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Musings of an Energy Nerd

How Balanced Ventilation Systems Become Exhaust-Only

Joe Nagan sends out another one of his humorous e-mails

Joe Nagan poses near the fresh air intake and exhaust termination for an HRV system. It looks like the two ports are separated by the necessary horizontal distance, and are high enough above grade so they won't be blocked by snow. So far, this inspection is going well.
Image Credit: All photos: Joe Nagan
View Gallery 5 images
Joe Nagan poses near the fresh air intake and exhaust termination for an HRV system. It looks like the two ports are separated by the necessary horizontal distance, and are high enough above grade so they won't be blocked by snow. So far, this inspection is going well.
Image Credit: All photos: Joe Nagan
What's that in the fresh air intake? Oh, just a wasps' nest. Just like Mike Mulligan's steam shovel, your neglected HRV is feeling very sad.
Image Credit: Image #3: Virginia Lee Burton
This photo shows Nagan providing emergency medical care for an ailing HRV. Nagan has lots of photos of clogged air intake screens.

Unlike the homes of our great-grandparents, the homes of most Americans are served by an array of automatic appliances and systems.

When our great-grandparents returned home after a three-day absence, they would need to haul a bucket of water from the spring and light a fire in the kitchen stove before they could brew tea. Today’s homes, of course, have electricity for lighting, a furnace for warmth, an air conditioner for cooling, a water heater for showers, and internet access for Googling.

If one of our modern services is interrupted, residents usually notice within a few hours. Teenagers are often the first to sense that something has conked out. “Dad, there’s no hot water,” they shout, or “Mom! The internet is down!” When the furnace shuts down in January or the air conditioner breaks in July, it doesn’t take long for someone to figure out that something is wrong.

We love our comforts, so most of our appliances are coddled and cared for when they get sick. Yet no one notices the poor HRV in the basement, even when it’s broken. (Remember the children’s classic, Mike Mulligan and His Steam Shovel? Someone should write a children’s story titled Amy, the Aging HRV. The main character would surely be sad, neglected, and lonely.)

The last time I discussed neglected HRVs was in a 2012 blog called Broken Ventilation Equipment Goes Unnoticed for Years. My article was inspired by an e-mail from Joe Nagan, an energy consultant from Wisconsin who is known for his sense of humor. Like other consultants who make site visits, Nagan has a good collection of stories from the field. This week, his latest HRV tale landed in my In box.

An inspector’s eyes

The setting for Nagan’s story is a handsome older house in Door Country, Wisconsin. Although Nagan was summoned to provide advice about a roof leak, he couldn’t resist walking around the house and checking things out. (If you inspect a lot of houses, you can’t turn off your inspector’s eyes.)

“It’s my nature to take a stroll around the house — any house,” Nagan wrote in his recent e-mail. “You never know what you may find. I somehow am attracted to ventilation stuff. Not sure why, but I take a shine to it. So I’m walking around this home, and I stumble across what look like two hoods from a ventilation system. (I’m not sure, because I haven’t been in the house yet.) Anyway, it looks from the ground where I’m standing that there’s something in one of the hoods.”

The mysterious obstruction was blocking the fresh air intake of a heat-recovery ventilator (HRV). “I usually find stuff in the intake hoods, as apparently pests tend to enjoy fresh air more than people living in the homes do.”

In this case, the obstruction was a very large wasps’ nest (see Image #2, below). In a followup e-mail, Nagan wrote, “Insects enjoy the weather protection provided by the hood, but most of all they enjoy the fresh air. If they were to hang around the exhaust vent, they would have to smell all the crap people have in their home. They’re smarter than we are. They like fresh air. I have yet to find any people stuck in the fresh air ducting. They’re all in the house with the plug-in air-fresheners and the junk.”

Have you checked your intake lately?

Nagan copied a few people on his e-mail; one of the recipients was Mac Pearce, the well-known microbiologist and mold expert from St. Paul, Minnesota. Responding to Nagan’s anecdote, Pearce supplied one of his own.

Pearce wrote, “My cousin Bob was proudly showing off his new (two-year-old) palace. As we walked around the outside, I asked if he had an HRV. After a moment of confusion he replied that he thought so. I decided to take an opportunity for a teaching moment. As we strolled to the rear of the house I described the HRV intake and the clumps of plant fiber that often clog it. When we got there, though, it was clean as a whistle — and he lives in the woods!”

Some consultants are irked when their predictions prove to be wrong, but Pearce took this setback in stride. He was sure that close inspection of the HRV would reveal a dirty filter. “We took a trip to the basement,” Pearce continued. “The Venmar hung in the utility room. Bob told me he didn’t know what it was or what it did and that he’d never touched the thing. I popped the door to show him the clogged filter. Again, the filter was spotless — like new!”

Hmm — what’s going on here? “Then I looked to the left and saw the dangling power cord,” Pearce wrote. “The thing had never been operated. That’s one way to maintain your equipment in mint condition, eh?”

Well, it started out as a balanced system

Nagan sees HRV problems all the time. “For years now, you’ve heard me whine about no one taking care of whole-house ‘balanced’ ventilation systems,” Nagan wrote in his e-mail. “After a while they morph into exhaust-only systems due to lack of awareness and ‘nobody cares’ type of no-maintenance. The intake gets restricted or plugged, while the exhaust side is unrestricted and is sucking on the building.”

According to Nagan, air intake grilles often get plugged by “debris in the air getting caught on the screen or mesh. What can also happen is that the hardware cloth or screen gets damp after the outside air temperature drops towards evening, primarily in the summer and fall. Everything on earth then gets a bit damp at night. If there’s any air being drawn in, the particles start accumulating on the screen, just like dirt particles on an electrostatic air cleaner.”

While Nagan points his finger at ignorant homeowners — “it is my firm belief that homeowners know ‘nothing-about-nothing,’ and then some” — I don’t think it’s reasonable to expect homeowners to pay attention to HRV maintenance. Since this problem is pervasive, the construction industry needs to come up with a technical solution to the problem; blaming the homeowner won’t solve anything.

Are the cobbler’s children ill-shod?

I’m sure that a few ventilation experts are reading this story, thinking, “Why can’t homeowners clean the intake screens of their HRVs?”

If you are such an expert, I have a a few questions for you. First, when was the last time you attached a garden hose to your water heater and flushed out the scale that accumulates at the bottom of the tank? (You know that it’s a good idea to do that once a year, don’t you?) Second, when was the last time that you disassembled your clothes dryer’s exhaust duct to clean out all the lint? What? You haven’t done that recently?

Full employment

Nagan may be cynical, but his sense of humor is intact. Here’s how he described his emotions after discovering the wasps’ nest: “As I stood there, as legally excited as I could get, I could hear Kate Smith singing away to the tune of ‘God Bless America.’ ”

After all, Nagan is an energy consultant. He concluded, “We won’t likely run out of work for quite a while.”

Martin Holladay’s previous blog: “Universal Design.”

Click here to follow Martin Holladay on Twitter.


  1. Dan Kolbert | | #1

    You enjoyed that far too much, Martin.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Dan Kolbert
    I do enjoy my job. Friends like Joe Nagan sometimes make it easy.

  3. Lucy Foxworth | | #3

    I thought the wasp's nest was
    I thought the wasp's nest was a bad toupee.

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Lucy Foxworth
    Among the many objects that have gotten caught in the air intake screen of a ventilation system -- including dog hair, grass clippings, autumn leaves, maple seeds, and road dust -- I'm sure that at some point, at some house, somewhere, a missing toupee was part of the fibrous mix.

  5. User avater
    Stephen Sheehy | | #5

    lost profit opportunity
    Why wouldn't the HTC installer regularly send an e-mail to the customer, reminding him/her that it was time to do routine maintenance? Lots of people either can't or won't change filters or clean intakes.

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to Stephen Sheehy
    Your suggestion (regular e-mail reminders sent by the HRV installer to the homeowner, informing the homeowner that the time has come for a maintenance task) is worth considering. It certainly can't hurt, and would probably nudge some homeowners to check their air intake screens.

    Other suggestions I have heard include:

    -- Providing each homeowner with an operating manual for their house (a three-ring binder including all of the owner's manuals for all of the home's appliances).

    -- Including a wall-mounted interface (designed to be installed in the living room) with LED lights that come on when an HRV maintenance task needs to be attended to.

    -- Installing air monitoring equipment that sounds an alarm when indoor air quality is sub-standard.

    All of these suggestions have a few advantages and many drawbacks. Concerning your suggestion of an e-mail reminder -- here are a few problems:

    -- Everybody's e-mail In box is already full of junk and commercial messages, so most homeowners ignore these e-mails.

    -- Even if a homeowner reads the e-mail, he will probably ignore the instructions. "Change your filter now!" doesn't sound particularly urgent if you are busy.

    -- Homeowers change their e-mail addresses all the time without notifying their contractors.

    -- Homes are sold; the new homeowner will never get that e-mail.

    -- Homeowners get divorced; if the husband was supposed to get the e-mail, he may be in the Bahamas with his new squeeze in two years.

  7. User avater
    Stephen Sheehy | | #7

    reply to martin
    Having the installer communicate with the homeowner, by e-mail, snail mail, telephone or whatever, provides a chance for the installer to provide a useful service and get paid for it.

  8. But Why? | | #8

    A simple air flow monitor
    A simple air flow monitor connected to a warning light (that had the hvac companies name and number on it) would work in most cases. At least until some snot nose kid picks the stick off or somebody paints ove the whole thing

  9. Eric Kessler | | #9

    How can we ... by Design, mitigate the risks of clogs
    After I built my last home, it did not take birds 2 years to find my kitchen stove exhaust vent as a favorate place to have a nest. that first year we let them have it because my daughter and I heard little baby birds... but in the fall I fixed that problem.

    So from a design and installation perspective, I can see placing my intake and exhaust ports close enough to reach so I don't have to climb a ladder 20' up... but I image some really smart inventor already have some super duper cover or "something" to install as you install my intake and exhause vents for my ERV/HRV on the side of the house??? Any ideas or solutions to mitigate some of these bug and debris from clogging my ERV intake?

    That said, I'm not saying to skip the regular PM that I enjoy; I am that guy by nature with a hose permanently attached to my water heater for my regular quick flushing. So I look forward to enjoying some PM on my ERV once I get the new house finished :).

  10. User avater
    Albert Rooks | | #10

    Exterior grills
    So far this Zehnder exterior grill is working fine on our installation in our building. The louver and screen don't have any bugs in them yet ... so no honey for morning coffee. Maybe next year.

  11. Kevin Dickson, MSME | | #11

    Twinfresh and Panasonic
    I'm not convinced there is a "necessary distance" between intake and outlet. Panasonic puts both through the same 6" hole, and it looks to me like it will work well:

    Here's the hailproof stainless grille from my Twinfresh Comfo

  12. User avater GBA Editor
    Martin Holladay | | #12

    Response to Kevin Dickson
    Thanks for posting the photos of your Twinfresh Comfo.

    You wrote, "I'm not convinced there is a 'necessary distance' between intake and outlet."

    You're right that some HRV and ERV manufacturers have come up with a way to use a single penetration for both the fresh air intake duct and the exhaust duct. The traditional method, however, requires two penetrations -- and many manufacturers specify a minimum distance between these two penetrations. Building codes require that equipment be installed in accordance with manufacturers' instructions -- so that when such minimum distances are specified, they should be adhered to.

  13. Robert Connor | | #13

    Why not place the intake and
    Why not place the intake and outlet on Opposite sides of a house? I always wonder why that is not done with high efficiency furnaces. Why does every house with one have those funny looking plastic pipes at the side? It would be a lot less noticeable if one pipe was on one side and the other on the opposite side. Is that allowed?

  14. User avater GBA Editor
    Martin Holladay | | #14

    Response to Robert Connor
    Q. "Why not place the intake and outlet on opposite sides of a house?"

    A. You can do that. Some builders do it that way.

    The main reason that your suggested approach isn't more common is that most homeowners prefer this type of penetration to be located on a side of the house that is not visible from the street (in other words, they want the penetrations to be on the least-visited and least-seen side of the house). In such a case, the penetrations usually end up on the same side of the house.

  15. User avater
    John Semmelhack | | #15

    Intake + Exhaust on opposite sides...
    Another reason not to put ERV/HRV intake/exhaust on opposite sides of the house is for performance. Opposite side of the house location would typically require some serious lengths of ductwork. The ductwork connecting the ERV to the outside is carrying air that is much closer to the outside temperature than the inside temperature, so you have unwanted heat transfer along the entire length of those ducts. For every 10' of duct connected to the outside, you are reducing your ERV "system" performance by 2-3% (ballpark)'s a little like deflating the tires in your Pruis to 20Psi...

  16. Jonathan Beers | | #16

    Response to Robert Connor
    Robert asked: "Why not place the intake and outlet on Opposite sides of a house? I always wonder why that is not done with high efficiency furnaces."

    Jon's reply: The intake and exhaust should be next to each other so that wind pressures are the same on both of them. Otherwise the safety switch could be fooled into sensing a blocked vent and would shut off the furnace. I mentioned this and some other common furnace venting mistakes in this 1994 article in Home Energy Magazine:

    1. User avater
      Nils Bird | | #17

      What about an earth tube/s intake ten feet below the slab and 80 ft. away and an outlet via the ridge vent or similar opening at the highest point on the house? The stack effect might be strong enough so that opening or closing vents would suffice to ventilate. Opening the vent on your high efficiency wood burning appliance might help ventilate as well.

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