How to Provide the Best-Cost, Not Least-Cost, Option on New and Retrofit ERVs
The wheel potentially makes the difference in performance
What separates energy recovery ventilators (ERV) from each other is the desiccant wheel and options; however, many contractors sell customers on price or commodity-type features that don’t vary much between brands.
Selling end-users just an ERV without realizing the options or wheel’s long-term performance benefits cheats a facility out its full energy-saving potential, which is ironically the premise for energy recovery in the first place.
Instead of controls, blowers, and other standard ERV commodities that don’t vary much between brands, it’s the wheel’s attributes that should lead a sales presentation to an owner. The wheel potentially makes the difference in performance specifications such as efficiency, energy savings, and the resultant payback over the long term.
The ERV should be billed as the “lungs of the building” and sold as a critical component to IAQ. Whether it’s for retrofit or new construction, choosing and presenting the best documented ERV performance, can help contractors win jobs and appear more professional over the competition.
For example, an ERV wheel’s static pressure drop specification of 1-inch w.g. compared to another wheel’s 0.6-inch w.g. doesn’t sound like a big difference. However, the energy savings in just fan energy could amount to thousands of dollars in energy savings over the ERV’s lifecycle. That’s certainly enough impetus to override the poorer performing, less expensive ERV that saves a project a few hundred dollars.
In another example, the assumption that a 6-inch (150-mm) deep desiccant wheel in an ERV has a greater pressure drop than a 3/4-inch (20-mm) deep wheel could be totally false. Depth doesn’t necessary mean a greater pressure drop, but it definitely could result in a significantly higher enthalpy performance. A 6-inch thick aluminum wheel using a molecular sieve desiccant and a self-cleaning fluted design, for example, will allow minimally-obstructed air passage compared to a nonmetallic wheel with silica gel desiccant and a compressed flute design that is more restrictive with airflow. The aluminum wheel/molecular sieve design has less of a static pressure drop even though it could be five or six times thicker (resulting in more latent and sensible heat transfer) than its nonmetallic wheel/silica gel counterpart. These substantiated specifications are many times overlooked in favor of a slight price difference between the two wheel types.
The more airflow restrictive design of nonmetallic wheels can collect debris, act as a filter, and require periodic maintenance. This adds static pressure in addition to the more compressed flute design. Just a 1/4-inch w.g. static pressure drop from a dirty wheel can significantly affect fan performance and even result in surging.
A recent case in point is a North Carolina contractor who chose the least expensive ERV, but still complied with the engineer’s specification. The conscientious engineer ran a calculation and found that the least expensive ERV had only a latitude of just 1/4-inch w.g. of static pressure before fan surging would occur. Since ERV wheels typically aren’t cleaned as often as they should, especially in school buildings where maintenance departments are understaffed, a moderately dirty wheel can easily gain 1/4-inch w.g. of static pressure and affect fan performance. Luckily, the engineer spotted this and mandated a wheel with more latitude.
The aforementioned flute design of aluminum wheels with molecular sieve desiccant also saves maintenance costs because they tend to not accumulate debris and don’t require periodic cleaning. However, nonmetallic wheels with silica gel desiccant can collect debris and clog in a few months, depending on the outdoor and indoor environments. Cleaning a nonmetallic wheel requires individual removal and significant personnel handling time. While building maintenance personnel are likely to perform the periodic 5-minute MERV media filter(s) replacement in the air handlers, shutting down a unit and allotting hours to clean a nonmetallic desiccant wheel every three months doesn’t happen as often as it should.
Additionally, shutting a unit off for cleaning during occupied periods may violate IAQ codes. If the wheel isn’t cleaned, the unit eventually shuts down on overload and violates building codes anyway. It might sound outlandish, but manufacturers regularly see building management system (BMS) historical recordings indicating air handlers were shut down for days or weeks, either by overload cutoff switches or by maintenance people, because there was no air movement due to clogged desiccant wheels.
ERV OPTIONS THAT PAY FOR THEMSELVES
Like any type of mechanical equipment, underserviced ERVs can slowly degrade in performance unbeknownst to the end-user. Energy-consuming dirty filters or wheels many times aren’t detected for months or even years in some cases. Some manufacturers’ solutions offer an on-board anemometer sensor option that compares static pressure differences before and after the wheel. Measured in inches w.g., the static pressure performance is typically detailed in an on-board, externally-mounted readout gauge. Maintenance departments can be trained to periodically monitor this optional troubleshooting sensor.
Historical recording, especially with variable speed wheel controls that include a BACnet board for Internet communication, are invaluable options for troubleshooting past operation, but many ERV specifications don’t include them for price-cutting reasons.
Another major wheel desiccant material design consideration is the potential for silica gel to cause cross contamination of exhaust air and incoming outdoor air. This occurs because silica gel pores become adsorbed with contaminants, which in turn decreases latent heat transfer. Molecular sieve materials aren’t susceptible to contaminant adsorption because the smaller pore size only allows the transfer of the water vapor from the exhaust to supply air streams.
Furthermore, cross contamination is no longer acceptable to health care facilities concerned with the alarming trend of hospital acquired infections (HAI). Desiccant media with anti-microbial coatings can also help minimize airborne biological contaminants.
Beside anti-microbial treatments, other desiccant media options include anti-corrosion and anti-stick coatings. These bring added value to the owner regarding IAQ.
Another consideration is the National Fire Protection Association (NFPA) fire rating. The aluminum wheel has a NFPA 90A rating of <25 and 50 for smoke and flame certification. In order for many nonmetallic wheels to be accepted under the NFPA standard they were instead certified under UL 900 testing. Additionally, some nonmetallic materials may release toxic off-gassing during a fire that could affect occupants.
Another wheel consideration is frosting, which is predominantly a northern climate challenge. Wheels with an exhaust side humidity sensor and VFD options can run at slower rpms to control frosting. Other frost prevention methods are more expensive energy-wise, such as gas-fired or electric heating to preheat the air to prevent frosting. Fossil fuel-based heating can raise ERV operational costs significantly in winter versus slowing the wheel with a VFD. A wheel with equal sensible and latent transfer can mitigate this concern.
A humidity sensor can also activate an energy-saving, wheel stop-jog economizer mode when ambient outdoor conditions are comfortable and heat recovery is not needed, such as 65°F and 60 percent rh.
ERV REPLACEMENT TIPS BEYOND WHEELS AND OPTIONS
Retrofitting an existing ERV requires different specification procedures, especially if the building envelope, environment, number of occupants, or function has changed since the original unit was installed. If a packaged unit replacement is part of the retrofit, the humidity load reduction of the ERV may allow for a smaller packaged unit than its predecessor.
Another good reason for replacing an existing ERV is technology advancements in the last 15 years, especially on the control side. Today’s controls are decidedly more sophisticated and energy saving than those controlling ERVs from 15 years ago. Older ERVs were probably designed to pump in “X” amount of air and the wheel speed didn’t vary. Since building environments change constantly, ERV options such as VFDs can adapt wheel speeds to match the requirements and save energy in the process.
In another retrofit scenario, an existing direct expansion (DX) rooftop system can be combined with an ERV to reduce its cooling and heating loads. This can significantly cut energy costs of a rooftop unit unsuccessfully attempting to maintain comfortable rh levels. ERV manufacturers have streamlined this scenario by designing some of their ERV product line to easily bolt onto an existing rooftop along with other installation aids.
One challenge the ERV industry must face is post installation follow-up to assure original operating specifications are maintained. Most ERVs operate well at start-up; however, HVAC contractors should offer at least an annual check-up to see if the system is operating at optimal conditions.
All ERV wheels and their options are not the same. Choosing an ERV to accompany a rooftop HVAC system, or replacing an existing ERV with a higher performing desiccant wheel can not only save operating costs, but also upfront capital costs, because the new rooftop system can be sized smaller due to the smaller load presented by the ERV system. The result is the contractor appears more professional and has an edge over the price-cutting contractor when the end-user realizes the design’s long-term operational cost savings.
SIDEBAR: WHEEL REPLACEMENT REQUIRES RER, NOT ROI CALCULATIONS
It’s not return on investment (ROI) calculations that are important in wheel replacement, but instead recovery efficiency ratio (RER), according to the Air Conditioning, Heating, and Refrigeration Institute (AHRI) and its Guideline V, “Calculating the Efficiency of Energy Recovery Ventilators and Its Effect on Efficiency and Sizing of Building HVAC Systems.”
Accounting purely for high efficiency and capital cost in wheel replacement, without regard to static pressure, may in fact defeat attempts by the building owner to maximize long-term energy savings and ROI.
Instead, a wheel replacement selection should be reviewed for its RER. The RER takes into consideration the efficiency and the static pressure of a desiccant wheel replacement. Not calculating the RER could result in tens of thousands of dollars in lost energy savings over the course of the desiccant wheel’s lifecycle.
Intended for service contractors, engineers, and building owners, Guideline V provides a means for calculating the impact of applied energy recovery equipment on the energy efficiency of the HVAC system at a single selected operating condition.
More simply, the calculations comprehensively take a host of factors into consideration, such as geographical climate, fan/motor efficiency, exhaust air transfer ratio (EATR), pressure drop, energy recovery methodology, and many other parameters. Guideline V also allows service contractors to compare energy recovery components and arrive at a comprehensive savings for heating and cooling, rather than just a manufacturer’s efficiency rating that doesn’t consider all variables.
Publication date: 11/9/2015