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The Problem with HRVs

When integrated with the HVAC system, most E/HRV's don't deliver the desired ventilation air. A new design seeks to solve this problem, but we need your input.

The Problem with HRVs
by Srikanth Puttagunta, PE
 

Builders believe that if they install an Energy Recovery or Heat Recovery Ventilator (E/HRV) they have ensured good indoor air quality. In reality, that's seldom the case.

To deliver the needed fresh air, an E/HRV must be installed in a way that guarantees balanced airflows—where the intake and exhaust airstreams move equal volumes. But the design of these units makes proper installation difficult and, when integrated with the HVAC system (as most are), almost ensures that they fail to work as advertised.

Of course, an E/HRV costs more than other ventilation strategies, so if it doesn't do what it's intended to, the builder has wasted that extra money.

Steven Winter Associates is collaborating with a major manufacturer to develop an ERV that solves the shortcomings of conventional units. We have completed the second prototype. We hope to have a final design by Fall of 2019 with commercial availability sometime in 2020.

Before moving to the final design, however, we would like input from the EEBA audience to make sure the product will meet your needs. We have included a link to a short survey at the end of this article.

The three major issues that lead to unbalanced airflow with today's E/HRV's are:

1. Typical duct configurations

2. The frost prevention controls they use

3. Installs that make them difficult to properly maintain

Duct Issues

Let’s start with typical duct configurations.

If the E/HRV's intake and exhaust ducts aren't similar lengths with the same number of bends, their resistance to airflow will vary, throwing the unit out of balance. To compensate, some manufacturers provide static pressure taps that let the installer adjust the unit's fans during installation. That compensation might be sufficient for an E/HRV that has its own ductwork, but it won't be for the majority that are integrated with the HVAC system's air handler unit (AHU). The following two scenarios explain why.

Scenario 1. Here, the E/HRV pulls stale air from the return duct, then delivers tempered outdoor air further downstream in the same duct, closer to the AHU. For this to work, the E/HRV needs to run in sync with the AHU fan. However, this also results in unbalanced air flow, as the larger AHU fan will impact the smaller E/HRV fans. In this case, result will be more supply than exhaust air.

The unit could be adjusted at startup to compensate for this imbalance (though we don’t commonly see this in the field). The problem is that most of today's AHUs have two-stage or variable-speed fans, so the E/HRV can only be balanced under one of those fan speed conditions.

Scenario 2. Here, the E/HRV pulls stale air from the return ductwork and delivers tempered outdoor air to the AHU's supply ductwork. While manufacturers recommend the AHU fan run in unison with the E/HRV, most don't require it and with the AHU off, the unit may end up ventilating the AHU but not the rest of the home.

Frost Prevention

Next, let’s look at cold climate frost prevention controls.

When the outdoor air falls below a certain temperature (which varies with the E/HRV model), the core will be at risk of freezing. Manufacturers prevent this in a variety of ways, none of which are ideal.

  • On/off cycling. When temperatures fall below the frost threshold, the unit switches off for a set period of time (in really cold conditions, this could be 20 minutes or so each hour) to give the core a chance to warm up.
  • Air recirculation. Here, the outdoor intake and exhaust ports are closed, and indoor air redirected through the core's outdoor air pathway to warm it up. During this period, no whole-house ventilation is provided.
  • Exhaust only. Some units run in exhaust-only for a period of time allowing the core to warm back up. During this period make-up air will be supplied through leaks in the building envelope.
 

In each case, the home has poor or no ventilation during frost prevention. Alternatively, an electric resistance pre-heater can be installed in the outdoor air duct to prevent frost from forming in the core. This maintains continuous airflow but is energy intensive.

Install Errors

A proper E/HRV installation leaves enough space around the unit for regular maintenance, which includes periodically changing the air filters and removing the core for cleaning. Based on what we see in the field, I wonder how many builders and installers understand this. Many installations make it difficult to access the filter and core, while in other cases access is blocked by ductwork and plumbing that was installed later.

You would also assume installers know how to connect the ducts, but I've seen a lot of problems here as well, including supply and exhaust ducts hooked up to the wrong sides of the E/HRV. I also see a lot of flex duct that's not pulled tight, creating static pressures that can severely restrict airflow through either side of the unit or both.

Our Solution

With support from the DOE's Building America program and industry partners, Steven Winter Associates is developing an integrated ERV that will make balanced ventilation easier in homes. Our design includes the following improvements.

1. Simplified installation through a better form factor.

The unit will connect directly to the return side of an air handler and will pull stale air from the air handler's return ductwork. Not only does this avoid the drawbacks of each configuration type, but the fact that the ERV unit only needs two duct connections rather than four makes proper installation easier.

The unit is sized for mechanical rooms with standard ceiling heights. For an up-flow configuration, the total combined height of the ventilation unit, the air handler, and a standard supply plenum will be less than 8 feet. Maintenance access for the core and filters is also from the front, so it matches the service area required for the AHU.

2. Fans that ensure balanced ventilation under constantly varying conditions (varying AHU fan speeds, outdoor winds and indoor pressure changes, for example).

We are incorporating ECM fans. Nothing new here, right? A lot of E/HRVs have ECM fans. But rather than the typical constant torque ECM fan we are using constant flow fans, which will maintain roughly a ± 5cfm airflow range. This also allows the unit to be configured with MERV 13+ filtration.

3. Better frost prevention

The system is designed to maintain balanced whole-house ventilation during the frost prevention cycle without using electric resistance pre-heat. It does this by using a modulating damper to mix a small amount of air from the AHU supply duct with the outdoor air to pre-temper it above the core's frost point. Overall airflow through the outdoor air pathway of the core is increased, but the portion of outdoor air to exhaust air remains balanced.

Have I piqued your interest? Then take a look at the drawing.

As we continue to make refinements to the components and controls, we hope you will assist us with some feedback. This will help us ensure that the final product truly meets the industry's needs. We have posted a short questionnaire online that will help us better understand your approach to whole-house ventilation as well as what features we need to prioritize.

The survey takes five minutes or less and can be completed anonymously. We thank you for your interest and look forward to hearing from you.

Srikanth Puttagunta, PE, is a Principal Mechanical Engineer with Steven Winter Associates, Inc.

Comments

re: The Problem with HRVs

Wednesday, April 10, 2019 5:06:10 PM

Here in Napa Valley and Northern California in general, we are increasingly concerned with paths for fire to get into the building.  Intake vents on outside walls and roofs present special problems, where intake can bring combustion air into the building in spite of our best attempts to protect with building with non combustible exteriors.  Any optional combined shut down/ firesafe exterior vent locations would be So welcome!

 

re: The Problem with HRVs

Wednesday, April 10, 2019 7:21:19 PM

The article touched on the problem, but did not correctly identify the problem:  You should NEVER confuse or connect your fresh air system with your furnace or AC system.  They serve completely different functions, and using the same ductwork could be possible in some cases, but is very counter-productive in most.  

re: The Problem with HRVs

Wednesday, April 10, 2019 7:36:22 PM

Another BIG problem with HRVs (and ERVs) is that they cannot balance your range fan, which is by far the biggest source of ventilation-related heat loss.   For more than fifteen years we have had the solution for all of the problems associated with ventilation and heat exchange, but because we are not selling anything, nobody is listening.  In most climates in the US, the HRV or ERV uses more energy to move the air than it would have cost to simply re-condition the air, IF you are using an efficient HVAC system.  You need to ask the right questions before you can get the right answers, and most of you seem to have your heads in the sand, continuing to do things the way you always have, and simply adding one more expensive gadget when someone wants to sell it to you.  My sincere apologies to those of you who are doing better than that.  

Paul Szymkiewicz
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re: The Problem with HRVs

Wednesday, April 10, 2019 10:00:40 PM

Ha! What happened to opening windows?  OK, OK, I get it, not practical in every climate or every season.

Agree with Ted on the need to divorce ventilation from your air handler (should we start calling it HAC system, not HVAC?), unless there is a clever way to use the ductwork while avoiding the AH.

Yes, those 400, 500, or even 700+cfm kitchen range hoods are monsters.  I have one in my house (400cfm on highest setting) and use it for two purposes: short-lived cooking odor evacuation and once-a-day or as needed general house ventilation. Weather permitting (meaning absence of extremes), I open a window in the opposite end of the house and crank up the range hood to 400cfm for maybe 10-15 minutes.  That's 4 small bedrooms worth of fresh air volume.  I am happy.

My ERV is still doing it's ASHRAE 62.2 homework (with ever changing task description) pushing maybe 80cfm for 30min per hour.  But nothing beats that distinctly fresh breath of air after a range hood exercise.  I am just a fresh air junkie.  Call the HVAC cops.

I guess the above anecdote just illustrates the importance of considering user behavior when designing systems for houses.  Not all of us behave the same way inside their homes.  I am sure there are even stranger practices taking place out there.  To me, and this is just my biased opinion, one of the strangest behaviors in many US homes is not opening windows.  I've observed neighborhoods I've lived in so far, and without doing a thorough sampling analysis, maybe about 5% or fewer homes can be observed with open windows even when the weather is perfect for it.  And so far, very, very few of those homes are likely to have an E/HRV installed - all the neighborhoods I've lived in had homes built in early 2000's or before.  How do these people survive?!  I couldn't, I need fresh air.

So Ted, I hope that in your call for simplicity by "re-condition the air" you mean to open the windows, wait a while, then close, and crank up the AC?

Are you saying that my ERV's 0.78 kWh daily energy consumption (65W for 12 hours) is more than say a 2kW heat pump running for how long daily?

Still, your frustration with the immense difficulty of re-educating the public is well appreciated.  I remember a client we did a remodel for who had a PhD in a STEM field.  I proposed a small upgrade to his early 1990's bath fan to replace with a DC motor static-compensating fan.  He was very stead-fast in his rejection which included a statement that his HVAC system did the job of ventilating, and that he would never open the windows because the air outside was so much worse (suburban central NC neighborhood).  The beliefs and behaviors we learned in our formative years firmly stay with us and are almost impossible to replace.

 

Danny Gough
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re: The Problem with HRVs

Thursday, April 11, 2019 7:28:09 AM

As I read the article, the thought came to me as to why precise balanced flow is essential for the proper operation of the H/ERV. I understand negative pressure can cause backdrafting. But most new homes in my climate have sealed combustion.

Positive pressure WRT outdoors is not an issue (in my hood with forgiving building assemblies and actually has some advantages.

The performance of imbalanced flows can be calculated and those results can go directly into the load calculation. No muss, no fuss. So I don't understand loosing sleep over a wee bit of imbalance.

Secondly, if one cannot design and commission ventilation ducts for desired flow, then don't expect them to design the ducts for heating and cooling with any success. 

In re, "balanced ventilation under constantly varying conditions (varying AHU fan speeds,...." , this practice needs to be banned. Varying indoor air flow destroys the performance of terminals. This represents a complete disconnect between the box designers and system designers. It is becoming apparent that box designers know little if anything about how their equipment should be used in a system. Some day we will recognize this was a really bad idea. Thankfully, the recently published Low Load Homes Manual (LLH) from ACCA begins this discussion.

Additionally, why re-invent the wheel. https://www.renewaire.com/wp-content/uploads/2019/01/Submittal_BR70_Jan2019.pdf

One huge potential problem with H/ERV's that very few people are discussing is the elevated risk of microbial growth from increased Aw in the core. One of my colleagues from Madison WI says she has NEVER tested an H/ERV that didn't have active mold growing inside.

I thought we were systems thinkers.

 

 

 

Danny Gough
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re: The Problem with HRVs

Thursday, April 11, 2019 7:28:09 AM

As I read the article, the thought came to me as to why precise balanced flow is essential for the proper operation of the H/ERV. I understand negative pressure can cause backdrafting. But most new homes in my climate have sealed combustion.

Positive pressure WRT outdoors is not an issue (in my hood with forgiving building assemblies and actually has some advantages.

The performance of imbalanced flows can be calculated and those results can go directly into the load calculation. No muss, no fuss. So I don't understand loosing sleep over a wee bit of imbalance.

Secondly, if one cannot design and commission ventilation ducts for desired flow, then don't expect them to design the ducts for heating and cooling with any success. 

In re, "balanced ventilation under constantly varying conditions (varying AHU fan speeds,...." , this practice needs to be banned. Varying indoor air flow destroys the performance of terminals. This represents a complete disconnect between the box designers and system designers. It is becoming apparent that box designers know little if anything about how their equipment should be used in a system. Some day we will recognize this was a really bad idea. Thankfully, the recently published Low Load Homes Manual (LLH) from ACCA begins this discussion.

Additionally, why re-invent the wheel. https://www.renewaire.com/wp-content/uploads/2019/01/Submittal_BR70_Jan2019.pdf

One huge potential problem with H/ERV's that very few people are discussing is the elevated risk of microbial growth from increased Aw in the core. One of my colleagues from Madison WI says she has NEVER tested an H/ERV that didn't have active mold growing inside.

I thought we were systems thinkers.

 

 

 

Jim Ingman
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re: The Problem with HRVs

Thursday, April 11, 2019 9:07:36 PM

Thank you for sharing!  I enjoyed hearing your insight as we are a builder that installs HRV's as a standard practice.  Being in a Northern climate, we often struggle with the use of the product in cold weather conditions.  Not saying that the HRV does not bring a lot of value to the home and structure but when it can not be operational when needed due to outside weather conditions, the value and productivity of the unit decreases to the homeowner.  If I was looking into improving the product, I would look at finding a solution to the fan speeds to insure the unit can stay balanced.  Finding a way to reduce duct work would be great.  Lastly, finding an ability to keep the unit operational in cold conditions would ensure the unit is valuable.  I have often wondered if the duct work could be integrated with a portion of the main trunk line acting like a coil to warm the air enough to ensure the HRV can keep operating.  Just a outside the box thought.  

Thank you for the insight!

Lynette
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re: The Problem with HRVs

Friday, June 28, 2019 9:43:33 AM

Hello,  I’m a homeowner in NW Montana.  I have a  small stucco home with lathe and plaster interior walls.   I installed a central heating system that uses propane.  I also have a crawls space that I had encapsulated.  The crawl space is ventilated into the basement, not the outside.  I have a moisture issue in the winter.  The moisture used to collect on the windows - in the back of the house I could wipe enough off the windows to soak a full size bath towel.  If not wiped it would eventually run right down the walls in little streams.  We solved that issue by putting plastic over the interior part of the windows in the winter.  However, now any interior wall that doesn’t get adequate ventilation will become too moist (condensation) and begin to mildew in the winter.  The area behind a bookcase was ridiculous.  I’ve been trying to solve this issue without having to rip out walls and reinsulate.  Is there any hope?  Would an HRV solve this issue?  I do not have an AC unit, but I have considered installing one if that matters. 

 

Lynette
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re: The Problem with HRVs

Friday, June 28, 2019 9:45:37 AM

SK
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re: The Problem with HRVs

Monday, July 1, 2019 5:54:05 PM

We had an HRV installed three years ago.  Setup is in the conditioned air space of the finished basement furnace room.    Just recently noticed condensation and mold growth inside the flex ducts where they attach to the HRV box.  So the microbial growth mentioned by Danny Gough above, doesn't just happen in the core of the unit.   No wonder my mold allergies are worse since having this thing installed.    I'm about to disassemble the HRV, cap the hard ducts where the flex duct was connected, and throw the whole thing out.  Opening and closing  a window is easier, and far cheaper than the HRV has cost us.  

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