy recovery ventilator (ERV) is to recover energy when there is a fresh air requirement. When fresh air is pumped into a building, conditioned air is exhausted back outside to equalize the pressure. The air leaving a building has energy in it – warm energy in winter and cool energy in summer. Ruskin ERVs capture about 70 percent of the energy from the air leaving the building and puts it back into fresh air entering the building. This recapturing of energy means rooftop units do not have to work as hard, resulting in lower utility bills. The rooftop unit may also be sized smaller since it does not have to work as hard.

Additionally, workplace and school environments can have a high concentration of people and need plenty of fresh air. Adding fresh air lowers the CO2 levels in a building and helps keep occupants alert and healthy. ERVs also help control odors since stale air is exhausted out of the building as fresh air is brought in. ERVs also manage humidity by keeping moist air inside during winter and outside during summer.

ERVs can be critical for schools to comply with ASHRAE 90.1, 2013. For example, a school in New York City with more than 1000 CFM design supply air and more than 70 percent outside air, requires Energy Recovery with 50% or greater effectiveness. Generally, around 15 CFM per student needs to be brought in.

The first check would be if more than 1,000 CFM is being moved through the HVAC system. If so, divide total CFM by the outside air to determine if the system is bringing in at least 70 percent outside air.

For example, a local school has an 8,000 CFM HVAC system and 400 students.

Here are the steps to determine if 50 percent recovery is required:

• The HVAC system delivers 8,000 CFM, so it is over the 1,000 CFM minimum.
• 400 students require approximately 6,000 CFM of fresh air (400 x 15 CFM = 6,000 CFM)
• 6,000 CFM / 8,000 CFM = 75%

In this case, the school’s HVAC system must bring in over 70 percent of outside air, so some type of Energy Recovery is required. A great solution would be to use ERVs from Ruskin since they achieve well over the 50 percent effectiveness required by ASHRAE 90.1 2013.

 

Let’s Look at Some Examples:

Scenario 1: Calculations show a new job in Miami, Florida, requires a 12.5-ton rooftop unit that is circulating approximately 5,000 CFM. There is a 35 percent outside air requirement, so 1,750 CFM of fresh air is being brought in. In this example, an ERV is again required because in the Miami area, at 35% outdoor air and equal to or greater than 5500 total design airflow, Energy Recovery is required per ASHRAE 90.1.  It also helps the economics by downsizing to a 10 ton rooftop because of the reduced load on the HVAC system. When sized properly, the payback period of the initial cost of an ERV is typically less than one year.

Scenario 2: A new installation where there will be no outside air brought in. Therefore, there is no reason for an ERV if no outside air is being brought in.

Scenario 3: A local school in New York City is being renovated that typically occupies 2,000 students. Four new 25-ton rooftop units are specified with a total of 40,000 CFM, with 30,000 CFM of fresh air. As the 30,000 CFM is brought into the building, a resulting 30,000 CFM is exhausted back outside to equalize the pressure. According to ASHRAE 90.1, 50 percent of the energy being exhausted in the 30,000 CFM needs to be recovered. An ERV would be perfect for this application since it would recover well over 50 percent of the energy and provide long-term utility bill savings.

A Closer Look at ERV Economics

Example #1: 20-Ton HVAC System with 25% Outside Air – Miami

Rooftop System with ERV		Rooftop System without ERV
Cost of 15-Ton DX System	$15,000	Cost of 20-Ton DX System	$20,000
Cost of ERV (5 Tons)	$6,000	Cost of ERV	$0
Rebate on ERV	($2,000)	Rebate on ERV	$0
Total Upfront Cost	$19,000	Total Upfront Cost	$20,000
Yearly Operating Savings – 12 hours x 5 days	$1,330	Yearly Operating Savings – 12 hours x 5 days	$0
Lifetime Savings (15 Years)	$19,950	Lifetime Savings (15 Years)	$0

Example #2: 20-Ton HVAC System with 25% Outside Air – Milwaukee

Building HVAC System with ERV		Building HVAC System without ERV
Cost of 15-Ton DX System	$15,000	Cost of 20-Ton DX System	$20,000
Cost of ERV (5 Tons)	$6,000	Cost of ERV	$0
Rebate on ERV	($2,500)	Rebate on ERV	$0
Total Upfront Cost	$18,500	Total Upfront Cost	$20,000
Yearly Operating Savings – 12 hours x 5 days	$1,883	Yearly Operating Savings – 12 hours x 5 days	$0
Lifetime Savings (15 Years)	$28,245	Lifetime Savings (15 Years)	$0

Examples above assume $1000 per ton, $3 per cfm for energy recovery, $1 per therm, $.08 kWh, 2000 cfm outdoor air.
Source: Airxchange

Type of ERVs and When They Work Best

Unitized ERVs work well with both Retrofits and New construction rooftop units. These fit in all major brands of 2-30 Ton rooftops and have adjustable legs. Plug and play installation, Metasys® controls, along with no roof penetration make these ERVs the most admired in the industry.

MiniVents are approximately 75% effective in indoor applications that require <1200 CFM of air typically found in classrooms, meeting rooms and small retail outlets. These wheeled MiniVents are designed specifically for indoor installation and can be floor mounted or ceiling mounted. A removable panel easily allows access to the enthalphy wheel and filters as needed.

Standard ERVs provide more flexibility than packaged MiniVents. These ERVs are used to recover exhaust air energy and reintroduce into the conditioned space. Besides the fact that they operate in a 600-6,200 CFM window, these can be used both indoor and outdoor. These are Metasys® ready with quickstep controls and the units are powder coated. Units achieve superior energy recovery in accordance with AHRI 1060-2000 & ASHRAE 90.1 compliance standards.

Full-Feature ERVs with heating and cooling work well both in Retrofits or New construction rooftop units.

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