The Ons & Offs of Energy Recovery Ventilation Effectiveness

About the Webinar

Energy recovery ventilation (ERV) systems exchange energy between outgoing exhaust air and incoming outdoor (ventilation) air to reduce heating and cooling energy while meeting ventilation requirements. ERVs have the potential to eliminate the majority of the energy used for heating and cooling ventilation air. However, many of these systems fall short of achieving their design effectiveness due to a variety of problems such as improper design, installation, or operation and maintenance issues. According to a 2010 Minnesota Market Assessment Report, only 48%of systems operate properly immediately after installation, with an overall average efficiency of 42%.

The Center for Energy and Environment’s recent field study characterized energy recovery systems in Minnesota commercial and institutional buildings and identified common problems that diminish the effectiveness of ERVs. Join this webinar to learn more about study findings, recommendations for utility services, and guidance for operational enhancement in the field. 

During the live webinar the audience had many great questions that time did not allow us to address. Answers to these questions below from principal investigator and presenter Josh Quinnell.

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The Ons & Offs of ERV Effectiveness​


Answers to audience questions

In your first Results slide (p. 27?) - does this speak to the ERVs included in the study (a sample), or is this a characterization of installations statewide (a bigger population)?
Of the three data sets, one included the entire state, one was limited to CenterPoint territory (25% of the state), and one was focused on the metro area. So we did have some information about the units throughout the state, but beyond that detailed geographic information wasn’t available.
Manufacturers-- what can they do to help alleviate some of these issues upstream?
The manufacturers already do quite a lot, especially considering how quickly the technology and their delivery has come along in the last 15 years. Their documentation is generally very good. Many of them have very clear recommendations on design, sequencing, and background information at various technical levels. I think they should push strongly for ERVs as essential components in HVAC systems, promote the responsible downsizing of associated heating & cooling equipment where applicable. Communicate more openly about annualized net energy savings compared to design performance, and the implications (cost and performance) of various system-level choices (e.g. flows, bypass, controls). 

I asked the question regarding performance, practical application and code requirements.  I would love to hear more on the economics of energy efficiency equipment (code intent vs reality).
I have two points on costs. It is hard to generalize a comparison of the cost of an ERV to the cost of cooling equipment because there are so many options and costs are often driven by other factors. Nonetheless RSMeans data suggests in a lot of situations that the cost of energy recovery is about the same or less than the cost of cooling capacity. This suggests that if one displaces cooling tonnage with an ERV they can possibly reduce or at least keep first costs constant. That doesn’t consider operational energy savings, which on the cooling side may be a wash in Minnesota.

However heating savings is where ERVs save energy. Speaking strictly in terms of total enthalpy wheels, cost-effectiveness is proportionate to size. A 20,000 cfm is an order of magnitude cheaper per cfm than a 2000 cfm unit. At the high end, the big units should have paybacks of about 3-4 years if operated perfectly. Units that run 24x7 might payback in a year. At the mid-range 5k-20k payback might be more like 4-6 years, or 2-3 on a 24x7 operation. At the low end <5000 cfm, the costs of ERVs get high per cfm and paybacks get long 7-9+ years.

Either of these arguments supports energy recovery on a cost basis. If you combine both ideas, they are very attractive, even if poor implementation “costs” some savings. It is important to recognize that adding ER does introduce more system components (moving parts, sensors, controls) so in order to claim savings, facilities need to have a plan for managing the risks associated with more critical equipment and sophisticated systems.

For my work, I would like to know more about new implementations of energy recovery and their cost effectiveness/payback, especially in industrial buildings with large fresh air requirements. However, I understand this was slightly off topic from the purpose of the presentation as it focused on operation and effectiveness of existing units.
ERVs become very attractive as 1) OA requirements increase and 2) operating hours increase. Total enthalpy wheels get cheaper per cfm as size increases.  See previous answer for some more info.

Can you talk more about the "balanced" OA supply/exhaust? With positive building pressurization, this doesn't seem feasible...Is code forcing purchase of equipment that isn't gaining the intended efficiency? 
To add a bit of clarification to my live answer. This is not a problem with code but with specification. It is perfectly fine to have ERVs with unbalanced flows. That choice just needs to be specified so that it can inform savings estimates and operating expectations. In fact, the best wheel in our study was constrained such that supply flow is 1.5 to 2X exhaust flow. In an unconstrained scenario a larger ERV would have even more savings, but this particular unit is still high performing and delivering on promised savings.
Did you find a difference in performance between fixed plate and rotating wheel ERVs?  Did the media material impact performance?

I’ll answer this question in two parts. First, we only measured one fixed plate heat exchanger so my perspective from the field is limited. When it comes to comparing normalized net savings among the different units in the study, the specific details of an implementation have a larger bearing on net recovery than do the heat exchanger performance ratings (i.e. wheels vs plates). I would say substituting a total enthalpy wheel for the flat plate in our study would have improved net recovered energy. There are three factors that would increase recovery 1) the ability to shed humidity from outside air during cooling season, 2) a more aggressive frost control strategy, and 3) an inherently higher sensible effectiveness.
Secondly in terms of the media and geometries specific to the different enthalpy wheels, yes they have a bearing on their heat/moisture transfer & pressure performance. All else equal, the unit with the higher rated effectiveness will save more energy. However all else is not equal and the 10% difference in potential design performance, I think, is only one factor to consider in the choice of an ERV unit.
Can you comment on ERVs that don’t have the ability to economize?

I would need more information about the particular system. Wheel based systems always have some ability to economize; the wheel can be stopped. They still incur some energy costs, but it is a net gain. For fixed plates there is nothing to turn off so the only option is to reroute flow around them (bypass). If this isn’t possible, I would assume there was some major design constraint that prevented such a design. Depending on the scope and abilities of the system(s) in the building, I am sure one could develop a strategy to improve economizing performance. Please contact me and we can discuss this further.
Did you analyze cost-effectiveness at the per SF level, beyond the range of energy cost savings on a per building basis that you shared in your slides?
Yeah, in our report we break down the costs on a $/cfm basis. On the cooling side these costs are very small (consistent with the other results). On the heating side they are about 0.2 to 0.5 $/cfm at 3130 hr/yr.

Related Links
Research project page
Full Report: Energy Recovery in Minnesota Commercial and Institutional Buildings: Expectations and Performance

This project supported in part by a grant from the Minnesota Department of Commerce, Division of Energy Resources through the Conservation Applied Research and Development (CARD) program. And with co-funding by CEE in support of its nonprofit mission to advance research, knowledge dissemination, and program design in the field of energy efficiency.